Eco-engineering urban infrastructure for marine and coastal biodiversity: Which interventions have the greatest ecological benefit?

Scale-dependent topographic complexity underpins abundance and spatial distribution of ecosystem engineers on natural and artificial structures

In response to ongoing coastal urbanization, it is critical to develop effective methods to improve the biodiversity and ecological sustainability of artificial shorelines. Enhancing the topographic complexity of coastal infrastructure through the mimicry of natural substrata may facilitate the establishment of ecosystem engineering species and associated biogenic habitat formation. However, interactions between ecosystem engineers and their substratum are likely determined by organismal size and resource needs, thus making responses to topography highly scale-dependent. Here, we assessed the topographic properties (rugosity, surface area, micro-surface orientations) that underpin the abundance and distribution of two ecosystem engineers (fucoids, limpets) across six spatial scales (1-500 mm). Furthermore, we assessed the 'biogenic' rugosity created by barnacle matrices across fine scales (1-20 mm). Field surveys and 3D scanning, conducted across natural and artificial substrata, showed major effects of rugosity and associated topographic variables on ecosystem engineer assemblages and spatial occupancy, while additional abiotic environmental factors (compass direction, wave exposure) and biotic associations only had weak influences. Natural substrata exhibited ≤67 % higher rugosity than artificial ones. Fucoid-covered patches were predominantly associated with high-rugosity substrata and horizontal micro-surfaces, while homescars of limpets (≥15 mm shell length) predominated on smoother substratum patches. Barnacle-driven rugosity homogenized substrata at scales ≤10 mm. Our findings suggest that scale-dependent rugosity is a key driver of fucoid habitat formation and limpet habitat use, with wider eco-engineering applications for mimicking ecologically impactful topography on coastal infrastructure.

Influence of habitat features on the colonisation of native and non-indigenous species

Marine artificial structures provide substrates on which organisms can settle and grow. These structures facilitate establishment and spread of non-indigenous species, in part due to their distinct physical features (substrate material, movement, orientation) compared to natural habitat analogues such as rocky shores, and because following construction, they have abundant resources (space) for species to colonise. Despite the perceived importance of these habitat features, few studies have directly compared distributions of native and non-indigenous species or considered how functional identity and associated environmental preferences drive associations. We undertook a meta-analysis to investigate whether colonisation of native and non-indigenous species varies between artificial structures with features most closely resembling natural habitats (natural substrates, fixed structures, surfaces oriented upwards) and those least resembling natural habitats (artificial materials, floating structures, downfacing or vertical surfaces), or whether functional identity is the primary driver of differences. Analyses were done at global and more local (SE Australia) scales to investigate if patterns held regardless of scale. Our results suggest that functional group (i.e., algae, ascidians. barnacles, bryozoans, polychaetes) rather than species classification (i.e., native or non-indigenous) are the main drivers of differences in communities between different types of artificial structures. Specifically, there were differences in the abundance of ascidians, barnacles, and polychaetes between (1) upfacing and downfacing/vertical surfaces, and (2) floating and fixed substrates. When differences were detected, taxa were most abundant on features least resembling natural habitats. Results varied between global and SE Australian analyses, potentially due to reduced variability across studies in the SE Australian dataset. Thus, the functional group and associated preferences of the highest threat NIS in the area should be considered in design strategies (e.g., ecological engineering) to limit their establishment on newly built infrastructure.

Effects of poly(3-hydroxybutyrate) [P(3HB)] coating on the bacterial communities of artificial structures

The expanding urbanization of coastal areas has led to increased ocean sprawl, which has had both physical and chemical adverse effects on marine and coastal ecosystems. To maintain the health and functionality of these ecosystems, it is imperative to develop effective solutions. One such solution involves the use of biodegradable polymers as bioactive coatings to enhance the bioreceptivity of marine and coastal infrastructures. Our study aimed to explore two main objectives: (1) investigate PHA-degrading bacteria on polymer-coated surfaces and in surrounding seawater, and (2) comparing biofilm colonization between surfaces with and without the polymer coating. We applied poly(3-hydroxybutyrate) [P(3HB)) coatings on concrete surfaces at concentrations of 1% and 6% w/v, with varying numbers of coating cycles (1, 3, and 6). Our findings revealed that the addition of P(3HB) indeed promoted accelerated biofilm growth on the coated surfaces, resulting in an occupied area approximately 50% to 100% larger than that observed in the negative control. This indicates a remarkable enhancement, with the biofilm expanding at a rate roughly 1.5 to 2 times faster than the untreated surfaces. We observed noteworthy distinctions in biofilm growth patterns based on varying concentration and number of coating cycles. Interestingly, treatments with low concentration and high coating cycles exhibited comparable biofilm enhancements to those with high concentrations and low coating cycles. Further investigation into the bacterial communities responsible for the degradation of P(3HB) coatings identified mostly common and widespread strains but found no relation between the concentration and coating cycles. Nevertheless, this microbial degradation process was found to be highly efficient, manifesting noticeable effects within a single month. While these initial findings are promising, it's essential to conduct tests under natural conditions to validate the applicability of this approach. Nonetheless, our study represents a novel and bio-based ecological engineering strategy for enhancing the bioreceptivity of marine and coastal structures.

Complexity-functioning relationships differ across different environmental conditions

Habitat complexity is widely considered an important determinant of biodiversity, and enhancing complexity can play a key role in restoring degraded habitats. However, the effects of habitat complexity on ecosystem functioning - as opposed to biodiversity and community structure - are relatively poorly understood for artificial habitats, which dominate many coastlines. With Greening of Grey Infrastructure (GGI) approaches, or eco-engineering, increasingly being applied around the globe, it is important to understand the effects that modifying habitat complexity has on both biodiversity and ecological functioning in these highly modified habitats. We assessed how manipulating physical (primary substrate) and/or biogenic habitat (bivalves) complexity on intertidal artificial substrata affected filtration rates, net and gross primary productivity (NPP and GPP, respectively) and community respiration (CR) - as well as abundance of filter feeders and macro-algae and habitat use by cryptobenthic fish across six locations in three continents. We manipulated both physical and biogenic complexity using 1) flat or ridged (2.5 cm or 5 cm) settlement tiles that were either 2) unseeded or seeded with oysters or mussels. Across all locations, increasing physical and biogenic complexity (5 cm seeded tiles) had a significant effect on most ecological functioning variables, increasing overall filtration rates and community respiration of the assemblages on tiles but decreasing productivity (both GPP and NPP) across all locations. There were no overall effects of increasing either type of habitat complexity on cryptobenthic fish MaxN, total time in frame or macro-algal cover. Within each location, there were marked differences in the effects of habitat complexity. In Hobart, we found higher filtration, filter feeder biomass and community respiration on 5 cm tiles compared to flat tiles. However, at this location, both macro-algae cover and GPP decreased with increasing physical complexity. Similarly in Dublin, filtration, filter feeder biomass and community respiration were higher on 5 cm tiles compared to less complex tiles. In Sydney, filtration and filter feeder biomass were higher on seeded than unseeded tiles, and fish MaxN was higher on 5 cm tiles compared to flat tiles. On unseeded tiles in Sydney, filter feeder biomass also increased with increasing physical complexity. Our findings suggest that GGI solutions via increased habitat complexity are likely to have trade-offs among potentially desired functions, such as productivity and filtration rates, and variable effects on cryptobenthic fish communities. Importantly, our results show that the effects of GGI practices can vary markedly according to the environmental context and therefore should not be blindly and uniformly applied across the globe.

Surface integrity could limit the potential of concrete as a bio-enhanced material in the marine environment

Coastal sprawl is among the main drivers of global degradation of shallow marine ecosystems. Among artificial substrates, quarry rock can have faster recruitment of benthic organisms compared to traditional concrete, which is more versatile for construction. However, the factors driving these differences are poorly understood. In this context, this study was designed to compare the intertidal and subtidal benthic and epibenthic assemblages on concrete and artificial basalt boulders in six locations of Madeira Island (northeastern Atlantic, Portugal). To assess the size of the habitat, the shorelines in the study area were quantified using satellite images, resulting in >34 % of the south coast of Madeira being artificial. Benthic assemblages differed primarily between locations and secondarily substrates. Generally, assemblages differed between substrates in the subtidal, with lower biomass and abundance in concrete than basalt. We conclude that these differences are not related to chemical effects (e.g., heavy metals) but instead to a higher detachment rate of calcareous biocrusts from concrete, as surface abrasion is faster in concrete than basalt. Consequently, surface integrity emerges as a factor of ecological significance in coastal constructions. This study advances knowledge on the impact and ecology of artificial shorelines, providing a baseline for future research towards ecological criteria for coastal protection and management.

Variable effects of substrate colour and microtexture on sessile marine taxa in Australian estuaries

Concrete infrastructure in coastal waters is increasing. While adding complex habitat and manipulating concrete mixtures to enhance biodiversity have been studied, field investigations of sub-millimetre-scale complexity and substrate colour are lacking. Here, the interacting effects of 'colour' (white, grey, black) and 'microtexture' (smooth, 0.5 mm texture) on colonisation were assessed at three sites in Australia. In Townsville, no effects of colour or microtexture were observed. In Sydney, spirorbid polychaetes occupied more space on smooth than textured tiles, but there was no effect of microtexture on serpulid polychaetes, bryozoans and algae. In Melbourne, barnacles were more abundant on black than white tiles, while serpulid polychaetes showed opposite patterns and ascidians did not vary with treatments. These results suggest that microtexture and colour can facilitate colonisation of some taxa. The context-dependency of the results shows that inclusion of these factors into marine infrastructure designs needs to be carefully considered.

A new sampling device for metabarcoding surveillance of port communities and detection of non-indigenous species

Metabarcoding techniques are revolutionizing studies of marine biodiversity. They can be used for monitoring non-indigenous species (NIS) in ports and harbors. However, they are often biased by inconsistent sampling methods and incomplete reference databases. Logistic constraints in ports prompt the development of simple, easy-to-deploy samplers. We tested a new device called polyamide mesh for ports organismal monitoring (POMPOM) with a high surface-to-volume ratio. POMPOMS were deployed inside a fishing and recreational port in the Mediterranean alongside conventional settlement plates. We also compiled a curated database with cytochrome oxidase (COI) sequences of Mediterranean NIS. COI metabarcoding of the communities settled in the POMPOMs captured a similar biodiversity than settlement plates, with shared molecular operational units (MOTUs) representing ca. 99% of reads. 38 NIS were detected in the port accounting for ca. 26% of reads. POMPOMs were easy to deploy and handle and provide an efficient method for NIS surveillance.

Habitat structure shapes temperate reef assemblages across regional environmental gradients

Intertidal artificial habitats are proliferating, but are generally simpler in structure and host lower biodiversity than natural rocky reefs. Eco-engineering aims to enhance the biodiversity of coastal infrastructure, often through physical structural modifications that mimic topographic properties of natural shores. Relationships between biotic assemblages and structural properties of natural and artificial reefs have been extensively studied at sampling scales of up to 1 m2. But evidence that quantified local structural variation has an appreciable influence on biotic assemblages, at a shore-wide scale across regional environmental gradients, is lacking. Here we addressed this knowledge gap with an observational study at 32 natural and artificial intertidal reef sites in Wales, UK. We used multivariate community analysis and permutation tests to examine associations between local physical structure, regional environmental variables and sessile biotic assemblages. A potential influence of local habitat structure on assemblage composition was evident across regional-scale environmental gradients. Compared to natural sites, artificial reefs had lower taxonomic richness, distinct and more variable assemblage composition, and different physical structure. After removing the effect of habitat (natural or artificial), canonical correspondence analysis showed that environmental variables (wave exposure, sea surface temperature and salinity variation), along with two metrics of physical structure (standard deviation in log-transformed detrended roughness and skewness of surface verticality, both at 0.5 m scale), explained 40 % of the variation in assemblage composition among sites. The two structural metrics independently explained 14.5 % of the variation. Associations identified between individual taxa and environmental variables indicated that sites with a higher proportion of horizontal surfaces hosted more canopy macroalgae, which in turn support other algae and invertebrates. Our findings provide evidence to inform scaling-up of structural eco-engineering interventions from experimental contexts to enhance the biodiversity of coastal infrastructure across regional extents.

The influence of environmental context on community composition in artificial rockpools associated with seawalls

Artificial structures have become widespread features of coastal marine environments, and will likely proliferate further over the coming decades. These constitute new hard substrata in the marine environment which provide a fundamentally different habitat than natural shores. Eco-engineering solutions aim to ameliorate these differences by combining ecological knowledge and engineering criteria in the construction and modification of artificial substrata. Vertipools™ are artificial bolt-on rockpools intended for deployment on seawalls, where they have been shown to provide biodiversity benefits. In this study, a total of 32 Vertipools were retrofitted on eight seawalls in different environmental contexts (estuarine vs marine and urban vs rural) along the Irish Sea coastline, and were exposed to the environment for a period of two years. After two years, there were no differences in species richness, species-abundance distributions, diversity, or community composition between the specific environmental contexts examined here. Site-level variation was significant, and communities on Vertipools deployed in marine contexts were more variable in general than those in estuarine contexts. Community composition differed significantly between structural sections of the Vertipools, indicating that different sections provide specific microhabitats for colonisation. This study indicates that Vertipools provide biodiversity benefits in a variety of environmental contexts, and therefore are broadly viable as an eco-engineering solution.

The impact of habitat complexity on the structure of marine sessile communities and larvae supply

Coastal infrastructure replaces complex and heterogeneous natural habitats with flat, two-dimensional concrete walls, reducing refuges against predation, which modifies the composition and identity of the dominant species in sessile communities. This modification in the community structure can also change the reproductive propagules available in plankton, affecting the recruitment dynamics in communities from natural habitats nearby. Here, we tested the combined effects of the habitat type (simple vs. complex with holes) and predation on the diversity, larval production, and structure of sessile communities from a recreational marina. Complex substrates showed a larger biomass and a greater abundance of solitary organisms, mainly ascidians and bivalves, that benefited from refuges. Barnacles and calcified encrusting bryozoans dominated simple, flat substrates. The difference in dominance affected the pool of larvae produced by the communities. After eight months, communities growing on flat substrates produced more barnacle larvae than those from complex substrates, where larvae of ascidians were more abundant. However, this difference disappeared after 18 months of community development. The difference in the pool of larvae between simple and complex substrates did not affect the structure of the community on flat substrates nearby, which was determined by the predation regime. In the studied region, communities in artificial environments are under intense predation control, suppressing eventual recruitment differences in communities developing in flat substrates. Large interventions that modify habitat topography, creating refuges in the subtidal zone, can change the dynamic of the sessile communities in artificial habitats and, consequently, the larval supply in the vicinities. However, differences in larval supply will only translate in distinct sessile communities when the scale of intervention encompasses large areas, and other processes do not buffer the differences in recruitment.

Metrics matter: Multiple diversity metrics at different spatial scales are needed to understand species diversity in urban environments

Worldwide, natural habitats are being replaced by artificial structures due to urbanisation. Planning of such modifications should strive for environmental net gain that benefits biodiversity and ecosystems. Alpha (α) and gamma (γ) diversity are often used to assess 'impact' but are insensitive metrics. We test several diversity measures across two spatial scales to compare species diversity in natural and artificial habitats. We show γ-diversity indicates equivalency in biodiversity between natural and artificial habitats, but natural habitats support greater taxon (α) and functional richness. Within-site β-diversity was also greater in natural habitats, but among-site β-diversity was greater in artificial habitats, contradicting the commonly held view that urban ecosystems are more biologically homogenous than natural ecosystems. This study suggests artificial habitats may in fact provide novel habitat for biodiversity, challenges the applicability of the urban homogenisation concept and highlights a significant limitation of using just α-diversity (i.e., multiple metrics are needed and recommended) for assessing environmental net gain and attaining biodiversity conservation goals.

Artificial fish nurseries can restore certain nursery characteristics in marine urban habitats

Port areas are subjected to multiple anthropic pressures that directly impact residing marine communities and deprive them of most of their essential ecological functions. Several global projects aim to rehabilitate certain ecosystem functions in port areas, such as a fish nursery function, by installing artificial fish nurseries (AFN). In theory, AFNs increase fish biodiversity and juvenile fish abundance in port areas, but studies on this subject remain scarce. Thus, the present study aimed to examine whether the use of such AFNs could restore part of the nursery function of natural habitats by increasing fish and juvenile abundance, and by decreasing predation intensity compared to bare docks. Two years of monitoring on AFNs showed they hosted 2.1 times more fish than on control docks and up to 2.4 more fish juveniles. Fish community structures were influenced by both treatment (AFN and Control) and year of monitoring. In general, AFNs hosted a greater taxonomic diversity of fish than controls. The predation intensity around these structures was significantly lower in the AFNs than in controls. Part of the definition of a fish nursery was thus verified, indicating that AFNs might be an effective restoration tool. However, we also noted that total fish abundance and Young of the Year (YOY) abundance decreased in controls, possibly due to a concentration effect. Further detailed monitoring is necessary to distinguish between these effects.

Population structure and reproductive states of the dogwhelk Nucella lapillus differ between artificial structures and natural rocky shores

Artificial structures are an increasingly common feature of coastal marine environments. These structures are poor surrogates of natural rocky shores, and generally support less diverse communities and reduced population sizes. Little is known about sub-lethal effects of such structures in terms of demographic properties and reproductive potential, both of which may influence the dynamics and long-term viability of populations. This study examines the population structure, reproductive states and embryo production of Nucella lapillus populations on artificial structures and natural shores in Ireland and Wales. Population density was measured twice at six natural shores and six artificial structures: once in winter and once in spring. At each sampling, the shell height of 100 individuals from each site was measured. Monthly collections of adult specimens and egg capsules were made at each site from November-January and from March-May, in order to determine sex ratios, reproductive states, and embryo abundances. Artificial structures supported larger individuals and very few juveniles compared to natural shores. Between December and January, natural shores experienced a distinctive pulse in spawning activity followed by a decline in the proportion of females in a reproductive state, whereas on artificial structures the proportion of reproductive females remained relatively stable. Differences observed may be due to a lack of microhabitats on artificial structures, along with subtle variations in structure slope. Eco-engineering interventions, including the addition of refugia such as cracks and crevices, may allow N. lapillus populations on artificial structures to approximate those on natural shores.

Intertidal biodiversity and physical habitat complexity on historic masonry walls: A comparison with modern concrete infrastructure and natural rocky cliffs

Maritime built heritage (e.g., historic seawalls) represents an important component of coastal infrastructure around the world. Despite this, the ecological communities supported by these structures are poorly understood. At seven locations across the UK, we compared the biodiversity and physical habitat characteristics of (1) historic (pre-1900s) masonry walls, (2) concrete walls, and (3) natural rocky cliffs. Historic masonry walls were found to support significantly more species than concrete walls, and in some locations, more diverse communities than nearby rocky cliffs. Nevertheless, community composition remained distinct between the three habitat types at each location. We also found that historic masonry walls provided substantially more cryptic space (i.e., crevices) than both concrete walls and rocky cliffs, and this is positively associated with the ecological value of these structures. Overall, our results suggest that the unique physical properties of historic masonry walls make them an important component of habitat diversity along developed coastlines.

The elephant in the room: Introduced species also profit from refuge creation by artificial fish habitats

Increasingly, ecological rehabilitation is envisioned to mitigate and revert impacts of ocean sprawl on coastal marine biodiversity. While in the past studies have demonstrated the positive effects of artificial fish habitats in port areas on fish abundance and diversity, benthic colonization of these structures has not yet been taken into consideration. This could be problematic as they may provide suitable habitat for Non-Indigenous Species (NIS) and hence facilitate their spreading. The present study aimed to examine communities developing on artificial fish habitats and to observe if the number of NIS was higher than in surrounding equivalent habitats. The structures were colonized by communities that were significantly different compared to those surrounding the control habitat, and they were home to a greater number of NIS. As NIS can cause severe ecological and economical damages, our results imply that in conjunction with the ecosystem services provided by artificial fish habitats, an ecosystem disservice in the form of facilitated NIS colonization may be present. These effects have not been shown before and need to be considered to effectively decide in which situations artificial structures may be used for fish rehabilitation.

The population structure, sex ratio and reproductive potential of limpets (Patella spp.) on natural shores and artificial structures in the Irish Sea

Artificial structures often support depauperate communities compared to natural rocky shores. Understanding variation in ecological success across shore types, particularly regarding habitat-forming species or those with structuring roles, is important to determine how artificial structure proliferation may influence ecosystem functioning and services. We investigated the population structure, sex ratio and reproductive potential of limpets on natural shores and artificial structures on Irish Sea coasts. Limpets were generally less abundant and Patella vulgata populations were often male dominated on artificial structures compared to natural shores, suggesting that shore type may influence these factors. P. vulgata length varied across sites within the Irish Sea (nested in coast and shore type) in autumn/winter, as well as temporally across sites along the Welsh coast. There was no difference in the proportion of P. vulgata in advanced stages of gonad development across shore types. The results suggest that rip-rap artificial structures may provide a habitat comparable to natural shores, however, the addition of ecological engineering interventions on artificial structures may allow limpet populations to better approximate those on natural shores.

Facilitation of non-indigenous ascidian by marine eco-engineering interventions at an urban site

Marine artificial structures often support lower native species diversity and more non-indigenous species (NIS), but adding complex habitat and using bioreceptive materials have the potential to mitigate these impacts. Here, the interacting effects of structural complexity (flat, complex with pits) and concrete mixture (standard, or with oyster shell or vermiculite aggregate) on recruitment were assessed at two intertidal levels at an urban site. Complex tiles had less green algal cover, oyster shell mixtures had less brown (Ralfsia sp.) algal cover. At a low tidal elevation, the non-indigenous ascidian Styela plicata dominated complex tiles. Additionally, mixtures with oyster shell supported higher total cover of sessile species, and a higher cover of S. plicata. There were no effects of complexity or mixture on biofilm communities and native and NIS richness. Overall, these results suggest that habitat complexity and some bioreceptive materials may facilitate colonisation by a dominant invertebrate invader on artificial structures.

Engineering coastal structures to centrally embrace biodiversity

Global environmental factors (e.g., extreme weather, climate action failure, natural disasters, human environmental damage) increasingly threaten coastal communities. Shorelines are often hardened (seawalls, bulkheads) to prevent flooding and erosion and protect coastal communities. However, hardened shorelines lead to environmental degradation and biodiversity loss. Developmental pressures that are growing in scale, scope, and complexity necessitate the development of sustainable solutions to work with, rather than against, nature. Such nature-based solutions (NBS) provide protection and improve environmental quality and enhance biodiversity. To further this pressing need into action, the US Army Corps of Engineers (USACE) began the Engineering With Nature (EWN) initiative to balance economic, environmental, and social benefits through collaboration with partners and stakeholders. This work shows how engineering practice can be advanced through structured decision-making and landscape architecture renderings that include ecological sciences and NBS into an integrated approach for enhancing biodiversity in coastal marine environments. This integrated approach can be applied when designing new infrastructure projects or modifying or repairing existing infrastructure. To help communicate designs incorporating NBS, drawings, and renderings showcasing EWN concepts can aid decision-making. Our experiences with implementing EWN in practice have revealed that involving landscape architects can play a crucial role in successful collaboration and lead to solutions that protect coastal communities while preserving or enhancing biodiversity.

Adding functions to marine infrastructure: Pollutant accumulation, physiological and microbiome changes in sponges attached to floating pontoons inside marinas

The rate of introduction of man-made habitats in coastal environments is growing at an unprecedented pace, as a consequence of the expansion of urban areas. Floating installations, due to their unique hydrodynamic features, are able to provide great opportunities for enhancing water detoxification through the use of sessile, filtering organisms. We assessed whether the application of sponges to floating pontoons could function as a tool for biomonitoring organic and inorganic pollutants and for improving water quality inside a moderately contaminated marina in the NW Mediterranean. Fragments of two common Mediterranean sponges (Petrosia (Petrosia) ficiformis and Ircinia oros) were fixed to either suspended natural fibre nets beneath a floating pontoon or to metal frames deployed on the sea bottom. We assessed the accumulation of organic and inorganic contaminants in sponge fragments and, in order to provide an insight into their health status, we examined changes in their metabolic and oxidative stress responses and associated microbiomes. Fragments of both sponge species filtered out pollutants from seawater on both support types, but generally showed a better physiological and metabolic status when fixed to nets underneath the pontoon than to bottom frames. P. (P) ficiformis maintained a more efficient metabolism and exhibited a lower physiological stress levels and higher stability of the associated microbiome in comparison with I. oros. Our study suggests that the application of sponges to floating pontoon represents a promising nature-based solution to improve the ecological value of urban environments.

Complexity-biodiversity relationships on marine urban structures: reintroducing habitat heterogeneity through eco-engineering

Urbanization is leading to biodiversity loss through habitat homogenization. The smooth, featureless surfaces of many marine urban structures support ecological communities, often of lower biodiversity, distinct from the complex natural habitats they replace. Eco-engineering (design for ecological co-benefits) seeks to enhance biodiversity and ecological functions on urban structures. We assessed the benefits to biodiversity of retrofitting four types of complex habitat panels to an intertidal seawall at patch (versus flat control panels) and site (versus unmodified control seawalls and reference rocky shores) scales. Two years after installation, patch-scale effects of complex panels on biodiversity ranged from neutral to positive, depending on the protective features they provided, though all but one design (honeycomb) supported unique species. Water-retaining features (rockpools) and crevices, which provided moisture retention and cooling, increased biodiversity and supported algae and invertebrates otherwise absent. At the site scale, biodiversity benefits ranged from neutral at the high- and mid-intertidal to positive at the low-intertidal elevation. The results highlight the importance of matching eco-engineering interventions to the niche of target species, and environmental conditions. While species richness was greatest on rockpool and crevice panels, the unique species supported by other panel designs highlights that to maximize biodiversity, habitat heterogeneity is essential.

This article is part of the theme issue ‘Ecological complexity and the biosphere: the next 30 years’.

Ecological complexity and the biosphere: the next 30 years

Global warming, habitat loss and overexploitation of limited resources are leading to alarming biodiversity declines. Ecosystems are complex adaptive systems that display multiple alternative states and can shift from one to another in abrupt ways. Some of these tipping points have been identified and predicted by mathematical and computational models. Moreover, multiple scales are involved and potential mitigation or intervention scenarios are tied to particular levels of complexity, from cells to human–environment coupled systems. In dealing with a biosphere where humans are part of a complex, endangered ecological network, novel theoretical and engineering approaches need to be considered. At the centre of most research efforts is biodiversity, which is essential to maintain community resilience and ecosystem services. What can be done to mitigate, counterbalance or prevent tipping points? Using a 30-year window, we explore recent approaches to sense, preserve and restore ecosystem resilience as well as a number of proposed interventions (from afforestation to bioengineering) directed to mitigate or reverse ecosystem collapse. The year 2050 is taken as a representative future horizon that combines a time scale where deep ecological changes will occur and proposed solutions might be effective.

This article is part of the theme issue ‘Ecological complexity and the biosphere: the next 30 years’.

Conceptualisation of multiple impacts interacting in the marine environment using marine infrastructure as an example

The human population is increasingly reliant on the marine environment for food, trade, tourism, transport, communication and other vital ecosystem services. These services require extensive marine infrastructure, all of which have direct or indirect ecological impacts on marine environments. The rise in global marine infrastructure has led to light, noise and chemical pollution, as well as facilitation of biological invasions. As a result, marine systems and associated species are under increased pressure from habitat loss and degradation, formation of ecological traps and increased mortality, all of which can lead to reduced resilience and consequently increased invasive species establishment. Whereas the cumulative bearings of collective human impacts on marine populations have previously been demonstrated, the multiple impacts associated with marine infrastructure have not been well explored. Here, building on ecological literature, we explore the impacts that are associated with marine infrastructure, conceptualising the notion of correlative, interactive and cumulative effects of anthropogenic activities on the marine environment. By reviewing the range of mitigation approaches that are currently available, we consider the role that eco-engineering, marine spatial planning and agent-based modelling plays in complementing the design and placement of marine structures to incorporate the existing connectivity pathways, ecological principles and complexity of the environment. Because the effect of human-induced, rapid environmental change is predicted to increase in response to the growth of the human population, this study demonstrates that the development and implementation of legislative framework, innovative technologies and nature-informed solutions are vital, preventative measures to mitigate the multiple impacts associated with marine infrastructure.

Greening the city: Thriving for biodiversity and sustainability

Urban population and urbanisation are increasing rapidly, mainly in developing countries, usually at the expense of green and blue areas. This trend will decrease the ecosystems' capacity to supply ecosystem services (ES) and threaten human wellbeing. Therefore, it is key to establish greening policies in urbanising areas, which are essential to improve the liveability of cities. Restoring and developing green and blue infrastructures using nature-based solutions is vital to improving urban biodiversity and urban ecosystems. Healthy urban ecosystems have a high capacity to supply regulating (e.g., air, noise, climate and water regulation), provisioning (e.g., food, medicinal plants, biomass) and cultural (e.g., recreation, landscape aesthetics, social cohesion) ES. This multifunctionality can provide diverse environmental, social and economic benefits to urban residents, hence contributing to the sustainability of urban areas. However, urban green and blue areas are also associated with ecosystem disservices (e.g., plant allergies or poisoning, emission of biogenic volatile organic compounds, unpleasant smells), tradeoffs (e.g., increased water consumption, wildfire risk, associated management costs) and implementation barriers (e.g., political motivation, lack of knowledge, time and workload). Overall, the SI published 8 articles from different parts of the world, such as China, the USA, Italy or Spain, focused on important aspects of greening the city (e.g., green roofs, green walls, green infrastructures, sustainable mobility).

Material type influences the abundance but not richness of colonising organisms on marine structures

Urbanisation of coastal areas and growth in the blue economy drive the proliferation of artificial structures in marine environments. These structures support distinct ecological communities compared to natural hard substrates, potentially reflecting differences in the materials from which they are constructed. We undertook a meta-analysis of 46 studies to compare the effects of different material types (natural or eco-friendly vs. artificial) on the colonising biota on built structures. Neither the abundance nor richness of colonists displayed consistent patterns of difference between artificial and natural substrates or between eco-friendly and standard concrete. Instead, there were differences in the abundance of organisms (but not richness) between artificial and natural materials, that varied according to material type and by functional group. When compared to biogenic materials and rock, polymer and metal supported significantly lower abundances of total benthic species (in studies assessing sessile and mobile species together), sessile invertebrates and corals (in studies assessing these groups individually). In contrast, non-indigenous species were significantly more abundant on wood than metal. Concrete supported greater abundances of the general community, including habitat-forming species, compared to wood. Our results suggest that the ecological requirements of the biological community, alongside economic, logistic and engineering factors should be considered in material selection for multifunctional marine structures that deliver both engineering and ecological (enhanced abundance and diversity) benefits.

Climate-Friendly Seafood: The Potential for Emissions Reduction and Carbon Capture in Marine Aquaculture

Aquaculture is a critical food source for the world's growing population, producing 52% of the aquatic animal products consumed. Marine aquaculture (mariculture) generates 37.5% of this production and 97% of the world's seaweed harvest. Mariculture products may offer a climate-friendly, high-protein food source, because they often have lower greenhouse gas (GHG) emission footprints than do the equivalent products farmed on land. However, sustainable intensification of low-emissions mariculture is key to maintaining a low GHG footprint as production scales up to meet future demand. We examine the major GHG sources and carbon sinks associated with fed finfish, macroalgae and bivalve mariculture, and the factors influencing variability across sectors. We highlight knowledge gaps and provide recommendations for GHG emissions reductions and carbon storage, including accounting for interactions between mariculture operations and surrounding marine ecosystems. By linking the provision of maricultured products to GHG abatement opportunities, we can advance climate-friendly practices that generate sustainable environmental, social, and economic outcomes.

Temperature-buffering by oyster habitat provides temporal stability for rocky shore communities

Intertidal rocky shores are considered among the most thermally stressful marine ecosystems, where many species live close to their upper thermal limit and depend on access to cool microclimates to persist through heat events. In such environments, the provision of cool microclimates by habitat-forming species enables persistence of associated species during high temperature events. We assessed whether, by maintaining cool microclimates through heat events, habitat formed by rock oysters (Saccostrea cucullata) provides temporal stability to associated invertebrate communities over periods of extreme temperatures. On three tropical rocky shores of Hong Kong, which experiences a monsoonal climate, we compared changes in microclimates and invertebrate communities associated with oyster and bare rock habitats between the cool and hot seasons. Oyster habitats were, across both seasons, consistently characterised by lower maximum temperatures and greater thermal stability than bare rock habitats. Invertebrate communities in the bare rock habitat were less diverse and abundant in the hot than the cool season, but communities in the cooler habitats provided by oysters did not display temporal change. These results demonstrate that microclimates formed by oysters provide temporal stability to associated communities across periods of temperature change and are key determinants of species distributions in thermally stressful environments. The conservation and restoration of oyster habitats may, therefore, build resilience in associated ecological communities subject to ongoing environmental change.

The heterogeneous effects of urbanisation and institutional quality on greenhouse gas emissions in Belt and Road Initiative countries

The rise of urbanisation in Belt and Road Initiative (BRI) countries that contribute to the disruption of the ecosystem, which would affect global sustainability, is a pressing concern. This study provides new evidence of the impact of urbanisation and institutional quality on greenhouse gas (GHG) emissions in the selected 48 BRI countries from the years 1984 to 2017. The models of this study are inferred by using panel regression model and panel quantile regression model to meet the objectives of our study as it contemplates unobserved country heterogeneity. From the panel regression model, the findings indicate that although urbanisation in BRI supports the 'life effect' hypothesis that could dampen the environment quality, this effect could be reduced through better institutional quality. Using the quantile regression method, this study concludes that one-size-fits-all strategies to reduce GHG emissions in countries with different GHG emissions levels are improbable to achieve success for all. Hence, GHG emissions control procedures should be adjusted differently across high-emission, middle-emission and low-emission countries. Based on these results, this study provides novel intuitions for policymakers to wisely plan the urbanisation blueprints to eradicate unplanned urbanisation and improve institutional quality in meeting pollution mitigation goals.

Design, production, and validation of the biological and structural performance of an ecologically engineered concrete block mattress: A Nature-Inclusive Design for shoreline and offshore construction

Over the past decade, the scientific community has studied, experimented, and published a notable body of literature on the ecological enhancement of coastal and marine infrastructure (CMI). The Nature-Inclusive Design (NID) approach refers to methods and technologies that can be integrated into the design and construction of CMI to create a suitable habitat for native species (or communities) whose natural habitat has been degraded or reduced. To examine the compliance of new environmentally sensitive technologies with structural requirements and fiscal restraints, while providing ecosystem and habitat value, this paper presents the findings of a structural-economical-biological analysis of ecologically engineered Articulated Concrete Block Mattresses (ACBMs). To evaluate the structural and biological performance of the Ecological Articulated Concrete Block Mattresses, a pilot project was deployed in April 2017 at Port Everglades, Florida, USA, and evaluated against controls of adjacent artificial structures and smooth-surface concrete blocks and monitored over a period of two years. The elements of ecological enhancement implemented in the fabrication and design of the ecologically enhanced ACBMs were comprised of bio-enhancing concrete additives and science-based designs. Based on the results of this study, these design alterations have increased the richness and diversity of sessile assemblages compared to control blocks and adjacent artificial structures and supported a higher abundance of mobile species. This ecological improvement was achieved within the operational limitations of conventional manufacturing and installation technologies, while complying with strict structural requirements for standard concrete marine construction. The results supported the working hypothesis and demonstrated that modifications of concrete composition, surface texture, and macro-design have the potential to increase the ecological value of concrete-based CMI and promote a more sustainable and adaptive approach to coastal and marine development in an era of climate resilience-building. Integr Environ Assess Manag 2022;18:148-162. © 2021 SETAC.

Integrating Engineering With Nature® strategies and landscape architecture techniques into the Sabine-to-Galveston Coastal Storm Risk Management Project

Damaging storm events frequently impact the Texas coast. In response, the US Army Corps of Engineers Galveston District (SWG) has undertaken the Sabine-to-Galveston (S2G) Coastal Storm Risk Management (CSRM) Project. This approximately $3.9B project includes numerous measures across several counties of the upper Texas coast, including levees, floodwalls, and pump stations. In June 2019, SWG leadership enlisted a team including the paper authors to integrate Engineering With Nature (EWN) strategies into this infrastructure project. EWN strategies intentionally align natural and engineering processes to efficiently and sustainably deliver economic, environmental, and social benefits through collaboration. The first step in this process was to develop potentially relevant EWN strategies. A collaborative workshop included visits to project sites and working sessions where the project team reviewed challenges associated with each site, generated an array of EWN strategies, and began to test design concepts based on those strategies through collaborative drawing sessions. Afterward, prioritized ideas were refined and evaluated in terms of property acquisition, estimated cost, logistics, stakeholder and sponsor interest, constructability, aesthetics, recreational opportunities, and ecological benefit. Design concepts considered feasible for integration into the broader S2G project included horizontal levees, inland floodwater storage areas that double as wildlife habitat, and strategic placement of sediment berms to reduce storm impacts and provide marsh substrate. All these concepts should achieve intended CSRM outcomes while enhancing environmental and social benefits. This assimilation of EWN strategies and landscape architecture techniques into a large CSRM study illustrates a method for expanding overall project value and producing infrastructure that benefits coastal communities. Integr Environ Assess Manag 2022;18:63-73. © 2021 SETAC.

Greater functional similarity in mobile compared to sessile assemblages colonizing artificial coastal habitats

Among anthropogenic habitats built in the marine environment, floating and non-floating structures can be colonized by distinct assemblages. However, there is little knowledge whether these differences are also reflected in the functional structure. This study compared the functional diversity of sessile and mobile invertebrate assemblages that settle over three months on floating vs. non-floating artificial habitats, in two Chilean ports. Using morphological, trophic, behavioral, and life history traits, we found differences between mobile and sessile assemblages regarding the effect of the type of habitat on the functional diversity. Compared to sessile assemblages, a greater functional similarity was observed for mobile assemblages, which suggests that their dispersal capacity enables them to balance the reduced connectivity between settlement structures. No traits, prevailing or selected in one or the other habitat type, was however clearly identified; a result warranting for further studies focusing on more advanced stages of community development.

Habitat Complexity Affects the Structure but Not the Diversity of Sessile Communities on Tropical Coastal Infrastructure

Increasing human population, urbanisation, and climate change have resulted in the proliferation of hard coastal infrastructure such as seawalls and breakwaters. There is increasing impetus to create multifunctional coastal defence structures with the primary function of protecting people and property in addition to providing habitat for marine organisms through eco-engineering - a nature-based solutions approach. In this study, the independent and synergistic effects of physical complexity and seeding with native oysters in promoting diversity and abundances of sessile organisms were assessed at two locations on Penang Island, Malaysia. Concrete tiles with varying physical and biological complexity (flat, 2.5 cm ridges and crevices, and 5 cm ridges and crevices that were seeded or unseeded with oysters) were deployed and monitored over 12 months. The survival of the seeded oysters was not correlated with physical complexity. The addition of physical and biological complexity interacted to promote distinct community assemblages, but did not consistently increase the richness, diversity, or abundances of sessile organisms through time. These results indicate that complexity, whether physical or biological, is only one of many influences on biodiversity on coastal infrastructure. Eco-engineering interventions that have been reported to be effective in other regions may not work as effectively in others due to the highly dynamic conditions in coastal environment. Thus, it is important that other factors such as the local species pools, environmental setting (e.g., wave action), biological factors (e.g., predators), and anthropogenic stressors (e.g., pollution) should also be considered when designing habitat enhancements. Such factors acting individually or synergistically could potentially affect the outcomes of any planned eco-engineering interventions.

Spatially Variable Effects of Artificially-Created Physical Complexity on Subtidal Benthos

In response to the environmental damage caused by urbanization, Nature-based Solutions (NbS) are being implemented to enhance biodiversity and ecosystem processes with mutual benefits for society and nature. Although the field of NbS is flourishing, experiments in different geographic locations and environmental contexts have produced variable results, with knowledge particularly lacking for the subtidal zone. This study tested the effects of physical complexity on colonizing communities in subtidal habitats in two urban locations: (1) Plymouth, United Kingdom (northeast Atlantic) and (2) Tel Aviv, Israel (eastern Mediterranean) for 15- and 12-months, respectively. At each location, physical complexity was manipulated using experimental tiles that were either flat or had 2.5 or 5.0 cm ridges. In Plymouth, biological complexity was also manipulated through seeding tiles with habitat-forming mussels. The effects of the manipulations on taxon and functional richness, and community composition were assessed at both locations, and in Plymouth the survival and size of seeded mussels and abundance and size of recruited mussels were also assessed. Effects of physical complexity differed between locations. Physical complexity did not influence richness or community composition in Plymouth, while in Tel Aviv, there were effects of complexity on community composition. In Plymouth, effects of biological complexity were found with mussel seeding reducing taxon richness, supporting larger recruited mussels, and influencing community composition. Our results suggest that outcomes of NbS experiments are context-dependent and highlight the risk of extrapolating the findings outside of the context in which they were tested.

Making marinas bivalve friendly for enhanced biodiversity outcomes

Natural coastlines are being replaced by artificial structures (pilings, pontoons, breakwaters), with negative environmental impacts, particularly in marinas. Ropes seeded with mussels (Mytilus galloprovincialis) were added to artificial structures in a marina, using aquaculture techniques, to reduce the colonisation of invasive taxa. After 6-months, droplines beneath pontoons had the highest seeded mussel survival and growth, richness of native and invasive taxa, and proportion of invasive to native taxa, compared with the other interventions. Mussel ropes on the intertidal structures (pilings and breakwaters) supported higher biomass of native taxa, whereas mussel ropes on subtidal structures (pontoons and breakwaters) had reduced biomass of invasive taxa, relative to the unseeded ropes. Droplines had the greater biomass of mussels, while mussel ropes placed under pontoons, and in subtidal gabion baskets limited the biomass but not the diversity of invasive species. Further study is required to determine whether these interventions can be upscaled to improve both the native biodiversity and functioning of marinas.

Artificial refuges for wildlife conservation: what is the state of the science?

Artificial refuges are human-made structures that aim to create safe places for animals to breed, hibernate, or take shelter in lieu of natural refuges. Artificial refuges are used across the globe to mitigate the impacts of a variety of threats on wildlife, such as habitat loss and degradation. However, there is little understanding of the science underpinning artificial refuges, and what comprises best practice for artificial refuge design and implementation for wildlife conservation. We address this gap by undertaking a systematic review of the current state of artificial refuge research for the conservation of wildlife. We identified 224 studies of artificial refuges being implemented in the field to conserve wildlife species. The current literature on artificial refuges is dominated by studies of arboreal species, primarily birds and bats. Threatening processes addressed by artificial refuges were biological resource use (26%), invasive or problematic species (20%), and agriculture (15%), yet few studies examined artificial refuges specifically for threatened (Vulnerable, Endangered, or Critically Endangered) species (7%). Studies often reported the characteristics of artificial refuges (i.e. refuge size, construction materials; 87%) and surrounding vegetation (35%), but fewer studies measured the thermal properties of artificial refuges (18%), predator activity (17%), or food availability (3%). Almost all studies measured occupancy of the artificial refuges by target species (98%), and over half measured breeding activity (54%), whereas fewer included more detailed measures of fitness, such as breeding productivity (34%) or animal body condition (4%). Evaluating the benefits and impacts of artificial refuges requires sound experimental design, but only 39% of studies compared artificial refuges to experimental controls, and only 10% of studies used a before-after-control-impact (BACI) design. As a consequence, few studies of artificial refuges can determine their overall effect on individuals or populations. We outline a series of key steps in the design, implementation, and monitoring of artificial refuges that are required to avoid perverse outcomes and maximise the chances of achieving conservation objectives. This review highlights a clear need for increased rigour in studies of artificial refuges if they are to play an important role in wildlife conservation.

Among-genotype responses of the coral Pocillopora acuta to emersion: implications for the ecological engineering of artificial coastal defences

Stony corals are promising transplant candidates for the ecological engineering of artificial coastal defences such as seawalls as they attract and host numerous other organisms. However, seawalls are exposed to a wide range of environmental stressors associated with periods of emersion during low tide such as desiccation and changes in salinity, temperature, and solar irradiance. All of these variables have known deleterious effects on coral physiology, growth, and fitness. In this study, we performed parallel experiments (in situ and ex situ) to examine among-genotype responses of Pocillopora acuta to emersion by quantifying growth, photophysiological metrics (F_v/F_m, non-photochemical quenching [NPQ], endosymbiont density, and chlorophyll [chl] a concentration) and survival, following different emersion periods. Results showed that coral fragments emersed for longer durations (>2 h) exhibited reduced growth and survival. Endosymbiont density and NPQ, but not F_v/F_m and chl a concentration, varied significantly among genotypes across different durations of emersion. Overall, the ability of P. acuta to tolerate emersion for up to 2 h suggests its potential to serve as a 'starter species' for transplantation efforts on seawalls. Further, careful characterisation and selection of genotypes with a high capacity to withstand emersion can help maximise the efficacy of ecological engineering using coral transplants.

The role of anthropogenic habitats in freshwater mussel conservation

Anthropogenic freshwater habitats may provide undervalued prospects for long-term conservation as part of species conservation planning. This fundamental, but overlooked, issue requires attention considering the pace that humans have been altering natural freshwater ecosystems and the accelerated levels of biodiversity decline in recent decades. We compiled 709 records of freshwater mussels (Bivalvia, Unionida) inhabiting a broad variety of anthropogenic habitat types (from small ponds to large reservoirs and canals) and reviewed their importance as refuges for this faunal group. Most records came from Europe and North America, with a clear dominance of canals and reservoirs. The dataset covered 228 species, including 34 threatened species on the IUCN Red List. We discuss the conservation importance and provide guidance on how these anthropogenic habitats could be managed to provide optimal conservation value to freshwater mussels. This review also shows that some of these habitats may function as ecological traps owing to conflicting management practices or because they act as a sink for some populations. Therefore, anthropogenic habitats should not be seen as a panacea to resolve conservation problems. More information is necessary to better understand the trade-offs between human use and the conservation of freshwater mussels (and other biota) within anthropogenic habitats, given the low number of quantitative studies and the strong biogeographic knowledge bias that persists.

Artificial shorelines lack natural structural complexity across scales

From microbes to humans, habitat structural complexity plays a direct role in the provision of physical living space, and increased complexity supports higher biodiversity and ecosystem functioning across biomes. Coastal development and the construction of artificial shorelines are altering natural landscapes as humans seek socio-economic benefits and protection from coastal storms, flooding and erosion. In this study, we evaluate how much structural complexity is missing on artificial coastal structures compared to natural rocky shorelines, across a range of spatial scales from 1 mm to 10 s of m, using three remote sensing platforms (handheld camera, terrestrial laser scanner and uncrewed aerial vehicles). Natural shorelines were typically more structurally complex than artificial ones and offered greater variation between locations. However, our results varied depending on the type of artificial structure and the scale at which complexity was measured. Seawalls were deficient at all scales (approx. 20–40% less complex than natural shores), whereas rock armour was deficient at the smallest and largest scales (approx. 20–50%). Our findings reinforce concerns that hardening shorelines with artificial structures simplifies coastlines at organism-relevant scales. Furthermore, we offer much-needed insight into how structures might be modified to more closely capture the complexity of natural rocky shores that support biodiversity.

Root Systems Research for Bioinspired Resilient Design: A Concept Framework for Foundation and Coastal Engineering

The continuous increase in population and human migration to urban and coastal areas leads to the expansion of built environments over natural habitats. Current infrastructure suffers from environmental changes and their impact on ecosystem services. Foundations are static anchoring structures dependent on soil compaction, which reduces water infiltration and increases flooding. Coastal infrastructure reduces wave action and landward erosion but alters natural habitat and sediment transport. On the other hand, root systems are multifunctional, resilient, biological structures that offer promising strategies for the design of civil and coastal infrastructure, such as adaptivity, multifunctionality, self-healing, mechanical and chemical soil attachment. Therefore, the biomimetic methodology is employed to abstract root strategies of interest for the design of building foundations and coastal infrastructures that prevent soil erosion, anchor structures, penetrate soils, and provide natural habitat. The strategies are described in a literature review on root biology, then these principles are abstracted from their biological context to show their potential for engineering transfer. After a review of current and developing technologies in both application fields, the abstracted strategies are translated into conceptual designs for foundation and coastal engineering. In addition to presenting the potential of root-inspired designs for both fields, this paper also showcases the main steps of the biomimetic methodology from the study of a biological system to the development of conceptual technical designs. In this way the paper also contributes to the development of a more strategic intersection between biology and engineering and provides a framework for further research and development projects.

Designing (for) Urban Food Webs

Interest is growing in designing resilient and ecologically rich urban environments that provide social and ecological benefits. Regenerative and biocentric designs fostering urban ecological habitats including food webs that provide ecosystem services for people and wildlife increasingly are being sought. However, the intentional design of urban landscapes for food webs remains in an early stage with few precedents and many challenges. In this paper, we explore the potential to design (for) urban food webs through collaborations between designers and ecologists. We start by examining the ecology and management of Jamaica Bay in New York City as a case study of an anthropogenic landscape where ecosystems are degraded and the integrity of extant food webs are intertwined with human agency. A subsequent design competition focusing on ecological design and management of this large-scale landscape for animal habitat and ecosystem services for people illustrates how designers approach this anthropogenic landscape. This case study reveals that both designing urban landscapes for food webs and directly designing and manipulating urban food webs are complicated and challenging to achieve and maintain, but they have the potential to increase ecological health of, and enhance ecosystem services in, urban environments. We identify opportunities to capitalize on species interactions across trophic structures and to introduce managed niches in biologically engineered urban systems. The design competition reveals an opportunity to approach urban landscapes and ecological systems creatively through a proactive design process that includes a carefully crafted collaborative approach to constructing ecologically functioning landscapes that can integrate societal demands. As designers increasingly seek to build, adapt, and manage urban environments effectively, it will be critical to resolve the contradictions and challenges associated with human needs, ecosystem dynamics, and interacting assemblages of species. Ecologists and designers are still discovering and experimenting with designing (for) urban food webs and fostering species interactions within them. We recommend generating prototypes of urban food webs through a learning-by-doing approach in urban development projects. Design and implementation of urban food webs also can lead to research opportunities involving monitoring and experiments that identify and solve challenges of food-web construction while supporting and encouraging ongoing management.

Making seawalls multifunctional: The positive effects of seeded bivalves and habitat structure on species diversity and filtration rates

The marine environment is being increasingly modified by the construction of artificial structures, the impacts of which may be mitigated through eco-engineering. To date, eco-engineering has predominantly aimed to increase biodiversity, but enhancing other ecological functions is arguably of equal importance for artificial structures. Here, we manipulated complexity through habitat structure (flat, and 2.5 cm, 5 cm deep vertical and 5 cm deep horizontal crevices) and seeding with the native oyster (Saccostrea glomerata, unseeded and seeded) on concrete tiles (0.25 m × 0.25 m) affixed to seawalls to investigate whether complexity (both orientation and depth of crevices) influences particle removal rates by suspension feeders and colonisation by different functional groups, and whether there are any ecological trade-offs between these functions. After 12 months, complex seeded tiles generally supported a greater abundance of suspension feeding taxa and had higher particle removal rates than flat tiles or unseeded tiles. The richness and diversity of taxa also increased with complexity. The effect of seeding was, however, generally weaker on tiles with complex habitat structure. However, the orientation of habitat complexity and the depth of the crevices did not influence particle removal rates or colonising taxa. Colonisation by non-native taxa was low compared to total taxa richness. We did not detect negative ecological trade-offs between increased particle removal rates and diversity and abundance of key functional groups. Our results suggest that the addition of complexity to marine artificial structures could potentially be used to enhance both biodiversity and particle removal rates. Consequently, complexity should be incorporated into future eco-engineering projects to provide a range of ecological functions in urbanised estuaries.

Assessment of the Performance of an Artificial Reef Made of Modular Elements through Small Scale Experiments

Artificial reefs have proven to be an optimal and effective solution in stabilizing coastlines around the world. They are submerged structures that imitate the protection service provided by natural reefs accomplishing the functions of dissipating wave energy and protecting beach morphology, but also being an ecological solution. In this paper, 2D small-scale experiments were performed to analyze the hydrodynamic, morphological, and ecological behavior of an artificial reef constructed of modular elements. Two typical beach-dune profiles were constructed in a wave flume over which two locations of an artificial reef were tested. From these tests, transmission coefficients were obtained as well as the beach profile response to the presence of the artificial reef. These results are used to discuss about the hydrodynamic, morphological, and ecological performance of the artificial reef. The proposed artificial reef showed good morphological performance while its hydrodynamic function had limited success. In turn, the ecologic performance was theoretically addressed.

Emerging Solutions to Return Nature to the Urban Ocean

Urban and periurban ocean developments impact 1.5% of the global exclusive economic zones, and the demand for ocean space and resources is increasing. As we strive for a more sustainable future, it is imperative that we better design, manage, and conserve urban ocean spaces for both humans and nature. We identify three key objectives for more sustainable urban oceans: reduction of urban pressures, protection and restoration of ocean ecosystems, and support of critical ecosystem services. We describe an array of emerging evidence-based approaches, including greening grayinfrastructure, restoring habitats, and developing biotechnologies. We then explore new economic instruments and incentives for supporting these new approaches and evaluate their feasibility in delivering these objectives. Several of these tools have the potential to help bring nature back to the urban ocean while also addressing some of the critical needs of urban societies, such as climate adaptation, seafood production, clean water, and recreation, providing both human and environmental benefits in some of our most impacted ocean spaces.

Evolutionary responses of marine organisms to urbanized seascapes

Many of the world's major cities are located in coastal zones, resulting in urban and industrial impacts on adjacent marine ecosystems. These pressures, which include pollutants, sewage, runoff and debris, temperature increases, hardened shorelines/structures, and light and acoustic pollution, have resulted in new evolutionary landscapes for coastal marine organisms. Marine environmental changes influenced by urbanization may create new selective regimes or may influence neutral evolution via impacts on gene flow or partitioning of genetic diversity across seascapes. While some urban selective pressures, such as hardened surfaces, are similar to those experienced by terrestrial species, others, such as oxidative stress, are specific to aquatic environments. Moreover, spatial and temporal scales of evolutionary responses may differ in the ocean due to the spatial extent of selective pressures and greater capacity for dispersal/gene flow. Here, we present a conceptual framework and synthesis of current research on evolutionary responses of marine organisms to urban pressures. We review urban impacts on genetic diversity and gene flow and examine evidence that marine species are adapting, or are predicted to adapt, to urbanization over rapid evolutionary time frames. Our findings indicate that in the majority of studies, urban stressors are correlated with reduced genetic diversity. Genetic structure is often increased in urbanized settings, but artificial structures can also act as stepping stones for some hard-surface specialists, promoting range expansion. Most evidence for rapid adaptation to urban stressors comes from studies of heritable tolerance to pollutants in a relatively small number of species; however, the majority of marine ecotoxicology studies do not test directly for heritability. Finally, we highlight current gaps in our understanding of evolutionary processes in marine urban environments and present a framework for future research to address these gaps.

Loss of coastal ecosystem spatial connectivity and services by urbanization: Natural-to-urban integration for bay management

Urbanization has negative consequences for the integrity of ecosystems and services they provide, by reducing their extent and quality in both aquatic and terrestrial environments. Few studies have explored how urban infrastructure expansion affects the spatial connectivity of coastal ecosystems by provoking their fragmentation and loss. Here we explore changes in the spatial connectivity of coastal ecosystems due to urbanization, analyzing ecosystem extent and concatenation with urban infrastructures (shared perimeter) in four bays of the Coquimbo region of northern Chile (from 29°S to 32°S) as model systems. Increase in natural-to-urban concatenation patterns were observed in most urbanized bays; sandy beaches and wetlands were the habitats most connected with urban infrastructures like roads and coastal artificial defenses. Availability of ecosystem services is compromised by progressive loss of natural connectivity and poor governance structure, which seems to confer high vulnerability to urbanized bays with future urban expansion. Complementary actions are proposed to reduce the vulnerability of coastal urban systems, considering 1) investment in nature-based infrastructures for coastal defenses, 2) restoration-rehabilitation of natural (remnant) urban ecosystems and eco-engineering of current artificial infrastructures, focusing on reestablishment of biodiversity patterns and habitat connectivity, and 3) limitation of coastal town and village expansion. Management strategies can improve coastal adaptation to natural hazards, stabilizing changes in the natural-urban concatenation mosaic present in coastal urban systems like bays.

Provision of refugia and seeding with native bivalves can enhance biodiversity on vertical seawalls

Recent studies have suggested that increasing habitat complexity of artificial seawalls by modifying surface heterogeneity could enhance exploitable habitat and therefore species richness and abundance. We tested the effects of adding complex tiles (with crevices/ledges) of different heterogeneity (i.e., flat tiles resembling the seawall vs. tiles with crevices of 2.5 cm or 5.0 cm depth) and seeding with native rock oysters, Saccostrea cuccullata (unseeded vs. seeded) on species richness and abundances of intertidal marine organisms on two vertical seawalls in Hong Kong. Tiles were affixed to the mid-intertidal zone of the seawalls for 12 months. The results showed that the tiles with crevices had greater species richness and cover of sessile epifauna than flat tiles. Seeding tiles with S. cuccullata also facilitated natural recruitment of the same species. Our results support the hypothesis that using eco-engineering to increase habitat complexity can enhance the biodiversity of intertidal marine organisms on seawalls.

Ecological enhancement of coastal engineering structures: Passive enhancement techniques

The rock type used in coastal engineering structures impacts biodiversity, but its effect has been understudied to date. We report here on whether different combinations of rock material and rock mass properties can improve habitat suitability and early phase ecological outcomes on coastal engineering structures. We examine two coastal engineering schemes that used different granites during construction. At site one, Shap granite boulders with a high number of cm-dm² surface features (e.g. ledges) were deliberately positioned during construction (called passive enhancement), to a) maximise the provision of cm-dm scale intertidal habitat and b) determine which scale of habitat is best for ecological enhancement. At site two, Norwegian granite boulders were installed without passive enhancement, allowing for a direct comparison. Passive positioning of Shap granite boulders led to an increase in limpet (Patella vulgata, Linnaeus, 1758) abundance within two years but few limpets were recorded on the non-enhanced Norwegian granite. Positioning of boulder thus exerts a strong control on the mm and mm-dm scale geomorphic features present, with clear ecological benefits when suitable features are selected for and optimally positioned (i.e. passive enhancement) to maximise habitat features. An EcoRock scoring matrix was developed to aid in the selection of the most ecologically suitable rock materials for coastal engineering worldwide; this can help improve habitat provision on engineered structures in a rapidly warming world.

A geometric basis for surface habitat complexity and biodiversity

Structurally complex habitats tend to contain more species and higher total abundances than simple habitats. This ecological paradigm is grounded in first principles: species richness scales with area, and surface area and niche density increase with three-dimensional complexity. Here we present a geometric basis for surface habitats that unifies ecosystems and spatial scales. The theory is framed by fundamental geometric constraints between three structure descriptors-surface height, rugosity and fractal dimension-and explains 98% of surface variation in a structurally complex test system: coral reefs. Then, we show how coral biodiversity metrics (species richness, total abundance and probability of interspecific encounter) vary over the theoretical structure descriptor plane, demonstrating the value of the theory for predicting the consequences of natural and human modifications of surface structure.

The state factor model and urban forest restoration

A ‘state factor’ model of ecosystems can serve as a conceptual framework for researching and managing urban ecosystems. This approach provides alternative goals and narratives to those derived from historically grounded dichotomies between nature and culture, which can reify constructions of human influence as inherently destructive. The integration of human behaviour and state factors is critical to the application of a state factor model to urban ecosystems. We emphasize the role of culture in co-producing urban ecosystems and the importance of feedbacks between urban ecosystems and state factors. We advocate for ecosystem models that encourage local agency and actions that enhance the capacity of cities to constructively adapt to environmental change. We contrast this approach to efforts intended to minimize human impacts on ecosystems. The usefulness of the state factor model for informing such efforts is assessed through a consideration of the norms and practices of urban forest restoration in New York City. Despite the limitations and challenges of applying a state factor model to urban ecosystems, it can inform comparative research within and between cities and offers an intuitive framework for understanding the ecological conditions created in cities by human behaviour.