Marine subsidies have multiple effects on coastal food webs

A seascape dichotomy in the role of small consumers for coral reef energy fluxes

Biogeochemical fluxes through ecological communities underpin the functioning of ecosystems worldwide. These fluxes are often heavily influenced by small-bodied consumers, such as insects, worms, mollusks, or small vertebrates, which transfer energy and nutrients from autotrophic sources to larger animals. Although coral reefs are one of the most productive ecosystems in the world, we know relatively little about how small consumers make energy available to larger predators and how their roles may vary across reefs. Here, we use community-scale collections of small, bottom-dwelling ("cryptobenthic") reef fishes along with size spectrum analyses, stable isotopes, and demographic modeling to examine their role in harnessing and transferring carbon in two distinct coral reef habitats. Using a comprehensive dataset from Mo'orea (French Polynesia), we demonstrate that, despite only being separated by a narrow reef crest, forereef and backreef habitats harbor distinct communities of cryptobenthic fishes that play vastly divergent roles in carbon transfer. Forereef communities in Mo'orea are depauperate, largely consisting of predatory and planktivorous species that have comparatively high standing biomass (both individually and collectively). In these communities, the combination of size spectra and isotope values suggests important contributions of pelagic subsidies, but the rate of biomass production and turnover (i.e., the rate at which biomass is replenished) is relatively low. In contrast, cryptobenthic fish communities in the backreef are characterized by high abundances of the smallest bodied species, forming a traditional bottom-heavy trophic pyramid that is fueled by benthic autotrophs. In these communities, benthic productivity fuels rapid production and turnover of fish biomass, while pelagic energy channels are notably less productive. Our integrative approach demonstrates the utility of combining multiple methods (e.g., isotopically informed demographic models) to trace energy fluxes through small consumer communities in complex ecosystems. Furthermore, our results highlight that coral reef productivity dynamics are highly habitat-dependent and the role of the smallest coral reef consumers may be most pronounced in shallow systems with limited connectivity to the open ocean.

Beach wrack: Discussing ecological roles, risks, and sustainable bioenergy and agricultural applications

The equilibrium of the marine ecosystem is currently threatened by several constraints, among which climate change and anthropogenic activities stand out. Indeed, these factors favour the growth of macroalgae, which sometimes end up stranded on the beaches at the end of their life cycle, forming what is known as beach wrack. Despite its undeniable important ecological role on beaches, as it is an important source of organic matter (OM), and provides food and habitat for several invertebrates, reptiles, small mammals, and shorebirds, the overaccumulation of beach wrack is often associated with the release of greenhouse gases, negatively impacting tourist activities, and generating economic expenses for its removal. Although currently beach wrack is mainly treated as a waste, it can be used for numerous potential applications in distinct areas. This review aimed at providing a solid point of view regarding the process of wrack formation, its spatiotemporal location, as well as its importance and risks. It also contains the current advances of the research regarding sustainable alternatives to valorise this organic biomass, that range from bioenergy production to the incorporation of wrack in agricultural soils, considering a circular economy concept. Although there are some concerns regarding wrack utilisation, from its variable availability to a possible soil contamination with salts and other contaminants, this review comprises the overall beneficial effects of the incorporation of this residue particularly in the organic agricultural model, strengthening the conversion of this wasted biomass into a valuable resource.

Spatial synchrony cascades across ecosystem boundaries and up food webs via resource subsidies

Cross-ecosystem subsidies are critical to ecosystem structure and function, especially in recipient ecosystems where they are the primary source of organic matter to the food web. Subsidies are indicative of processes connecting ecosystems and can couple ecological dynamics across system boundaries. However, the degree to which such flows can induce cross-ecosystem cascades of spatial synchrony, the tendency for system fluctuations to be correlated across locations, is not well understood. Synchrony has destabilizing effects on ecosystems, adding to the importance of understanding spatiotemporal patterns of synchrony transmission. In order to understand whether and how spatial synchrony cascades across the marine-terrestrial boundary via resource subsidies, we studied the relationship between giant kelp forests on rocky nearshore reefs and sandy beach ecosystems that receive resource subsidies in the form of kelp wrack (detritus). We found that synchrony cascades from rocky reefs to sandy beaches, with spatiotemporal patterns mediated by fluctuations in live kelp biomass, wave action, and beach width. Moreover, wrack deposition synchronized local abundances of shorebirds that move among beaches seeking to forage on wrack-associated invertebrates, demonstrating that synchrony due to subsidies propagates across trophic levels in the recipient ecosystem. Synchronizing resource subsidies likely play an underappreciated role in the spatiotemporal structure, functioning, and stability of ecosystems.

Inter- and intrapopulation resource use variation of marine subsidized western fence lizards

Marine resource subsidies alter consumer dynamics of recipient populations in coastal systems. The response to these subsidies by generalist consumers is often not uniform, creating inter- and intrapopulation diet variation and niche diversification that may be intensified across heterogeneous landscapes. We sampled western fence lizards, Sceloporus occidentalis, from Puget Sound beaches and coastal and inland forest habitats, in addition to the lizards' marine and terrestrial prey items to quantify marine and terrestrial resource use with stable isotope analysis and mixing models. Beach lizards had higher average δ13C and δ15N values compared to coastal and inland forest lizards, exhibiting a strong mixing line between marine and terrestrial prey items. Across five beach sites, lizard populations received 20-51% of their diet from marine resources, on average, with individual lizards ranging between 7 and 86% marine diet. The hillslope of the transition zone between marine and terrestrial environments at beach sites was positively associated with marine-based diets, as the steepest sloped beach sites had the highest percent marine diets. Within-beach variation in transition zone slope was positively correlated with the isotopic niche space of beach lizard populations. These results demonstrate that physiography of transitional landscapes can mediate resource flow between environments, and variable habitat topography promotes niche diversification within lizard populations. Marine resource subsidization of Puget Sound beach S. occidentalis populations may facilitate occupation of the northwesternmost edge of the species range. Shoreline restoration and driftwood beach habitat conservation are important to support the unique ecology of Puget Sound S. occidentalis.

Predator-driven behavioural shifts in a common lizard shape resource-flow from marine to terrestrial ecosystems

Foraging decisions shape the structure of food webs. Therefore, a behavioural shift in a single species can potentially modify resource-flow dynamics of entire ecosystems. To examine this, we conducted a field experiment to assess foraging niche dynamics of semi-arboreal brown anole lizards in the presence/absence of predatory ground-dwelling curly-tailed lizards in a replicated set of island ecosystems. One year after experimental translocation, brown anoles exposed to these predators had drastically increased perch height and reduced consumption of marine-derived food resources. This foraging niche shift altered marine-to-terrestrial resource-flow dynamics and persisted in the diets of the first-generation offspring. Furthermore, female lizards that displayed more risk-taking behaviours consumed more marine prey on islands with predators present. Our results show how predator-driven rapid behavioural shifts can alter food-web connectivity between oceanic and terrestrial ecosystems and underscore the importance of studying behaviour-mediated niche shifts to understand ecosystem functioning in rapidly changing environments.

Loss of sea turtle eggs drives the collapse of an insular reptile community

Marine subsidies are vital for terrestrial ecosystems, especially low-productivity islands. However, the impact of losing these subsidies on the terrestrial food web can be difficult to predict. We analyzed 23 years of survey data from Orchid Island to assess the consequences of the abrupt loss of an important marine subsidy. After climate-driven beach erosion and predator exclusion efforts resulted in the abrupt loss of sea turtle eggs from the terrestrial food web, predatory snakes altered their foraging habitats. This increased predation on other reptile species in inland areas, resulting in population declines in most terrestrial reptile species. Comparisons with sea turtle-free locations where lizard populations remained stable supported these findings. Our study emphasizes the cascading effects of generalist predators and the unintended consequences of single-species conservation, highlighting the importance of understanding species interconnectedness and considering potential ripple effects in marine-dependent insular ecosystems.

The ecological effect of large-scale coastal natural and cultivated seaweed litter decay processes: An overview and perspective

Seaweeds are important components of marine ecosystems and can form a large biomass in a few months. The decomposition of seaweed litter provides energy and material for primary producers and consumers and is an important link between material circulation and energy flow in the ecosystem. However, during the growth process, part of the seaweed is deposited on the sediment surface in the form of litter. Under the joint action of the environment and organisms, elements enriched in seaweed can be released back into the environment in a short time, causing pollution problems. The cultivation yield of seaweed worldwide reached 34.7 million tons in 2019, but the litter produced during the growth and harvest process has become a vital bottleneck that restricts the further improvement of production and sustainable development of the seaweed cultivation industry. Seaweed outbreaks worldwide occur frequently, producing a mass of litter and resulting in environmental pollution on coasts and economic losses, which have negative effects on coastal ecosystems. The objective of this review is to discuss the decomposition process and ecological environmental effects of seaweed litter from the aspects of the research progress on seaweed litter; the impact of seaweed litter on the environment; and its interaction with organisms. Understanding the decomposition process and environmental impact of seaweed litter can provide theoretical support for coastal environmental protection, seaweed resource conservation and sustainable development of the seaweed cultivation industry worldwide. This review suggests that in the process of large-scale seaweed cultivation and seaweed outbreaks, ageing or falling litter should be cleared in a timely manner, mature seaweed should be harvested in stages, and dried seaweed produced after harvest and washed up on shore should be handled properly to ensure the benefits of environmental protection provided by seaweed growth and sustainable seaweed resource development.

Quantifying seaweed and seagrass beach deposits using high-resolution UAV imagery

Macroalgae and seagrass wash ashore by tidal waters and episodic events and create an ocean-to-land transport of carbon and nutrients. On land, these deposits (beach wrack) are consumed by macrofauna, remineralized by microorganisms, or washed back to the sea, during which recycling of carbon and nitrogen affect the biochemical cycles in coastal zones. Manual quantification of beach wracks is time-consuming and often difficult due to complex topography and remote locations. Here, we present a novel method using Unoccupied Aerial Vehicle (UAV) photogrammetry combined with in situ measurements of carbon and nitrogen contents of wrack to quantify marine carbon and nutrient deposits in beach zones. The UAV method was tested against placed cubes ranging from 125 to 88,218 cm³ and demonstrated a high accuracy (R² > 0.99) for volume acquisition when compared to manual measurements. Also, the UAV-based assessments of the cross-sectional area of beach deposits demonstrated a high accuracy when compared to manual and high-precision GNSS (Global Navigation Satellite System) measurements without significant differences between the methods. This demonstrated that UAVs can provide detailed spatial maps, three-dimensional (3D) surface models, and accurate volumetric assessments of beach wrack deposits. In three case studies, combined with carbon and nitrogen measures, total organic carbon and nitrogen deposits in beach wracks were quantified ranging from 4.3 to 9.7 and from 0.3 to 0.5 kg per meter coastline, respectively. In conclusion, this UAV method demonstrated an effective tool to quantify ecosystem carbon and nitrogen deposits relevant to ecosystem assessments and quantification of blue carbon stocks. The method is optimal when the terrain below beach wrack deposits is known, as in the case with before-and-after or repeated surveys. Further, UAVs display strong time- and cost-effective advantages over manual methods which is amplified with increasing project scale. We propose it as a valuable method for multiple scientific and commercial applications related to environmental monitoring and management, including marine resource exploration and exploitation.

The role of inputs of marine wrack and carrion in sandy-beach ecosystems: a global review

Sandy beaches are iconic interfaces that functionally link the ocean with the land via the flow of organic matter from the sea. These cross-ecosystem fluxes often comprise uprooted seagrass and dislodged macroalgae that can form substantial accumulations of detritus, termed 'wrack', on sandy beaches. In addition, the tissue of the carcasses of marine animals that regularly wash up on beaches form a rich food source ('carrion') for a diversity of scavenging animals. Here, we provide a global review of how wrack and carrion provide spatial subsidies that shape the structure and functioning of sandy-beach ecosystems (sandy beaches and adjacent surf zones), which typically have little in situ primary production. We also examine the spatial scaling of the influence of these processes across the broader land- and seascape, and identify key gaps in our knowledge to guide future research directions and priorities. Large quantities of detrital kelp and seagrass can flow into sandy-beach ecosystems, where microbial decomposers and animals process it. The rates of wrack supply and its retention are influenced by the oceanographic processes that transport it, the geomorphology and landscape context of the recipient beaches, and the condition, life history and morphological characteristics of the macrophyte taxa that are the ultimate source of wrack. When retained in beach ecosystems, wrack often creates hotspots of microbial metabolism, secondary productivity, biodiversity, and nutrient remineralization. Nutrients are produced during wrack breakdown, and these can return to coastal waters in surface flows (swash) and aquifers discharging into the subtidal surf. Beach-cast kelp often plays a key trophic role, being an abundant and preferred food source for mobile, semi-aquatic invertebrates that channel imported algal matter to predatory invertebrates, fish, and birds. The role of beach-cast marine carrion is likely to be underestimated, as it can be consumed rapidly by highly mobile scavengers (e.g. foxes, coyotes, raptors, vultures). These consumers become important vectors in transferring marine productivity inland, thereby linking marine and terrestrial ecosystems. Whilst deposits of organic matter on sandy-beach ecosystems underpin a range of ecosystem functions and services, they can be at variance with aesthetic perceptions resulting in widespread activities, such as 'beach cleaning and grooming'. This practice diminishes the energetic base of food webs, intertidal fauna, and biodiversity. Global declines in seagrass beds and kelp forests (linked to global warming) are predicted to cause substantial reductions in the amounts of marine organic matter reaching many beach ecosystems, likely causing flow-on effects for food webs and biodiversity. Similarly, future sea-level rise and increased storm frequency are likely to alter profoundly the physical attributes of beaches, which in turn can change the rates at which beaches retain and process the influxes of wrack and animal carcasses. Conservation of the multi-faceted ecosystem services that sandy beaches provide will increasingly need to encompass a greater societal appreciation and the safeguarding of ecological functions reliant on beach-cast organic matter on innumerable ocean shores worldwide.

The social system of the lava lizard, Microlophus atacamensis: the interplay between social structure and social organization

Background

Animal social systems can be described through four main components: social structure, social organization, mating system, and care system. Social structure describes the relationships between individuals in a population, while social organization describes the group composition, size, and spatiotemporal variation of a population. We use the frameworks of social structure and social organization to study the social system of Microlophus atacamensis, a lizard found in the rocky intertidal zone along the Chilean coast. The area M. atacamensis inhabits poses specific challenges stemming from their use of two distinct habitat types in the intertidal zone: they forage in the cool areas near the water’s edge and use large rocks in more inland areas for basking and refuge.

Methods

Our assessment of their social system focused on two separate populations: one to characterize social structure by means of focal observations and social network analysis, and a second to assess social organization via home range and core area analyses. Further, we examined the social system in two habitat types that comprise the intertidal zone: cobble fields and interspersed large rocks.

Results

Social network analysis revealed an interconnected network with a few highly central individuals. Body size influenced the outcomes of aggressive interactions, with interactions being more common in cobble fields where males had more associates and more repeated interactions than females. Spatial analyses revealed that the social organization of M. atacamensis is characterized by (1) high home range overlap, specifically in the cobble fields and (2) relatively exclusive core areas dispersed across both habitat types.

Conclusion

A social system is composed of both cooperative and competitive behaviors. While our study focused on competitive interactions, the extent and influence of cooperative behaviors is still unclear and merits future research. We suggest that M. atacamensis has a variable social system in which territoriality on large rocks affects access to stationary resources in that habitat (e.g., basking sites and refuges), while competition in the cobble fields could lay the foundation for a system of dominance relationships controlling access to variable food resources in cobble field areas of the intertidal zone.

Grazing preference and isotopic contributions of kelp to Zostera marina mesograzers

In seagrass food webs, small invertebrate mesograzers often exert top-down control on algal epiphytes growing on seagrass blades, which in turn releases the seagrass from competition for light and nutrients. Yet, nearshore habitat boundaries are permeable, and allochthonous subsidies can provide alternative food sources to in-situ production in seagrass meadows, which may in turn alter mesograzer-epiphyte interactions. We examined the contribution of allochthonous kelp (Nereocystis luetkeana), autochthonous epiphytic macroalgal (Smithora naiadum), Ulva lactuca, and seagrass production to mesograzer diets in a subtidal Zostera marina (eelgrass) meadow. In both choice feeding experiments and isotopic analysis, mesograzer diets revealed a preference for allochthonous N. luetkeana over Z. marina, S. naiadum, and U. lactuca. Notably, Idotea resecata showed an ~20x greater consumption rate for N. luetkeana in feeding experiments over other macrophytes. In the meadow, we found a positive relationship between epiphytic S. naiadum and gammarid amphipod biomass suggesting weak top-down control on the S. naiadum biomass. Epiphyte biomass may be driven by bottom-up factors such as environmental conditions, or the availability and preference of allochthonous kelp, though further work is needed to disentangle these interactions. Additionally, we found that gammarid and caprellid amphipod biomass were positively influenced by adjacency to kelp at seagrass meadow edges. Our findings suggest that N. luetkeana kelp subsidies are important to the diets of mesograzers in Z. marina meadows. Spatial planning and management of marine areas should consider trophic linkages between kelp and eelgrass habitats as a critical seascape feature if the goal is to conserve nearshore food web structure and function.

Scale-dependent effects of marine subsidies on the island biogeographic patterns of plants

Although species richness can be determined by different mechanisms at different spatial scales, the role of scale in the effects of marine inputs on island biogeography has not been studied explicitly. Here, we evaluated the potential influence of island characteristics and marine inputs (seaweed wrack biomass and marine-derived nitrogen in the soil) on plant species richness at both a local (plot) and regional (island) scale on 92 islands in British Columbia, Canada. We found that the effects of subsidies on species richness depend strongly on spatial scale. Despite detecting no effects of marine subsidies at the island scale, we found that as plot level subsidies increased, species richness decreased; plots with more marine-derived nitrogen in the soil hosted fewer plant species. We found no effect of seaweed wrack at either scale. To identify potential mechanisms underlying the decrease in diversity, we fit a spatially explicit joint species distribution model to evaluate species level responses to marine subsidies and effects of biotic interactions among species. We found mixed evidence for competition for both light and nutrients, and cannot rule out an alternative mechanism; the observed decrease in species richness may be due to disturbances associated with animal-mediated nutrient deposits, particularly those from North American river otters (Lontra canadensis). By evaluating the scale-dependent effects of marine subsidies on island biogeographic patterns of plants and revealing likely mechanisms that act on community composition, we provide novel insights on the scale dependence of a fundamental ecological theory, and on the rarely examined links between marine and terrestrial ecosystems often bridged by animal vectors.

Wrack enhancement of post-hurricane vegetation and geomorphological recovery in a coastal dune

Coastal ecosystems such as sand dunes, mangrove forests, and salt marshes provide natural storm protection for vulnerable shorelines. At the same time, storms erode and redistribute biological materials among coastal systems via wrack. Yet how such cross-ecosystem subsidies affect post-storm recovery is not well understood. Here, we report an experimental investigation into the effect of storm wrack on eco-geomorphological recovery of a coastal embryo dune in north-eastern Florida, USA, following hurricane Irma. We contrasted replicated 100-m2 wrack-removal and unmanipulated (control) plots, measuring vegetation and geomorphological responses over 21 months. Relative to controls, grass cover was reduced 4-fold where diverse storm wrack, including seagrass rhizomes, seaweed, and wood, was removed. Wrack removal was also associated with a reduction in mean elevation, which persisted until the end of the experiment when removal plots had a 14% lower mean elevation than control plots. These results suggest that subsides of wrack re-distributed from other ecosystem types (e.g. seagrasses, macroalgae, uplands): i) enhances the growth of certain dune-building grasses; and ii) boosts the geomorphological recovery of coastal dunes. Our study also indicates that the practice of post-storm beach cleaning to remove wrack–a practice widespread outside of protected areas–may undermine the resilience of coastal dunes and their services.

A hurricane alters the relationship between mangrove cover and marine subsidies

As global change alters the composition and productivity of ecosystems, the importance of subsidies from one habitat to another may change. We experimentally manipulated black mangrove (Avicennia germinans) cover in ten large plots and over five years (2014-2019) quantifying the effects of mangrove cover on subsidies of floating organic material (wrack) into coastal wetlands. As mangrove cover increased from zero to 100%, wrack cover and thickness decreased by ~60%, the distance that wrack penetrated into the plots decreased by ~70%, and the percentage of the wrack trapped in the first six m of the plot tripled. These patterns observed during four "normal" years disappeared in a fifth year following Hurricane Harvey (2017), when large quantities of wrack were pushed far into the interior of all the plots, regardless of mangrove cover. Prior to the storm, the abundance of animals collected in grab samples increased with wrack biomass. Wrack composition did not affect animal abundance or composition. Experimental outplants of two types of wrack (red algae and seagrass) revealed that animal abundance and species composition varied between the fringe and interior of the plots, and between microhabitats dominated by salt marsh versus mangrove vegetation. The importance of wrack to overall carbon stocks varied as a function of autochthonous productivity: wrack inputs (per m² ) based on survey data were greater than aboveground plant biomass in the plots (42 × 24 m) dominated by salt marsh vegetation, but decreased to 5% of total aboveground biomass in plots dominated by mangroves. Our results illustrate that increasing mangrove cover decreases the relative importance of marine subsidies into the intertidal at the plot level, but concentrates subsidies at the front edge of the mangrove stand. Storms, however, may temporarily override mangrove attenuation of wrack inputs. Our results highlight the importance of understanding how changes in plant species composition due to global change will impact marine subsidies and exchanges among ecosystems, and foster a broader understanding of the functional interdependence of adjacent habitats within coastal ecosystems.

Impacts of Warming on Reciprocal Subsidies Between Aquatic and Terrestrial Ecosystems

Cross-ecosystem subsidies are important as their recipients often rely on them to supplement in situ resource availability. Global warming has the potential to alter the quality and quantity of these subsidies, but our knowledge of these effects is currently limited. Here, we quantified the biomass and diversity of the invertebrates exchanged between freshwater streams and terrestrial grasslands in a natural warming experiment in Iceland. We sampled invertebrates emerging from the streams, those landing on the water surface, ground-dwelling invertebrates falling into the streams, and those drifting through the streams. Emerging invertebrate biomass or diversity did not change with increasing temperature, suggesting no effect of warming on aquatic subsidies to the terrestrial environment over the 1-month duration of the study. The biomass and diversity of aerial invertebrates of terrestrial origin landing on the streams increased with temperature, underpinned by increasing abundance and species richness, indicating that the greater productivity of the warmer streams may attract more foraging insects. The biomass of ground-dwelling invertebrates falling into the streams also increased with temperature, underpinned by increasing body mass and species evenness, suggesting that soil warming leads to terrestrial communities dominated by larger, more mobile organisms, and thus more in-fall to the streams. The biomass and diversity of terrestrial invertebrates in the drift decreased with temperature, however, underpinned by decreasing abundance and species richness, reflecting upstream consumption due to the higher energetic demands of aquatic consumers in warmer environments. These results highlight the potential for asynchronous responses to warming for reciprocal subsidies between aquatic and terrestrial environments and the importance of further research on warming impacts at the interface of these interdependent ecosystems.

Marine subsidy promotes spatial and dietary niche variation in an omnivore, the Keen's mouse (Peromyscus keeni)

Marine-derived resource subsidies can generate intrapopulation variation in the behaviors and diets of terrestrial consumers. How omnivores respond, given their multiple trophic interactions, is not well understood. We sampled mice (Peromyscus keeni) and their food sources at five sites on three islands of the Central Coast of British Columbia, Canada, to test predictions regarding variation in the spatial behavior and consumption of marine-subsidized foods among individuals. About 50% of detections (n = 27 recaptures) occurred at traps closest to shoreline (25 m), with capture frequencies declining significantly inland (up to 200 m). Stable isotope signatures (δ 13C and δ 15N), particularly δ 15N, in plant foods, forest arthropod prey, and mouse feces were significantly enriched near shorelines compared with inland, while δ 13C patterns were more variable. Bayesian isotope mixing models applied to isotope values in mouse hair indicated that over one-third (35-37%) of diet was comprised of beach-dwelling arthropods, a marine-derived food source. Males were more abundant near the shoreline than females and consumed more marine-derived prey, regardless of reproductive status or availability of other food sources. Our results identify how multiple pathways of marine nutrient transfer can subsidize terrestrial omnivores and how subsets of recipient populations can show variation in spatial and dietary response.

Diet of a threatened endemic fox reveals variation in sandy beach resource use on California Channel Islands

The coastal zone provides foraging opportunities for insular populations of terrestrial mammals, allowing for expanded habitat use, increased dietary breadth, and locally higher population densities. We examined the use of sandy beach resources by the threatened island fox (Urocyon littoralis) on the California Channel Islands using scat analysis, surveys of potential prey, beach habitat attributes, and stable isotope analysis. Consumption of beach invertebrates, primarily intertidal talitrid amphipods (Megalorchestia spp.) by island fox varied with abundance of these prey across sites. Distance-based linear modeling revealed that abundance of giant kelp (Macrocystis pyrifera) wrack, rather than beach physical attributes, explained the largest amount of variation in talitrid amphipod abundance and biomass across beaches. δ¹³C and δ¹⁵N values of fox whisker (vibrissae) segments suggested individualism in diet, with generally low δ¹³C and δ¹⁵N values of some foxes consistent with specializing on primarily terrestrial foods, contrasting with the higher isotope values of other individuals that suggested a sustained use of sandy beach resources, the importance of which varied over time. Abundant allochthonous marine resources on beaches, including inputs of giant kelp, may expand habitat use and diet breadth of the island fox, increasing population resilience during declines in terrestrial resources associated with climate variability and long-term climate change.

Allochthonous resources are less important for faunal communities on highly productive, small tropical islands

Ecosystems are interconnected by energy fluxes that provide resources for the inhabiting organisms along the transition zone. Especially where in situ resources are scarce, ecosystems can become highly dependent on external resources. The dependency on external input becomes less pronounced in systems with elevated in situ production, where only consumer species close to the site of external input remain subsidized, whereas species distant to the input site rely on the in situ production of the ecosystem. It is largely unclear though if this pattern is consistent over different consumer species and trophic levels in one ecosystem, and whether consumer species that occur both proximate to and at a distance from the input site differ in their dependency on external resource inputs between sites. Using stable isotope analysis, we investigated the dependency on external marine input for common ground-associated consumer taxa on small tropical islands with high in situ production. We show that marine input is only relevant for strict beach-dwelling taxa, while the terrestrial vegetation is the main carbon source for inland-dwelling taxa. Consumer species that occurred both close (beach) and distant (inland) to the site of marine input showed similar proportions of marine input in their diets. This supports earlier findings that the relevance of external resources becomes limited to species close to the input site in systems with sufficient in situ production. However, it also indicates that the relevance of external input is also species-dependent, as consumers occurring close and distant to the input site depended equally strong or weak on marine input.

Global change in marine coastal habitats impacts insect populations and communities

Salt marsh and mangrove coastal ecosystems provide critical ecosystem services, but are being lost at an alarming rate. Insect communities in these ecosystems are threatened by human impacts, including sea level rise, habitat loss, external inputs including nutrients, metals, and hydrocarbons, as well as weather events, such as hurricanes. While some disturbances are felt throughout the food web (e.g. hurricanes), others are mediated by impacts on the dominant plants (e.g. nutrient subsidies). The impacts of these disturbances on insects/spiders and their rate of recovery is dependent on trophic level, life history, and diet breadth. While we understand impacts of single disturbances relatively well, we have very little understanding of how multiple disturbances interact to affect insect communities.

Species identity drives ecosystem function in a subsidy-dependent coastal ecosystem

Declines in species diversity carry profound implications for ecosystem functioning. Communities of primary producers and consumers interact on evolutionary as well as ecological time scales, shaping complex relationships between biodiversity and ecosystem functioning. In subsidized ecosystems, resource inputs are independent of consumer actions, offering a simplified view of the relationship between species diversity and function for higher trophic levels. With food webs supported by substantial but variable inputs of detritus from adjacent marine ecosystems, sandy beaches are classic examples of subsidized ecosystems. We investigated effects of consumer species diversity and identity on a key ecological function, consumption of kelp wrack from nearshore giant kelp (Macrocystis pyrifera) forests. We assessed effects of species richness on kelp consumption by experimentally manipulating richness of six common species of invertebrate detritivores in laboratory mesocosms and conducting field assays of kelp consumption on beaches. Consumer richness had no effect on kelp consumption in the field and a slight negative effect in laboratory experiments. Kelp consumption was most strongly affected by the species composition of the detritivore community. Species identity and body size of intertidal detritivores drove variation in kelp consumption rates in both experiments and field assays. Our results provide further evidence that species traits, rather than richness per se, influence ecosystem function most, particularly in detrital-based food webs with high functional redundancy across species. On sandy beaches, where biodiversity is threatened by rising sea levels and expanding development, our findings suggest that loss of large-bodied consumer species could disproportionally impact ecosystem function.

Collembola (Hexapoda) as Biological Drivers between Land and Sea

Macroalgae debris accumulated onshore function as points of interaction between marine and terrestrial ecological systems, but knowledge of the importance of detritivores facilitating the introduction of organic matter via the detritus pathway into neighbouring ecosystems, is still poorly understood. In particular, not much is known about biodiversity patterns and the colonisation of macroalgal debris by terrestrial, detritivorous soil microarthropods in the harsh environmental conditions in the subpolar Arctic region. We hypothesised that (i) soil microarthropods of the coastal tundra, including Collembola, can cross the ecosystem boundary and colonise decaying and freshly exposed macroalgae; and (ii) various inundation regimes by sea water, microhabitat stability and decaying of macroalgae drive distribution patterns of collembolan species. Our results suggest that environmental filtering influences collembolan species’ distributions across the examined gradient and induces sorting of species according to their functional traits, including dispersal ability, resistance to disturbance and environmental tolerance.

The effects of resource subsidy duration in a detritus-based stream ecosystem: A mesocosm experiment

Most resource subsidies are temporally variable, dynamically affecting the consumer populations, community structures and ecosystem functions of recipient ecosystems. Temporally variable resource subsidies are characterized by the duration, magnitude, timing and frequency of resource subsidy inputs. These different characteristics may have different mechanisms by which to affect recipient ecosystems. Few studies have examined the duration of resource subsidy inputs on recipient ecosystems, although there exist previous studies focusing on magnitude, timing and frequency. We provide the first experimental test of the effects of subsidy duration on a stream ecosystem by using an outdoor mesocosm experiment, in which we directly manipulated the subsidy duration (pulsed vs. prolonged) of terrestrial invertebrate input into the mesocosm. Given the same overall amount of terrestrial invertebrate subsidy was added, a prolonged subsidy allowed large-stage fish to effectively monopolize the subsidy over small-stage fish, which led small-stage fish to maintain their predation pressure on in-situ prey, that is, benthic invertebrates. On the other hand, a pulsed subsidy allowed small-stage fish to increase their feeding rate of the subsidy and to become away from foraging in-situ prey. Consequently, weaker indirect positive effects on in-situ benthic prey and leaf break-down rate were found with the prolonged versus pulsed subsidy. However, these indirect effects varied by the dominant benthic prey species, which differed in edibility for fish. Such predator-specific vulnerability of benthic prey can be important in mediating trophic cascades in detritus-based stream food webs. Phenological events that generate temporal subsidies (e.g. salmon spawning run and arthropod emergence) can be synchronized (pulsed) or desynchronized (prolonged) within and among species, depending on the degree of spatial and temporal environmental heterogeneity. The effects of subsidy duration would thus be important to better understand ecological processes in spatially and temporally coupled ecosystems.

Rapid Dietary Shift in Podarcis siculus Resulted in Localized Changes in Gut Function

AbstractNatural dietary shifts offer the opportunity to address the nutritional physiological characters required to thrive on a particular diet. Here, we studied the nutritional physiology of Podarcis siculus, with populations on Pod Mrčaru, Croatia, that have become omnivorous and morphologically distinct (including the development of valves in the hindgut) from their insectivorous source population on Pod Kopište. We compared gut structure and function between the two island populations of this lizard species and contrasted them with an insectivorous mainland out-group population in Zagreb. On the basis of the adaptive modulation hypothesis, we predicted changes in gut size and structure, digestive enzyme activities, microbial fermentation products (short-chain fatty acids [SCFAs]), and plant material digestibility concomitant with this dietary change. The Pod Mrčaru population had heavier guts than the mainland population, but there were no other differences in gut structure. Most of the enzymatic differences we detected were between the island populations and the out-group population. The Pod Mrčaru lizards had higher amylase and trehalase activities in their hindguts compared with the Pod Kopište population, and the Pod Kopište lizards had greater SCFA concentrations in their hindguts than the omnivorous Pod Mrčaru population. Interestingly, the differences between the Pod Mrčaru and Pod Kopište populations are primarily localized to the hindgut and are likely influenced by microbial communities and a higher food intake by the Pod Mrčaru lizards. Although subtle, the changes in hindgut digestive physiology impact the digestibility of plant material in adult lizards-Pod Mrčaru lizards had higher digestibility of herbivorous and omnivorous diets fed over several weeks in the laboratory than did their source population.

Seabird droppings: Effects on a global and local level

Seabirds, with approximately 1 billion specimens, are the main exchangers of nutrients between Terrestial and Marine Systems and they have become an emerging interest group because of their effects on the planet's ecosystem. This review paper aims to highlight the impact of seabird droppings at different trophic levels, their occurrence, ecological risks and effects on soil, water, atmosphere and biota at global and local level to try to understand the ecological and climatic changes associated with the activities of these birds. Seabirds they have a very marked influence on the ecosystems where they form their colonies since, in addition to their function as predators, alongside with their depositions, they condition the primary producers and, consequently, the rest of the food chain. Their excrements contain large amounts of N, P and trace elements, most of which are bioavailable. In this study, besides bringing together the different works on nutrients and trace elements in excrements and differentiating some terms referring to these excrements, a brief historical overview of their importance for agriculture is made. In addition, the impacts produced by these birds on the ecosystem are also analysed according to two levels, at a global and local level. At each of these levels, a current state of the effects on the different compartments of the ecosystems is made, from the biota to the soils, the water or the atmosphere. This review supports the idea that more studies are needed both at the atmospheric level and in the terrestrial or marine environment for a better understanding of the changes these birds generate.

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.

Foraging behaviour of four avian species feeding on the same temporarily available prey

Low tide events provide terrestrial predators with ephemeral, but predictable and abundant sources of prey. Understanding the relationships between tidal cycles, prey availability, and predator abundances is vital to characterizing the ecological relationship between terrestrial predators and their marine prey. Here, we describe the foraging tactics of four common bird species in western North America — Bald Eagles (Haliaeetus leucocephalus (Linnaeus, 1766)), Great Blue Herons (Ardea herodias Linnaeus, 1758), Glaucous-winged Gulls (Larus glaucescens J.F. Naumann, 1840), and Northwestern Crows (Corvus caurinus S.F. Baird, 1858) — feeding on the same transiently accessible fish species, the plainfin midshipman (Porichthys notatus Girard, 1854). We conducted avian predator surveys at breeding beaches of plainfin midshipman across multiple years and sites. Our census data showed that Bald Eagle and Great Blue Heron abundances were higher when the tides were receding than incoming at Ladysmith Harbour, British Columbia, Canada, but the opposite trend was found for total predator abundance at a second site in Dabob Bay, Washington, USA. Glaucous-winged Gull abundance decreased over the course of the plainfin midshipman breeding season (April–July), whereas the abundances of the other three species remained stable. Our data suggest that the foraging activities of birds in the intertidal zones of western North America are linked with the tidal cycles, corresponding to periods of high prey vulnerability.

Consumer Responses to Experimental Pulsed Subsidies in Isolated versus Connected Habitats

AbstractIncreases in consumer abundance following a resource pulse can be driven by diet shifts, aggregation, and reproductive responses, with combined responses expected to result in faster response times and larger numerical increases. Previous work in plots on large Bahamian islands has shown that lizards (Anolis sagrei) increased in abundance following pulses of seaweed deposition, which provide additional prey (i.e., seaweed detritivores). Numerical responses were associated with rapid diet shifts and aggregation, followed by increased reproduction. These dynamics are likely different on isolated small islands, where lizards cannot readily immigrate or emigrate. To test this, we manipulated the frequency and magnitude of seaweed resource pulses on whole small islands and in plots within large islands, and we monitored lizard diet and numerical responses over 4 years. We found that seaweed addition caused persistent increases in lizard abundance on small islands regardless of pulse frequency or magnitude. Increased abundance may have occurred because the initial pulse facilitated population establishment, possibly via enhanced overwinter survival. In contrast with a previous experiment, we did not detect numerical responses in plots on large islands, despite lizards consuming more marine resources in subsidized plots. This lack of a numerical response may be due to rapid aggregation followed by disaggregation or to stronger suppression of A. sagrei by their predators on the large islands in this study. Our results highlight the importance of habitat connectivity in governing ecological responses to resource pulses and suggest that disaggregation and changes in survivorship may be underappreciated drivers of pulse-associated dynamics.

Marine subsidies mediate patterns in avian island biogeography

The classical theory of island biogeography, which predicts species richness using island area and isolation, has been expanded to include contributions from marine subsidies, i.e. subsidized island biogeography (SIB) theory. We tested the effects of marine subsidies on species diversity and population density on productive temperate islands, evaluating SIB predictions previously untested at comparable scales and subsidy levels. We found that the diversity of terrestrial breeding bird communities on 91 small islands (approx. 0.0001–3 km²) along the Central Coast of British Columbia, Canada were correlated most strongly with island area, but also with marine subsidies. Species richness increased and population density decreased with island area, but isolation had no measurable influence. Species richness was negatively correlated with marine subsidy, measured as forest-edge soil δ¹⁵N. Density, however, was higher on islands with higher marine subsidy, and a negative interaction between area and subsidy indicates that this effect is stronger on smaller islands, offering some support for SIB. Our study emphasizes how subsidies from the sea can shape diversity patterns on islands and can even exceed the importance of isolation in determining species richness and densities of terrestrial biota.

Tri-trophic interactions: bridging species, communities and ecosystems

A vast body of research demonstrates that many ecological and evolutionary processes can only be understood from a tri-trophic viewpoint, that is, one that moves beyond the pairwise interactions of neighbouring trophic levels to consider the emergent features of interactions among multiple trophic levels. Despite its unifying potential, tri-trophic research has been fragmented, following two distinct paths. One has focused on the population biology and evolutionary ecology of simple food chains of interacting species. The other has focused on bottom-up and top-down controls over the distribution of biomass across trophic levels and other ecosystem-level variables. Here, we propose pathways to bridge these two long-standing perspectives. We argue that an expanded theory of tri-trophic interactions (TTIs) can unify our understanding of biological processes across scales and levels of organisation, ranging from species evolution and pairwise interactions to community structure and ecosystem function. To do so requires addressing how community structure and ecosystem function arise as emergent properties of component TTIs, and, in turn, how species traits and TTIs are shaped by the ecosystem processes and the abiotic environment in which they are embedded. We conclude that novel insights will come from applying tri-trophic theory systematically across all levels of biological organisation.

Predator-induced collapse of niche structure and species coexistence

Biological invasions are both a pressing environmental challenge and an opportunity to investigate fundamental ecological processes, such as the role of top predators in regulating biodiversity and food-web structure. In whole-ecosystem manipulations of small Caribbean islands on which brown anole lizards (Anolis sagrei) were the native top predator, we experimentally staged invasions by competitors (green anoles, Anolis smaragdinus) and/or new top predators (curly-tailed lizards, Leiocephalus carinatus). We show that curly-tailed lizards destabilized the coexistence of competing prey species, contrary to the classic idea of keystone predation. Fear-driven avoidance of predators collapsed the spatial and dietary niche structure that otherwise stabilized coexistence, which intensified interspecific competition within predator-free refuges and contributed to the extinction of green-anole populations on two islands. Moreover, whereas adding either green anoles or curly-tailed lizards lengthened food chains on the islands, adding both species reversed this effect-in part because the apex predators were trophic omnivores. Our results underscore the importance of top-down control in ecological communities, but show that its outcomes depend on prey behaviour, spatial structure, and omnivory. Diversity-enhancing effects of top predators cannot be assumed, and non-consumptive effects of predation risk may be a widespread constraint on species coexistence.

Functional consumers regulate the effect of availability of subsidy on trophic cascades in the Yellow River Delta, China

Understanding the environmental context where heterogeneous ecological processes affect biotic interactions is a key aim of ecological research. However, mechanisms underlying spatial variation in trophic interactions linked to resource availability across ecosystem gradients remains unclear. We experimentally manipulated the interactive effects of predator fish and quantitative gradient of leaf detritus on macroinvertebrates and benthic algae. We found that non-linear changes in the strength of trophic cascades were strongly linked to the retention rates of experimental leaf detritus and also determined by predatory consumers. Retention rate of leaf detritus influenced the recruitment of predatory invertebrates and foraging preference of predators, accounting for largely the variations in shift of strengthening and weakening trophic cascades. Our results highlight the importance to identify joint processes of recruitment and foraging responses of functional consumer in understanding the impacts of both anthropogenic and natural alterations in subsidy on trophic interaction of coastal food webs.

Warming intensify CO2 flux and nutrient release from algal wrack subsidies on sandy beaches

Algal wrack subsidies underpin most of the food web structure of exposed sandy beaches and are responsible of important biogeochemical processes that link marine and terrestrial ecosystems. The response in decomposition of algal wrack deposits to global warming has not been studied in ocean-exposed sandy beaches to date. With this aim, passive open top chambers (OTCs) were used to increase soil temperature within the range predicted by the IPCC for western Europe (between 0.5 and 1.5°C), following the hypothesis that the biogeochemical processing of macroalgal wrack subsidies would accelerate in response to temperature increase. The effect of temperature manipulation on three target substrates: fresh and aged macroalgae, and bare sand, was tested. Results indicated that a small warming (<0.5°C) affected the wrack decomposition process through traceable increases in soil respiration through CO2 flux, inorganic nutrients within the interstitial environment (N and P), sediment organic contents measured through the amount of proteins and microbial pool through the total soil DNA. The different responses of soil variables in the studied substrates indicated that the decomposition stage of stranded macroalgae influences the biogeochemical processing of organic matter in sandy beaches. Thus, CO2 fluxes, releases of organic and inorganic nutrients and microbial activity intensify in aged wrack deposits. Our results predict that expected global warming will increase the release of inorganic nutrients to the coastal ocean by 30% for the N (21 Gg/year) and 5.9% for P (14 Gg/year); that increase for the flow of C to the atmosphere as CO2 was estimated in 8.2% (523 Gg/year). This study confirms the key role of sandy beaches in recycling ocean-derived organic matter, highlighting their sensitivity to a changing scenario of global warming that predicts significant increases in temperature over the next few decades.

Functional plasticity in vertebrate scavenger assemblages in the presence of introduced competitors

Introduced species may suppress or enhance ecological functions, or they may have neutral effects in ecosystems where they replace or complement native species. Few studies, however, have explicitly tested for these trajectories, and for the effect these might have for native species. In this study, we experimentally test the trajectory and scale of change in the function of 'carrion removal' at different carrion loads along ocean beaches in Eastern Australia that have different numbers of introduced red foxes (Vulpes vulpes) and several species of native raptors. We hypothesized that the 'positive' effect of foxes on carrion removal would be greatest at high carrion loads, because competition for resources between native and introduced species is lower. Scavenger abundance, fox occurrences, and carrion consumption by these species differed widely between locations and times. Despite distinct spatial differences in the structure of vertebrate scavenger assemblages, total carrion consumption was not significantly different between locations at any carrion load. This lack of variation in functional rates indicates potential functional plasticity in the scavenger assemblage and possible functional accommodation of red foxes. Neutral fox effects on ecological functions or the ecosystem more broadly are, however, very unlikely to extend beyond carrion consumption.