A trophic cascade regulates salt marsh primary production

Connecting the dots: Managing species interaction networks to mitigate the impacts of global change

Global change is causing unprecedented degradation of the Earth's biological systems and thus undermining human prosperity. Past practices have focused either on monitoring biodiversity decline or mitigating ecosystem services degradation. Missing, but critically needed, are management approaches that monitor and restore species interaction networks, thus bridging existing practices. Our overall aim here is to lay the foundations of a framework for developing network management, defined here as the study, monitoring, and management of species interaction networks. We review theory and empirical evidence demonstrating the importance of species interaction networks for the provisioning of ecosystem services, how human impacts on those networks lead to network rewiring that underlies ecosystem service degradation, and then turn to case studies showing how network management has effectively mitigated such effects or aided in network restoration. We also examine how emerging technologies for data acquisition and analysis are providing new opportunities for monitoring species interactions and discuss the opportunities and challenges of developing effective network management. In summary, we propose that network management provides key mechanistic knowledge on ecosystem degradation that links species- to ecosystem-level responses to global change, and that emerging technological tools offer the opportunity to accelerate its widespread adoption.

Ecological associations of the coastal marsh periwinkle snail Littoraria irrorata: field and laboratory evidence of vegetation habitat preferences

Coastal salt marshes serve as the margin between terrestrial and marine biomes, provide a variety of important services, and are dynamic ecosystems characterized by keystone species that shape trophic networks. In coastal salt marshes of the Atlantic and Gulf Coasts of the United States, marsh periwinkle snails (Littoraria irrorata) exhibit high abundance and form critical trophic pathways as important herbivores and detritivores. Specifically, snails forage on Spartina alterniflora and associated fungal growth, for which L. irrorata may act as a top-down control on plant growth. Yet, L. irrorata occupies other salt marsh plants, suggesting its habitat niche may be broader than previously reported. Here, we documented snail densities and size distributions in a Louisiana (USA) salt marsh composed of multiple marsh graminoids and report the results of behavioral choice experiments designed to test snail habitat preferences as a potential mechanism underlying their field distribution. We observed higher snail densities on S. alterniflora stalks (283 snails m-2) than other plant species, however, snails were highly abundant on S. patens (116 snails m-2), Juncus roemerianus (95 snails m-2), and Distichlis spicata (57 snails m-2) with densities comparable or higher on all species than reported on S. alterniflora in other studies along the U.S. Atlantic and Gulf coasts. Snails found on S. alterniflora and J. roemerianus, both plants with tall and rigid stalks, were also larger than snails found on other plant species. In species preference experiments, snails preferred S. alterniflora over S. patens and D. spicata, but no clear preferences were observed between S. alterniflora and J. roemerianus, nor between any combinations of S. patens, D. spicata, and J. roemerianus. Finally, we found that snails preferred senescing and dead S. alterniflora tissue over fresh S. alterniflora. Interpreting these results in tandem, this study suggests L. irrorata snails have consistent patterns of field distributions that match their habitat preferences, and future studies should test potential processes driving snail habitat selection, such as dietary habits and predator refugia (i.e., climbing sturdy stalks to avoid aquatic predators). Considering the abundance and trophic role of L. irrorata in coastal salt marshes, snail behavior may be a key modulator for salt marsh trophic networks.

Salt marsh macrofauna: An overview of functions and services

Salt marshes are globally important blue carbon ecosystems, providing essential services such as coastal protection, carbon sequestration, nutrient cycling, and biodiversity support. Among their key inhabitants, macrofauna play critical roles in sustaining ecosystem health and resilience through processes like bioturbation, nutrient cycling, organic matter turnover, and trophic interactions, which in turn support ecosystem services such as fisheries and coastal community livelihoods. Despite their contributions, no comprehensive review has yet focused exclusively on the diverse roles and services of salt marsh macrofauna. This review aims to address this gap by synthesizing current research, supported by a bibliometric analysis revealing significant growth in studies since the year 2000, especially those addressing ecosystem services and climate resilience. We provide an in-depth assessment of macrofaunal functions in bioturbation, nutrient cycling, organic matter dynamics, greenhouse gas regulation, primary and secondary production, and food web interactions. Additionally, we examine the ecosystem services provided, such as provisioning, regulating, and cultural services, and explore the impact of environmental stressors on macrofaunal communities. Finally, this review identifies significant knowledge gaps, offering strategic insights for future research and serving as a vital reference for advancing coastal management and salt marsh conservation strategies.

Abundance, Size Structure, and Growth of the Invasive Blue Crab Callinectes sapidus in the Lesina Lagoon, Southern Adriatic Sea

The fishery biology of the invasive Atlantic blue crab Callinectes sapidus in the Mediterranean Sea outside the eastern sectors of the basin has been only recently investigated. Here we studied the population of C. sapidus in the Lesina Lagoon (Adriatic Sea, SE Italy). In total, 838 crabs were captured monthly between February 2021 and January 2022 using fyke nets. Abundances varied seasonally with catches per unit effort ranging between 0 and 1.76 crabs fyke nets-1 d-1 in winter and summer. Spatial abundances estimated in summer by a Carle-Strub procedure ranged between 0.06 and 0.64 crabs m-2. The sex ratio (♂/♀) was close to 1:1; males prevailed only in August and September; ovigerous females occurred from April to August. The males' size at morphological maturity was smaller than females (110.6-112.3 mm vs. 122.1-123.1 mm). Seasonal von Bertalanffy growth parameters indicated that, compared with males, females showed a shorter maximum lifespan (5 vs. 8 years), a higher growth coefficient K (0.6 vs. 0.4 y-1) and growth performance index Ф' (4.6 vs. 4.3), while maximum sizes CW∞ (237.8 vs. 232.6 mm) and seasonality indices C (0.62 vs. 0.57) were similar. Furthermore, females showed higher natural and fishing mortalities and exploitation rate. We discussed the results of the present study in the context of the available literature to provide a valuable basis for the implementation of standardized Mediterranean-scale management plans, matching exploitation of C. sapidus with sustainable conservation of coastal ecosystems.

Mesopredator release moderates trophic control of plant biomass in a Georgia salt marsh

Predators regulate communities through top-down control in many ecosystems. Because most studies of top-down control last less than a year and focus on only a subset of the community, they may miss predator effects that manifest at longer timescales or across whole food webs. In southeastern US salt marshes, short-term and small-scale experiments indicate that nektonic predators (e.g., blue crab, fish, terrapins) facilitate the foundational grass, Spartina alterniflora, by consuming herbivorous snails and crabs. To test both how nekton affect marsh processes when the entire animal community is present, and how prior results scale over time, we conducted a 3-year nekton exclusion experiment in a Georgia salt marsh using replicated 19.6 m2 plots. Our nekton exclusions increased densities of plant-grazing snails and juvenile deposit-feeding fiddler crab and, in Year 2, reduced predation on tethered juvenile snails, indicating that nektonic predators control these key macroinvertebrates. However, in Year 3, densities of mesopredatory benthic mud crabs increased threefold in nekton exclusions, erasing the tethered snails' predation refuge. Nekton exclusion had no effect on Spartina biomass, likely because the observed mesopredator release suppressed grazing snail densities and elevated densities of fiddler crabs, whose burrowing alleviates soil stresses. Structural equation modeling supported the hypotheses that nektonic predators and mesopredators control invertebrate communities, with nektonic predators having stronger total effects on Spartina than mud crabs by controlling densities of species that both suppress (grazers) and facilitate (fiddler crabs) plant growth. These findings highlight that salt marshes can be resilient to multiyear reductions in nektonic predators if mesopredators are present and that multiple pathways of trophic control manifest in different ways over time to mediate community dynamics. These results highlight that larger scale and longer-term experiments can illuminate community dynamics not previously understood, even in well-studied ecosystems such as salt marshes.

Long-term data reveal that grazer density mediates climatic stress in salt marshes

Understanding how climate and local stressors interact is paramount for predicting future ecosystem structure. The effects of multiple stressors are often examined in small-scale and short-term field experiments, limiting understanding of the spatial and temporal generality of the findings. Using a 22-year observational dataset of plant and grazer abundance in a southeastern US salt marsh, we analyzed how changes in drought and grazer density combined to affect plant biomass. We found: (1) increased drought severity and higher snail density both correlated with lower plant biomass; (2) drought and snail effects interacted additively; and, (3) snail effects had a threshold, with additive top-down effects only occurring when snails were present at high densities. These results suggest that the emergence of multiple stressor effects can be density dependent, and they validate short-term experimental evidence that consumers can augment environmental stress. These findings have important implications for predicting future ecosystem structure and managing natural ecosystems.

Top-predator recovery abates geomorphic decline of a coastal ecosystem

The recovery of top predators is thought to have cascading effects on vegetated ecosystems and their geomorphology1,2, but the evidence for this remains correlational and intensely debated3,4. Here we combine observational and experimental data to reveal that recolonization of sea otters in a US estuary generates a trophic cascade that facilitates coastal wetland plant biomass and suppresses the erosion of marsh edges-a process that otherwise leads to the severe loss of habitats and ecosystem services5,6. Monitoring of the Elkhorn Slough estuary over several decades suggested top-down control in the system, because the erosion of salt marsh edges has generally slowed with increasing sea otter abundance, despite the consistently increasing physical stress in the system (that is, nutrient loading, sea-level rise and tidal scour7-9). Predator-exclusion experiments in five marsh creeks revealed that sea otters suppress the abundance of burrowing crabs, a top-down effect that cascades to both increase marsh edge strength and reduce marsh erosion. Multi-creek surveys comparing marsh creeks pre- and post-sea otter colonization confirmed the presence of an interaction between the keystone sea otter, burrowing crabs and marsh creeks, demonstrating the spatial generality of predator control of ecosystem edge processes: densities of burrowing crabs and edge erosion have declined markedly in creeks that have high levels of sea otter recolonization. These results show that trophic downgrading could be a strong but underappreciated contributor to the loss of coastal wetlands, and suggest that restoring top predators can help to re-establish geomorphic stability.

Impacts of Climate Change on Marine Foundation Species

Marine foundation species are the biotic basis for many of the world's coastal ecosystems, providing structural habitat, food, and protection for myriad plants and animals as well as many ecosystem services. However, climate change poses a significant threat to foundation species and the ecosystems they support. We review the impacts of climate change on common marine foundation species, including corals, kelps, seagrasses, salt marsh plants, mangroves, and bivalves. It is evident that marine foundation species have already been severely impacted by several climate change drivers, often through interactive effects with other human stressors, such as pollution, overfishing, and coastal development. Despite considerable variation in geographical, environmental, and ecological contexts, direct and indirect effects of gradual warming and subsequent heatwaves have emerged as the most pervasive drivers of observed impact and potent threat across all marine foundation species, but effects from sea level rise, ocean acidification, and increased storminess are expected to increase. Documented impacts include changes in the genetic structures, physiology, abundance, and distribution of the foundation species themselves and changes to their interactions with other species, with flow-on effects to associated communities, biodiversity, and ecosystem functioning. We discuss strategies to support marine foundation species into the Anthropocene, in order to increase their resilience and ensure the persistence of the ecosystem services they provide.

Shorebirds-driven trophic cascade helps restore coastal wetland multifunctionality

Ecosystem restoration has traditionally focused on re-establishing vegetation and other foundation species at basal trophic levels, with mixed outcomes. Here, we show that threatened shorebirds could be important to restoring coastal wetland multifunctionality. We carried out surveys and manipulative field experiments in a region along the Yellow Sea affected by the invasive cordgrass Spartina alterniflora. We found that planting native plants alone failed to restore wetland multifunctionality in a field restoration experiment. Shorebird exclusion weakened wetland multifunctionality, whereas mimicking higher predation before shorebird population declines by excluding their key prey - crab grazers - enhanced wetland multifunctionality. The mechanism underlying these effects is a simple trophic cascade, whereby shorebirds control crab grazers that otherwise suppress native vegetation recovery and destabilize sediments (via bioturbation). Our findings suggest that harnessing the top-down effects of shorebirds - through habitat conservation, rewilding, or temporary simulation of consumptive or non-consumptive effects - should be explored as a nature-based solution to restoring the multifunctionality of degraded coastal wetlands.

Top ten priorities for global saltmarsh restoration, conservation and ecosystem service research

Coastal saltmarshes provide globally important ecosystem services including 'blue carbon' sequestration, flood protection, pollutant remediation, habitat provision and cultural value. Large portions of marshes have been lost or fragmented as a result of land reclamation, embankment construction, and pollution. Sea level rise threatens marsh survival by blocking landward migration where coastlines have been developed. Research-informed saltmarsh conservation and restoration efforts are helping to prevent further loss, yet significant knowledge gaps remain. Using a mixed methods approach, this paper identifies ten research priorities through an online questionnaire and a residential workshop attended by an international, multi-disciplinary network of 35 saltmarsh experts spanning natural, physical and social sciences across research, policy, and practitioner sectors. Priorities have been grouped under four thematic areas of research: Saltmarsh Area Extent, Change and Restoration Potential (including past, present, global variation), Spatio-social contexts of Ecosystem Service delivery (e.g. influences of environmental context, climate change, and stakeholder groups on service provisioning), Patterns and Processes in saltmarsh functioning (global drivers of saltmarsh ecosystem structure/function) and Management and Policy Needs (how management varies contextually; challenges/opportunities for management). Although not intended to be exhaustive, the challenges, opportunities, and strategies for addressing each research priority examined here, providing a blueprint of the work that needs to be done to protect saltmarshes for future generations.

Herbivory limits success of vegetation restoration globally

Restoring vegetation in degraded ecosystems is an increasingly common practice for promoting biodiversity and ecological function, but successful implementation is hampered by an incomplete understanding of the processes that limit restoration success. By synthesizing terrestrial and aquatic studies globally (2594 experimental tests from 610 articles), we reveal substantial herbivore control of vegetation under restoration. Herbivores at restoration sites reduced vegetation abundance more strongly (by 89%, on average) than those at relatively undegraded sites and suppressed, rather than fostered, plant diversity. These effects were particularly pronounced in regions with higher temperatures and lower precipitation. Excluding targeted herbivores temporarily or introducing their predators improved restoration by magnitudes similar to or greater than those achieved by managing plant competition or facilitation. Thus, managing herbivory is a promising strategy for enhancing vegetation restoration efforts.

Effects of herbivore on seagrass, epiphyte and sediment carbon sequestration in tropical seagrass bed

Herbivores strongly affect the ecological structure and functioning in seagrass bed ecosystems, but may exhibit density-dependent effects on primary producers and carbon sequestration. This study examined the effects of herbivorous snail (Cerithidea rhizophorarum) density on snail intraspecific competition and diet, dominant seagrass (Thalassia hemprichii) and epiphyte growth metrics, and sediment organic carbon (SOC). The growth rates of the herbivorous snail under low density (421 ind m-2) and mid density (842 ind m-2) were almost two times of those at extremely high density (1684 ind m-2), indicating strong intraspecific competition at high density. Herbivorous snails markedly reduced the epiphyte biomass on seagrass leaves. Additionally, the seagrass contribution to herbivorous snail as food source under high density was about 1.5 times of that under low density, while the epiphyte contribution under low density was 3 times of that under high density. A moderate density of herbivorous snails enhanced leaf length, carbon, nitrogen, total phenol and flavonoid contents of seagrasses, as well as surface SOC content and activities of polyphenol oxidase and β-glucosidase. However, high density of herbivorous snails decreased leaf glucose, fructose, detritus carbon, and total phenols contents of seagrasses, as well as surface SOC content and activities of polyphenol oxidase and β-glucosidase. Therefore, the effects of herbivorous snail on seagrass, epiphyte and SOC were density-dependent, and moderate density of herbivorous snail could be beneficial for seagrasses to increase productivity. This provided theoretical guidance for enhancing carbon sink in seagrass bed and its better conservation.

Effects of anthropogenic magnetic fields on the behavior of a major predator of the intertidal and subtidal zones, the velvet crab Necora puber

With the progress of the offshore renewable energy sector and electrical interconnection projects, a substantial rise in the number of submarine power cables is expected soon. Such cables emit either alternating or direct current magnetic fields whose impact on marine invertebrates is currently unknown and hardly studied. In this context, this study aimed to assess potential short-term exposure (30 min) effects of both alternating and direct magnetic fields of increasing intensity (72-304 μT) on the behavior of the high-ecological value velvet crab (Necora puber). Three experiments were designed to evaluate whether the strongest magnetic field intensities induce crabs' attraction or repulsion responses, and whether foraging and sheltering behaviors may be modified. We extracted from video analyses several variables as the time budgets crabs spent immobile, moving, feeding, or sheltering as well as total and maximal distance reached in the magnetic field (MF) gradient. The crabs exposed to artificial MF did not exhibit significant behavioral changes compared with those exposed to the "natural" MF. Overall, our results suggest that, at such intensities, artificial magnetic fields do not significantly alter behaviors of N. puber. Nevertheless, future studies should be conducted to examine the effects of longer exposure periods and to detect potential habituation or resilience processes.

Multi-elemental consumer-driven nutrient cycling when predators feed on different prey

Predators play a fundamental role in cycling nutrients through ecosystems, by altering the amount and compositions of waste products and uneaten prey parts available to decomposers. Different prey can vary in their elemental content and the deposition of elements in predator waste can vary depending on which elements are preferentially retained versus eliminated as waste products. We tested how feeding on different prey (caterpillars, cockroaches, crickets, and flies) affected the concentrations of 23 elements in excreta deposited by wolf spider across 2 seasons (spring versus fall). Spider excreta had lower concentrations of carbon and higher concentrations of many other elements (Al, B, Ba, K, Li, P, S, Si, and Sr) compared to prey remains and whole prey carcasses. In addition, elemental concentrations in unconsumed whole prey carcasses and prey remains varied between prey species, while spider excreta had the lowest variation among prey species. Finally, the concentrations of elements deposited differed between seasons, with wolf spiders excreting greater concentrations of Fe, Mg, Mn, Mo, S, and V in the fall. However, in the spring, spiders excreted higher concentrations of Al, B, Ba, Ca, Cd, Cu, K, P, Na, Si, Sr, and Zn. These results highlight that prey identity and environmental variation can determine the role that predators play in regulating the cycling of many elements. A better understanding of these convoluted nutritional interactions is critical to disentangle specific consumer-driven effects on ecosystem function.

Beyond Kuhnian paradigms: Normal science and theory dependence in ecology

The Structure of Scientific Revolutions by Thomas Kuhn has influenced scientists for decades. It focuses on a progression of science involving periodic, fundamental shifts-revolutions-from one existing paradigm to another. Embedded in this theory is the concept of normal science, that is, scientists work within the confines of established theory, a process often compared to a type of puzzle-solving. This Kuhnian aspect of scientific research has received little attention relative to the much-scrutinized concepts of revolutions and paradigms. We use Kuhn's normal science framework to reflect on the way ecologists practice science. This involves a discussion of how theory dependence influences each step of the scientific method, specifically, how past experiences and existing research frameworks guide the way ecologists acquire knowledge. We illustrate these concepts with ecological examples, including food web structure and the biodiversity crisis, emphasizing that the way one views the world influences how that person engages in scientific research. We conclude with a discussion of how Kuhnian ideas inform ecological research at practical levels, such as influences on grant funding allocation, and we make a renewed call for the inclusion of philosophical foundations of ecological principles in pedagogy. By studying the processes and traditions of how science is carried out, ecologists can better direct scientific insight to address the world's most pressing environmental problems.

Responses to simultaneous anthropogenic and biological stressors were mixed in an experimental saltmarsh ecosystem

Coastal ecosystems are essential for absorbing and bouncing back from the impacts of climate change, yet accelerating climate change is causing anthropogenically-derived stressors in these ecosystems to grow. The effects of stressors are more difficult to foresee when they act simultaneously, however, predicting these effects is critical for understanding ecological change. Spartina alterniflora (Spartina), a foundational saltmarsh plant key to coastal resilience, is subject to biological stress such as herbivory, as well as anthropogenic stress such as chemical pollution. Using saltmarsh mesocosms as a model system in a fully factorial experiment, we tested whether the effects of herbivory and two chemicals (oil and dispersant) were mediated or magnified in combination. Spartina responded to stressors asynchronously; ecophysiology responded negatively to oil and herbivores in the first 2-3 weeks of the experiment, whereas biomass responded negatively to oil and herbivores cumulatively throughout the experiment. We generally found mixed multi-stressor effects, with slightly more antagonistic effects compared to either synergistic or additive effects, despite significant reductions in Spartina biomass and growth from both chemical and herbivore treatments. We also observed an indirect positive effect of oil on Spartina, via a direct negative effect on insect herbivores. Our findings suggest that multi-stressor effects in our model system, 1) are mixed but can be antagonistic more often than expected, a finding contrary to previous assumptions of primarily synergistic effects, 2) can vary in duration, 3) can be difficult to discern a priori, and 4) can lead to ecological surprises through indirect effects with implications for coastal resilience. This leads us to conclude that understanding the simultaneous effects of multiple stressors is critical for predicting foundation-species persistence, discerning ecosystem resilience, and managing and mitigating impacts on ecosystem services.

Seasonal and Zonal Succession of Bacterial Communities in North Sea Salt Marsh Sediments

Benthic microbial communities of intertidal zones perform important biogeochemical processes and provide accessible nutrients for higher organisms. To unravel the ecosystem services of salt marsh microbial communities, we analyzed bacterial diversity and metabolic potential along the land–sea transition zone on seasonal scales on the German North Sea Island of Spiekeroog. Analysis of bacterial community was based on amplicon sequencing of 16S rRNA genes and –transcripts. Insights into potential community function were obtained by applying the gene prediction tool tax4fun2. We found that spatial variation of community composition was greater than seasonal variations. Alphaproteobacteria (15%), Gammaproteobacteria (17%) and Planctomycetes (11%) were the most abundant phyla across all samples. Differences between the DNA-based resident and RNA-based active communities were most pronounced within the Planctomycetes (17% and 5%) and Cyanobacteriia (3% and 12%). Seasonal differences were seen in higher abundance of Gammaproteobacteria in March 2015 (25%) and a cyanobacterial summer bloom, accounting for up to 70% of the active community. Taxonomy-based prediction of function showed increasing potentials for nitrification, assimilatory nitrate and sulfate reduction from sea to land, while the denitrification and dissimilatory sulfate reduction increased towards the sea. In conclusion, seasonal differences mainly occurred by blooming of individual taxa, while the overall community composition strongly corresponded to locations. Shifts in their metabolism could drive the salt marsh’s function, e.g., as a potential nitrogen sink.

Extensive spatial impacts of oyster reefs on an intertidal mudflat community via predator facilitation

Habitat engineers make strong and far-reaching imprints on ecosystem processes. In intertidal mudflats, the dominant primary producer, microphytobenthos (MPB), often forms high biomass patches around oyster reefs. We evaluate multiple hypotheses linking MPB with oyster reefs, including oyster biodeposition, meiofaunal grazing, and abiotic factors, aiming to help predict effects of reef removal or proliferation. We quantify spatial patterns of an Atlantic mudflat community and its environment around two large Crassostrea reefs before experimentally sacrificing one reef via burning. MPB biomass was enriched surrounding living oyster reefs although infaunal biomass and individual sizes were low. Structural equation modelling best supported the hypothesis that crab predation intensity, which decayed with distance from the reefs, locally freed MPB from grazing. Our results suggest that Crassostrea reef expansion may enrich local MPB patches and redirect trophic energy flows away from mudflat infauna, with potential implications for the sustainability of local fisheries and bird conservation.

Biota-mediated carbon cycling-A synthesis of biotic-interaction controls on blue carbon

Research into biotic interactions has been a core theme of ecology for over a century. However, despite the obvious role that biota play in the global carbon cycle, the effects of biotic interactions on carbon pools and fluxes are poorly understood. Here we develop a conceptual framework that illustrates the importance of biotic interactions in regulating carbon cycling based on a literature review and a quantitative synthesis by means of meta-analysis. Our study focuses on blue carbon ecosystems-vegetated coastal ecosystems that function as the most effective long-term CO₂ sinks of the biosphere. We demonstrate that a multitude of mutualistic, competitive and consumer-resource interactions between plants, animals and microbiota exert strong effects on carbon cycling across various spatial scales ranging from the rhizosphere to the landscape scale. Climate change-sensitive abiotic factors modulate the strength of biotic-interaction effects on carbon fluxes, suggesting that the importance of biota-mediated carbon cycling will change under future climatic conditions. Strong effects of biotic interactions on carbon cycling imply that biosphere-climate feedbacks may not be sufficiently represented in current Earth system models. Inclusion of new functional groups in these models, and new approaches to simplify species interactions, may thus improve the predictions of biotic effects on the global climate.

Can the native faunal communities be restored from removal of invasive plants in coastal ecosystems? A global meta-analysis

Coastal ecosystems worldwide are being threatened by invasive plants in the context of global changes. However, how invasive plants influence native faunal communities and whether native faunal communities can recover following the invader removals/controls across global coastal ecosystems are still poorly understood. Here, we present the first global meta-analysis to quantify the impacts of Spartina species invasions on coastal faunal communities and further to evaluate the outcomes of Spartina species removals on faunal community recovery based on 74 independent studies. We found that invasive Spartina species generally decreased the biodiversity (e.g., species richness), but increased coastal faunal abundance (e.g., individual number) and fitness (e.g., biomass), though the effect on abundance was insignificant. The pattern of influence was strongly dependent on habitat types, faunal taxa, trophic levels, and feeding types. Specifically, Spartina species invasion of mudflats caused greater impacts than invasion of vegetated habitats. Insects and birds at higher trophic levels were strongly affected by invasive Spartina, indicating that invasive plant effects can cascade upward along the food chain. Additionally, impacts of Spartina invasions were more obvious on food specialists such as herbivores and carnivores. Furthermore, our analyses revealed that invader removals were overall beneficial for native faunal communities to recover from the displacement caused by Spartina invasions, but this recovery process depended on specific removal measure and time. For example, the long-term waterlogging had strong negative impacts on faunal recovery, so it should not be encouraged. Our findings suggest that invasive plants could have contrasting effects on functional responses of native faunal communities. Although invasive plant removals could restore native faunal communities, future functional restorations of invaded ecosystems should take the legacy effects of invasive species on native communities into account. These findings provide insightful implications for future scientific controls of invasive species and ecosystem restoration under intensifying global changes.

Do predators have a role to play in wetland ecosystem functioning? An experimental study in New England salt marshes

The historical ecological paradigm of wetland ecosystems emphasized the role of physical or "bottom-up" factors in maintaining functions and services. However, recent studies have shown that the loss of predators in coastal salt marshes can lead to a significant reduction in wetland extent due to overgrazing of vegetation by herbivores. Such studies indicate that consumers or "top-down" factors may play a much larger role in the maintenance of wetland ecosystems than was previously thought. The objective of this study was to evaluate whether altering top-down control by manipulating the presence of predators can lead to measurable changes in salt marsh ecosystem properties. Between May and August of 2015 and 2016, we established exclosure and enclosure cages within three New England coastal wetland areas and manipulated the presence of green crab predators to assess how they and their fiddler and purple marsh crab prey affect changes in ecosystem properties. Predator presence was associated with changes in soil nitrogen and aboveground biomass at two of the three field sites, though the magnitude and direction of these effects varied from site to site. Further, path analysis results indicate that across field sites, a combination of bottom-up and top-down factors influenced changes in measured variables. These results challenge the growing consensus that consumers have strong effects, indicating instead that predator impacts may be highly context-dependent.

Top-down control of foundation species recovery during coastal wetland restoration

Restoration has been increasingly adopted to halt trends in coastal wetland loss globally. Existing restoration often assumes that once abiotic stress is relieved, disturbances are prevented, and invasive species are eradicated, coastal wetlands will recover if propagules of native species are supplied either through natural dispersal or planting. Whether other factors including consumers can help explain the often suboptimal performance of existing restoration remains poorly understood. In a series of field experiments in the Yangtze estuary, we examined the relative importance of abiotic stress and crab grazing in regulating the recovery of the native foundation plant species Scirpus mariqueter in salt marsh areas where exotic cordgrass was successfully eradicated. We found that grazing by herbivorous crabs, rather than abiotic stress, was the primary obstacle restricting the recovery of planted Scirpus. This negative effect of crab grazing varied predictably across elevation and was strongest at low elevations where abiotic conditions were positive for Scirpus. These findings highlight that i) measures to control crab grazing are needed to enhance the success of Scirpus restoration, even in areas where abiotic conditions are set to be optimal, and ii) restoration measures purely focused on reducing abiotic stress could be ineffective or suboptimal in field conditions, likely jeopardizing restoration investment and success. Since top-down control of foundation plant species is common in many coastal wetlands and can be especially important in degraded systems where herbivores are abundant, we urge that future coastal wetland restoration assesses for the impacts of grazers and, when present, apply intervention measures.

A global occurrence database of the Atlantic blue crab Callinectes sapidus

The Atlantic blue crab Callinectes sapidus is a portunid native to the western Atlantic, from New England to Uruguay. The species was introduced in Europe in 1901 where it has become invasive; additionally, a significant northward expansion has been emphasized in its native range. Here we present a harmonized global compilation of C. sapidus occurrences from native and non-native distribution ranges derived from online databases (GBIF, BISON, OBIS, and iNaturalist) as well as from unpublished and published sources. The dataset consists of 40,388 geo-referenced occurrences, 39,824 from native and 564 from non-native ranges, recorded in 53 countries. The implementation of quality controls imposed a severe reduction, in particular from online databases, of the records selected for inclusion in the dataset. In addition, a technical validation procedure was used to flag entries showing identical coordinates but different year of record, in-land occurrences and those located close to the coast. Similarly, a flagging system identified entries outside the known distribution of the species, or associated with unsuccessful introductions.

Measurement(s)	geographic location
Technology Type(s)	digital curation
Factor Type(s)	location • year of record
Sample Characteristic - Organism	Callinectes sapidus
Sample Characteristic - Environment	marine biome
Sample Characteristic - Location	Globe

Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.14077112

Community-wide consequences of nonconsumptive predator effects on a foundation species

Predators can exert nonconsumptive effects (NCEs) on prey, which often take place through prey behavioural adjustments to minimise predation risk. As NCEs are widespread in nature, interest is growing to determine whether NCEs on a prey species can indirectly influence several other species simultaneously, thus leading to changes in community structure. In this study, we investigate whether a predator can exert NCEs on a foundation species and indirectly affect community structure. Through laboratory experiments, we first tested whether the predatory marine snail Acanthina monodon exerts negative NCEs on larviphagy (consumption of pelagic larvae) and phytoplankton filtration rates of the mussel Perumytilus purpuratus, an intertidal foundation species. These hypotheses stem from the notion that mussels may decrease feeding activities in the presence of predator cues to limit detection by predators. Afterwards, a field experiment tested whether the presence of A. monodon near mussel beds leads to higher colonisation rates of invertebrates that reproduce through pelagic larvae (expected under a lower larviphagy in P. purpuratus) and to a lower algal biomass on P. purpuratus shells (expected under a lower metabolite excretion in the mussels), thereby changing the community structure of the species typically found in P. purpuratus beds. The laboratory experiments revealed that waterborne cues from A. monodon limit the larviphagy and filtration rates of P. purpuratus. In turn, the field experiment showed that A. monodon cues led to greater abundances of barnacles and bivalves and a lower algal biomass in P. purpuratus beds, thus altering community structure. Overall, this study shows that a predator can indirectly affect community structure through NCEs on an invertebrate foundation species. As invertebrate foundation species are ubiquitous worldwide, understanding predator NCEs on these organisms could help to better understand community regulation in systems structured by such species.

Unraveling the Gordian Knot: Eight testable hypotheses on the effects of nutrient enrichment on tidal wetland sustainability

The position of tidal wetlands at the land-sea interface makes them especially vulnerable to the effects of nutrient discharges and sea level rise (SLR). Experimental studies of coastal wetland nutrient additions report conflicting results among and within habitats, highlighting the importance of site-specific factors, and how spatial and temporal scaling modulates responses. This suite of influences as SLR accelerates creates a "Gordian Knot" that may compromise coastal habitat integrity. We present eight testable hypotheses here to loosen this knot by identifying critical modulators about nutrient form, soil type and porosity, physiochemical gradients, and eco-evolutionary responses that may control the impacts of nutrient enrichment on coastal wetland sustainability: (1) the delivery and form of the nutrient shapes the ecosystem response; (2) soil type mediates the effects of nutrient enrichment on marshes; (3) belowground responses cannot be solely explained by phenotypic responses; (4) shifting zones of redox and salinity gradients modulate nutrient enrichment impacts; (5) eco-evolutionary processes can drive responses to nutrient availability; (6) nutrient enrichment leads to multiple changed ecosystem states; (7) biogeography trumps a plant's plastic responses to nutrient enrichment; and, (8) nutrient-enhanced wetlands are more susceptible to additional (and anticipated) anthropogenic changes. They provide a framework to investigate and integrate the urgently needed research to understand how excess nutrients threaten the sustainability of coastal wetlands, and wetlands in general. While there is no single 'right way' to test these hypotheses, including a combination of complex and simple, highly-replicated experiments is essential.

Consumer regulation of the carbon cycle in coastal wetland ecosystems

Despite escalating anthropogenic alteration of food webs, how the carbon cycle in ecosystems is regulated by food web processes remains poorly understood. We quantitatively synthesize the effects of consumers (herbivores, omnivores and carnivores) on the carbon cycle of coastal wetland ecosystems, 'blue carbon' ecosystems that store the greatest amount of carbon per unit area among all ecosystems. Our results reveal that consumers strongly affect many processes of the carbon cycle. Herbivores, for example, generally reduce carbon absorption and carbon stocks (e.g. aboveground plant carbon by 53% and aboveground net primary production by 23%) but may promote some carbon emission processes (e.g. litter decomposition by 32%). The average strengths of these effects are comparable with, or even times higher than, changes driven by temperature, precipitation, nitrogen input, CO₂ concentration, and plant invasions. Furthermore, consumer effects appear to be stronger on aboveground than belowground carbon processes and vary markedly with trophic level, body size, thermal regulation strategy and feeding type. Despite important knowledge gaps, our results highlight the powerful impacts of consumers on the carbon cycle and call for the incorporation of consumer control into Earth system models that predict anthropogenic climate change and into management strategies of Earth's carbon stocks. This article is part of the theme issue 'Integrative research perspectives on marine conservation'.

Ecological impacts of human-induced animal behaviour change

A growing body of literature has documented myriad effects of human activities on animal behaviour, yet the ultimate ecological consequences of these behavioural shifts remain largely uninvestigated. While it is understood that, in the absence of humans, variation in animal behaviour can have cascading effects on species interactions, community structure and ecosystem function, we know little about whether the type or magnitude of human-induced behavioural shifts translate into detectable ecological change. Here we synthesise empirical literature and theory to create a novel framework for examining the range of behaviourally mediated pathways through which human activities may affect different ecosystem functions. We highlight the few empirical studies that show the potential realisation of some of these pathways, but also identify numerous factors that can dampen or prevent ultimate ecosystem consequences. Without a deeper understanding of these pathways, we risk wasting valuable resources on mitigating behavioural effects with little ecological relevance, or conversely mismanaging situations in which behavioural effects do drive ecosystem change. The framework presented here can be used to anticipate the nature and likelihood of ecological outcomes and prioritise management among widespread human-induced behavioural shifts, while also suggesting key priorities for future research linking humans, animal behaviour and ecology.

Sea-level rise and the emergence of a keystone grazer alter the geomorphic evolution and ecology of southeast US salt marshes

Human disturbances, climate change, and their combined effects on species distributions and environmental conditions are increasingly modifying the organization of our world’s oceans, forests, grasslands, wetlands, tundras, and reefs. Here, we reveal that these contemporary conditions can trigger the emergence of novel keystone species. Across the southeastern US coastal plain, sea-level rise is outpacing salt marsh vertical accretion, causing these grasslands to be tidally inundated for longer and softening marsh substrates to levels optimal for crab burrowing. Using field experiments, measurements, surveys, and models, we show that these conditions amplify the burrowing and grazing effects of a previously inconspicuous crab, enabling it to redefine predator–prey interactions, eco-geomorphic feedbacks, and the mechanisms by which salt marshes are responding to climate change.

Keystone species have large ecological effects relative to their abundance and have been identified in many ecosystems. However, global change is pervasively altering environmental conditions, potentially elevating new species to keystone roles. Here, we reveal that a historically innocuous grazer—the marsh crab Sesarma reticulatum—is rapidly reshaping the geomorphic evolution and ecological organization of southeastern US salt marshes now burdened by rising sea levels. Our analyses indicate that sea-level rise in recent decades has widely outpaced marsh vertical accretion, increasing tidal submergence of marsh surfaces, particularly where creeks exhibit morphologies that are unable to efficiently drain adjacent marsh platforms. In these increasingly submerged areas, cordgrass decreases belowground root:rhizome ratios, causing substrate hardness to decrease to within the optimal range for Sesarma burrowing. Together, these bio-physical changes provoke Sesarma to aggregate in high-density grazing and burrowing fronts at the heads of tidal creeks (hereafter, creekheads). Aerial-image analyses reveal that resulting “Sesarma-grazed” creekheads increased in prevalence from 10 ± 2% to 29 ± 5% over the past <25 y and, by tripling creek-incision rates relative to nongrazed creekheads, have increased marsh-landscape drainage density by 8 to 35% across the region. Field experiments further demonstrate that Sesarma-grazed creekheads, through their removal of vegetation that otherwise obstructs predator access, enhance the vulnerability of macrobenthic invertebrates to predation and strongly reduce secondary production across adjacent marsh platforms. Thus, sea-level rise is creating conditions within which Sesarma functions as a keystone species that is driving dynamic, landscape-scale changes in salt-marsh geomorphic evolution, spatial organization, and species interactions.

Succession of macrofaunal communities and environmental properties along a gradient of smooth cordgrass Spartina alterniflora invasion stages

The exotic species smooth cordgrass (Spartina alterniflora) is recognized as an important invasive species in China, introduced about 40 years ago. The consistent smooth cordgrass invasion significantly modified the coastal ecosystem. Understanding the ecological succession and mechanisms of wetland soil ecosystems is essential for biological conservation after the landscape change resulting from the smooth cordgrass invasion. In this study, five different invasion stages of a 16-year smooth cordgrass invasion sequence were identified in a coastal wetland as no invasion, initial invasion, young invasion, mature invasion, and senescing invasion. The succession of macrofaunal communities and environments were investigated along the gradient of invasion stages. The infauna decreased, and the epifauna increased along the invasion sequence. The significant differences of the communities were detected among the mud flats experiencing different invasion stages. The initial and young invasion stages of smooth cordgrass possibly promote the macrofaunal biodiversity, but biodiversity decreased at mature and senescing invasion stages. The ecological effect of smooth cordgrass invasion on macrofauna depended on the species' traits and the invasion stage. The environmental properties co-varied with invasion stages, and varied significantly among selected habitats. Total organic carbon (TOC), total nitrogen, and the carbon-nitrogen ratio (C/N) strongly related to the smooth cordgrass coverage, stem density, and height. C/N was identified as the key factor for shaping the environment by principal components analysis, and TOC for regulating the macrofaunal community by canonical correspondence analysis. The succession of macrofaunal communities should be considered as a comprehensive response to the variations on environmental properties co-varying with smooth cordgrass invasion in coastal wetlands.

A Mutualism Between Unattached Coralline Algae and Seagrasses Prevents Overgrazing by Sea Turtles

Seagrass meadows are threatened biodiversity hot spots that provide essential ecosystem services. Green sea turtles may overgraze meadows, further enhancing seagrass decline. However, we observed an unexpected, remarkable recovery of seagrasses in a previously overgrazed meadow with abundant unattached branched coralline algae, suggesting that turtle grazing had ceased. We hypothesize that this recovery is due to an effective grazing-protection mutualism, in which the spiny coralline algae structures protect the seagrass meadows from overgrazing, while the seagrasses protect the algae from removal by currents and waves. Removing coralline algae from recovered seagrass plots allowed the turtles to resume grazing, while addition of coralline algae to grazed plots caused cessation of grazing. Coralline algae that were placed on bare sand were quickly displaced by wave action, whereas those placed in grazed or ungrazed seagrass remained. Our experiments demonstrate a grazing-protection mutualism, which likely explains the witnessed recovery of an overgrazed seagrass meadow. To our knowledge, this is the first account of a plant–plant grazing-protection mutualism in an aquatic environment. Our findings show that grazing-protection mutualisms can be vital for the maintenance and recovery of ecosystems shaped by habitat-structuring foundation species, and highlight the importance of mutualisms in coastal ecosystems. As seagrasses, sea turtles and coralline algae share habitats along tropical shores worldwide, the mutualism may be a global phenomenon. Overgrazing is expected to increase, and this mutualism adds a new perspective to the conservation and restoration of these valuable ecosystems.

Effects of Changing Vegetation Composition on Community Structure, Ecosystem Functioning, and Predator–Prey Interactions at the Saltmarsh-Mangrove Ecotone

Decreasing frequency of freeze events due to climate change is enabling the poleward range expansion of mangroves. As these tropical trees expand poleward, they are replacing herbaceous saltmarsh vegetation. Mangroves and saltmarsh vegetation are ecosystem engineers that are typically viewed as having similar ecosystem functions. However, few studies have investigated whether predation regimes, community structure, and ecosystem functions are shifting at the saltmarsh-mangrove ecotone. In this study, we manipulated predator access to marsh and mangrove creekside habitats to test their role in mediating vegetation and invertebrate structure and stability in a two-year experiment. We also conducted a survey to evaluate how shifting vegetation is modifying structural complexity, invertebrate communities, and ecosystem functioning at the ecotone. Excluding larger (> 2 cm diameter) predators did not affect vegetation or invertebrate structure or stability in either saltmarsh or mangrove habitats. The survey revealed that the two habitat types consistently differ in structural metrics, including vegetation height, inter-stem distance, and density, yet they support similar invertebrate and algal communities, soil properties, and predation rates. We conclude that although mangrove range expansion immediately modifies habitat structural properties, it is not altering larger predator consumptive effects, community stability, community composition, or some other ecosystem functions and properties at the ecotone.

Population genetics and historical demographic inferences of the blue crab Callinectes sapidus in the US based on microsatellites

The native range of the blue crab Callinectes sapidus spans Nova Scotia to northern Argentina. In the US, it constitutes a keystone species in estuarine habitats of the Atlantic coast and Gulf of Mexico (GOM), serving as both predator and prey to other species, and also has historically represented a multi-billion dollar fishery. Knowledge relevant to effective management and monitoring of this ecologically and economically important species, such as levels of population genetic differentiation and genetic diversity, is necessary. Although several population genetics studies have attempted to address these questions in one or more parts of its distribution, conflicting results and potential problems with the markers used, as well as other issues, have obscured our understanding on them. In this study, we examined large-scale genetic connectivity of the blue crab in the US, using 16 microsatellites, and genotyped individuals from Chesapeake Bay, in the US Atlantic, and from nine localities along the US GOM coast. Consistent with the high long-distance dispersal potential of this species, very low levels of genetic differentiation were detected for the blue crab among the ten US localities examined, suggesting it constitutes a large panmictic population within this region. Estimations of genetic diversity for the blue crab appear to be high in the US, and provide a baseline for monitoring temporal changes in this species. Demographic analyses indicate a recent range expansion of the US population, probably during the Holocene. In addition, capitalizing on published microsatellite data from southern Brazil, our analyses detected high genetic differentiation between localities in the US and Brazil. These results point to the need for examination of genetic diversity and differentiation along the area spanning the US to southern Brazil.

Supporting Spartina: Interdisciplinary perspective shows Spartina as a distinct solid genus

In 2014, a DNA-based phylogenetic study confirming the paraphyly of the grass subtribe Sporobolinae proposed the creation of a large monophyletic genus Sporobolus, including (among others) species previously included in the genera Spartina, Calamovilfa, and Sporobolus. Spartina species have contributed substantially (and continue contributing) to our knowledge in multiple disciplines, including ecology, evolutionary biology, molecular biology, biogeography, experimental ecology, biological invasions, environmental management, restoration ecology, history, economics, and sociology. There is no rationale so compelling to subsume the name Spartina as a subgenus that could rival the striking, global iconic history and use of the name Spartina for over 200 yr. We do not agree with the subjective arguments underlying the proposal to change Spartina to Sporobolus. We understand the importance of both the objective phylogenetic insights and of the subjective formalized nomenclature and hope that by opening this debate we will encourage positive feedback that will strengthen taxonomic decisions with an interdisciplinary perspective. We consider that the strongly distinct, monophyletic clade Spartina should simply and efficiently be treated as the genus Spartina.

Parasites enhance resistance to drought in a coastal ecosystem

Parasites are more diverse and numerous than their hosts and commonly control population dynamics. Whether parasites also regulate key ecosystem processes, such as resistance to climate stress, is unclear. In southern U.S. salt marshes, drought interacts synergistically with keystone grazing to generate extensive ecosystem die-off. Field manipulations of parasite prevalence and salt stress in sediments in healthy marshes demonstrated that trematode parasites, by suppressing feeding activity of grazers that overgraze on drought-stressed plants, have the potential to slow the rate of ecosystem loss. Surveys along 1,000 km of coastline during an intense drought event revealed parasitism is common in grazers on die-off borders and that increasing infection prevalence along marsh die-off borders is negatively correlated with per capita grazing. Combined, results from this field experiment and survey suggested, but did not show, that parasites could affect rates of drought-driven salt marsh die-off. To test whether parasites can indeed protect marshes under real drought conditions, we experimentally manipulated parasite prevalence in grazers over a month-long period on active die-off borders in three North Carolina marshes. Experimentally reducing parasite prevalence markedly increased the rate of plant ecosystem decline, an effect that scaled positively with prevalence. Thus parasites, by generating a trophic cascade, indirectly enhanced ecosystem resistance to overgrazing under intense drought in these North Carolina marshes. The generality of these results across the entire range of this keystone grazer in the southeastern United States needs to be tested, employing both experiments and extensive surveys that examine how the rate of ecosystem decline is mediated by parasitism. Given the ubiquity of parasites in ecosystems, our results suggest that more research effort should be invested in examining the possible roles for parasitism in regulating ecosystem function and stability.

Estimating Aboveground Biomass and Its Spatial Distribution in Coastal Wetlands Utilizing Planet Multispectral Imagery

Coastal salt marshes are biologically productive ecosystems that generate and sequester significant quantities of organic matter. Plant biomass varies spatially within a salt marsh and it is tedious and often logistically impractical to quantify biomass from field measurements across an entire landscape. Satellite data are useful for estimating aboveground biomass, however, high-resolution data are needed to resolve the spatial details within a salt marsh. This study used 3-m resolution multispectral data provided by Planet to estimate aboveground biomass within two salt marshes, North Inlet-Winyah Bay (North Inlet) National Estuary Research Reserve, and Plum Island Ecosystems (PIE) Long-Term Ecological Research site. The Akaike information criterion analysis was performed to test the fidelity of several alternative models. A combination of the modified soil vegetation index 2 (MSAVI2) and the visible difference vegetation index (VDVI) gave the best fit to the square root-normalized biomass data collected in the field at North Inlet (Willmott’s index of agreement d = 0.74, RMSE = 223.38 g/m2, AICw = 0.3848). An acceptable model was not found among all models tested for PIE data, possibly because the sample size at PIE was too small, samples were collected over a limited vertical range, in a different season, and from areas with variable canopy architecture. For North Inlet, a model-derived landscape scale biomass map showed differences in biomass density among sites, years, and showed a robust relationship between elevation and biomass. The growth curve established in this study is particularly useful as an input for biogeomorphic models of marsh development. This study showed that, used in an appropriate model with calibration, Planet data are suitable for computing and mapping aboveground biomass at high resolution on a landscape scale, which is needed to better understand spatial and temporal trends in salt marsh primary production.