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The impacts of ocean deoxygenation on biodiversity and ecosystem function are well established in temperate regions, and here we illustrate how the study of hypoxia in tropical ecosystems can offer insights of general importance. We first describe how mechanisms of resilience have developed in response to naturally occurring hypoxia across three tropical ecosystems: coral reefs, seagrass beds, and mangrove forests. We then suggest that the vulnerability of these systems to deoxygenation lies in interactions with other stressors that are increasing rapidly in the Anthropocene. Finally, we advocate for the adoption of a broader community- and ecosystem-level perspective that incorporates mutualisms, feedbacks, and mechanisms of self-rescue and recovery to develop a better predictive understanding of the effects of deoxygenation in coastal ecosystems.

Significance 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

Habitat heterogeneity is considered a primary causal driver underpinning patterns of diversity, yet the universal role of heterogeneity in structuring biodiversity is unclear due to a lack of coordinated experiments testing its effects across geographic scales and habitat types. Furthermore, key species interactions that can enhance heterogeneity, such as facilitation cascades of foundation species, have been largely overlooked in general biodiversity models. Here, we performed 22 geographically distributed experiments in different ecosystems and biogeographical regions to assess the extent to which variation in biodiversity is explained by three axes of habitat heterogeneity: the amount of habitat, its morphological complexity, and capacity to provide ecological resources (e.g. food) within and between co-occurring foundation species. We show that positive and additive effects across the three axes of heterogeneity are common, providing a compelling mechanistic insight into the universal importance of habitat heterogeneity in promoting biodiversity via cascades of facilitative interactions. Because many aspects of habitat heterogeneity can be controlled through restoration and management interventions, our findings are directly relevant to biodiversity conservation., Species interactions that can enhance habitat heterogeneity such as facilitation cascades of foundation species have been overlooked in biodiversity models. This study conducted 22 geographically distributed experiments in different ecosystems and biogeographical regions to assess the extent to which biodiversity is explained by three axes of habitat heterogeneity in facilitation cascades.

Ocean deoxygenation threatens the persistence of coastal ecosystems worldwide. Despite an increasing awareness that coastal deoxygenation impacts tropical habitats, there remains a paucity of empirical data on the effects of oxygen limitation on reef-building corals. To address this knowledge gap, we conducted laboratory experiments with ecologically important Caribbean corals Acropora cervicornis and Orbicella faveolata. We tested the effects of continuous exposure to conditions ranging from extreme deoxygenation to normoxia (~ 1.0 to 6.25 mg L−1 dissolved oxygen) on coral bleaching, photophysiology, and survival. Coral species demonstrated markedly different temporal resistance to deoxygenation, and within a species there were minimal genotype-specific treatment effects. Acropora cervicornis suffered tissue loss and mortality within a day of exposure to severe deoxygenation (~ 1.0 mg L−1), whereas O. faveolata remained unaffected after 11 days of continuous exposure to 1.0 mg L−1. Intermediate deoxygenation treatments (~ 2.25 mg L−1, ~ 4.25 mg L−1) elicited minimal responses in both species, indicating a low oxygen threshold for coral mortality and coral resilience to oxygen concentrations that are lethal for other marine organisms. These findings demonstrate the potential for variability in species-specific hypoxia thresholds, which has important implications for our ability to predict how coral reefs may be affected as ocean deoxygenation intensifies. With deoxygenation emerging as a critical threat to tropical habitats, there is an urgent need to incorporate deoxygenation into coral reef research, management, and action plans to facilitate better stewardship of coral reefs in an era of rapid environmental change.

Loss of oxygen in the global ocean is accelerating due to climate change and eutrophication, but how acute deoxygenation events affect tropical marine ecosystems remains poorly understood. Here we integrate analyses of coral reef benthic communities with microbial community sequencing to show how a deoxygenation event rapidly altered benthic community composition and microbial assemblages in a shallow tropical reef ecosystem. Conditions associated with the event precipitated coral bleaching and mass mortality, causing a 50% loss of live coral and a shift in the benthic community that persisted a year later. Conversely, the unique taxonomic and functional profile of hypoxia-associated microbes rapidly reverted to a normoxic assemblage one month after the event. The decoupling of ecological trajectories among these major functional groups following an acute event emphasizes the need to incorporate deoxygenation as an emerging stressor into coral reef research and management plans to combat escalating threats to reef persistence., How acute deoxygenation events affect tropical marine ecosystems remains poorly understood. This study integrates analyses of coral reef benthic communities with microbial community sequencing to show how a deoxygenation event rapidly altered a shallow tropical coral reef ecosystem in the Caribbean.

Foundation species structure communities by creating habitat and modifying environmental conditions, and there is increasing interest in how foundation species, such as corals and mangroves, interact with one another as these interactions can have cascading effects on diversity and abundance of associated organisms. Given recent reports of corals living on or between mangrove roots under the canopy, we hypothesized that mangroves can serve as a refuge for corals from stresses such as high solar irradiance and temperatures that are associated with the adjacent shallow reef. Using field surveys and a reciprocal transplant experiment, we tested the effects of light and habitat (e.g., reef or mangrove) on coral community structure (i.e., coral species richness, abundance, and diversity) and condition (i.e., level of bleaching, tissue loss, and mortality). The surveys revealed higher coral richness in mangroves than on the adjacent reef, indicating that mangroves can serve as refugia for numerous coral species. Our experimental manipulation of light in mangrove and reef habitats indicated that light intensity is a key environmental parameter mediating coral bleaching and survival, with mangrove habitats providing a refuge from the light stress experienced on nearby shallow reefs. Moreover, our experiment revealed that reef corals bleached less than mangrove corals following transplantation, regardless of whether they were transplanted into mangrove or reef habitats. We suggest that the lower coral richness of the shallow reef is the result of the extreme environmental conditions that select for a subset of coral species able to tolerate these conditions. The facilitative interactions that allow mangroves to act as coral refugia by reducing environmental stress will likely become increasingly important with global climate change.

Este artículo contiene 7 páginas, 3 figuras, 1 tabla., The global distribution of primary production and consumption by humans (fisheries) is well-documented, but we have no map linking the central ecological process of consumption within food webs to temperature and other ecological drivers. Using standardized assays that span 105° of latitude on four continents, we show that rates of bait consumption by generalist predators in shallow marine ecosystems are tightly linked to both temperature and the composition of consumer assemblages. Unexpectedly, rates of consumption peaked at midlatitudes (25 to 35°) in both Northern and Southern Hemispheres across both seagrass and unvegetated sediment habitats. This pattern contrasts with terrestrial systems, where biotic interactions reportedly weaken away from the equator, but it parallels an emerging pattern of a subtropical peak in marine biodiversity. The higher consumption at midlatitudes was closely related to the type of consumers present, which explained rates of consumption better than consumer density, biomass, species diversity, or habitat. Indeed, the apparent effect of temperature on consumption was mostly driven by temperature-associated turnover in consumer community composition. Our findings reinforce the key influence of climate warming on altered species composition and highlight its implications for the functioning of Earth’s ecosystems., We acknowledge funding from the Smithsonian Institution and the Tula Foundation.

Coral reefs are suffering unprecedented declines worldwide. Most studies focus on stressors such as rising temperatures, nutrient pollution, overfishing, and ocean acidification as drivers of this degradation. However, recent mass mortality events associated with low oxygen on coral reefs indicate that oxygen is a critical factor that can be limiting in reef environments. Here, we present evidence that integrates across disciplines and perspectives to reveal how natural and anthropogenic factors drive variation in oxygen at multiple scales on coral reefs. This variation, in turn, limits essential processes such as productivity, respiration, and calcification on reefs and often plays a role in the outcome of interactions between corals and their competitors, pathogens, and mutualists. Moreover, the apparent effects of temperature, eutrophication, acidification, and other stressors on corals are commonly mediated by oxygen. As a consequence, the imprint of oxygen variation is evident in many patterns including reef biodiversity, coral bleaching, colony morphology, and fish behavior. We suggest that the structure and dynamics of coral reefs can be fully understood only by considering the ubiquitous role of oxygen, and we identify critical areas of future oxygen research to guide the study and management of coral reefs in a changing world.

Many Caribbean coral reefs are heavily degraded, yet their pre-human, natural states are often assumed or estimated using space-for-time substitution approaches. Here we use an 11-hectare suite of fossilised mid-Holocene (7.2–5.6 ka) fringing reefs in Caribbean Panama to define natural variation in hard coral community structure before human-impact to provide context to the states of the same reefs today. We collected bulk samples from four trenches dug into the mid-Holocene fossil reef and surficial bulk samples from 2–10 m depths on five adjacent modern reefs extending over 5 km. Analysis of the abundances of coral taxa in fossil bulk samples define the Historical Range of Variation (HRV) in community structure of the reefs. When compared to the community structure of adjacent modern reefs, we find that most coral communities today fall outside the HRV, identifying them as novel ecosystems and corroborating the well-documented transition from acroporid-dominated Caribbean reefs to reefs dominated by stress-tolerant taxa (Porites and Agaricia). We find one modern reef, however, whose community composition remains within the HRV showing that it has not transitioned to a novel state. Reef-matrix cores extracted from this reef reveal that the coral community has remained in this state for over 800 years, suggesting long-term stability and resistance to the region-wide shift to novel states. Without these data to provide historical context, this potentially robust and stable reef would be overlooked since it does not fulfil expectations of what a Caribbean coral reef should look like in the absence of humans. This example illustrates how defining past variation using the fossil record can improve our understanding of modern degradation and guide conservation.

Degradation of coastal water quality in the form of low dissolved oxygen levels (hypoxia) can harm biodiversity, ecosystem function, and human wellbeing. Extreme hypoxic conditions along the coast, leading to what are often referred to as "dead zones," are known primarily from temperate regions. However, little is known about the potential threat of hypoxia in the tropics, even though the known risk factors, including eutrophication and elevated temperatures, are common. Here we document an unprecedented hypoxic event on the Caribbean coast of Panama and assess the risk of dead zones to coral reefs worldwide. The event caused coral bleaching and massive mortality of corals and other reef-associated organisms, but observed shifts in community structure combined with laboratory experiments revealed that not all coral species are equally sensitive to hypoxia. Analyses of global databases showed that coral reefs are associated with more than half of the known tropical dead zones worldwide, with >10% of all coral reefs at elevated risk for hypoxia based on local and global risk factors. Hypoxic events in the tropics and associated mortality events have likely been underreported, perhaps by an order of magnitude, because of the lack of local scientific capacity for their detection. Monitoring and management plans for coral reef resilience should incorporate the growing threat of coastal hypoxia and include support for increased detection and research capacity.

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 (&gt, 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.

Accurate, rapid, and comprehensive biodiversity assessments are critical for investigating ecological processes and supporting conservation efforts. Environmental DNA (eDNA) surveys show promise as a way to effectively characterize fine-scale patterns of community composition, but most studies to date have evaluated its effectiveness in single habitats and for conspicuous taxonomic groups in temperate ecosystems. We tested whether a single PCR survey of eDNA in seawater using a broad metazoan primer could identify differences in community composition between five adjacent habitats at 19 sites across a tropical Caribbean bay in Panama. We paired this effort with visual fish surveys to compare methods for a conspicuous taxonomic group. eDNA revealed a tremendous diversity of animals (8,586 operational taxonomic units), including many small taxa that would be undetected in traditional in situ surveys. Fish comprised only 0.07% of the taxa detected by a broad COI primer, yet included 43 species not observed in the visual survey. eDNA revealed significant differences in fish and invertebrate community composition across adjacent habitats and areas of the bay driven in part by taxa known to be habitat-specialists or tolerant to wave action. Our results demonstrate the ability of broad eDNA surveys to identify biodiversity patterns in the ocean.

Facilitation cascades occur when a habitat-forming species facilitates another habitat-forming species with synergistic effects on biodiversity. This review summarizes the current state of knowledge of facilitation cascades in marine environments, describing (1) the geographic distribution of studies documenting facilitation cascades; (2) the range of habitats and organisms involved; (3) the mechanisms underpinning their establishment; and (4) the sources of their spatial and temporal patterns and variability. We synthesize this knowledge to identify ways in which facilitation cascades could be integrated with other ecological theories regarding community assembly and function, and also to explore how they may be integrated into conservation and management strategies. Finally, we identify critical knowledge gaps and future research directions for improving our understanding of the mechanisms regulating the establishment and persistence of facilitation cascades in marine environments.

Ecology is a young discipline that needs to develop into a predictive science to confront the challenges of human population pressures and habitat degradation. Basic ecology has disproportionately focused on undisturbed, charismatic ecosystems, species and academic questions, leaving gaps in its ability to inform the conservation and management of degraded, threatened ecosystems. Foundation species-dependent organisms have been studied at the expense of the habitat-forming species that build and maintain communities. We used cobble beaches as a model system to discuss the consequences of this disparity on translational ecology. We suggest that the historic development of ecology has led to an academic discipline ill-suited for proactive conservation. We propose that the incorporation of foundation species and a hierarchical organization theory, into the conceptual framework of ecology, will improve its predictive ability and successful application in conservation and the restoration of degraded ecosystems.

Fish communities associated with coral reefs worldwide are threatened by overexploitation and other human impacts such as bleaching events that cause habitat degradation. We assessed the fish community on coral reefs on the Caribbean coast of Panama, as well as those associated with mangrove and seagrass habitats, to explore the influences of habitat cover, connectivity and environmental characteristics in sustaining biomass, richness and trophic structure in a degraded tropical ecosystem. Overall, 94 % of all fishes across all habitat types were of small body size (≤11 cm), with communities dominated by fishes that usually live in habitats of low complexity, such as Pomacentridae (damselfishes) and Gobiidae (gobies). Moreover, total fish biomass was very low, small fishes from low trophic levels were over-represented, and top predators were under-represented relative to other Caribbean reefs. For example, herbivorous/omnivorous/detrivorous fishes (trophic level 2-2.7) comprised 37 % of total fish biomass, with the diminutive parrotfish Scarus iseri comprising 72 % of the parrotfish biomass. However, the abundance of sponges and proximity of mangroves were found to be important positive drivers of reef fish richness, biomass and trophic structure on a given reef, presumably by promoting functional processes of ecosystems. The masked goby (Coryphopterus personata) was a strong indicator of reef degradation, apparently benefiting from the reduced density of large predators on local reefs. The damselfish Abudefduf saxatilis was more common on reefs with high sponge cover, and also to proximity to mangroves. Our study suggests that a diverse fish community can persist on degraded coral reefs, and that the availability of habitat forming organisms other than corals, including sponges and mangroves, and their arrangement on the landscape, is critical to the maintenance of functional processes in these ecosystems.

Bioerosion, the breakdown of hard substrata by organisms, is a fundamental and widespread ecological process that can alter habitat structure, biodiversity and biogeochemical cycling. Bioerosion occurs in all biomes of the world from the ocean floor to arid deserts, and involves a wide diversity of taxa and mechanisms with varying ecological effects. Many abiotic and biotic factors affect bioerosion by acting on the bioeroder, substratum, or both. Bioerosion also has socio-economic impacts when objects of economic or cultural value such as coastal defences or monuments are damaged. We present a unifying definition and advance a conceptual framework for (a) examining the effects of bioerosion on natural systems and human infrastructure and (b) identifying and predicting the impacts of anthropogenic factors (e.g. climate change, eutrophication) on bioerosion. Bioerosion is responding to anthropogenic changes in multiple, complex ways with significant and wide-ranging effects across systems. Emerging data further underscore the importance of bioerosion, and need for mitigating its impacts, especially at the dynamic land-sea boundary. Generalised predictions remain challenging, due to context-dependent effects and nonlinear relationships that are poorly resolved. An integrative and interdisciplinary approach is needed to understand how future changes will alter bioerosion dynamics across biomes and taxa.

Facilitation cascades generated by co-occurring foundation species can enhance the abundance and diversity of associated organisms. However, it remains poorly understood how differences among native and invasive species in their ability to exploit these positive interactions contribute to emergent patterns of community structure and biotic acceptance. On intertidal shorelines in New England, we examined the patterns of coexistence between the native mud crabs and the invasive Asian shore crab in and out of a facilitation cascade habitat generated by mid intertidal cordgrass and ribbed mussels. These crab species co-occurred in low intertidal cobbles adjacent to the cordgrass–mussel beds, despite experimental findings that the dominant mud crabs can kill and displace Asian shore crabs and thereby limit their successful recruitment to their shared habitat. A difference between the native and invasive species in their utilization of the facilitation cascade likely contributes to this pattern. Only the Asian shore crabs inhabit the cordgrass–mussel beds, despite experimental evidence that both species can similarly benefit from stress amelioration in the beds. Moreover, only Asian shore crabs settle in the beds, which function as a nursery habitat free of lethal mud crabs, and where their recruitment rates are particularly high (nearly an order of magnitude higher than outside beds). Persistence of invasive adult Asian shore crabs among the dominant native mud crabs in the low cobble zone is likely enhanced by a spillover effect of the facilitation cascade in which recruitment-limited Asian shore crabs settle in the mid intertidal cordgrass–mussel beds and subsidize their vulnerable populations in the adjacent low cobble zone. This would explain why the abundances of Asian shore crabs in cobbles are doubled when adjacent to facilitation cascade habitats. The propensity for this exotic species to utilize habitats created by facilitation cascades, despite the lack of a shared evolutionary history, contributes to species coexistence and the acceptance of invasives into a diverse community.

Understanding ecosystem resilience to human impacts is critical for conserva- tion and restoration. The large-scale die-off of New England salt marshes was triggered by overfishing and resulted from decades of runaway crab grazing. In 2009, however, cordgrass began to recover, decreasing die-off ;40% by 2010. We used surveys and experiments to test whether plant-substrate feedbacks underlie marsh resilience. Initially, grazer-generated die-off swept through the cordgrass, creating exposed, stressful peat banks that inhibited plant growth. This desertification cycle broke when banks eroded and peat transitioned into mud with fewer herbivores, less grazing, and lower physical stress. Cordgrass reestablished in these areas through a feedback where it engineered a recovery zone by further ameliorating physical stresses and facilitating additional revegetation. Our results reveal that feedbacks can play a critical role in rapid, reversible ecosystem shifts associated with human impacts, and that the interplay of facilitative and consumer interactions should be incorporated into resilience theory.

Coastal areas are among the world's most productive and highly affected ecosystems. Centuries of human activity on coastlines have led to overexploitation of marine predators, which in turn has led to cascading ecosystem-level effects. Human effects and approaches to mediating them, however, differ regionally due to gradients in biotic and abiotic factors. Salt marsh die-off on Cape Cod, Massachusetts (U.S.A.), triggered by a recreational-fishing-induced trophic cascade that has released herbivorous crabs from predator control, has been ongoing since 1976. Similar salt marsh die-offs have been reported in Long Island Sound and Narragansett Bay (U.S.A.), but the driving mechanism of these die-offs has not been examined. We used field experiments to assess trophic interactions and historical reconstructions of 24 New England marshes to test the hypotheses that recreational fishing and predator depletion are a regional trigger of salt marsh die-off in New England and that die-offs in Long Island Sound and Narragansett Bay are more recent than those on Cape Cod. Predator depletion was the general trigger of marsh die-off and explained differences in herbivorous crab abundance and the severity of die-off across regions. Die-offs in Long Island Sound and Narragansett Bay are following a trajectory similar to die-off on Cape Cod, but are approximately 20 years behind those on Cape Cod. As a result, die-off currently affects 31.2% (SE 2.2) of low-marsh areas in Long Island Sound and Narragansett Bay, less than half the severity of die-off on Cape Cod. Our results contribute to the growing evidence that recreational fishing is an increasing threat to coastal ecosystems and that studying the effects of human activity at regional scales can provide insight into local effects and aid in early detection and potential remediation. Ontogenia Regional de un Incremento en la Mortandad en una Marisma Salada de Nueva Inglaterra Resumen Las zonas costeras se encuentran entre los ecosistemas mas productivos y mas afectados del mundo. Los siglos de actividad humana sobre las lineas costeras han ocasionado la sobreexplotacion de depredadores marinos, lo que ha llevado a efectos en cascada a nivel de ecosistema. Sin embargo, los efectos humanos y las aproximaciones para mediarlos varian regionalmente debido a los gradientes en los factores bioticos y abioticos. El incremento en la mortandad en una marisma salada en Cape Cod, Massachusetts (E.U.A.), iniciada por una cascada trofica inducida por pesca recreativa que ha liberado a los cangrejos herbivoros del control por depredadores, ha estado sucediendo desde 1976. Se han reportado incrementos similares en las marismas saladas de Long Island Sound y Narragansett Bay (E.U.A), pero el mecanismo que las causa no ha sido examinado. Usamos experimentos de campo para estudiar las interacciones troficas y reconstrucciones historicas de 24 marismas de Nueva Inglaterra para probar la hipotesis de que la pesca recreativa y la disminucion de depredadores son un detonante regional del incremento en la mortandad de las marismas saladas en Nueva Inglaterra y que los incrementos en Long Island Sound y Narragansett Bay son mas recientes que los de Cape Cod. La disminucion de depredadores fue el detonante general del incremento en mortandad de las marismas y explico las diferencias entre la abundancia de cangrejos herbivoros y la severidad del incremento a lo largo de las regiones. Los incrementos en Long Island Sound y Narragansett Bay estan siguiendo una trayectoria similar al de Cape Cod, pero estan aproximadamente 20 anos atras del de esa localidad. Como resultado, el incremento en la mortandad actualmente afecta 31.2% (SE 2.2) de las areas de marismas bajas en Long Island Sound y Narragansett Bay, menos de la mitad de la severidad del incremento en Cape Cod. Nuestros resultados contribuyen a la creciente evidencia de que la pesca recreativa es una amenaza que va en aumento para los ecosistemas costeros y que el estudio de los efectos de la actividad humana en escalas regionales puede proporcionar una introspectiva a los efectos locales y auxiliar en la deteccion temprana y la remediacion potencial.

Understanding human impacts on ecosystems is critical for conservation, but can be complicated by interactions between multiple impacts occurring at different times. Using historical patterns of ditch construction on Cape Cod, Massachusetts, we tested the hypothesis that mosquito ditches have exacerbated salt marsh die-offs. Ditching activities occurred in the 1930s and were followed by post-World War II shoreline development, which created > 90% of current shoreline infrastructure on Cape Cod. Recently, predator depletion caused by recreational fishing has allowed populations of a native herbivorous crab (Sesarma reticulatum) to increase dramatically, triggering herbivore-driven cordgrass (Spartina alterniflora) die-off at developed sites. Depression-era mosquito ditching had little effect for decades, but accelerated subsequent die-offs by expanding cordgrass habitat. Despite occurring decades apart, ditching interacted synergistically with shoreline development and recreational fishing to devastate ~ 5...

Foundation species provide many important ecosystem functions including the provision of habitat for diverse communities, but their degradation and mortality has the potential to compromise these roles. Corals are widely recognized foundation species that create reef habitats that are hotspots for biodiversity. However, the impact of global reef degradation on overall patterns of biodiversity remains difficult to predict because of our limited knowledge of mechanistic relationships between reef structure and community composition. We examined the resilience of invertebrate abundance and biodiversity on reefs following a recent coral mass mortality event on the Caribbean coast of Panama. First, we surveyed mobile invertebrate communities at both healthy and degraded reef sites and found that dead coral habitats support invertebrate assemblages that can be more diverse and abundant than live coral habitats and that coral habitat (whether live or dead) in turn supports higher diversity and abundance than structurally simple sand areas without coral. Second, we experimentally tested mechanisms of reef habitat suitability for invertebrate colonization by manipulating coral mortality and structural complexity. We found that the abundance and species richness of mobile invertebrates were significantly affected by substrate complexity rather than whether coral was live or dead. However, we detected shifts in species identity between live and dead coral. Moreover, the sensitivity of the community to reef structural complexity indicates that the ability of degraded coral reefs to sustain invertebrate assemblages is unlikely to persist if declines in reef complexity outpace recovery of living corals to the reef. Our findings suggest that the biodiversity‐sustaining function of reefs has the potential to persist following coral disturbance at the scale of entire reefs and that some metrics of community structure are therefore resilient to events of foundation species mortality.

Overexploitation of predators has been linked to the collapse of a growing number of shallow-water marine ecosystems. However, salt-marsh ecosystems are often viewed and managed as systems controlled by physical processes, despite recent evidence for herbivore-driven die-off of marsh vegetation. Here we use field observations, experiments, and historical records at 14 sites to examine whether the recently reported die-off of northwestern Atlantic salt marshes is associated with the cascading effects of predator dynamics and intensive recreational fishing activity. We found that the localized depletion of top predators at sites accessible to recreational anglers has triggered the proliferation of herbivorous crabs, which in turn results in runaway consumption of marsh vegetation. This suggests that overfishing may be a general mechanism underlying the consumer-driven die-off of salt marshes spreading throughout the western Atlantic. Our findings support the emerging realization that consumers play a dominant role in regulating marine plant communities and can lead to ecosystem collapse when their impacts are amplified by human activities, including recreational fishing.

Natural stress gradients can reduce predation intensity and increase prey abundances. Whether the harsh conditions of anthropogenic habitat degradation can similarly reduce predation intensity and structure community dynamics remains largely unexplored. Oxygen depletion in coastal waters (hypoxia) is a form of degradation that has recently emerged as one of the greatest threats to coastal ecosystems worldwide due to increased rates of eutrophication and climate change. I conducted field experiments and surveys to test whether relaxed predation could explain the paradoxically high abundance of clams that have sustained a fishery in a degraded estuary with chronic hypoxic conditions. Hypoxia reduced predation on all experimental species but enhanced the long-term survivorship of only sufficiently hypoxia-tolerant prey due to periodic extreme conditions. As a consequence, only the harvested quahog clam (Mercenaria mercenaria) thrived in hypoxic areas that were otherwise rendered dead zones with depauperate diversity and low abundances of other species. This suggests that enhanced populations of some key species may be part of a predictable nonlinear community response that sustains ecosystem services and masks overall downward trends of habitat degradation.

Ecological studies of plant distributions along environmental gradients, such as plant zonation in salt marshes, have primarily focused on abiotic stress and plant interactions (competition and facilitation). A decades-old paradigm is that the stressful and benign boundaries of salt marsh plants are determined by abiotic stress and competition, respectively. Although consumers have long been recognized as mediating algal and sessile animal zonation in the rocky intertidal, their role in generating plant zonation in salt marshes remains largely unexplored. We examined the zonation of two annual succulents, Salicornia europaea and Suaeda salsa, along an elevation gradient in a northern Chinese salt marsh, with and without manipulating the common herbivorous crab Helice tientsinensis. Salicornia occupies waterlogged, low-salinity habitats, whereas Suaeda dominates non-waterlogged, hypersaline habitats at higher elevations. We first conducted a pot experiment crossing salinity, waterlogging, and competition, followed by a field experiment with removal of competitors, and found that neither waterlogging nor salinity stress explained the absence of either species from the other's zone, while Suaeda competitively excluded Salicornia from the upper non-waterlogged zone. We then conducted field and lab herbivory experiments, which showed that Helice preferentially grazed Suaeda at waterlogged low elevations and that Helice grazing on Suaeda increased with waterlogging. These results reveal that while competition plays a role in the zonation by excluding Salicornia from the upper Suaeda zone, crab grazing limits the success of Suaeda in the lower Salicornia zone. These findings challenge the idea that plant interactions and abiotic stress are sufficient to explain marsh zonation in all cases, and highlight an overlooked role of consumers, a role potentially general across diverse intertidal ecosystems. Future models of plant distributions should consider how consumer pressure interacts with plant interactions and abiotic stress across environmental gradients.

Instances of mutualism and commensalism between alpheid shrimp and other marine invertebrates and fish are common in tropical waters. In this study, we present evidence that the temperate alpheid big-clawed snapping shrimp (Alpheus heterochaelis) participates in a symbiosis with the black-clawed mud crab (Panopeus herbstii), which constructs and maintains burrows in salt marshes of mid-Atlantic states, U.S.A. We surveyed eight mid-Atlantic salt marshes and found that 11% of occupied crab burrows (n = 1042) also housed Alpheus. Feeding trials showed that whereas Panopeus readily consumes other marsh Crustacea (i.e., grass shrimp and fiddler crabs), it will not prey on snapping shrimp. Alpheus caged in the marsh without access to mud crab lairs could not construct burrows. These results suggest that alpheid shrimps, thought to be limited in distribution to unvegetated mudflats and oyster reefs, may expand their intertidal range by living commensally with Panopeus herbstii in salt marsh habitats.

Mounting evidence indicates that spatial interactions are important in structuring coastal ecosystems. Until recently, however, most of this work has been focused on seemingly exceptional systems that are characterized by regular, self-organized patterns. In this review, we document that interactions that operate at long distances, beyond the direct neighborhood of individual organisms, are more common and have much more far-reaching implications for coastal ecosystems than was previously realized. We review studies from a variety of ecosystem types—including cobble beaches, mussel beds, coral reefs, seagrass meadows, and mangrove forests—that reveal a startling interplay of positive and negative interactions between habitats across distances of up to a kilometer. In addition to classical feeding relations, alterations of physical conditions constitute an important part of these long-distance interactions. This entanglement of habitats has crucial implications for how humans manage coastal ecosystems, and evaluations of anthropogenic impact should explicitly address long-distance and system-wide effects before we deem these human activities to be causing little harm.

The combined acoustic behavior of soniferous organisms living on benthic habitats from coral to oyster reefs produces habitat-specific soundscapes. These soundscapes are predicted to have a role in the settlement behavior of fish and invertebrate larvae searching for a benthic habitat on which to settle at the conclusion of their pelagic phase. Given the frequency-specific sound detection abilities of these organisms, only a portion of the soundscape will have the frequency and amplitude characteristics to serve as potential navigational and habitat selection cues. We recorded the soundscapes of four coral reefs in Caribbean Panama for six weeks and predicted the sounds most likely to be used as cues by larval fishes by using knowledge of their hearing sensitivity. Next we used an individual-based model to test the relationship between the temporal characteristics of the acoustic cues and settlement success. To do this we created cue time-series that represented the temporal variability observed at the fo...

Predator depletion on Cape Cod (USA) has released the herbivorous crab Sesarma reticulatum from predator control leading to the loss of cordgrass from salt marsh creek banks. After more than three decades of die-off, cordgrass is recovering at heavily damaged sites coincident with the invasion of green crabs ( Carcinusmaenas ) into intertidal Sesarma burrows. We hypothesized that Carcinus is dependent on Sesarma burrows for refuge from physical and biotic stress in the salt marsh intertidal and reduces Sesarma functional density and herbivory through consumptive and non-consumptive effects, mediated by both visual and olfactory cues. Our results reveal that in the intertidal zone of New England salt marshes, Carcinus are burrow dependent, Carcinus reduce Sesarma functional density and herbivory in die-off areas and Sesarma exhibit a generic avoidance response to large, predatory crustaceans. These results support recent suggestions that invasive Carcinus are playing a role in the recovery of New England salt marshes and assertions that invasive species can play positive roles outside of their native ranges.

Nearly all coral reef fishes pass through a pelagic larval stage that concludes when they locate and select appropriate reef habitat. At least some species use the reef soundscape—the collection of sounds produced by reef-dwelling organisms—to achieve this task that is necessary for survival. Since larval fishes in the open ocean are inherently difficult to study, investigating their acoustic environment can allow predictions about their behavior in response to these cues. Further, we can predict the roles of different frequency bands in aiding long-distance navigation or short-distance habitat selection. We recorded the soundscapes at four reefs in Caribbean Panama for six weeks to inform the identity and variability of the most common sounds. We next predict the frequency-dependent transmission loss using an acoustic propagation model calibrated with acoustic transects. Including knowledge of source sound levels allows us to elucidate the sound fields created by low frequency fish and high frequency inv...

Belowground herbivory is commonly overlooked as a mechanism of top-down control in vegetated habitats, particularly in aquatic ecosystems. Recent research has revealed that increased densities of the herbivorous crab Sesarma reticulatum have led to runaway herbivory and widespread salt marsh die-off on Cape Cod, Massachusetts, USA. Aboveground herbivory is a major driver of this cordgrass habitat loss, but the role of belowground grazing is poorly understood. Sesarma live in communal burrows typically consisting of 1-2 openings and containing 2-3 crabs. However, at die-off sites, burrow complexes can cover > 90% of the low marsh zone, with crab densities as high as 50 crabs/m2 and burrow opening densities of 170 openings/m2. The magnitude of belowground Sesarma activity in association with salt marsh die-off provides an excellent opportunity to extend our knowledge of belowground herbivory impacts in coastal wetlands. Since Sesarma burrows allow access to cordgrass roots and rhizomes, and Sesarma are frequently restricted to burrows by thermal stress and predation, we hypothesized that belowground herbivory would be widespread in die-off areas. We experimentally demonstrate that Sesarma readily eat belowground roots and rhizomes in addition to aboveground cordgrass leaves. We then partitioned above- and belowground herbivory with field manipulations and found that belowground grazing is not only common, but can cause total plant mortality. Additional experiments revealed that plants remain vulnerable to belowground herbivory even after reaching a size refuge from aboveground grazing. This suggests that belowground herbivory contributes to salt marsh die-offs and adds to growing evidence that belowground herbivory is a widespread structuring force in plant communities that can limit habitat persistence.

Die-offs of cordgrass are pervasive throughout western Atlantic salt marshes, yet understanding of the mechanisms precipitating these events is limited. We tested whether herbivory by the native crab, Sesarma reticulatum, is generating die-offs of cordgrass that are currently occurring on Cape Cod, Massachusetts (U.S.A.), by manipulating crab access to cordgrass transplanted into die-off areas and healthy vegetation. We surveyed 12 Cape Cod marshes to investigate whether the extent of cordgrass die-off on creek banks, where die-offs are concentrated, was related to local Sesarma grazing intensity and crab density. We then used archived aerial images to examine whether creek bank die-off areas have expanded over the past 2 decades and tested the hypothesis that release from predation, leading to elevated Sesarma densities, is triggering cordgrass die-offs by tethering crabs where die-offs are pervasive and where die-offs have not yet been reported. Intensity of crab grazing on transplanted cordgrass was an order of magnitude higher in die-off areas than in adjacent vegetation. Surveys revealed that Sesarma herbivory has denuded nearly half the creek banks in Cape Cod marshes, and differences in crab-grazing intensity among marshes explained >80% of variation in the extent of the die-offs. Moreover, the rate of die-off expansion and area of marsh affected have more than doubled since 2000. Crab-tethering experiments suggest that release from predation has triggered elevated crab densities that are driving these die-offs, indicating that disruption of predator-prey interactions may be generating the collapse of marsh ecosystems previously thought to be exclusively under bottom-up control.

BACKGROUND: Our understanding of the functional consequences of changes in biodiversity has been hampered by several limitations of previous work, including limited attention to trophic interactions, a focus on species richness rather than evenness, and the use of artificially assembled communities. METHODOLOGY AND PRINCIPAL FINDINGS: In this study, we manipulated the density of an herbivorous snail in natural tide pools and allowed seaweed communities to assemble in an ecologically relevant and non-random manner. Seaweed species evenness and biomass-specific primary productivity (mg O(2) h(-1) g(-1)) were higher in tide pools with snails because snails preferentially consumed an otherwise dominant seaweed species that can reduce biomass-specific productivity rates of algal assemblages. Although snails reduced overall seaweed biomass in tide pools, they did not affect gross primary productivity at the scale of tide pools (mg O(2) h(-1) pool(-1) or mg O(2) h(-1) m(-2)) because of the enhanced biomass-specific productivity associated with grazer-mediated increases in algal evenness. SIGNIFICANCE: Our results suggest that increased attention to trophic interactions, diversity measures other than richness, and particularly the effects of consumers on evenness and primary productivity, will improve our understanding of the relationship between diversity and ecosystem functioning and allow more effective links between experimental results and real-world changes in biodiversity.

Though species interactions across local environmental gradients are well studied, the way in which species interactions shift between different habitats on a landscape scale has received less attention. We hypothesised that interactions among a suite of shoreline plant species shift across a hydrodynamic-exposure gradient, leading to generation of apparently distinct habitat types (e.g. bare cobble beaches, vegetated cobble beaches, fringing marshes and salt marshes). We examined hydrodynamic forcing and found that it was strongly correlated with shoreline habitat type. A trans- plant experiment revealed that all plants were rapidly crushed and abraded in bare cobble areas with high hydrodynamic energy and were out-competed by grasses in the low-energy salt marshes. Vegetated cobble beaches are inhabited by the largest amount of plant species under intermediate conditions, where hydrodynamic energy is sufficiently low to allow the establishment of the ecosys- tem engineer Spartina and sufficiently high to prevent the monopolisation of space by competitively dominant, but stress intolerant grasses. Experimentally reducing physical and biotic stresses (hydro- dynamics and interspecific competition) on bare cobble beaches and salt marshes, respectively, enabled forbs to persist across the whole gradient. These results demonstrate that the outcome of interspecific interactions at landscape scales is driven by background physical conditions, and that this can result in the development of what are considered distinct habitats.

We integrated across individual, population, community, and ecosystem levels to understand the impact of environmental stress by tracking the foundation species Mytilus edulis in the hypoxic estuary Narragansett Bay, Rhode Island, USA. Our initial surveys revealed that the mussels occurred in nine extensive (2-28 ha) dense (814-9943 individuals/m2) subtidal reefs that attracted a diverse suite of predators (sea stars, crabs, gastropods). Hypoxia occurred in the summer of 2001, and a mussel transplant experiment revealed overall reduced growth rates of individuals, and higher mortality rates among larger mussels. At the population level, large decreases in densities and cover of mussels were correlated with dissolved oxygen concentrations, leading to extinction at one site and reductions of over an order of magnitude at others. Within one year, seven of the eight remaining populations were edged to extinction, and the previously extinct population was recolonized. At the community level, a predator exclusion experiment indicated that predation was an unimportant source of mussel mortality during the hypoxic period, in part due to the emigration of sea stars, as predicted by the Consumer Stress Model. However, mussels were too intolerant to hypoxia to have a net benefit from the predation refuge. The seasonal (summer) occurrence of hypoxia allowed sea stars to return following a lag, as predicted by a stress return time model, and the resumption of predation contributed to the subsequent extinction of mussel populations. At the ecosystem level, the initial filtration rate of the mussel reefs was estimated at 134.6 x 10(6) m3/d, equivalent to filtering the volume of the bay 1.3 times during the 26-d average residence time. That function was reduced by >75% following hypoxia. The effect of hypoxia on each level of organization had consequences at others. For example, size-specific mortality and decreased growth of individuals, and reduced filtration capacity of reefs, indicated a loss of the ability of mussels to entrain planktonic productivity and potential to control future eutrophication and hypoxia. Our study quantified patterns of loss and identified pathways within an integrative framework of feedbacks, summarized in a conceptual model that is applicable to similar foundation species subjected to environmental stress.

The complex structure of biogenic habitats including coral reefs, hemlock forests, and seagrass meadows are widely recognized for the diverse communities they shelter. As human impacts accelerate the loss of such foundation species, we need to identify the general characteristics of previously unrecognized species that can also fulfill the role of habitat provider. We conducted surveys and experiments to test whether the slate-pencil urchin (Eucidaris galapagensis) acts as a foundation species on subtidal walls in the Galapagos archipelago. The spines of slate-pencil urchins are more than 90% encrusted with a diverse epifauna, and a single urchin can host over 20 species (e.g., sponges, ascidians, bryozoans, corals, molluscs, worms, and crustaceans). Like many other foundation species, urchins can provide substrate and a refuge from predators. Urchins are consistently abundant throughout the Galapagos subtidal, and the total surface area of urchin spines can rival that of the primary rock substrate, which is significant since substrate availability can limit small-scale species richness in this system. Our experimental manipulation of spine configuration and exclusion of predators revealed that urchins also enhance epifaunal diversity by providing a predation refuge. Unlike previously recognized foundation species, however, the urchin habitat is modular and mobile, and has the potential to redistribute associated epifauna. Characterizing how previously overlooked foundation species can act as reservoirs of biodiversity in ecosystems, such as the Galapagos where other foundation species such as coral have declined, has important implications for how we identify foundation species, predict ecosystem stability, and prioritize conservation efforts.