A Pleistocene legacy structures variation in modern seagrass ecosystems.

Distribution of Earth's biomes is structured by the match between climate and plant traits, which in turn shape associated communities and ecosystem processes and services. However, that climate-trait match can be disrupted by historical events, with lasting ecosystem impacts. As Earth's environment changes faster than at any time in human history, critical questions are whether and how organismal traits and ecosystems can adjust to altered conditions. We quantified the relative importance of current environmental forcing versus evolutionary history in shaping the growth form (stature and biomass) and associated community of eelgrass (Zostera marina), a widespread foundation plant of marine ecosystems along Northern Hemisphere coastlines, which experienced major shifts in distribution and genetic composition during the Pleistocene. We found that eelgrass stature and biomass retain a legacy of the Pleistocene colonization of the Atlantic from the ancestral Pacific range and of more recent within-basin bottlenecks and genetic differentiation. This evolutionary legacy in turn influences the biomass of associated algae and invertebrates that fuel coastal food webs, with effects comparable to or stronger than effects of current environmental forcing. Such historical lags in phenotypic acclimatization may constrain ecosystem adjustments to rapid anthropogenic climate change, thus altering predictions about the future functioning of ecosystems.

Climate drives the geography of marine consumption by changing predator communities.

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.

The biogeography of community assembly: latitude and predation drive variation in community trait distribution in a guild of epifaunal crustaceans.

While considerable evidence exists of biogeographic patterns in the intensity of species interactions, the influence of these patterns on variation in community structure is less clear. Studying how the distributions of traits in communities vary along global gradients can inform how variation in interactions and other factors contribute to the process of community assembly. Using a model selection approach on measures of trait dispersion in crustaceans associated with eelgrass (Zostera marina) spanning 30° of latitude in two oceans, we found that dispersion strongly increased with increasing predation and decreasing latitude. Ocean and epiphyte load appeared as secondary predictors; Pacific communities were more overdispersed while Atlantic communities were more clustered, and increasing epiphytes were associated with increased clustering. By examining how species interactions and environmental filters influence community structure across biogeographic regions, we demonstrate how both latitudinal variation in species interactions and historical contingency shape these responses. Community trait distributions have implications for ecosystem stability and functioning, and integrating large-scale observations of environmental filters, species interactions and traits can help us predict how communities may respond to environmental change.

The impacts of mangrove range expansion on wetland ecosystem services in the southeastern United States: Current understanding, knowledge gaps, and emerging research needs.

Climate change is transforming ecosystems and affecting ecosystem goods and services. Along the Gulf of Mexico and Atlantic coasts of the southeastern United States, the frequency and intensity of extreme freeze events greatly influence whether coastal wetlands are dominated by freeze-sensitive woody plants (mangrove forests) or freeze-tolerant grass-like plants (salt marshes). In response to warming winters, mangroves have been expanding and displacing salt marshes at varying degrees of severity in parts of north Florida, Louisiana, and Texas. As winter warming accelerates, mangrove range expansion is expected to increasingly modify wetland ecosystem structure and function. Because there are differences in the ecological and societal benefits that salt marshes and mangroves provide, coastal environmental managers are challenged to anticipate the effects of mangrove expansion on critical wetland ecosystem services, including those related to carbon sequestration, wildlife habitat, storm protection, erosion reduction, water purification, fisheries support, and recreation. Mangrove range expansion may also affect wetland stability in the face of extreme climatic events and rising sea levels. Here, we review the current understanding of the effects of mangrove range expansion and displacement of salt marshes on wetland ecosystem services in the southeastern United States. We also identify critical knowledge gaps and emerging research needs regarding the ecological and societal implications of salt marsh displacement by expanding mangrove forests. One consistent theme throughout our review is that there are ecological trade-offs for consideration by coastal managers. Mangrove expansion and marsh displacement can produce beneficial changes in some ecosystem services, while simultaneously producing detrimental changes in other services. Thus, there can be local-scale differences in perceptions of the impacts of mangrove expansion into salt marshes. For very specific local reasons, some individuals may see mangrove expansion as a positive change to be embraced, while others may see mangrove expansion as a negative change to be constrained.

Meta-analysis of salt marsh vegetation impacts and recovery: a synthesis following the Deepwater Horizon oil spill.

Marine oil spills continue to be a global issue, heightened by spill events such as the 2010 Deepwater Horizon spill in the Gulf of Mexico, the largest marine oil spill in US waters and among the largest worldwide, affecting over 1,000 km of sensitive wetland shorelines, primarily salt marshes supporting numerous ecosystem functions. To synthesize the effects of the oil spill on foundational vegetation species in the salt marsh ecosystem, Spartina alterniflora and Juncus roemerianus, we performed a meta-analysis using data from 10 studies and 255 sampling sites over seven years post-spill. We examined the hypotheses that the oil spill reduced plant cover, stem density, vegetation height, aboveground biomass, and belowground biomass, and tracked the degree of effects temporally to estimate recovery time frames. All plant metrics indicated impacts from oiling, with 20-100% maximum reductions depending on oiling level and marsh zone. Peak reductions of ~70-90% in total plant cover, total aboveground biomass, and belowground biomass were observed for heavily oiled sites at the marsh edge. Both Spartina and Juncus were impacted, with Juncus affected to a greater degree. Most plant metrics had recovery time frames of three years or longer, including multiple metrics with incomplete recovery over the duration of our data, at least seven years post-spill. Belowground biomass was particularly concerning, because it declined over time in contrast with recovery trends in most aboveground metrics, serving as a strong indicator of ongoing impact, limited recovery, and impaired resilience. We conclude that the Deepwater Horizon spill had multiyear impacts on salt marsh vegetation, with full recovery likely to exceed 10 years, particularly in heavily oiled marshes, where erosion may preclude full recovery. Vegetation impacts and delayed recovery is likely to have exerted substantial influences on ecosystem processes and associated species, especially along heavily oiled shorelines. Our synthesis affords a greater understanding of ecosystem impacts and recovery following the Deepwater Horizon oil spill, and informs environmental impact analysis, contingency planning, emergency response, damage assessment, and restoration efforts related to oil spills.

Repeated Genetic and Adaptive Phenotypic Divergence across Tidal Elevation in a Foundation Plant Species.

AbstractMicrogeographic genetic divergence can create fine-scale trait variation. When such divergence occurs within foundation species, then it might impact community structure and ecosystem function and cause other cascading ecological effects. We tested for parallel microgeographic trait and genetic divergence in Spartina alterniflora, a foundation species that dominates salt marshes of the US Atlantic and Gulf coasts. Spartina is characterized by tall-form (1-2 m) plants at lower tidal elevations and short-form (<0.5 m) plants at higher tidal elevations, yet whether this trait variation reflects plastic and/or genetically differentiated responses to these environmental conditions remains unclear. In the greenhouse, seedlings raised from tall-form plants grew taller than those from short-form plants, indicating a heritable difference in height. When we reciprocally transplanted seedlings back into the field for a growing season, composite fitness (survivorship and seed production) and key plant traits (plant height and biomass allocation) differed interactively across origin and transplant zones in a manner indicative of local adaptation. Further, a survey of single nucleotide polymorphisms revealed repeated, independent genetic differentiation between tall- and short-form Spartina at five of six tested marshes across the native range. The observed parallel, microgeographic genetic differentiation in Spartina likely underpins marsh health and functioning and provides an underappreciated mechanism that might increase capacity of marshes to adapt to rising sea levels.

Recruitment enhancement varies by taxonomic group and oyster reef habitat characteristics.

The rapid loss of coastal and estuarine biogenic habitats has reduced the delivery of valuable ecosystem services, resulting in calls for increased habitat restoration. Yet, a lack of information on how key habitat characteristics (e.g., area, vertical relief, age) influence the ability of restored habitats to deliver these ecosystem services hinders efforts to maximize the return on restoration investments. We conducted a meta-analysis to assess the influence of reef type (natural or restored), taxa, and restored reef size, vertical relief, age, and tidal zone on the presence and magnitude of recruitment enhancement for nekton (i.e., fish and swimming crabs). Both intertidal and subtidal reefs, as well as restored and natural reefs, enhanced nekton recruitment, though there was variation among taxonomic groups with reef types. Recruitment enhancement was more common across taxa on restored (six families) than on natural (one family) reefs. Resident nekton families were more consistently enhanced than transient families. Nekton enhancement varied with a number of restored reef characteristics. Recruitment enhancement increased with greater reef size across taxa, decreased with higher vertical relief for two families, showed maximum recruitment around a single intertidal reef age for one family, and showed minimum recruitment around a single subtidal reef age for three families. Understanding variation across species in response to key design elements will improve restoration success and enhance return on investment. Moving forward, we recommend studies that vary reef habitat characteristics independently and in combination to identify how variation in these characteristics interact to influence nekton recruitment enhancement by oyster reefs.

Plant response to fungal root endophytes varies by host genotype in the foundation species Spartina alterniflora.

PREMISE: Root-associated fungi provide a wide range of functions for their host plants, including nutrient provisioning, pathogen protection, and stress alleviation. In so doing, they can markedly influence host-plant structural and physiological traits, although the degree to which these effects vary within particular plant host species is not well understood. METHODS: We conducted a 7-month common-garden inoculation experiment to test the potential effects of a marine fungus (Lulwoana sp.) on the phenotypic traits of different genotypes of the host, the salt marsh plant species Spartina alterniflora. Lulwoana belongs to the dark septate endophytes (DSE), a polyphyletic group of fungi that are commonly found colonizing healthy plant roots, though their ecological role remains unclear. RESULTS: We documented significant impacts of Lulwoana on S. alterniflora morphology, biomass, and biomass allocation. For most traits in our study, these impacts varied significantly in direction and/or magnitude across S. alterniflora genotypes. Effects that were consistent across genotype were generally negative. Plant response was not predicted by the percentage of roots colonized, consistent with findings that dark septate endophytes do not necessarily influence plant growth responses through direct contact with roots. CONCLUSIONS: The observed changes in stem height, biomass, and biomass allocation have important effects on plant competitive ability, growth, and fitness, suggesting that plant-fungal interactions have community and ecosystem level effects in salt marshes.

Genetic diversity and phenotypic variation within hatchery-produced oyster cohorts predict size and success in the field.

The rapid growth of the aquaculture industry to meet global seafood demand offers both risks and opportunities for resource management and conservation. In particular, hatcheries hold promise for stock enhancement and restoration, yet cultivation practices may lead to enhanced variation between populations at the expense of variation within populations, with uncertain implications for performance and resilience. To date, few studies have assessed how production techniques impact genetic diversity and population structure, as well as resultant trait variation in and performance of cultivated offspring. We collaborated with a commercial hatchery to produce multiple cohorts of the eastern oyster (Crassostrea virginica) from field-collected broodstock using standard practices. We recorded key characteristics of the broodstock (male : female ratio, effective population size), quantified the genetic diversity of the resulting cohorts, and tested their trait variation and performance across multiple field sites and experimental conditions. Oyster cohorts produced under the same conditions in a single hatchery varied almost twofold in genetic diversity. In addition, cohort genetic diversity was a significant positive predictor of oyster performance traits, including initial size and survival in the field. Oyster cohorts produced in the hatchery had lower within-cohort genetic variation and higher among-cohort genetic structure than adults surveyed from the same source sites. These findings are consistent with "sweepstakes reproduction" in oysters, even when manually spawned. A readily measured characteristic of broodstock, the ratio of males to females, was positively correlated with within-cohort genetic diversity of the resulting offspring. Thus, this metric may offer a tractable way both to meet short-term production goals for seafood demand and to ensure the capacity of hatchery-produced stock to achieve conservation objectives, such as the recovery of self-sustaining wild populations.

Nonconsumptive effects of a predator weaken then rebound over time.

Predators can influence prey traits and behavior (nonconsumptive effects [NCEs]), often with cascading effects for basal resources and ecosystem function. But critiques of NCE experiments suggest that their duration and design produce results that describe the potential importance of NCEs rather than their actual importance. In light of these critiques, we re-evaluated a toadfish (predator), crab (prey), and oyster (resource) NCE-mediated trophic cascade. In a 4-month field experiment, we varied toadfish cue (NCE) and crab density (approximating variation in predator consumptive effects, CE). Toadfish initially benefitted oyster survival by causing crabs to reduce consumption. But this NCE weakened over time (possibly due to prey hunger), so that after 2 months, crab density (CE) dictated oyster survivorship, regardless of cue. However, the NCE ultimately re-emerged on reefs with a toadfish cue, increasing oyster survivorship. At no point did the effect of toadfish cue on mud crab foraging behavior alter oyster population growth or sediment organic matter on the reef, which is a measure of benthic-pelagic coupling. Instead, both decreased with increasing crab density. Thus, within a system shown to exhibit strong NCEs in short-term experiments (days) our study supported predictions from theoretical models: (a) within the generation of individual prey, the relative influence of NCEs appears to cycle over longer time periods (months); and (b) predator CEs, not NCEs, drive longer-term resource dynamics and ecosystem function. Thus, our study implies that the impacts of removing top predators via activities such as hunting and overfishing will cascade to basal resources and ecosystem properties primarily through density-mediated interactions.

Predators, environment and host characteristics influence the probability of infection by an invasive castrating parasite.

Not all hosts, communities or environments are equally hospitable for parasites. Direct and indirect interactions between parasites and their predators, competitors and the environment can influence variability in host exposure, susceptibility and subsequent infection, and these influences may vary across spatial scales. To determine the relative influences of abiotic, biotic and host characteristics on probability of infection across both local and estuary scales, we surveyed the oyster reef-dwelling mud crab Eurypanopeus depressus and its parasite Loxothylacus panopaei, an invasive castrating rhizocephalan, in a hierarchical design across >900 km of the southeastern USA. We quantified the density of hosts, predators of the parasite and host, the host's oyster reef habitat, and environmental variables that might affect the parasite either directly or indirectly on oyster reefs within 10 estuaries throughout this biogeographic range. Our analyses revealed that both between and within estuary-scale variation and host characteristics influenced L. panopaei prevalence. Several additional biotic and abiotic factors were positive predictors of infection, including predator abundance and the depth of water inundation over reefs at high tide. We demonstrate that in addition to host characteristics, biotic and abiotic community-level variables both serve as large-scale indicators of parasite dynamics.

Effects of intraspecific diversity on survivorship, growth, and recruitment of the eastern oyster across sites.

Intraspecific diversity, particularly of foundation species, can significantly affect population, community, and ecosystem processes. Examining how genetic diversity relates to demographic traits provides a key mechanistic link from genotypic and phenotypic variation of taxa with complex life histories to their population dynamics. We conducted a field experiment to assess how two metrics of intraspecific diversity (cohort diversity, the number of independent juvenile cohorts created from different adult source populations, and genetic relatedness, genetic similarity among individuals within and across cohorts) affect the survivorship, growth, and recruitment of the foundation species Crassostrea virginica. To assess the effects of both cohort diversity and genetic relatedness on oyster demographic traits under different environmental conditions, we manipulated juvenile oyster diversity and predator exposure (presence/absence of a cage) at two sites differing in resource availability and predation intensity. Differences in predation pressure between sites overwhelmingly determined post-settlement survivorship of oysters. However, in the absence of predation (i.e., cage treatment), one or both metrics of intraspecific diversity, in addition to site, influenced long-term survivorship, growth, and recruitment. While both cohort diversity and genetic relatedness were negatively associated with long-term survivorship, genetic relatedness alone showed a positive association with growth and cohort diversity alone showed a positive association with recruitment. Thus, our results demonstrate that in the absence of predation, intraspecific diversity can affect multiple demographic traits of a foundation species, but the relative importance of these effects depends on the environmental context. Moreover, the magnitude and direction of these effects vary depending on the diversity metric, cohort diversity or genetic relatedness, suggesting that although they are inversely related in this system, each captures sufficiently different components of intraspecific diversity. Given the global loss of oyster reef habitat and rapid decline in oyster population size, our results are particularly relevant to management and restoration. In addition, aquaculture, which commonly excludes predators during early life history stages, may benefit from incorporation of oyster cohort diversity into standard practice.

The biogeography of trophic cascades on US oyster reefs.

Predators can indirectly benefit prey populations by suppressing mid-trophic level consumers, but often the strength and outcome of trophic cascades are uncertain. We manipulated oyster reef communities to test the generality of potential causal factors across a 1000-km region. Densities of oyster consumers were weakly influenced by predators at all sites. In contrast, consumer foraging behaviour in the presence of predators varied considerably, and these behavioural effects altered the trophic cascade across space. Variability in the behavioural cascade was linked to regional gradients in oyster recruitment to and sediment accumulation on reefs. Specifically, asynchronous gradients in these factors influenced whether the benefits of suppressed consumer foraging on oyster recruits exceeded costs of sediment accumulation resulting from decreased consumer activity. Thus, although predation on consumers remains consistent, predator influences on behaviour do not; rather, they interact with environmental gradients to cause biogeographic variability in the net strength of trophic cascades.

Predatory fish sounds can alter crab foraging behaviour and influence bivalve abundance.

The risk of predation can have large effects on ecological communities via changes in prey behaviour, morphology and reproduction. Although prey can use a variety of sensory signals to detect predation risk, relatively little is known regarding the effects of predator acoustic cues on prey foraging behaviour. Here we show that an ecologically important marine crab species can detect sound across a range of frequencies, probably in response to particle acceleration. Further, crabs suppress their resource consumption in the presence of experimental acoustic stimuli from multiple predatory fish species, and the sign and strength of this response is similar to that elicited by water-borne chemical cues. When acoustic and chemical cues were combined, consumption differed from expectations based on independent cue effects, suggesting redundancies among cue types. These results highlight that predator acoustic cues may influence prey behaviour across a range of vertebrate and invertebrate taxa, with the potential for cascading effects on resource abundance.

Host and parasite recruitment correlated at a regional scale.

Drivers of large-scale variability in parasite prevalence are not well understood. For logistical reasons, explorations of spatial patterns in parasites are often performed as observational studies. However, to understand the mechanisms that underlie these spatial patterns, standardized and controlled comparisons are needed. Here, we examined spatial variability in infection of an important fishery species and ecosystem engineer, the oyster (Crassostrea virginica) by its pea crab parasite (Zaops ostreus) across 700 km of the southeastern USA coastline. To minimize the influence of host genetics on infection patterns, we obtained juvenile oysters from a homogeneous source stock and raised them in situ for 3 months at multiple sites with similar environmental characteristics. We found that prevalence of pea crab infection varied between 24 and 73% across sites, but not systematically across latitude. Of all measured environmental variables, oyster recruitment correlated most strongly (and positively) with pea crab infection, explaining 92% of the variability in infection across sites. Our data ostensibly suggest that regional processes driving variation in oyster recruitment similarly affect the recruitment of one of its common parasites.

Genetic relatedness influences plant biomass accumulation in eelgrass (Zostera marina).

In multispecies assemblages, phylogenetic relatedness often predicts total community biomass. In assemblages dominated by a single species, increasing the number of genotypes increases total production, but the role of genetic relatedness is unknown. We used data from three published experiments and a field survey of eelgrass (Zostera marina), a habitat-forming marine angiosperm, to examine the strength and direction of the relationship between genetic relatedness and plant biomass. The genetic relatedness of an assemblage strongly predicted its biomass, more so than the number of genotypes. However, contrary to the pattern observed in multispecies assemblages, maximum biomass occurred in assemblages of more closely related individuals. The mechanisms underlying this pattern remain unclear; however, our data support a role for both trait differentiation and cooperation among kin. Many habitat-forming species interact intensely with conspecifics of varying relatedness; thus, genetic relatedness could influence the functioning of ecosystems dominated by such species.

The effect of warming on seagrass wasting disease depends on host genotypic identity and diversity.

Temperature increases due to climate change have affected the distribution and severity of diseases in natural systems, causing outbreaks that can destroy host populations. Host identity, diversity, and the associated microbiome can affect host responses to both infection and temperature, but little is known about how they could function as important mediators of disease in altered thermal environments. We conducted an 8-week warming experiment to test the independent and interactive effects of warming, host genotypic identity, and host genotypic diversity on the prevalence and intensity of infections of seagrass (Zostera marina) by the wasting disease parasite (Labyrinthula zosterae). At elevated temperatures, we found that genotypically diverse host assemblages had reduced infection intensity, but not reduced prevalence, relative to less diverse assemblages. This dilution effect on parasite intensity was the result of both host composition effects as well as emergent properties of biodiversity. In contrast with the benefits of genotypic diversity under warming, diversity actually increased parasite intensity slightly in ambient temperatures. We found mixed support for the hypothesis that a growth-defense trade-off contributed to elevated disease intensity under warming. Changes in the abundance (but not composition) of a few taxa in the host microbiome were correlated with genotype-specific responses to wasting disease infections under warming, consistent with the emerging evidence linking changes in the host microbiome to the outcome of host-parasite interactions. This work emphasizes the context dependence of biodiversity-disease relationships and highlights the potential importance of interactions among biodiversity loss, climate change, and disease outbreaks in a key foundation species.

Habitat traits and predation interact to drive abundance and body size patterns in associated fauna.

Habitat-forming organisms provide three-dimensional structure that supports abundant and diverse communities. Variation in the morphological traits of habitat formers will therefore likely influence how they facilitate associated communities, either via food and habitat provisioning, or by altering predator-prey interactions. These mechanisms, however, are typically studied in isolation, and thus, we know little of how they interact to affect associated communities. In response to this, we used naturally occurring morphological variability in the alga Sargassum vestitum to create habitat units of distinct morphotypes to test whether variation in the morphological traits (frond size and thallus size) of S. vestitum or the interaction between these traits affects their value as habitat for associated communities in the presence and absence of predation. We found morphological traits did not interact, instead having independent effects on epifauna that were negligible in the absence of predation. However, when predators were present, habitat units with large fronds were found to host significantly lower epifaunal abundances than other morphotypes, suggesting that large frond alga provided low-value refuge from predators. The presence of predators also influenced the size structure of epifaunal communities from habitat units of differing frond size, suggesting that the refuge value of S. vestitum was also related to epifauna body size. This suggests that habitat formers may chiefly structure associated communities by mediating size-selective predation, and not through habitat provisioning. Furthermore, these results also highlight that habitat traits cannot be considered in isolation, for their interaction with biotic processes can have significant implications for associated communities.

Ocean current patterns drive the worldwide colonization of eelgrass (Zostera marina).

Currents are unique drivers of oceanic phylogeography and thus determine the distribution of marine coastal species, along with past glaciations and sea-level changes. Here we reconstruct the worldwide colonization history of eelgrass (Zostera marina L.), the most widely distributed marine flowering plant or seagrass from its origin in the Northwest Pacific, based on nuclear and chloroplast genomes. We identified two divergent Pacific clades with evidence for admixture along the East Pacific coast. Two west-to-east (trans-Pacific) colonization events support the key role of the North Pacific Current. Time-calibrated nuclear and chloroplast phylogenies yielded concordant estimates of the arrival of Z. marina in the Atlantic through the Canadian Arctic, suggesting that eelgrass-based ecosystems, hotspots of biodiversity and carbon sequestration, have only been present there for ~243 ky (thousand years). Mediterranean populations were founded ~44 kya, while extant distributions along western and eastern Atlantic shores were founded at the end of the Last Glacial Maximum (~19 kya), with at least one major refuge being the North Carolina region. The recent colonization and five- to sevenfold lower genomic diversity of the Atlantic compared to the Pacific populations raises concern and opportunity about how Atlantic eelgrass might respond to rapidly warming coastal oceans.

A neighboring plant species creates associational refuge for consumer and host.

Examples of plant-animal and plant-plant associational defenses are common across a variety of systems, yet the potential for them to occur in concert has not been explored. In salt marshes in the Gulf of Mexico, the marsh periwinkle (Littoraria irrorata) is an abundant and conspicuous member of the community, climbing up the stems of marsh plants to remain out of the water at high tide. Though Littoraria are thought to primarily utilize stems of marsh cordgrass Spartina alterniflora as a source of food and refuge, Littoraria were more abundant in mixed assemblages of Spartina and Juncus roemerianus than in Spartina-only areas at the same tidal height. Mesocosm experiments confirmed that Juncus provided a refuge for Littoraria, with predation by Callinectes sapidus (but not Melongena corona) reduced when Juncus was present. However, Littoraria's utilization of Juncus as well as the effectiveness of Juncus as a refuge depended strongly on plant height: when Juncus was experimentally clipped to a shorter height than Spartina, snail abundance on Spartina and snail predation by crabs increased. Interestingly, this plant animal refuge led to a corresponding refuge for Spartina from Littoraria: Spartina plants lost less biomass to snail grazing when growing with Juncus in mesocosm and field experiments, and Spartina plants in natural assemblages were taller when growing with Juncus than when growing alone, even in the presence of abundant snails. This example highlights the potential importance of plant plant and plant-animal associational refuges in competitive plant assemblages.