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ABSTRACT Species conservation and management benefit from precise understanding of natural patterns of dispersal and genetic variation. Using recent advances in indirect genetic methods applied to both adult plants and dispersed seeds, we find that the mean seed dispersal in a threatened marine foundation plant (the eelgrassZostera marina) is approximately 100–200 m. This distance is surprisingly more similar to that of wind‐dispersed terrestrial seeds (~10s to 100s of meters) than the passive dispersal of marine propagules via currents (~10s to 100s of kilometres). Because nearshore marine plants likeZosteraare commonly distributed across strong selective gradients driven by bathymetry (depth) even within these restricted spatial scales, seeds are capable of dispersing to novel water depths and experiencing profound shifts in light availability, temperature and wave exposure. We documented strong phenotypic variation and genome‐wide differentiation among plants separated by approximately the spatial scale of mean realised dispersal. This result suggests genetic isolation by environment in response to depth‐related environmental gradients as one plausible explanation for this pattern. The ratio of effective to census size (or Ne/Nc) approximated 0.1%, indicating that a fraction of existing plants provides the genetic variation to allow adaptation to environmental change. Our results suggest that successful conservation of seagrass meadows that can adapt to microspatial and temporal variation in environmental conditions will be low without direct and persistent intervention using large numbers of individuals or a targeted selection of genotypes. 

Abstract 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 algaSargassum vestitumto create habitat units of distinct morphotypes to test whether variation in the morphological traits (frond size and thallus size) ofS. vestitumor 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 ofS. vestitumwas 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. 

Diversity within species can have community‐level effects similar in magnitude to those of species diversity. Intraspecific diversity in producers and consumers has separately been shown to affect trophic interactions, yet we have little understanding of how variation at these two levels could simultaneously affect trophic interactions. Salt marshes dominated by Spartina alterniflora are an ideal system in which to ask this question as this plant exhibits substantial genetically based trait variation. Further, herbivores can have sizable impacts on Spartina, but the impact of herbivore trait variation is not well understood. We conducted an experiment in a Massachusetts salt marsh to determine how herbivorous crab (Sesarma reticulatum) size diversity and Spartina genotypic diversity affect the plant community. Herbivore effects on plant traits varied by herbivore size, with large crabs generally having stronger impacts on plants. At times, the effect of small crabs on plant traits depended on plant genotypic diversity. The effects of crab size diversity (i.e., small and large crabs combined) were most often predicted by the independent effects of each size class, though there were synergistic effects on stem density, flowering stems, and mean stem height. Finally, we tested whether herbivore size or size diversity could have reciprocal effects on plant genotypic diversity. Small‐ and mixed‐crab treatments promoted plant genotypic richness, whereas large crabs did not. Our results demonstrate that intraspecific diversity at multiple trophic levels can have simultaneous and sometimes interactive effects on species interactions, highlighting the importance of variation within species for understanding species interactions and community processes. 

Abstract 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 marinaL.), 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 ofZ. marinain 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.