Habitat isolation interacts with top-down and bottom-up processes in a seagrass ecosystem.

Habitat loss is accelerating at unprecedented rates, leading to the emergence of smaller, more isolated habitat remnants. Habitat isolation adversely affects many ecological processes independently, but little is known about how habitat isolation may interact with ecosystem processes such as top-down (consumer-driven) and bottom-up (resource-driven) effects. To investigate the interactive influence of habitat isolation, resource availability and consumer distribution and impact on community structure, we tested two hypotheses using invertebrate and algal epibionts on temperate seagrasses, an ecosystem of ecological and conservation importance. First, we hypothesized that habitat isolation will change the structure of the seagrass epibiont community, and isolated patches of seagrass will have lower epibiont biomass and different epibiont community composition than contiguous meadows. Second, we hypothesized that habitat isolation would mediate top-down (i.e., herbivory) and bottom-up (i.e., nutrient enrichment) control for algal epibionts. We used observational studies in natural seagrass patches and experimental artificial seagrass to examine three levels of habitat isolation. We further manipulated top-down and bottom-up processes in artificial seagrass through consumer reductions and nutrient additions, respectively. We indeed found that habitat isolation of seagrass patches decreased epibiont biomass and modified epibiont community composition. This pattern was largely due to dispersal limitation of invertebrate epibionts that resulted in a decline in their abundance and richness in isolated patches. Further, habitat isolation reduced consumer abundances, weakening top-down control of algal epibionts in isolated seagrass patches. Nutrient additions, however, reversed this pattern, and allowed a top-down effect on algal richness to emerge in isolated habitats, demonstrating a complex interaction between patch isolation and top-down and bottom-up processes. Habitat isolation may therefore shape the relative importance of central processes in ecosystems, leading to changes in community composition and food web structure in marine habitats.

Predation shapes invertebrate diversity in tropical but not temperate seagrass communities.

The hypothesis that biotic interactions are stronger at lower relative to higher latitudes has a rich history, drawing from ecological and evolutionary theory. While this hypothesis suggests that stronger interactions at lower latitudes may contribute to the maintenance of contemporary patterns of diversity, there remain few standardized biogeographic comparisons of community effects of species interactions. Using marine seagrasses as a focal ecosystem of conservation importance and sessile marine invertebrates as model prey, we tested the hypothesis that predation is stronger at lower latitudes and can shape contemporary patterns of prey diversity. To further advance understanding beyond prior studies, we also explored mechanisms that likely underlie a change in interaction outcomes with latitude. Multiple observational and experimental approaches were employed to test for effects of predators, and the mechanisms that may underlie these effects, in seagrass ecosystems of the western Atlantic Ocean spanning 30° of latitude from the temperate zone to the tropics. In predator exclusion experiments conducted in a temperate and a tropical region, predation decreased sessile invertebrate abundance, richness and diversity on both natural and standardized artificial seagrass at tropical but not temperate sites. Further, predation reduced invertebrate richness at both local and regional scales in the tropics. Additional experiments demonstrated that predation reduced invertebrate recruitment in the tropics but not the temperate zone. Finally, direct observations of predators showed higher but variable consumption rates on invertebrates at tropical relative to temperate latitudes. Together, these results demonstrate that strong predation in the tropics can have consequential impacts on prey communities through discrete effects on early life stages as well as longer-term cumulative effects on community structure and diversity. Our detailed experiments also provide some of the first data linking large-scale biogeographic patterns, community-scale interaction outcomes and direct observation of predators in the temperate zone and tropics. Therefore, our results support the hypothesis that predation is stronger in the tropics, but also elucidate some of the causes and consequences of this variation in shaping contemporary patterns of diversity.