A trajectory of environmental filtering: Consequences of initial assembly and subsequent interactions for community structure in a dynamic ecosystem

Background/Question/Methods

The formation of communities largely depends on individual species’ abilities to disperse and to recognize their target habitat within a mixed landscape. Communities may retain their initial settlement assembly, but post-dispersal dynamics have the potential to neutralize or accentuate community differences between habitats. As habitats shift with climate change, associated species will vary in their ability to keep up and thrive in habitats out of context. The relative importance of initial settlement and subsequent interactions for resident species will influence the net outcome of emerging novel communities. Globally, mangroves are shifting poleward – displacing temperate grass marshes without changing net wetland extent – with unknown consequences for wetland inhabitants. Along one mangrove-marsh ecotone on Florida’s Atlantic coast, I investigate initial and subsequent consequences of habitat shifts for wetland crab communities. Crabs, like many wind- and water-dispersed species, depend on broad-scale passive movement for recruitment, so I expected communities within alternative wetlands to look most similar during early assembly and then diverge later in residency. Crab larval supply and settlement patterns were sampled with plankton nets and passive traps during spring recruitment (2013, 2014) to evaluate dispersal limitations and habitat associations. I then surveyed juvenile demographic distributions to determine subsequent habitat occupancy patterns. Simultaneous tethering studies evaluated survival patterns by size in each vegetation type. I then compared how initial settlement patterns and subsequent habitat use influence the populations that emerge from each habitat at the end of the nursery phase.

Results/Conclusions

Available larval supply indicated comparable species pools across the landscape, but settling crab communities were habitat-specific even during initial assembly. The consistency of habitat associations across latitudes indicates that crabs are responding to local mangrove emergence in the landscape even before wetland dominance shifts. Beyond initial differences in species composition, even species that occur in both habitats should utilize them differently. As expected due to increasing interactions with residency, juvenile blue crab survival in mangroves differed from grass more strongly in later/larger stages than at initial settlement. Thus, post-settlement interactions appear to magnify habitat-based differences throughout development. If inhabitant community composition initially differs by habitat and is enhanced throughout development, then changes in habitat composition accompanying climate change may profoundly alter natural communities even when there is no net ecosystem loss. Considering the relative strengths of initial settlement and subsequent environmental filtering as complimentary community-structuring ecological processes will help us understand the implications of global habitat modification on emerging communities.