Seascape and metacommunity processes regulate fish assemblage structure in coastal wetlands

Faunal complexity is an impediment to understanding the function of fragmented coastal wetlands. Conceiving faunal communities as part of a larger network of communities (or a metacommunity) helps to resolve this complexity by enabling simultaneous consideration of local environmental influences and 'regional' dispersal-driven processes. We assessed the role of local vs. regional factors on the fish assemblage structure of a wetland system comprising 20 tidal pools. In equivalent freshwater metacommunities, regional factors often override local influences, resulting in patterns of nestedness among patches as species and individuals are progressively filtered out along gradients of isolation. While the tidal pool assemblage was primarily structured by regional processes, patterns deviated from freshwater systems, as 2 faunal groups exhibited contrasting responses to tidal connectivity. A subset of typical estuary channel fauna was restricted to better connected pools at lower elevations, which connect to the estuary channel or other pools on most neap high tides. Frequent connections among these pools subsequently enabled sorting of species relative to preferred environmental condition (including depth and substrate). Contradicting models of nestedness, a distinct faunal group including salt marsh residents and juvenile marine-spawned taxa occurred in greater abundances in more isolated, higher elevation pools, which connect to the estuary channel or other pools only on larger spring high tides. These higher elevation pools represent a functionally unique seascape component, and colonisation by marine-spawned taxa seems to reflect an innate drive to ascend upstream gradients to access them. This illustrates how seemingly similar patches within coastal wetlands may perform considerably different nursery functions because of their position in the landscape. Together, metacommunity and seascape frameworks offer complementary perspectives in understanding the role of spatial ecology in structuring coastal ecosystem function and productivity.