Effects of Wetland Vegetation Fragmentation on Aquatic Invertebrate Dispersal and Colonization

Aquatic invertebrates rely on wetland vegetation as the physical structure on which they live, disperse and persist, but the wetland vegetation is being fragmented into smaller and more isolated patches, through drainage, land conversion and altered hydrology. This study focuses on the effects of fragmentation of the wetland vegetation, rather than the overall quantity of the vegetation, on the dispersal and colonization of aquatic invertebrates. A spatially explicit metapopulation simulation was a landscape of 100 × 100 units with a fixed 30% of landscape vegetated in 1 to 64 circular patches. Weak, moderate and strong dispersal were simulated on 300 different landscapes for each fragmentation level (dispersal distances of 5, 15 and 40 units). Patch connectivity was positively correlated with colonization, patch area negatively correlated with extinction, and metapopulation persistence and patch occupancy were observed at the end of 60-time steps. Occupancy of weak dispersers was sharply reduced (51% to 1%) and persistence of weak dispersers was reduced (58% to 10%) in the 64-patch landscapes compared to the 2-patch landscapes. Moderate dispersers exhibited a "hump" centered at four patches, while strong dispersers had little effect on occupancy, ranging above 76% at all fragmentation levels. The results show that fragmentation effects are not consistently negative and that the dispersal ability of a wetland species helps to determine whether fragmentation results in a fragmented wetland as a connected network of habitats or as a series of isolated habitats. Large patches of vegetation and stepping stone connections should therefore be retained in conservation planning, especially for those poorly dispersing taxa. The results are simulation-derived and offer predictions for field validation.