Consequences of fish for cladoceran, water beetle and macrophyte communities in a farmland pond landscape: implications for conservation

Traditionally, fish have been neglected in pond ecology and conservation studies and it has frequently been assumed that they have a detrimental impact on pond biodiversity and ecosystem function. In order to assess the consequences of fish for pond biodiversity and ecosystem structure we sampled a set of 40 small farmland ponds (20 with and 20 without fish) in eastern England and compared their water chemistry as well as assemblage characteristics (abundance, diversity, species composition) for three biological groups: cladocerans (zooplankton), water beetles and macrophytes. Water depth was significantly greater in fish ponds, while pond bottom oxygen levels and pH were significantly higher in the ponds without fish. The presence of fish significantly reduced the abundance of macrophytes and altered the community composition of cladocerans and macrophytes, but had no detectable influence on water beetles. Variation partitioning using environmental and spatial variables, indicated that all three biological groups were spatially structured. The inclusion of fish, however, reduced the importance attributed to space in the case of both cladocerans and macrophytes, suggesting that space effects for these two groups were at least partly the result of a spatially structured predator (i.e. fish). In most cases fish did not have an effect on cladoceran and water beetle alpha diversity (number of species, Shannon's and Simpson's index), although the opposite was true for macrophytes. Nevertheless, at the landscape level, gamma diversity (i.e. total number of species) was enhanced for all three biological groups. Our results suggest that fish, at least small, typical, pond-associated species, are an important component of heterogeneity in farmland pond networks, thereby increasing landscape scale diversity across several faunal and floral elements. Consequently, we propose that fish should be more fully included in future pond biodiversity surveys and conservation strategies.