Environmental and spatial filters of zooplankton metacommunities in shallow pools in high-elevation peatlands in the tropical Andes

Understanding metacommunity organisation in freshwater ecosystems is particularly important at high elevation in the tropical Andes, as this region is considered an important biodiversity hotspot with many endemic species threatened by climate change. High levels of dispersal at the regional scale and strong environmental filters at the local scale are thought to structure aquatic communities in these systems. However, metacommunity organisation patterns at the limit of aquatic life (>4,500ma.s.l.) remain largely unknown. Given the extreme environmental conditions experienced by organisms at high elevation, environment filters would be expected to play a greater role than spatial filters in community assemblages. We assessed the effects of environmental and spatial filters on zooplankton (Cladocera), to gain insight into metacommunity organisation in the shallow high-elevation pools of the tropical Andes. We sampled zooplankton communities and assessed nine environmental variables, for 200 pools from 20 peatlands in the Cordillera Real of Bolivia. We performed redundancy and variation partitioning analyses on both abundance and presence/absence data, to evaluate the contribution of environmental and spatial processes (geographic and elevation distances) to cladoceran metacommunity assembly at different spatial scales. We identified 18 cladoceran species, eight of which were endemic to the Andes. We found that both environment and spatial filters made a significant contribution to the structuring of cladoceran communities at higher spatial scales (i.e. intervalleys and within valleys), whereas only the environmental filter operated at small spatial scales (i.e. within peatlands). Elevation had a significant effect on cladoceran community variation (e.g. 6.9% within valleys, for abundance data), but not on species diversity. Similar patterns were obtained with presence/absence data. Macrophyte cover, pool area and pH were the most important variables explaining community variation and the species diversity of cladocerans at pool level. These results suggest that species sorting is probably the main process generating variation in cladoceran communities at all spatial scales. At larger spatial scales (intervalleys and within valleys), limited dispersal may also filter species composition. We identified factors significantly contributing to the variation observed, but their overall explanatory power was low. This finding highlights the difficulties involved in evaluating the response of aquatic organisms to their environment and suggests that other environmental variables should be considered. Moreover, frequent and unpredictable disturbances and high stochasticity probably operate simultaneously with environmental filters to structure zooplankton organisation in these unique and harsh aquatic systems