Functional diversity and redundancy of rotifer communities affected synergistically by top-down and bottom-up effects in tropical urban reservoirs

Abstract

Rotifers are one of the most important functional groups in lentic freshwaters, and mainly structured by predation or competition from crustaceans and food availability from phytoplankton. Their functional traits involving in defense and food utilization provides insights into the functional diversity and assembly of their communities. Tropical urban reservoirs are commonly subjected to intense fish culture and strong eutrophication, and rotifers become the dominant component of the zooplankton communities and play an important role in ecosystem functioning. Knowledge about the functional diversity and functional redundancy of rotifers is therefore of relevance for effective ecosystem management. In tropical reservoirs, we hypothesized that functional diversity and redundancy of rotifer communities are synergistically affected by top-down and bottom-up effects, and that they strongly change seasonally with environmental conditions at local scale as well as with connectivity between reservoirs at regional scale. To test these hypotheses, we investigated the seasonal dynamics of taxonomic and functional diversities in twenty-five tropical urban reservoirs. Traits of rotifers mainly associated with body size, feeding behavior, and defensive strategies were used to explore the drivers of functional diversity and functional redundancy. We found that, at the local scale, both taxonomic alpha and functional alpha diversities of rotifers increased from the wet to the dry season, whereas the functional alpha redundancy showed no significant change. Crustacean abundance (top-down effect) and phytoplankton biomass (bottom-up effect) were the dominant variables explaining the variation of taxonomic and functional alpha diversities. At the regional scale, both the taxonomic and the functional beta diversities of rotifer communities decreased from the wet to the dry season. However, functional beta redundancy increased, accompanied by a reduction in environmental distance, i.e., biological and physicochemical distance. Thus, weakened environmental heterogeneity and strengthened homogenization of biological selection resulted in a more similar functional trait composition than a more similar taxonomic composition between reservoirs.