Population studies of Asplanchna girodi, a predatory rotifer, and its prey in experimental communities

I examined the relative importance of predation, resource limitation and stochastic events in determining rotifer biomass and diversity using replicated outdoor tanks. Experiments were conducted using large outdoor tanks containing zooplankton communities with and without the predatory rotifer Asplanchna girodi.; The first experiment examined the effects of predation by Asplanchna on the prey rotifers when crustaceans were present. In this experiment, predation by Asplanchna had no apparent effect on the biomass of any rotifer group. There was, however, a strong negative correlation between non-predaceous crustaceans and the dominant rotifer group. This was apparently unrelated to the presence or absence of Asplanchna.; The second experiment examined the effects of predation by Asplanchna on the prey rotifers when crustaceans were absent. There was again no discernible effect of predation on rotifer biomass. Rotifer biomass in this experiment was not controlled by total phytoplankton biomass, although it may have been controlled by particular groups of phytoplankton. In addition, there was a negative correlation between two groups of rotifers, which suggests possible intra-rotifer competition.; Predation had a variable effect on diversity, possibly due to variable breadth in diet. In both experiments there was a high correlation between rotifer diversity and number of Asplanchna girodi in communities in which Asplanchna had a narrow diet (Spearman's rank correlation coefficient ranged from.59 to.75). In communities in which Asplanchna had a broader diet, the correlation was much lower (Spearman's rank correlation coefficients ranged from {dollar}-{dollar}.46 to.25). Crustaceans also had a variable effect on diversity for unknown reasons.; Variability among replicates was high in both experiments despite identical treatment, perhaps due to stochastic events. Two examples of stochastic events which may have affected these communities differentially are priority effects and exogenous nutrient input.; The lack of effect of predation on prey biomass was possibly due to the relatively low numbers of the predator. The factors controlling the abundance of Asplanchna girodi were examined in detail in the communities containing Asplanchna in the first experiment and in the two communities with the most Asplanchna in the second experiment. Details of the predator's diet were also examined. (Abstract shortened with permission of author.)...