Temporal variability and intensity of grazing: a mesocosm experiment.

Abstract

Grazing has long been recognised as a structuring force for plant assemblages. Most of this knowledge comes from experiments in which grazers have been excluded or their densities manipulated. However, the intensity of grazing can vary, in space and time. Recently, an increasing number of studies have stressed the importance of the variance around the mean of ecological processes, but the potential effects of temporal variability in grazing in marine systems have not yet been explored. We examined the separate effects of intensity and temporal variability of grazing by the gastropod Cantharidus purpureus (Gemelin, 1931) on algal assemblages in a mesocosm experiment. In replicated experiments, algal assemblages grown on artificial substrata were subject to grazing regimes with mean intensity and temporal variance as crossed factors. In the first experiment, the more variable regimes led to greater reductions in algal cover, regardless of the level of grazing intensity. In the second experiment, variability elicited a similar effect, but this effect was larger for the low-than for the high-intensity treatments. These results indicate that temporally variable grazing regimes may have greater effects on algal assemblages than those anticipated from changes in the mean intensity of grazing alone. Thus, we suggest that temporal variability is a potentially important aspect of grazing processes that should be examined and incorporated into predictive models.