Grazer impacts on algal community structure vary with the coastal upwelling regime

Community dynamics are often influenced by processes operating at large spatial scales. For example, the structure and dynamics of rocky intertidal communities depend not only on local factors, but also on the nearshore oceanic processes that affect the delivery of nutrients, propagules, and food particles. Conceptual models such as the grazer-reversal hypothesis predict that grazers will decrease the diversity of primary producers in nutrient-poor environments and increase diversity in nutrient-rich environments. To test this hypothesis, natural variation in the productivity of nearshore waters around the south island of New Zealand was used as a backdrop against which the occurrence of limpets, the dominant grazers, was experimentally manipulated. Limpets were either excluded or allowed access to replicate plots at five sites, two of relatively high nutrient availability and three of relatively low nutrient availability. Limpets had a negative effect on algal species richness and biomass at the nutrient-poor sites and little effect at the nutrient-rich sites, thus supporting the predictions of the grazer-reversal hypothesis. Results from this experiment suggest that in contrast to earlier results in the low zone, in the high zone of the rocky intertidal stronger bottom up effects (higher nutrients) did not “flow” up the food chain (to the herbivore level) to produce stronger top down effects. This finding is consistent with the idea that increasing environmental stress can alter the strength and direction of species interactions expected under a top-down/bottom-up scenario.