COS 124-7
Experimental determination of the effects of predation on the coexistence of two competing species of intertidal mussels in central Chile
Predation and competition, arguably the most studied interactions among species, are processes that often interact and affect species coexistence simultaneously. Although many empirical studies demonstrate that predation can promote coexistence between competing species through reducing the abundance of a dominant competitor (i.e. the predation hypothesis), theoretical work suggests that predation can have positive, negative or even no effect on the outcome of competition. Two mussels common to the mid-low intertidal zone of central Chile, Perumytilus purpuratus and Semimytilus algosus, compete for limited space, but previous studies, all conducted in absence of predation effects, have shown that the outcome of competition can vary and that the competitive hierarchy can even be reversed. Here we evaluate the role of predation (as a function of predator preference and attack rates) in mediating the effects of competition on mussel coexistence. At two sites, separated by 300 km, we transplanted mono- and poly-specific assemblages of adult mussels into plots that were either exposed or unexposed to local predators. We monitored species-specific adult mussel mortality rates and local predator abundance over a 6 month period to assess the interactive effect of predation and competition on mussel coexistence.
Results/Conclusions
Predator abundances differed among sites, however we observed a steady decrease in the abundance of both mussel species within the exposed plots within the first 5 days of the experiment at both sites, with nearly all mussels removed after 3 weeks time. There were no significant differences in the species-specific per capita mortality rates in exposed plots between the mono and poly-specific treatments suggesting that predators exhibitted no preference between the mussel prey. Within unexposed plots, both species of mussels experienced similarly low mortality rates at both sites, however Perumytilus experienced higher mortality in poly-specific plots than when under monospecific conditions. Because the net effect of local predation is intense but non-selective we conclude that the dynamics of coexistence between Perumytilus and Semimytilus should not be directionally influenced by consumers. Predators, however, may have an important role in the generation of free space and the reduction in the intensity of competition especially when local mussel recruitment is high. Finally we consider the role of variable recruitment, predation rates, and competition coefficients via theoretical scenarios to predict the parameters under which local coexistence can be achieved.