Parasites alter community structure

Abstract

Parasites often play an important role in modifying the physiology and behavior of their hosts and may, consequently, mediate the influence hosts have on other components of an ecological community. Along the northern Atlantic coast of North America, the dominant herbivorous snail Littorina littorea structures rocky intertidal communities through strong grazing pressure and is frequently parasitized by the digenean trematode Cryptocotyle lingua. We hypothesized that the effects of parasitism on host physiology would induce behavioral changes in L. littorea, which in turn would modulate L. littorea's influence on intertidal community composition. Specifically, we hypothesized that C. lingua infection would alter the grazing rate of L. littorea and, consequently, macroalgal communities would develop differently in the presence of infected versus uninfected snails. Our results show that uninfected snails consumed 40% more ephemeral macroalgal biomass than infected snails in the laboratory, probably because the digestive system of infected snails is compromised by C. lingua infection. In the field, this weaker grazing by infected snails resulted in significantly greater expansion of ephemeral macroalgal cover relative to grazing by uninfected snails. By decreasing the per-capita grazing rate of the dominant herbivore, C. lingua indirectly affects the composition of the macroalgal community and may in turn affect other species that depend on macroalgae for resources or habitat structure. In light of the abundance of parasites across systems, we suggest that, through trait-mediated indirect effects, parasites may be a common determinant of structure in ecological communities.

Fig. 1.

Change in the mass of macroalgae over the course of a 13-day laboratory experiment in compartments with no snails (i.e., control, n = 4), compartments with a single infected snail (n = 34), and compartments with a single uninfected snail (n = 23). Control compartments were not included in the statistical analysis and are presented here for reference. Columns are means ± 1 SE.

Fig. 2.

Change in percent cover of ephemeral macroalgae out of total macroalgal abundance (A) and community composition as summarized by the first principal component (B) over the 23–24 days of the field experiment. Columns are means ± 1 SE.