Grazer diversity and biogenic substrate heterogeneity interactively accelerate intertidal algal succession

Background/Question/Methods

Heterogeneity in environmental conditions has long been thought to promote coexistence by providing opportunity for niche partitioning. Likewise, environmental heterogeneity may intensify the expression of niche differences in diverse communities and therefore mediate ecological processes. In particular, we were interested in how habitat heterogeneity and grazer community composition interactively determine algal succession. We experimentally tested this idea in the field by manipulating heterogeneity in barnacle cover and the composition of three gastropod grazer taxa (two limpets, Lottia digitalis and Lottia scabra, and littorine snails) on a mid-high intertidal rock wall over the course of one year. Barnacle cover heterogeneity was achieved by removing all or no barnacles from a plot (low heterogeneity), or by removing barnacles from one half of a plot (high heterogeneity). Grazer composition was manipulated and maintained through species removals to establish monocultures, which were compared to intact grazer assemblages. 

Results/Conclusions

Barnacle cover heterogeneity revealed strong preferences in grazer habitat utilization. Littorine snails were found almost exclusively in areas with barnacles, while Lottia scabra exclusively preferred barnacle-free areas in the presence of the habitat generalist Lottia digitalis. Both grazer diversity and barnacle-free space impeded the establishment and accumulation of early successional micro- and macroalgae, but these effects were largely independent. However, the establishment of late successional perennial macroalgae was accelerated by grazing and substrate heterogeneity, and these effects depended on herbivore composition. Within intact grazer communities, perennials only established in plots with full barnacle cover, which was largely driven by the presence of Lottia digitalis. In treatments without Lottia digitalis perennials established fastest in the heterogeneous barnacle treatment. Grazer habitat utilization in heterogeneous barnacle treatments depended on grazer diversity, suggesting that environmental heterogeneity shifted the relative importance of herbivore species identity and complementarity as predictors of algal community dynamics.