A growing body of work illustrates the importance of plant diversity for primary productivity. The effects of herbivory on plant biomass and diversity are also well-documented. However, few studies have assessed the relative importance of these factors. Further, because many animals use plants as food and habitat, plant diversity can also affect the herbivore community, leading to potential complex interactions between the effects of plant diversity and herbivory on community productivity and resilience. Most experimental studies examining the effects of herbivores on recovery from disturbance do so without regard to the diversity or composition of the surrounding community, yet the surrounding community could affect recovery directly by influencing propagule supply or indirectly by altering ambient herbivore density or modifying local abiotic conditions. To determine the interactive effect of herbivory and surrounding algal diversity on the recovery of a rocky intertidal community, we conducted a 2 yr field experiment assessing algal recruitment and succession in small cleared patches. We nested two herbivore treatments, ambient and reduced abundances, within 1.5 m diameter monocultures and polycultures of four common algal species. To assess recovery, we compared the cover and density of seaweeds within experimental treatments to ambient control plots.

Results/Conclusions

Recovery of perennial algae was fastest in patches cleared within polycultures, but only when exposed to herbivores. The increased recovery rate of patches containing herbivores is likely due to the consumption of ephemeral algal species that compete for space with perennial species, but the reasons for the “diversity effect” are more complex. In polycultures, desiccation rates are lower, herbivore composition is altered, and propagule supply may be higher compared to monocultures, all of which could contribute the observed effect of diversity. We provide evidence in support of each of these mechanisms and including evidence that the composition of the grazer community affects algal diversity and cover. This, combined with previous work in this system showing that algal diversity and cover also affects the composition of the grazer community, suggest the potential presence of feedbacks between grazers and algal communities. The implications of these reciprocal effects between grazers and algal communities may exert profound effects of community diversity, stability and resilience.