Identity trumps diversity in determining the effects of a tropical marine herbivore community

Background/Question/Methods

Voracious herbivore communities often determine the structure and function of ecosystems. This is particularly true in the marine environment, where entire habitats may rapidly transform or disappear due to the presence or absence of a single important grazer or grazing community. The relative importance of species diversity and identity within herbivore guilds is central to our understanding of how they structure communities, yet our understanding of these characteristics is lacking for many marine ecosystems. Coral reefs are one such ecosystem, which is surprising given they harbor 30% of the diversity of all marine life and are vitally reliant upon the effectiveness of grazer communities. We used a long-term, two-level approach to explore the influence of diversity and identity on the function of an herbivore community in a protected coral reef environment. First, we conducted field-based assays to quantify variation in the dietary preferences among five common, native, herbivorous sea urchins. Second, we used an 8-treatment, 24-month manipulative field experiment to test and compare the relative influence of species identity and diversity on the “long-term” direct and indirect effects of this guild on natural coral reef benthic communities.

Results/Conclusions

Results of 100 field-based assays revealed variation in dietary preference, but also overlap, among all five evaluated urchins. For example, Tripneustes was the only taxon that preferred brown algae, whereas Heterocentrotus preferred turf and crustose corallines. Variation in dietary preferences indicated the potential for a positive relationship between diversity and function within the guild, possibly contingent, however, upon the degree of overlap observed when resources become scarce. Results of the manipulative experiment revealed strong differences among treatments. Biomass of macroalgae increased greatly when herbivores were excluded, indicating a strong influence of grazers.  Plots containing monospecific Heterocentrotus communities, however, were identical to full exclosures, confirming  results of assays which indicated this urchin did not target macroalgae. In contrast, all plots containing Echinothrix, both mono- and poly-specific, exhibited near-complete elimination of all accessible macroalgae. Polyspecific communities exhibited intermediate influence on macroalgal biomass. Overall, species identity (vs. diversity) more strongly influenced the effect of this guild on macroalgal biomass. Given that macroalgae compete with and replace the dominant foundation species (i.e., corals) in these systems, our results mirror observations in other marine ecosystems, emphasizing the potential for identity to trump diversity in determining the effects of herbivores on benthic community structure and function.