Functional redundancy and the non-additive impacts of marine consumers

Background/Question/Methods

Declines in global biodiversity have prompted ecologists to investigate the role of diversity in providing functional redundancy in communities.  We tested the effects of consumer identity and functional diversity on subtidal rock wall epifauna using two field experiments in the San Juan Islands, WA.  In the first, we manipulated urchin densities for three months to test the direct and indirect effects of urchin grazing on the creation of available space (the limiting resource).  In the second, we conducted a factorial removal of urchins and chitons for one year to test whether these consumers serve additive or non-additive roles in the maintenance of available space.  We interpreted experimental results in the context of diet analyses and structural equation models. 

Results/Conclusions

In the context of consumer diets, the results of the first experiment (urchin addition) demonstrated that urchins generate available space on subtidal rock walls by consuming macroalgae and invertebrate colonies.  Furthermore, chiton density increased after the experimental addition of urchins, and structural equation models suggest that chitons maintained patches of space free of microscopic algae and recruits of larger sessile taxa through consumption and/or physical disturbance.  In the second experiment, the removal of each functional group in isolation had no effect on the epifaunal community, but the removal of both consumers caused a decrease in available space and an increase in the cover clonal ascidians.  Together, these experiments suggest that urchins and chitons can be considered functionally redundant in the maintenance of space, but not the creation of space.  Facilitation and redundancy among consumers may contribute to the resiliency of urchin-mediated ‘barrens’, even if urchins do not persist.