Spatial covariance reverses the compound effect of multiple stressors on rocky shore biofilm

Understanding how multifactorial fluctuating environments affect species and communities remains one of the major challenges in ecology. Here, we use field experiments and simulations to assess the influence of nonlinear effects and spatial covariation of two important climate variables – warming and sediment deposition – on the biomass and photosynthetic activity of rocky intertidal biofilm. Simulations stemming from an experimentally derived response surface showed how the degree and direction of spatial covariation between warming and sediment deposition ultimately determined the nonlinear response of biofilm biomass (but not photosynthetic activity) to fluctuating levels of the two climate variables. Experimental results corroborated these predictions, probing the buffering effect of negative spatial covariation against extreme levels of warming and sediment deposition. Together, these results indicate that consideration of local-scale patterns of covariation between climate drivers can improve our understanding and ability to predict ecological responses to ongoing and future changes in Earth’s climate.