Compensation alters estimates of the number of species required to maintain ecosystem functioning across an emersion gradient: A case study with intertidal macroalgae

Whether more species are required to maintain ecosystem functioning as spatial scale increases or across environmental gradients has frequently been studied by examining whether different species drive ecosystem functioning in different sites. However, this approach does rule out the counterfactual scenario where a few species could potentially maintain ecosystem functioning across sites as this requires examining which species can (or cannot) compensate for the loss of others.Here, I used an observational study and a field-based transplant experiment to examine the effects of species loss on biomass productivity in an intertidal marine macroalgal system. I calculated the number of species required to maintain biomass productivity across four depth zones reflecting a water emersion gradient using two commonly used observational approaches. Then, I combined hypothetical simulated extinction scenarios with field-based transplant data of relative growth rates of all species across the four depth zones to explore how the number of species required to maintain biomass productivity across depth zones changed under counterfactual scenarios where species compensated for species loss.The observational analyses suggested that between three and four species were required to maintain productivity across the depth zones. The simulated extinction scenarios did not. Rather, decreases in biomass productivity due to the loss of some species (e.g. Fucus spiralis, Ascophyllum nodosum) were easily compensated by other species (e.g. Fucus vesiculosus). However, for some species like F. vesiculosus, the extinction simulations suggested that compensation would be unlikely.Commonly used observational approaches may overestimate the number of species required to maintain ecosystem functioning across environmental gradients and spatial scales.Read the free Plain Language Summary for this article on the Journal blog. Read the free Plain Language Summary for this article on the Journal blog.image