Recent work highlights that the existing understanding of influences of biodiversity on ecosystem function is primarily based on local scale studies, and that at local scales it is unclear if species richness is actually declining. What is clearer is that biodiversity is declining at large scales. Theoretical work using metacommunity models has explored whether biodiversity provides spatial insurance for ecosystem functioning by spatial exchanges among local systems in landscapes with spatiotemporal environmental heterogeneity. These find that local function is maximized at intermediate dispersal, where regional richness is high or intermediate and local richness is also high. In these models, local and regional richness are interlinked, together emerging from metacommunity process, including the degree of dispersal. This does not allow consideration of the direct effects of loss of regional richness due to habitat loss. Here we instead use a simple mainland-island model with environmental fluctuation in the local community to consider spatial insurance effects in a context where we can separate the effects of regional species richness on local ecosystem function.
In our model regional richness does indeed positively influence local function through spatial insurance, even though it has little impact on local richness. We find that the time scale of environmental fluctuations and the degree to which the local environment favors one species over the others, which were both fixed in previous studies, are key determiners of the strength of influence of regional richness. When the environment fluctuates quickly, changing regional richness has a smaller effect on the community biomass, as the community remains dominated by the species that is most often preferred by the local environment. When the environment fluctuates more slowly, local community biomass increases quickly at relative low regional richness while it increases slowly at high regional richness. In this slow fluctuation regime, the dominant species is constantly changing, and increasing richness increases the match between potential immigrating species and the local environment, at least up to a point, where even higher diversity and a tighter match results in a limited time to grow in biomass for each dominating species and is of no additional benefit in biomass. When the environment favors one species over the others more strongly, the positive effect of regional richness is more significant, as the match between species and environment is vital. Our study offers a theoretical foundation for the impacts of regional richness on local ecosystem function and community composition.