New paper on unifying models of temporal environmental stochasticity published in Oikos

Ecological communities are universally subject to temporal environmental stochasticity—random variation in environmental conditions over time that cause species’ vital rates to change. This can have both positive and negative effects on species richness. For example, negative effects can arise because stochasticity makes a species more vulnerable to extinction, whereas positive effects can arise because stochasticity reduces the strength of competition between species. Is the net effect on species richness typically negative or positive, and what mechanisms typically dominate? Answering this question is of paramount importance for our understanding of the effects of temporally varying environments on biodiversity. In a study just published in Oikos, Tak and Ryan together with James O’Dwyer from the University of Illinois tackled this topic using a novel mathematical approach that unified past models of temporal environmental stochasticity and generated new insights.

Our novel approach was to focus on one species and approximate the rest of the multispecies community as a single entity, thus converting a many-body problem into a one-body problem and greatly simplifying the mathematics. We developed methods for separating the effects of temporal environmental stochasticity into population-level effects and community-level effects, and found that the strength and direction of these effects varied in complex and surprising ways as a function of temporal correlation in the environmental stochasticity.

Our most tantalising result was that there is a threshold value of temporal correlation above which the net effect of temporal environmental stochasticity on species richness switches from positive to negative. The threshold is approximately twice the reciprocal of the rate at which new species enter the community. This points the way to a practical method for assessing whether temporal environmental stochasticity has a net positive or negative effect on biodiversity in reality, although further theoretical work is needed to test the generality of our result.

Species richness S for competitive model communities exposed to temporal environmental stochasticity, as a function of temporal autocorrelation T (measured in generations; horizontal axis) and the coefficient of variation cb (colours). Below a critical threshold of temporal autocorrelation indicated by the dashed grey line, species richness is typically greater than in the baseline scenario with no temporal environmental stochasticity (i.e., a neutral scenario), while to the right of the line species richness is typically lower than in the baseline scenario. (Other model parameters for this figure are community size J=10,000 individuals and new species input rate ν=0.01; the critical threshold is T=2/(Jν)).

Fung, T., J. P. O’Dwyer, and R. A. Chisholm. 2021. Effects of temporal environmental stochasticity on species richness: A mechanistic unification spanning weak to strong temporal correlations. Oikos (in press)