COS 227-2 - Connecting spatial scaling of biodiversity change to ecological processes: Allee effect, Janzen-Connell effect, target effect, and beyond

Abstract: Global biodiversity has been changing. We have reasons to expect that unprecedented extinctions, extirpations, and invasions have been taking place all over the world, but the overall picture of how fast and where these processes happen is complex: In some regions and organisms, we know that global diversity declines, but not much has been happening on an average local patch. This means that the rate and direction of biodiversity change depends on spatial grain (resolution, scale); plausible are completely opposite trends at different grains, for instance biodiversity losses at one grain but gains at another grain. Ecological mechanisms behind this discrepancy between fine and coarse grains are still unclear.

We hypothesized that the direction and magnitude of the relationship between biodiversity change (ΔS) and spatial grain (the average area of observation) depends on the magnitude of classical ecological processes such as the Allee effect, Janzen-Connell effect, and the “target effect” (from the equilibrium theory of island biogeography).

Specifically, we focused on the smallest level of a single individual of an organism. We linked this individual’s probability of death to population density in a larger geographic area. Similarly, we linked the individual’s dispersal potential to the area of the target location. Then, we upscaled these individual-level processes, and examined how they translate to spatial patterns of biodiversity change at coarser grains. We formalized, and then analyzed, our hypotheses mathematically. We complemented this analysis with simulations based on point patterns.

We found that spatial scaling of extinction rates depends on how individual-level probability of death scales with overall population density, and thus on rarity or commonness species. We thus theoretically demonstrate a low-level mechanism that can explain different rates of biodiversity change which several authors observed at different spatial grains in empirical data.