Larger areas have more species. Many deterministic and stochastic ecological processes operating at fundamentally different scales aggregate to produce the species-area relationship (SAR). Unfortunately this causal medley obscures the relationship between predictions from dynamic theory and field data snapshots and the application of insights from SARs to socio-ecological problems. Contemporary coexistence theory offers avenues for reconciliation by predicting which species will stably coexist in a given area—that is, the coexistence-area relationship (CAR)—versus which species occurrences are transient (SAR-CAR).
We sought to empirically measure the CAR in a community of annual plant species wherein competition for shared resources and consumption by a shared seed consumer (a harvester ant) couple plant population dynamics. We constructed a coexistence model, then parameterized it with field data from (i) competition and consumption experiments that quantified the indirect effects of species on one another through these two coupling pathways and (ii) mapping of ant nest locations as a measure of spatially-varying consumer pressure. Projecting the model allowed us to measure the coexistence potential of species from their invasion growth rates from small areas to the whole landscape, giving rise to the CAR. Finally, we used a simulations to decompose the CAR into the spatial coexistence mechanisms that underpin it, revealing the how spatial heterogeneity enables plant coexistence across scales.
Results/Conclusions
Plant population dynamics varied in space because ant consumers affected the absolute seed (fitness) loss to consumers and the relative differences in seed losses between our seven focal plant species. We found a strong competition-defense trade-off near ant nests: the best competitors lost so many seeds to ant consumption that their populations could not persist locally. Seed consumption rates declined with increasing distance from the ant nest for all plant species, but the slopes varied independently among plant species, dissolving the trade-off.
We found evidence for a positive CAR—increasing mutual invasibility with increasing area—in most (>90%) of our 21 plant species pairs, driven by the spatial storage effect. The CARs saturated rapidly such that stable coexistence was possible for very few (<10%) plant species pairs at the landscape scale. However, the measure of mutual invasibility also provides a metric for predicting the transient persistence time of non-coexisting species. Viewing SARs through the lens of coexistence theory offers a novel opportunity to not only understand how spatial heterogeneity stabilizes higher diversity in larger areas, but also to predict the number and identities of transient species across spatial scales.