Components of stochastic niche dynamics and their influence on the species abundance distribution

Background/Question/Methods

Whether and how patterns of species abundance reflect the dynamic processes involved in the formation and maintenance of communities remains an open question in ecology.  Recent studies focus on the relative roles of neutral and niche-based mechanisms in shaping these patterns. Neutral dynamics are based on demographic stochasticity and immigration and niche dynamics are generated by trait differences that lead to frequency dependent growth rates of competing populations. Recent studies have argued for similarity in species abundance distributions produced by neutral and niche dynamics, and in fact for these distributions to be identical in the limit of high diversity.  That work is based on stochastic niche models that ignore interactions across niches.  In contrast, our recent work shows that there can be important differences between niche and neutral species abundance distributions, even under high species richness, when a fully interactive competition model is used.  In such a model, the niche dynamics are complex, emerging out of the multispecies interactions.  Here we divide the emergent niche dynamics arising in an interactive stochastic niche model into components, and ask what role each component plays in shaping the species abundance distribution, and in particular in generating differences from the neutral case.

Results/Conclusions

We divide up the emergent niche dynamics into four components: 1) community-wide heterogeneity in intrinsic fitness, 2) inter-niche stabilization, 3) intra-niche heterogeneity in inter-niche interactions, and 4) intra-niche heterogeneity in intra-niche interactions.  We demonstrate the effect of each of these components of niche structure on the species abundance distribution by comparing across model scenarios that build up to the full niche dynamics by adding each of these components.  We show that the effect of intra-niche heterogeneity in inter-niche interactions can be approximated as additional, “effective” heterogeneity in fitness.  Its effect on the species abundance distribution declines as the number of niches increases, i.e. as the amount of coexistence driven by stabilization increases.  We also show that heterogeneity in intrinsic fitness—either fundamental or “effective”—and stabilization tend to have opposing influences on the species abundance distribution, so that fitness differences often mask the effects of stabilization.  These results indicate that although species abundance distributions produced by niche dynamics may differ substantially from the neutral case, it may be difficult to see the signature of stabilization in species abundance distributions, unless community-wide heterogeneity in intrinsic fitness is low, and stabilization is driving coexistence of most of the species present.