The relationship between species richness and productivity has been extensively studied in ecology. However, despite decades of empirical and theoretical research, it remains unclear how the relationship changes with the spatial scale of observation. A new theoretical study led by Tak and published in Theoretical Ecology sheds some light on this question by showing how, even in a neutral model, increasing the spatial scale results in non-trivial changes to the richness–productivity relationship.
The study analyses a neutral model representing a local community of species populations competing for finite resources. We derived a mathematical formula for the richness–productivity relationship in this model. The resulting curve has a unimodal shape, consistent with previous simulation-based studies. The rising part of the relationship is driven by a sampling effect—more individuals typically comprise a greater number of species (the “more-individuals effect“). The declining part of the relationship is driven by a greater proportion of the pool of propagules being of local origin, which dilutes species diversity arising from immigrant propagules (the “dilution effect“).
Our study’s main novel finding is that the peak of the unimodal richness–productivity relationship from the model shifts to the left as spatial scale increases. The underlying reason is that an increase in spatial scale results in a greater area-to-perimeter ratio, which increases the strength of the dilution effect relative to that of the more-individuals effect. Thus, the increasing phase of the richness–productivity relationship is predicted to be more prominent on smaller spatial scales, while the decreasing phase is predicted to be more prominent on larger scales. These findings can potentially account for some of the observed scale-dependence of richness–productivity relationships in nature.