The species-area relationship (SAR) has been a long-standing and successful model to predict and understand spatial variation in species numbers coexisting across local communities. Notwithstanding, there is much residual variation in SAR models so that local communities have less or more species than what is expected by their areas. Part of this variation can reflect how energy transfer efficiency across trophic levels within local communities hinders or enable different types of species to coexist. Body size is a key trait involved in species coexistence and energy flux given that efficiency is expected to strongly covary with the distribution of body sizes within a local community. As such, we predict that more energetically efficient communities should allow the coexistence of a greater number of species than what is expected by their areas. The goal of this study was to test this hypothesis by proposing a quantitative framework that integrates model slopes from size-abundance distributions, a proxy for bioenergetic efficiency within local communities, into SAR models. We applied the proposed framework to model spatial patterns in fish richness across 700 lakes in the province of Ontario, Canada. These data represent one of the largest and most detailed studies of lake-fish abundance-size distributions globally.
Results/Conclusions
The SAR model explained about 30% of the total variation in species richness. The slope of size- abundance distributions for each lake were calculated by using a bounded maximum likelihood method that estimates the best fitting model predicting the size of individual fish. The steepness of the slope estimates the efficiency in energy transfer. Lakes with steeper size–abundance relationships are less efficient in energy transfer because there is less energy available to support large organisms. Conversely, lakes with shallower slopes for their mass-abundance relationships are more efficient in transferring energy across trophic levels and able to sustain relatively greater proportion of relatively large individuals. We then integrated the size-abundances slopes as an additional predictor into the SAR model and found that the prediction was in the expected direction. Lakes with positive residual variation (i.e., greater number of species than predicted by lake-area) tended to have shallower abundance-size slopes in relation to lakes with negative residual variation. To our knowledge, this is the first study linking SAR models and bioenergetics at the level of local communities to explain species coexistence.