The size of the smallest species in an assembly is tightly linked to richness in worldwide fish distributions
We investigated body size and species richness patterns in world-wide freshwater fish distributions. The abiotic filtering hypothesis suggests that stressful environments promote a high degree of ecological similarity among species. Consequently, the body size range is expected to be limited in high stress environments, ultimately leading to reduced species richness. Abiotic filters are expected to affect extreme sizes most strongly. Thus, we hypothesize that there is a strong relationship between species richness and extreme body sizes. In particular, we hypothesize that the size of the smallest fish species in an ecoregion is negatively related to species richness, while the size of the largest species is positively related to species richness. Such a connection could of course also be caused by a pure sampling effect: drawing a larger number of species at random from all fish species leads to a higher chance of drawing extreme sizes than drawing a small number of species. Therefore, we used three simulation models as null models for comparing to our empirical results. We used world-wide freshwater fish database compiled data from different sources including FISHBASE and FEOW comprising 9,613 fish species in 27,929 individual occurrences across 300 ecoregions.
There is a stronger negative correlation between the size of the smallest species and species richness than expected from the simulation models, while the size of the largest species showed a weaker than expected positive relationship. Thus, the observed relationships between species richness and extreme body sizes cannot be explained completely by a sampling artifact, suggesting that they are consequences of ecological and evolutionary factors. Our results suggest that abiotic filters most strongly affect the smallest fish species. Further, our first simulation model suggests that the reduction in richness is due to the effect of abiotic filters on body sizes. We also suggest that the stronger than expected relationship between the size of the smallest species and richness is driven by a strong positive relationship between body size and latitude in the smallest fish in conjunction with the known positive relationship between latitude and richness. However, our second and third simulations show that the connection via latitude does not fully account for the strength of the empirical relationship, leaving room for additional processes at work that need to be investigated.