Body size distribution patterns and its underlying mechanism for global freshwater fish

Background/Question/Methods

Body size distributions are important ecological patterns that can provide insights into the assembly and functioning of species communities. Regardless of taxa the typical expectation and observation is a right-skewed distribution pattern due to more small than large-bodied species. However, different body size patterns have been documented for a variety of taxa, and processes causing the patterns are not well understood. We studied the body size distribution patterns of fresh water fish based on a global database. This database catalogs all known fish species in 375 irregular shaped and variable sized ecoregions. We extracted maximum body length from a different database. We consider mean, median, coefficient of variation, and skewness describing distribution patterns, and ask how they change with spatial scale, latitude, and other factors related to hypothesized underlying processes. These factors include variables such as total precipitation, area, and species richness.

Results/Conclusions

Mean and median body length increases at higher latitude, which agrees with the Bergman rule. However, the relationship may also be related to the distribution of trophic levels. At higher latitude there is an increase in average trophic level of species because of lower trophic levels dropping out. The body size distribution is right skewed for most ecoregions, as expected, and is highly variable at low latitudes. There is no relationship between skewness and spatial scale (as expressed in ecoregion size) at least in the range of area sizes we have available. This result contrasts with such a relationship having been shown for mammals. Furthermore, there is a strong positive correlation between skewness and species richness. Species richness itself is not related to area, which is likely an artifact of ecoregion delineation, but is related to total precipitation, which is more closely related to total amount of aquatic habitat available than terrestrial area.