Biodiversity modulates the size-abundance relationship in changing environments

Organismal abundance tends to decline with increasing body size. Metabolic theory links this size structure with energy use and productivity across levels of biological organization, predicting a size-abundance slope of -0.75 that is invariant across environments. We tested whether the size-abundance relationship is robust to a gradient of protist species richness (1 to 6 species), temperature (15 to 25° C), and time. Our results support the expected slope of -0.75 for the size-abundance relationship, but we found interactive effects indicating that the size-abundance relationship is not invariant. In high-richness communities, temperature increased the abundance of small protists more than that of the large protists, leading to a steeper size abundance slope. Temperature and size-dependent species interactions were identified as potential drivers. Understanding variation in size-abundance relationship hence provides novel insight into the underlying mechanisms shaping the size structure of ecological communities under environmental change.