Size spectra in freshwater streams are consistent across temperature and resource supply

Biological communities can be described using the individual size distribution (ISD), a remarkably consistent negative relationship between individual mass and abundance that can be described as a power law (N ~ Mλ). The parameter λ denotes the rate of decline in relative abundance from small to large individuals. Despite well-known influences of temperature on λ, interactive effects of temperature and resource supply on λ remain poorly understood. Leveraging data from 2.4 million individual body sizes in freshwater stream food webs, we test the hypothesis that λ varies as a function of temperature and resource supply. Surprisingly, despite varied environmental conditions and complete species turnover, there was no discernible impact on λ from temperature or resource supply. Instead, λ was consistently ~ -1.2 (sd = 0.04), differing from predictions of metabolic scaling theory, which assumes a value close to -2. This implies substantial subsidy inputs to large predators along with metabolic scaling coefficients shallower than assumed by the ¾ power law. Using simulations and mesocosm experiments, we provide support for a metabolic scaling coefficient of approximately ~0.4 for freshwater streams, along with a consistent influence of subsidies. The findings underscore remarkable consistency of individual size distributions in freshwater streams, likely driven by shallow metabolic scaling and large subsidies to large consumers.