Temperature and Resource Supply Drive Continental Variation in Size Structure of Freshwater Food Webs

Biological communities follow a remarkably consistent negative relationship between individual mass (M) and abundance (N), represented by a power law (N ~ M^λ). The parameter λ denotes the rate of decline in relative abundance from small to large individuals and serves as a proxy for energy transfer efficiency in food webs. Although warming is expected to affect λ, its influence remains uncertain, possibly due to interactions with resource supply. Using ~670,000 individual body sizes from stream food webs, we tested how temperature and resources shape λ. Temperature effects depended on resource supply (gross primary production [GPP] and organic matter standing stock [OM]) but contradicted expectations that λ becomes more negative with warming. At medium and high resources, λ becomes less negative under warming while no change occurred at low resources. Variation in OM, not GPP, drove these patterns, highlighting the role of external energy inputs and challenging the idea that large organisms are rarer at higher temperatures.