Widespread shifts in body size within populations and assemblages

Biotic responses to global change include directional shifts in organismal traits. Body size, an integrative trait that determines demographic rates and ecosystem functions, is thought to be shrinking in the Anthropocene. Here, we assessed the prevalence of body size change in six taxon groups across 5025 assemblage time series spanning 1960 to 2020. Using the Price equation to partition this change into within-species body size versus compositional changes, we detected prevailing decreases in body size through time driven primarily by fish, with more variable patterns in other taxa. We found that change in assemblage composition contributes more to body size changes than within-species trends, but both components show substantial variation in magnitude and direction. The biomass of assemblages remains quite stable as decreases in body size trade off with increases in abundance. Large organisms may be particularly susceptible to impacts from human activities, including hunting and harvesting, as well as the stress of climate change. Martins et al . analyzed body-size trends in plant and animal communities since 1960 from the BioTIME database and separated the effects of changes within species from those driven by species composition. They found that trends varied across communities, with marine fish more consistently shifted toward smaller body size. Mean body size changed within populations, but community-level trends were more affected by changes in the abundance of small- and large-bodied species. Biomass generally stayed stable over time, suggesting that more small individuals were present despite the loss of larger ones. —Bianca Lopez Analysis of plant and animal communities spanning from 1960 to 2020 and across six taxon groups reveals prevailing decreases in body size.