The Sizes of Life

Abstract Recent research has revealed the diversity and biomass of life on Earth, but how that biomass is distributed across body sizes remains unclear. We compile the present-day global body size-biomass spectra for the terrestrial, marine, and subterranean realms. To achieve this compilation, we pair biomass estimates with previously uncatalogued body size ranges across all free-living biological groups. These data show that diverse organism types converge on similar overall minimum and maximum sizes. We then propagate biomass and size uncertainties and provide statistical descriptions of body size-biomass spectra across and within major habitat realms. Power laws show exponentially decreasing abundance (exponent -0.9, R 2 =0.97) and nearly equal biomass (exponent 0.09, R 2 =0.56) across log size bins. Gaussian mixture models show small and large organisms outweigh other sizes by about one order magnitude ( R 2 =0.86 in the size-biomass spectrum), but one-to-two orders of magnitude uncertainty persists across all organisms. The results show that the global body size-biomass relationships may be bimodal, but additional data will be needed to clarify whether global-scale universal constraints or local forces shape these patterns.