Biodiversity By Organism Size And Aggregate Biomass On Earth

Life on Earth is disproportionately made up of tiny organisms and large organisms, as measured by the aggregate biomass of organisms of each size.

A. Median carbon biomass (log scale) per log size as a function of body size with 95% confidence bounds (black dotted curves) cumulated across biological groups from 1000 bootstraps over within-group biomass and body size error distributions. Groups were organized from the least massive at the bottom to the most massive at the top for visibility on the log scale (ordered from top left to bottom right in color legend for group identity). Group biomasses are stacked so each group’s biomass is represented by its upper y-axis location minus its lower y-axis location (not by the upper y-axis location alone).

 

B. Median biomass in linear biomass scale. Confidence bounds are not shown here because they are so large as to obscure the median patterns on the linear scale.

Recent research has revealed the diversity and biomass of life across ecosystems, but how that biomass is distributed across body sizes of all living things 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 existing and updated biomass estimates with previously uncatalogued body size ranges across all free-living biological groups. These data show that many biological groups share similar ranges of body sizes, and no single group dominates size ranges where cumulative biomass is highest. 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±0.02 S.D., R^2 = 0.97) and nearly equal biomass (exponent 0.09±0.01, R^2 = 0.56) across log size bins, which resemble previous aquatic size spectra results but with greater organismal inclusivity and global coverage. In contrast, a bimodal Gaussian mixture model describes the biomass pattern better (R2 = 0.86) and suggests small (~10^−15 g) and large (~10^7 g) organisms outweigh other sizes by one order magnitude (15 and 65 Gt versus ~1 Gt per log size). The results suggest that the global body size-biomass relationships is bimodal, but substantial one-to-two orders-of-magnitude uncertainty mean that additional data will be needed to clarify whether global-scale universal constraints or local forces shape these patterns.

Eden W. Tekwa, Katrina A. Catalano, Anna L. Bazzicalupo, Mary I. O’Connor, Malin L. Pinsky. "The sizes of life." 18(3) PLOS ONE e0283020 (2023). DOI: 10.1371/journal.pone.0283020 via Science Daily.