The predator-prey power law: Biomass scaling across terrestrial and aquatic biomes

A general scaling law for ecology Despite the huge diversity of ecological communities, they can have unexpected patterns in common. Hatton et al. describe a general scaling law that relates total predator and prey biomass in terrestrial and aquatic animal communities (see the Perspective by Cebrian). They draw on data from many thousands of population counts of animal communities ranging from plankton to large mammals, across a wide range of biomes. They find a ubiquitous pattern of biomass scaling, which may suggest an underlying organization in ecosystems. It seems that communities follow systematic changes in structure and dynamics across environmental gradients. Science, this issue 10.1126/science.aac6284; see also p. 1053 The number and size of predators and their prey scale across a broad range of terrestrial and aquatic animal communities. [Also see Perspective by Cebrian] INTRODUCTION A surprisingly general pattern at very large scales casts light on the link between ecosystem structure and function. We show a robust scaling law that emerges uniquely at the level of whole ecosystems and is conserved across terrestrial and aquatic biomes worldwide. This pattern describes the changing structure and productivity of the predator-prey biomass pyramid, which represents the biomass of communities at different levels of the food chain. Scaling exponents of the relation between predator versus prey biomass and community production versus biomass are often near ¾, which indicates that very different communities of species exhibit similar high-level structure and function. This recurrent community growth pattern is remarkably similar to individual growth patterns and may hint at a basic process that reemerges across levels of organization. RATIONALE We assembled a global data set for community biomass and production across 2260 large mammal, invertebrate, plant, and plankton communities. These data reveal two ecosystem-level power law scaling relations: (i) predator biomass versus prey biomass, which indicates how the biomass pyramid changes shape, and (ii) community production versus community biomass, which indicates how per capita productivity changes at a given level in the pyramid. Both relations span a wide range of ecosystems along large-scale biomass gradients. These relations can be linked theoretically to show how pyramid shape depends on flux rates into and out of predator-prey communities. In order to link community-level patterns to individual processes, we examined community size structure and, particularly, how the mean body mass of a community relates to its biomass. RESULTS Across ecosystems globally, pyramid structure becomes consistently more bottom-heavy, and per capita production declines with increasing biomass. These two ecosystem-level patterns both follow power laws with near ¾ exponents and are shown to be robust to different methods and assumptions. These structural and functional relations are linked theoretically, suggesting that a common community-growth pattern influences predator-prey interactions and underpins pyramid shape. Several of these patterns are highly regular (R2 > 0.80) and yet are unexpected from classic theories or from empirical relations at the population or individual level. By examining community size structure, we show these patterns emerge distinctly at the ecosystem level and independently from individual near ¾ body-mass allometries. CONCLUSION Systematic changes in biomass and production across trophic communities link fundamental aspects of ecosystem structure and function. The striking similarities that are observed across different kinds of systems imply a process that does not depend on system details. The regularity of many of these relations allows large-scale predictions and suggests high-level organization. This community-level growth pattern suggests a systematic form of density-dependent growth and is intriguing given the parallels it exhibits to growth scaling at the individual level, both of which independently follow near ¾ exponents. Although we can make ecosystem-level predictions from individual-level data, we have yet to fully understand this similarity, which may offer insight into growth processes in physiology and ecology across the tree of life. African large-mammal communities are highly structured. In lush savanna, there are three times more prey per predator than in dry desert, a pattern that is unexpected and systematic. [Photo: Amaury Laporte] Ecosystems exhibit surprising regularities in structure and function across terrestrial and aquatic biomes worldwide. We assembled a global data set for 2260 communities of large mammals, invertebrates, plants, and plankton. We find that predator and prey biomass follow a general scaling law with exponents consistently near ¾. This pervasive pattern implies that the structure of the biomass pyramid becomes increasingly bottom-heavy at higher biomass. Similar exponents are obtained for community production-biomass relations, suggesting conserved links between ecosystem structure and function. These exponents are similar to many body mass allometries, and yet ecosystem scaling emerges independently from individual-level scaling, which is not fully understood. These patterns suggest a greater degree of ecosystem-level organization than previously recognized and a more predictive approach to ecological theory.