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. |
