|Figure 1 from Hatton et al. 2015. "Predators include lion, hyena, and other large carnivores (20 to 140 kg), which compete for large herbivore prey from dik-dik to buffalo (5 to 500 kg). Each point is a protected area, across which the biomass pyramid becomes three times more bottom-heavy at higher biomass. This near ¾ scaling law is found to recur across ecosystems globally."|
|Figure 5 from Hatton et al. 2015. "Similar scaling links trophic structure and production.|
Each point is an ecosystem at a period in time (n = 2260 total from 1512 locations) along a biomass gradient. (A toP) An exponent k in bold (with 95% CI) is the least squares slope fit to all points n in each row of plots..."
Their finding is perhaps more remarkable because the scaling exponent has similarities to another possible law, metabolic scaling theory, particularly the ~0.75 exponent (or perhaps ~2/3, depending on who you talk to). It’s a bit difficult for me, particularly as someone biased towards believing in the complexities of nature, to explain how such a pattern could emerge from vastly different systems, different types of predators, and different abiotic conditions. The model they present is greatly
|Peter Yodzis' famous food web for|
the Benguela ecosystem.
There are multiple explanations to be explored. First, perhaps these consistent exponents represent a stable arrangement for such complex systems or consistency in patterns of density dependence. Consistent relationships sometimes are concluded to be statistical artefacts rather than actually driven by ecological processes (e.g. Taylor’s Law). Perhaps most interestingly, in such a general pattern, we can consider the values that don’t occur in natural systems. Macroecology is particularly good at highlighting the boundaries on relationships that are observed in natural systems, rather than always identifying predictable relationships. The biggest clues to understanding this pattern maybe in finding when (or if) systems diverge from the 0.75 scaling rule and why.
Ian A. Hatton, Kevin S. McCann, John M. Fryxell, T. Jonathan Davies, Matteo Smerlak, Anthony R. E. Sinclair, Michel Loreau. The predator-prey power law: Biomass scaling across terrestrial and aquatic biomes. Science. Vol. 349 no. 6252. DOI: 10.1126/science.aac6284