Assistant Professor University of Louisiana, United States
Background/Question/Methods
Across fields as diverse as economics, physics, geology, and sociology, many phenomena follow the '80/20 rule', such that at least 80% of consequences are attributable to 20% of causes. For example, >80% of wealth is owned by 20% of Americans, and 80% of earthquake damage is caused by 20% of earthquakes. The 80/20 rule results from highly skewed frequency distributions, often best fit by a lognormal or power law. Do ecosystem functions follow this same general pattern? That is, might 80% or more of ecosystem function be provided by 20% of the species in an ecological community? If so, this would provide a novel perspective on the relationship between biodiversity and ecosystem function, which thus far has been studied primarily by measuring the effect of species richness on function, often without accounting for the species-abundance distribution. Here, we explore this idea using two contrasting ecosystem functions, pollination by wild bee species and carbon storage by tropical forest trees, to ask (1) What are the frequency distributions of ecosystem function per species?, and (2) Does ecosystem function follow the 80/20 rule?
Results/Conclusions
We collected 7449 individual bees of 110 species visiting flowers of three plant species, and measured pollen deposition per flower visit by 753 individual bees. We estimated pollination function per bee species as flower visits times pollen grains deposited per visit. We used publicly available data from a 50 ha tropical forest plot in Barro Colorado Island, Panama to obtain the above-ground biomass of 207,719 individual trees of 298 species. We estimated carbon storage per tree species as the sum of individual tree biomasses times the C content of wood. Thus, one function (the pollination provided by a wild bee species) follows from the abundance of that species, and the other (carbon storage provided by tropical forest trees) follows from the biomass of that species. In both cases, ecosystem function per species was best fit by a lognormal frequency distribution. Both ecosystem functions followed the 80/20 rule: the proportion of pollination and carbon storage done by the top 20% of bee and tree species ranged from 80% to 90%. Our results suggest that, at least for these two functions at the scale at which they were measured here, the majority of ecosystem function is provided by a minority of species.
