I develop a general partitioning of the effects of species loss on any ecosystem function comprising the summed contributions of individual species. The approach partitions the difference in ecosystem function before vs. after species loss into additive components attributable to different fundamental mechanisms. Using the Price Equation from evolutionary biology, I show that three fundamental mechanisms cause ecosystem function to vary between sites: the “species richness effect” (SRE; random loss of species richness), the “species composition effect” (SCE; non-random loss of high- or low-functioning species), and the “context dependence effect” (CDE; post-loss changes in the functioning of the remaining species). The SCE and CDE are analogous to evolutionary processes: the SCE is analogous to natural selection, while the CDE is analogous to biased transmission. I go on to show how this ‘Price Equation partition’ can be re-expressed in terms of processes operating on the phenotypic traits that determine species’ functional contributions. This is analogous to re-expressing phenotypic evolution in terms of selection and transmission operating on the genotypes that determine individual phenotypes. Finally, I show how to extend the Price Equation partition to account for species gain as well as species loss.