Shifts of community composition and population density substantially affect ecosystem function despite invariant richness

There has been considerable focus on the impacts of environmental change on ecosystem functionarising from changes in species richness. However, environmental change may affect ecosystemfunction without affecting richness, most notably by affecting population densities and communitycomposition. Using a theoretical model, we find that, despite invariant richness, (1) small environ-mental effects may already lead to a collapse of function; (2) competitive strength may be a lessimportant determinant of ecosystem function change than the selectivity of the environmentalchange driver and (3) effects on ecosystem function increase when effects on composition are lar-ger. We also present a complementary statistical analysis of 13 data sets of phytoplankton andperiphyton communities exposed to chemical stressors and show that effects on primary produc-tion under invariant richness ranged from�75% to+10%. We conclude that environmental pro-tection goals relying on measures of richness could underestimate ecological impacts ofenvironmental change.