(a) Depiction of changes in an environmental condition or resource that affects the abundance of the species of interest across time from period t1 to period t2. (b) Depiction of the range of variability in traits present in two species. The red species exhibits relatively small amounts of trait variation relative to the blue species. (c) The responses of two species (red and blue) to changes in conditions depicted in panel (a). The dotted histogram represents the relationship between a response trait and abundance under conditions at t1, whereas the solid histogram represents this relationship at t2. The red species has limited capacity to shift its trait values and thus exhibits large changes in abundance when compared with the blue species, which has higher levels of trait variability, and thus more stable populations. (d) If we assume that the red species also has limited variability in its effects traits relative to the blue species, in situations where the relationship between effects traits and ecosystem functioning is not contingent on environmental conditions, we predict that the red species should cause greater stability in ecosystem functioning than the blue species. (e) However, if the relationship between effects traits and ecosystem functioning changes in different environments (t1 versus t2), it is the blue species with higher variability in effects traits that is predicted to stabilize ecosystem functioning.