Selection changes productivity over time

Experiments suggest that increasing species’ diversity increases productivity over short observation periods. At equilibrium, theory predicts that diversity may increases productivity, such as in communities of coexisting competitors. This suggests a disconnect with theory focusing on long term trends and experiments observing short-term population growth. To resolve this gap, we start from short term models of population growth and analyze the conditions under which diversity productivity relationships emerge. Using this approach, we show that selection (as it is formally defined in dynamic models) does not affect productivity. Instead, selection changes productivity over time. In contrast, niche complementarity can change productivity directly. We illustrate how to measure each of the terms in competition experiments. These results highlight how the effects of diversity on productivity can be obscured by our choice of response variable.