Abstract
Experiments suggest that increasing species’ diversity increases
productivity over short observation periods. Theory predicts that
diversity may increases productivity at equilibrium, such as in
communities of coexisting competitors. In effect, experiment and theory
are using different response variables. We illustrate how this gap can
obscure some of the mechanisms through which diversity shapes
productivity. To do this, we propose a formal link to dynamic models of
population growth. Using this link, 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.