Figure 3. Nutrient and consumer effects on relative exotic cover and richness.

Effect of nutrient addition and herbivore exclusion on the change in relative cover and richness of exotic plants. The Consumer by Nutrient Experiment (a and c) is a factorial combination of nutrient addition (Nut) and consumer exclusion (Fnc) replicated at 34 sites. The Multiple Nutrient Experiment (b and d) is a full factorial addition of nitrogen (N), phosphorus (P), and potassium with micronutrients (K) replicated at 37 sites. Plotted values are the treatment effects on the annual change in relative cover or richness of exotics per year estimated using mixed-effects models. Interactions (for example, N*P, N*K, P*K, and N*P*K) test for additivity, with significant positive or negative values indicating super- or sub-additivity, respectively. Exotic cover and richness was relativized by dividing by the total cover or richness in a plot multiplying by 100. Error bars are s.e. of the slope estimates. Slopes that are significantly different from zero are indicated as follows: ***P≤0.001, **P≤0.01 and *P≤0.05.