Productivity and disturbance are known to affect diversity but their exact roles have been much debated.The prevalent Intermediate Disturbance Hypothesis has been debated recently, in part due to mixed empirical support. Alternatively, the Dynamic Equilibrium Model (DEM) expects interaction between productivity and disturbance to predict a biodiversity maximum at matched, intermediate levels of disturbance and productivity. We tested the DEM using 40 outdoor aquatic mesocosms, inoculated from a homogenized seed bank. Disturbance treatments varied among eight increments, affecting 12-96% of substrate area. Primary productivity was manipulated by a gradient of fertilizer treatments aimed to oligotrophic-eutrophic phosphorus concentrations. Resulting net primary productivity was directly measured by diurnal changes in dissolved oxygen. Vegetation communities were quantified as percent cover (including filamentous algae) and final dry biomass (excluding filamentous algae) per species. Diversity was estimated as species richness, effective diversity, and evenness. Competing regression models were compared via AICc, where predictors were combinations of linear and quadratic productivity and disturbance.
Results/Conclusions
Diversity based on percent cover better accounted for algal growth than biomass measurements and revealed significant effects for evenness and diversity. The most plausible model for both measures of biodiversity was an additive relationship of quadratic productivity and quadratic disturbance. For evenness, this model meaningfully outperformed the null (dAICc=5.6) and found significant effects for productivity (p<0.01). Disturbance was found to be insignificant (p>0.1).
For diversity, this model slightly outperformed the null (dAICc=0.9). It found significant effects of productivity(p<0.01) and marginally significant effects of disturbance (p<0.06). For both models, the effect of productivity was several times greater than that of disturbance.
These findings provide mixed support of the DEM. The predicted quadratic relationships of productivity and disturbance were supported but did not statistically interact and represent only 1/3 of the variance in evenness (r2=0.35) and 1/4 of variance in diversity (r2 =0.26). We infer that diversity patterns are partially driven by productivity and disturbance, but those deterministic processes can be muted by stochastic effects (colonization sequence & timing, priority effects, etc.).