The importance of species interactions in harsh environments determined using formal links between theory and experimental data

Background/Question/Methods

The relative importance of environmental factors and species interactions in determining community structure is a central theme in ecology. Many influential ecological theories suggest that negative species interactions such as competition decrease in intensity, and therefore importance, along gradients of environmental stress. However, alternative theoretical work suggests that the importance of competition is largely independent of the intensity of competition, and depends instead on intrinsic population growth rates in a given environment. Although competition along environmental gradients has been reasonably well studied, traditional empirical approaches are poorly linked to competition theory, severely limit inference about competition intensity and rarely incorporate differences in estimates of population growth rates among environments. To overcome these problems, we fit theoretical models of competition to data from an efficient, response-surface experimental design to investigate interactions among marine bryozoans in both harsh and benign environments. Analytical links between theory and data allowed us to formally compare the suitability of different theoretical models for describing competition in each environment in the field. Furthermore, our estimates of population growth rates and competition coefficients can be compared directly among species and environments.

Results/Conclusions

As expected, parameter estimates from the best model fit show that population growth rates in the absence of competition were substantially lower in harsh environments for both species. For the encrusting bryozoan Watersipora subtorquata, a strong, positive, interspecific-competition coefficient that did not vary with environmental harshness suggests that interspecific competition will have stronger effects on population persistence in harsh environments because population growth rates in these environments are already low. In contrast, for the arborescent bryozoan Bugula neritina, interspecific facilitation (i.e. a strong, negative competition coefficient) in harsh environments has the potential to offset low population growth rates such that population persistence will depend strongly on the density of interspecific competitors in each environment. Our study supports theoretical work suggesting that the relative importance of competition among environments depends strongly on the magnitude of the direct effects of environmental harshness on population growth rates.