Mutualisms still play a relatively unimportant role in the theoretical understanding of community dynamics. Recent progress has occured as realistic models of mutualisms have been developed that allow for prediction of the role of environmental factors on mutualisms and the structure of mutualisms webs. However, few attempts have been made to understand what role mutualisms play in altering dynamics of communities with both consumer-resource and mutualistic interactions. As this topic is still in its infancy, the study of extraordinarily simple models systems can still provide insight into the role of mutualisms in community stability. Here, I discuss a series of analyses that investigate how strong mutualisms may be in providing enrichment that destabilizes predator-prey interactions. The models all involve simple Lotka-Volterra models of mutualisms constrained to represent stable, facultative mutualisms where one of the mutualists is consumed by a predator.
Results/Conclusions
In general, analyses of these models confirms previous work that enrichment generally destabilizes predator-prey interactions. However, the analysis demonstrates that the strength of that destabilization is nearly always weaker than that expected based on the enrichment itself. In particular, three measures of stability: eigenvalues, Hopf bifurcation points and minumum density of cycling populations, show that for equivalent equilibrium densities in the absence of predators, mutualism is a more stable form of enrichment than simply increasing the carrying capacity of the prey species. Furthermore, the stronger the positive feedback between the mutualists is, the weaker the effect of mutualist is on destabilizing the predator-prey model. The mechanism underlying this is that as predators reduce prey species the strength of the enrichment effect on the prey species is further weakened. These results suggests that in real systems, mutualists can provide a boost to prey densities without a significant destabilization of that interaction. When analyzing the productivity of intact systems and its effect on the stability of those systems, ecologists should seek to understand the how much of that productivity relies on mutualists.
