Recruitment limitation modifies the net effects of shared enemies on competitively inferior plants

Abstract

1. Generalist enemies often influence communities by differentially impacting species. The net effect of a shared enemy on the fitness of an individual depends upon the direct effects on that individual, its competitors and the nature of competition among individuals. 2. I present a stochastic model for predicting the net effect of shared enemies on competitively inferior individuals in plant communities that exhibit enemy-induced recruitment limitation. 3. When the probability of enemy-induced recruitment failure is included, it is not necessary that a shared enemy have a greater direct effect on the superior competitor to have a positive net effect on competitively inferior individuals. Nor is it sufficient. Specifically, a shared enemy can have a positive net effect on an inferior competitor even if it has a larger negative direct effect on the inferior competitor. Alternatively, a shared enemy may have a negative net effect on an inferior competitor even if it has a larger negative direct effect on the superior competitor. The net effect of the shared enemy on inferior competitors depends strongly on the trade-off betweens the gains the inferior competitor receives by the enemy-induced recruitment limitation of the superior competitor and the losses received in its ability to win sites through competition. 4. Recruitment limitation modifies the net effect of the shared enemy on the inferior competitor most strongly if dispersal is of short scale and effective fecundities are low. A spatially explicit simulation model was used to show that under these conditions, the individual level effects predicted by the analytical model scale up to the population level. The situations where these results are likely to be important in natural communities are discussed. 5. Synthesis. The effects of enemies and recruitment limitation on plant community structure have each been well studied individually. I find that these factors can interact to produce non-intuitive effects on community structure. Moreover, I find that the importance of this interaction depends strongly on the length scales through which plants interact with the environment. These results emphasize the importance of considering the role of spatial processes in models of plant communities. © 2007 The Author.