coexistence in guilds of competing mutualists

A number of very interesting empirical studies address the mechanisms that maintain coexistence among competitors which interact mutualistically with their shared resource (for example, Marr et al. 2001, Bronstein et al. 2003, Hoeksema and Kummel 2003, Morris et al 2003, Wilson et al. 2003). I study this question from a theoretical standpoint, asking how mutualistic effects of consumers on their resource populations may modify or interact with known mechanisms of competitive coexistence. With Brian Inouye at FSU, I am working on mutualisms that involve long-term obligate associations, such as between myrmecophytic plants and their patrolling ants. We’ve been excited to find that the positive effect of ant species on plant population dynamics can powerfully stabilize competitive coexistence. With Tom Miller at Rice, we are studying relationships that are more facultative, such as between ants and nectar-producing cacti. In this system, the positive ant-plant interaction can stabilize coexistence when the plant species is structured by size or developmental stage. These findings reinforce the importance of resource dynamics in competitors’ indirect interaction: mutualism between consumers and their shared resource is a novel coexistence mechanism that could operate in a wide variety of natural systems.