Situations where species each benefit the other are called mutualisms. Mutualisms are widespread, ranging from humans and their domesticated plants and animals, to flowers and their pollinators, and even to the bacteria in the digestive tracts of most animals. Most ecological research on mutualisms has considered only pairs of interacting species, even though we know that many of these systems have multiple partners on both sides of the mutualism. For example, many insects may pollinate a plant species, and many plant species may be pollinated by a single insect species. We will be examining multi-species mutualism in a model system-- the whistling thorn acacia trees of East Africa and the symbiotic ants that protect them. Does the fact that there are several ant species competing for host trees in this system result in adaptations by the acacias that encourage more protective ant species at the expense of others (poorer protectors)? Do tree responses change the "balance of power" among these ant species, altering their tendency to coexist? How does this balance change depending on the age of the tree, the richness of the soil, and the abundance of browsing animals? We will carry out a series of descriptive studies and controlled experiments designed to answer these questions. These studies will give us a much more realistic understanding of multi-species mutualisms that should be broadly applicable to diverse and useful mutualist systems.
