Short-term dynamics of an acacia ant community in Laikipia, Kenya

In monospecific stands of Acacia drepanolobium in Laikipia, Kenya, virtuall y all but the smallest trees are occupied by one of four species of ants. A lthough trees are a limiting resource, all four ant species are maintained in this system. Three separate lines of evidence confirm a linear dominance hierarchy among these four ants: (1) experimentally staged conflicts, (2) natural transitions among 1773 tagged trees over a 6-month period, and (3) the average sizes of trees occupied by ants of different species. Short-ter m dynamics during a drying period reveal that many smaller trees (<1 m) occ upied by dominant ants were subsequently abandoned, and that abandoned tree s had grown more slowly than those that were not abandoned. Height growth i ncrements over 6 months were generally independent of ant occupant, but inc reased with tree height. Among taller trees (>1 m), changes in ant occupati on congruent with the dominance hierarchy (i.e., transitions from more subo rdinate ant species to more dominant ant species) occurred on trees that gr ew faster than average. In contrast, the (less frequent) changes in ant occ upation "against" the direction of the dominance hierarchy occurred on tree s that grew more slowly than average. Observed correlations between tree vi gor and takeover direction suggest that colony growth of dominant ant speci es is either favored in more productive microhabitats, or that such colonie s differentially seek out healthier trees for conquest. Colonies of dominan t species may differentially abandon more slowly growing trees during (dry) periods of retrenchment, or suffer higher mortality on these trees. Subord inate ant species appear to move onto these abandoned trees and, to a lesse r extent, colonize new recruits in the sapling class. These data reveal tha t within a simple linear dominance hierarchy, shortterm variations exist th at may reveal underlying mechanisms associated with coexistence.