Friday, August 6, 2010

PS 96-66: Combined effects of exogenous and endogenous variability on the spatial distribution of ant communities in a forested ecosystem

Senay Yitbarek, University of Michigan Ann Arbor and John Vandermeer, University of Michigan.


Ecosystems are characterized by the degree to which relatively simple local interactions emerge to form large-scale macroscopic patterns. The patchiness of ecosystems has been attributed to exogenous processes, while in other cases endogenous processes are believed to produce non-random spatial patterns in ecosystems. Yet, relative few studies have attempted to examine the combined effects of exogenous and endogenous processes on the distribution of organisms across spatial and temporal scales. Here we aim to investigate whether spatial patterns of under-story tree species at a large spatial scale (18 ha) influences the formation of spatial patterns of ground foraging ant species at a much smaller spatial scale (20m x 20m). A total of 12 plots were established along borderlines of patches of dominant under-story tree species (red maple-witch hazel; red maple-black cherry; black cherry-witch hazel). To this extent, our study examined 1) Whether there is any signal in the ground foraging ant community that can be associated with the patches of different dominant under-story tree species and 2) Whether pattern formation of the ground foraging ants themselves depends on the particular dominant tree patch within which the ant community is located. We therefore estimated 1) population changes of ants over space and time, and 2) competitive interactions between ant species.


Our results demonstrate that spatial patterns of under-story trees species at a larger spatial scale influences the formation of spatial patterns of ground foraging ant species at a smaller spatial scale. At the regional scale, we found a statistically significant relationship for the occurrence of Camponotus noveboracensis, Formica subsericae, and Lasius alienus species in black cherry habitats. The presence of extra-floral nectarines, honeydew, and insect preys are associated with the presence of these ant species. In the witch-hazel plots, we similarly found a statistically significant relationship for Myrmica americana, Formica fusca, and Formica subericae species. The high abundance of Myrmica americana species in witch-hazel habitats suggests that these species are physiologically adapted to soil moisture variability. At smaller spatial scales, we observed mosaic ant patches changing rapidly over time. The formations of mosaic ant patches are the direct consequence of specialized foraging behaviors exhibited by these ants, in which some ant species are more efficient in exploiting limiting resources, while other species employ aggressive behavior to prevent competitors from dominating resources. Our study reveals that spatial self-organization operating at different scales has important implications for the distribution and structuring of ecological communities.