A little-known species of African ant prunes its home trees into leafy islands, preventing murderous takeover raids by other ant species but castrating the trees in the process, suggests a new study led by researchers at the University of California, Davis.

The findings, described in this Thursday's issue of the journal Nature with researchers from the University of Oregon and Mpala Research Centre in Kenya, have implications for two important questions in ecology.

First, why are ecological communities so diverse, and specifically, how do weaker competitors persist? In this community, the branches of neighboring acacia trees often touch, forming bridges for ant invasions, says lead author Maureen Stanton, a UC Davis professor of evolution and ecology. But Crematogaster nigriceps ants eliminate those bridges by chewing off their host tree's new growth. "This avoidance strategy allows these ant colonies to persist longer, almost as fugitives, in hostile neighborhoods," Stanton said.

Second, what might cause an organism living in a partnership to evolve into a parasite? Many specialized plant-ant species live cooperatively with their hosts -- the plants house and feed the ant colony, while the ants protect their hosts from herbivores, pathogens and competitors. Not so with C. nigriceps, Stanton said.

"Our field results suggest that this selfish pruning behavior has evolved because it increases the life span of C. nigriceps colonies, even though it removes all the host tree's flowers and stops the tree from reproducing," Stanton said. "There can be a fine line separating a mutualistic interaction from a parasitic one."

The study was funded by the National Science Foundation and the UC Davis Bridge Grant program.

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Contact: Maureen Stanton
mlstanton@ucdavis.edu
530-752-2405
University of California - Davis
6-Oct-1999