Palmer, an ecologist at the University of Florida, was walking past a fenced-off research site in Kenya when he noticed something curious: instead of thriving, acacia trees that were protected from leaf-eating elephants and giraffes were withering and dying.
“That struck me as paradoxical,” he said in a telephone interview this week from the site. “If you remove large herbivores, you should see more vigorous trees.”
Dr. Palmer and his colleagues investigated. Their findings, reported in Friday’s issue of the journal Science, add to the mounting evidence that relationships between plant and animal species can be far more complex than had been thought and that even seemingly benign interference can have devastating effects.
The acacias and a species of ant that colonize them live together in an arrangement called mutualism. The ants nest in the trees’ thorns and sip on their nectar; in return, they swarm out ferociously, ready to bite, when a tree is disturbed by an elephant, a giraffe or other grazing animal.
But somehow, Dr. Palmer said, the trees seem to sense when no one is munching on their leaves and, after a year or so, seemingly decide, “We are going to reduce our investment in ants” by not producing so many roomy thorns or so much tasty nectar. The ants’ responses — lassitude is one — eventually encourage wood-boring beetles to invade the trees. Soon their tunnels leave the trees sickly, dying or dead.
The finding shows that what looks like two-species mutualism may involve other species. And they offer new proof of the fragility of the web of life, a phenomenon observed, for example, when wolves vanish from mountain landscapes or sharks and other top marine predators are fished out of the marine food chain.
Without wolf predation, elk are freer to roam and eat more plants. Result: aspen begin to vanish. Similarly, the overfishing of sharks and similar large fish leave smaller, algae-eating fish free to graze unhindered on algae growing on (and feeding) coral. Result: dead coral.
Dr. Palmer said it was shocking to see how quickly the ant-acacia mutualism, evolved over thousands of years, “dissolved” once the herbivores were removed. Now, he said, he and his colleagues want to see if they can restore the old pattern by again allowing giraffes and elephants to feed on the trees.