The thorny acacia trees of East Africa live in close harmony with ant colonies, and each depends on the other for health and survival – but disrupting that relationship can lead to death and danger, scientists have discovered.
And that, they say, could threaten the habitats of Africa’s largest animals in many regions of the continent.
Normally, the huge swollen thorns on the branches of the scrubby trees provide housing for the ants, and they feed on rich nectar from the base of the acacia leaves. In exchange, the tiny biting insects guard and protect the trees by swarming out to repel big browsers like elephants and giraffes that would otherwise feed destructively on the acacia leaves.
It’s a classic example of what scientists call “mutualism,” the phenomenon of evolution that’s common everywhere in nature – think honeybees evolving to pollinate flowers, and flowers evolving to give nectar for the bees. It’s been going on throughout nature for millions of years.
But the system can go badly awry, and biologists from the University of Florida, Stanford and UC Davis have discovered in Kenya that if the acacias are fenced off to protect them from the browsing animals, all that mutualism disappears – the acacia thorns produce less nectar, the “good” ants grow weaker, tribes of nasty ants and wood-boring beetles take over, and the trees sicken and die prematurely.
“It can turn into a battleground out there,” said Todd Palmer of the University of Florida, who began his research as a graduate student at UC Davis and arrived in Kenya on Monday to continue his work. “Mutualism gives way to nature, red in tooth and claw, as Darwin knew very well.”
Palmer and his colleagues at Stanford and UC Davis are publishing their findings Friday in the journal Science.
He was reached by cell phone in Nairobi as he was preparing to head for the Mpala Research Centre near the market town of Nanyuki in Kenya’s central highlands, far from the deadly post-election tribal turbulence that has engulfed that once-stable nation.
“Things are quiet at Mpala,” Palmer said, “and our research goes on.”
One of his principal colleagues is Robert Pringle, a graduate student in ecology at Stanford’s Center for Conservation Biology; he is joining Palmer in Kenya this weekend to continue the study and pursue his own research into underground termites and the “cascade” of spiders and lizard populations those termites encourage.
The Mpala center holds a series of “exclosures” where 7,500-volt electrified fences surround areas of acacia trees to keep the big browsing animals away.
As Palmer and Pringle say, one would think that protecting the trees from foraging elephants, giraffes and gazelles would help the acacias to grow and their ant “bodyguards” to flourish.
But it ain’t necessarily so, as the old Gershwin song goes.
For nearly 12 years, Palmer and members of the team have monitored the fenced-in plots twice a year and have observed that as the beasts have been kept away, the whole ecology of the fenced areas has changed. The trees’ growth patterns trees slowed; many sickened, and their death rate doubled; they produced fewer thorns for the ants to use as homes and less nectar for the ants to eat.
Meanwhile, entire colonies of those ants – which typically number 100,000 on a single tree – began to shrink, and the few that remained were driven away by another ant species that occupied the territory. Those changes in the ant populations, in turn, allowed huge numbers of stem-boring beetles to invade the trees.
Thus ended the pastoral era of mutualism.
And that’s what could well happen in East Africa’s savannah, Palmer said. The elephants, giraffes and other large browsers are rapidly vanishing because of poaching and over-hunting, while land-hungry human populations are growing, and the animals’ natural habitats are being degraded, he said.
“Whenever you start fiddling with these wonderful mutual relationships like the acacias and the ants and elephants, you create all kinds of unintended consequences,” said Truman P. Young, a botanist at UC Davis who was Palmer’s professor and a leader of the Mpala experiment from the beginning.
“The elephants in this case are such big players everywhere in Africa that their presence or absence can alter whole ecosystems in many ways,” Young said.
Brian Fisher, an entomologist at the California Academy of Sciences who first studied the evolution of mutualism in ants and plants 20 years ago in Panama and Peru, called the work by Palmer and his colleagues “a wonderful study” because it showed so clearly how a single change in a biological system can so swiftly start a change in the course of evolution there.
“In this case,” Fisher explained, “you’d expect that removing the elephants would encourage the acacias to grow stronger and for their ant colonies to thrive. But here, with the elephants gone, there’s less pressure on the trees to defend themselves by providing homes and food for their guardian ants. Their large thorns become smaller, and the sources of the nectar dry up.
“So as the ‘good’ ants begin to leave, the ‘bad’ ants take over; and then in come the wood-boring beetles, the trees weaken and the whole system of mutualism falls apart.”
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