11 January 2008

The domino effect of extinction

Cosmos Online
The extinction of large animals such as giraffes and elephants could have knock-on effects that damage entire ecosystems, says a new study.
The domino effect of extinction

Power grazers: Giraffes around the ant-plant, Acacia drepanolobium. Credit: Todd Palmer

SYDNEY: The extinction of large animals such as giraffes and elephants could have knock-on effects that damage entire ecosystems, says a new study.

The research – which looked at the impacts of removing giraffe grazers on Africa’s acacia trees and ants that live alongside them – adds to the evidence that the extinction of one species can have a domino effect on many others.

You scratch my back, I’ll scratch yours

“With the human induced decline of big herbivores in Africa, it shows that the generally positive relationship between ants and plants essentially becomes antagonistic when big herbivores are removed,” said zoologist Todd Palmer of the University of Florida in Gainesville, USA.

Palmer is the co-author of the study which is published today in the U.S. journal Science.

To test the effect of removing large herbivores on the interactions of other species, Palmer’s team studied the unique mutualistic relationship between ants and acacias.

The Acacia drepanolobium tree produces hollow, swollen thorns that house three different ant species. These insects are reliant upon the nectar produced by the tree and in return, the ants swarm out to protect their host from the ravages of browsing herbivores.

However, a fourth ant mentioned in the study, is an antagonistic species, which does not nest in acacia thorns, but in stem cavities excavated by destructive, wood-boring beetles. An abundance of these ants and the beetles, has a negative effect on the growth and survival of the trees.

Palmer’s team tested three different plots of acacia trees in a Kenyan wildlife reserve. These plots were divided into blocks that were either totally unfenced or that had fencing that prevented the access of most animals greater than 15 kg in weight.

Over a period of 10 years they found that when giraffes, elephants and other large mammals were prevented from grazing on the trees, those trees began to produce fewer thorns for ants to live in and less nectar for them to eat.

This, the researchers write, induced a cascade of unforseen consequences.

A change in the community structure and competitive relationships between the ant species caused the once dominant “good mutualist” ant to starve and increase behaviour that was damaging for the tree – such as farming other harmful sap-sucking insects for nectar.

Easily disturbed equilibrium

Furthermore, the team found that the “bad antagonistic” ant species, which live in holes bored into the wood, became more abundant. This led to slower growth in the host trees – and twice as many of those trees died compared to the trees dominated by the mutualist nectar-eating species.

So what stops this antagonist ant species taking over in the first place? The study showed that the mutualist ants have a competitive edge over the antagonistic ants, keeping the system in an easily disturbed state of equilibrium.

Palmer said that the study highlights the importance of not just conserving species themselves, but also conserving the interactions between species.

Judith Bronstein, an ecologist at the University of Arizona in Tucson commended the research and said that it raises an important point.

“If we wish to guarantee the survival of species whose fates we care about, we have to stop thinking about them in isolation from their environments,” she said. “At the very least, we need to guarantee the health and welfare of the species that our favourite organisms depend upon, both directly and indirectly.”

Not everyone agrees, however. John Fox, a vegetation scientist at Curtin University in Western Australia, said that flaws in the experiment’s design may have meant the researchers were not measuring the effect of reduced herbivory of large mammals at all. He said that giraffes feed on acacia trees that are taller than the trees used in this study; which were between 1.8 and three metres tall.


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