US ecologists have highlighted the delicate balance at work in Nature with a study showing that the disappearance of elephants has knock-on effects on ants and ultimately the survival of trees.
Working in Kenya, University of Florida researcher Todd Palmer and his colleagues simulated the extinction of large grazing mammals such as elephants and giraffes by using fences to exclude the animals from patches of ground. Then, for the next ten years they monitored the effects on a species of Acacia tree, known as whistling thorns (Acacia drepanolobium) and the ants that inhabit the trees. The trees normally produce specialised hollow thorns and secretions of nectar to attract certain ant species which, in return for their thorny lodgings and the food, vigorously defend their host tree against other parasites and browsing animals.
But once the large animals were excluded things began to change. The species of ants inhabiting the trees changed, and the ant colony sizes reduced dramatically. The trees also became less healthy, grew less fast and were more than twice as likely to die compared with normally grazed trees.
So what provoked this dramatic change? The lack of grazing by elephants and other large animals resulted in the trees reducing their production of ant-incentives including the hollow thorns for them to live in and nectar rewards. Consequently the ants that would normally defend the trees, a species called Crematogaster mimosae, were lost and replaced with other ant species including one called Crematogaster sjostedti, which lives in cavities created in tree stems by long-horned beetles, which the ants themselves encourage to colonise the tree. But whilst it provides a home for the ants, the wood-weakening effects of the beetle larvae kills the trees. So, unpredictable as it would seem, if elephants and other large African mammals were to go extinct, trees and even ants would immediately suffer, highlighting the fragility and interdependence of every species in nature.
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