Big herbivores keep ants and trees together

If you thought your relationships were complicated, try being an acacia tree; or an ant, or a large herbivore. Nature, it turns out, is full of complicated relationships. And we mess with them at our peril.

If you thought your relationships were complicated, try being an acacia tree; or an ant, or a large herbivore. Nature, it turns out, is full of complicated relationships. And we mess with them at our peril.

Recently, I wrote about one such complex relationship – between large primates and large-seeded tropical fruit trees. It turns out that many such trees rely on large primates to distribute their seeds through the forest.

When the primates are killed off through hunting or habitat loss, the trees suffer too, making the primates’ comeback even more difficult.

But primates aren’t alone in having these mutually beneficial relationships. Nature is full of them, weaving a complex tapestry that, once unwound, can be difficult to put back together.

Consider large herbivores of the African savanna – animals like giraffes, antelopes and elephants. These animals are big eaters, requiring large quantities of plant matter to fuel their bodies. To reduce their odds of being eaten by these creatures, some plants have evolved defence mechanisms such as thorns or noxious tastes. And some, like the acacia tree, have developed intimate relationships with insects, such as ants, to be their defenders.

This concept is called mutualism, where, not unlike political parties in a minority government, organisms that might not otherwise get along very well work together for a mutual benefit. In the case of the acacia trees, several species of ant help keep roving mammals at bay. One dominant ant species in particular, Crematogaster mimosae, will swarm large herbivores to chase them off. In return, the trees offer the ants carbohydrate-rich nectar for food and hollow thorns in which to raise their young.

But it’s a tenuous relationship. And according to a recent paper published in the journal Science, it’s the loss of the antagonistic herbivores that can turn the relationship sour.

Common sense seems to indicate that when trees are fenced off from the creatures that eat them, the trees should thrive. But that’s not what happened when researchers in Kenya looked at acacia trees that had been protected from large herbivores since 1995. Compared to trees outside the fenced enclosures, the protected trees were growing more slowly and were far more likely to die.

It seems that when the herbivores were no longer a threat, the mutual relationship between the trees and the ants broke down. Without the herbivores around, the trees produced far less nectar for the ants and fewer hollow thorns in which to nest. This was particularly hard on Crematogaster mimosae, which, in turn, became a much weaker defender of acacia trees. Lacking the abundant nectar, these ants also started tending to other insects as sources of food, which likely increased stress on the acacia trees, causing them to grow more slowly.

While Crematogaster mimosae fared poorly when herbivores were removed, another ant species, Crematogaster sjostedti, flourished – but again to the detriment of the acacia trees. Instead of relying on hollow thorns to raise their young, this species uses cavities in the trees bored out by beetle larvae for its nests. In fact, the ants somehow actively encourage the beetles, whose invasive activities are harmful to the health of the trees. Simple changes in nature can often have profound and unexpected consequences.

This column is co-written by scientist and broadcaster David Suzuki and Faisal Moola, a scientist.

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