Big herbivores keep ants and trees together

Here’s your weekly Science Matters column by David Suzuki with Faisal Moola.

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 defense 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. In this case, loss of large herbivores, rather than being
beneficial to the trees they eat, actually caused the trees to grow
more slowly and die more often. As the researchers conclude, the
ongoing loss of herbivores due to human activities in Africa thus could
trigger cascading effects throughout ecosystems in which they occur.
That would certainly be bad news for the herbivores, but given nature’s
complex relationships, also the trees, the ants and ultimately, all of
us.

Take David Suzuki’s Nature Challenge and learn more at www.davidsuzuki.org.