If you thought your relationships were complicated, try being an acacia tree, or
an ant, or a large herbivore. Nature 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. Many such trees rely on primates
to distribute their seeds through the forest. When 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
Large herbivores of the African savanna, like giraffes, antelopes and
elephants are big eaters. 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 a relationship with ants
to be their defenders.
This concept, called mutualism, is not unlike political parties in a
minority government; organisms that might not otherwise get along work together
for a mutual benefit. In the case of the acacia trees, one dominant ant species
— Crematogaster mimosae — swarms 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. According to a recent paper published in
the journal Science, it’s the loss of the antagonistic herbivores that can turn
the relationship sour.
After trees were fenced off from creatures that eat them in Kenya in 1995
they grew slower and were more likely to die.
When the herbivores were no longer a threat, the trees produced far less
nectar for the ants and fewer hollow thorns in which to nest. Another ant
species began to flourish, but it’s a species that uses cavities in the trees
bored out by beetle larvae for its nests. In fact, the ants somehow encourage
the beetles, whose invasive activities are harmful to the trees.
Simple changes in nature can often have profound and unexpected
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.