Yahoo News 11 Jan 08;
The pressures on elephants, giraffes and other big mammals could have far-reaching effects on the ecosystem of the sub-Saharan savannah, including disrupting the relationship between acacia trees and insects, researchers said Thursday.
In an experiment that simulated the extinction of these animals, researchers fenced off patches of land with the umbrella-shaped whistling thorn tree, or Acacia drepanolobium, on the Kenyan savannah in eastern Africa so that the animals could not graze on them.
Over time, they found that the absence of these plant-eating animals upset the relationship between the tree and its ant parasites, ultimately compromising the health of the tree, and increasing tree mortality by half.
"Throughout sub-Saharan Africa, these large mammals are threatened by human population growth, habitat fragmentation, over-hunting and other degradation, so we have to wonder how their loss will affect these ecosystems," said Todd Palmer, the paper's lead author and an assistant professor of zoology at the University of Florida.
"The last thing you would think is that individual trees would start to suffer as well, and yet that's exactly what we see."
Acacia trees have swollen thorns that serve as nests for three species of biting ants. Healthy trees have hundreds of thorns, often containing more than 100,000 ants per tree.
It's a mutually beneficial relationship for the insects and the tree that probably evolved over millennia. The ants get shelter and they feed on the nectar from Acacia leaves.
The ants swarm over anything that threatens the tree, protecting it from the plant-eating animals that feed voraciously on it.
However, after a decade in an enclosure, the acacia trees in the experiment appeared to have adapted to their new environment. They had fewer swollen thorns, secreted less nectar and the thorns needed to repel predators were shorter.
But, unexpectedly, the trees also looked sickly compared to their unfenced counterparts. The fenced trees were twice as likely to die as the unfenced ones, and they grew 65 percent more slowly.
On closer inspection, the researchers found that the tree's "adaptations" appeared to have changed the balance of power between the tree's three resident ant species, which in turn left the tree more vulnerable to the ravages of a destructive, wood-boring beetle.
With less shelter and food, the formerly dominant ant species (Crematogaster mimosae) was weakened and eventually overrun by another ant species (Crematogaster sjostedti).
The former underdog parasite encourages the wood-boring beetle and uses cavities made by it as its home, so ultimately the changes in the ant pecking order proved detrimental to the health and viability of the tree, the paper said.
"You get a community-wide replacement of good ants with bad ants and the result is that the trees start doing poorly," said Palmer.
The findings underscore the complexities of ecosystems and illustrate how man-made changes to animal habitats could result in unexpected trickle-down effects, the researchers said in the paper published in the journal Science.
"If you get rid of the large mammal, it shifts the balance of power, because the trees default on their end of the bargain. When the trees opt out, their hard-working employees starve and grow weak, which causes them to lose out. So ironically, getting rid of the mammals causes individual trees to grow more slowly and die younger."
