Imagine that you are standing on a flat plain in eastern Africa. Your view is dominated by one species of stunted tree, the whistling thorn acacia. This landscape may look simple, even monotonous, but plant ecologists from UC Davis are demonstrating that this is far from the case. In the acacia highlands of Kenya , Professor Maureen Stanton, Truman Young and their colleagues are unraveling the intricate relationships between these trees and their insect inhabitants.
Caution is advised when approaching the whistling thorn. As protection against Kenya's many browsers such as antelope and giraffe, the acacia is armed with two kinds of thorns. Pale, paired stilettos, up to three inches long, are interspersed with modified thorn pairs joined at the base by a hollow bulbous swelling. Tiny holes pierce these swollen thorns, turning them into miniature flutes in the breeze, and giving the tree its name. Music isn't their only surprise. As Maureen Stanton and Truman Young know too well, even the slightest brush against a tree can result in a mass of biting ants streaming out of the holes in the swollen thorns.
The whistling thorn is always occupied by ants, some species of which live nowhere else. This co-existence suggests to ecologists that ant and tree have evolved a 'symbiotic' or mutually beneficial relationship . If so, ask Stanton and Young, then what is each partner getting out of it? Finding the answers involves painstaking observations of individual trees and their insect residents, as well as various experiments altering the number and species of ants on a tree. The most indispensable piece of equipment to researchers is a pair of latex gloves that help deter angry ants and their painful bites.
As far as benefits to the ants go, things seem fairly straightforward. The swollen thorns provide them with a nest site. The region's inhospitable "black cotton" soils that swell in the rain, while shrinking and cracking when it's dry, probably prevent ants from nesting underground, forcing them to colonize the trees. In addition, the acacia's leaves have special glands called "nectaries" that produce a sweet solution solely to feed the ants . In return for bed and breakfast, what does a whistling thorn get from its tenants ?
The answer is complicated by the fact that four different species of ants habitually live in acacias, although never at the same time due to violent intolerance of one another. The four species have markedly different relationships with their host. For example, their responses to disturbance vary considerably. Two species are highly militant, immediately swarming over and biting an intruder at the slightest shake of a branch. These ants can be effective at warding off hungry young giraffe, and thus provide the tree with a second line of defense in addition to thorns. In contrast, the slender black acacia ant is far less aggressive, taking refuge in the swollen thorns at the first sign of trouble. Is this more peaceable ant a 'free-loader'? Oddly, although its aggressive reactions are slow, it packs a very nasty sting when finally provoked. Whether or not a host tree gains protective benefits from this species remains to be seen. And that's only part of the story.
While more aggressive ants may deter browsers, they aren't all ideal tenants. Like herdsmen tending cattle, two of the species tend scale insects, eating the sweet secretions from the bugs. As all gardeners know, scale insects are anything but beneficial to a tree, sucking its life juice. It is not yet clear how detrimental it is to an acacia to support the "livestock" of these ants.
But the worst occupant of all might be one of the more warlike species whose worker ants nip off the growing tips of the tree, including the flower buds. Such pruning stimulates the growth of super-large nectaries for the ants, but effectively sterilizes the tree. This ant-induced infertility must surely outweigh the advantage of having browsers deterred.
The whistling thorn would not have evolved its ant-supporting
features if it didn't benefit significantly from at least one
of its residents. It remains uncertain, however, which species
of ant has a truly mutualistic partnership with the tree, and
which in contrast, is more like a parasite. It will take time
and careful study before scientists, latex in hand, have worked
out the complexities of the ant-acacia system.
Dept. of Anthropology, University of California, Davis, CA 95616
e-mail: kjstewart@ucdavis.edu