opportunity for cross- fertilization among colonies.
Because of normal evolution, the fungi cultivars
diverge genetically. Studies have shown that fungal
filaments (mycelia) from neighboring cultivars are
rejected by the fungi in any given colony, and the
rejection intensity is proportional to the genetic
difference between the fungi from the two colonies.
The fungus garden ants are the expert agriculturalists
of the insect world, and their labors pay off in
evolutionary fitness. The fungus symbiont digests
cellulose, an energy- rich compound that ants cannot
digest. Not only that, but the fungus is unaffected by
many, if not most, of the defense compounds contained
within leaves of many plant species. By eating the
nutritionally rich cultivated fungi, ants circumvent
many of the diverse defense compounds typical of
Neotropical plants while at the same time tapping into
the immense abundance of energy in rain forest leaves.Are Leaf- cutter Ants the Dominant
Herbivores of Neotropical Rain Forest?Leaf- cutter ants have often been assumed to be the major
herbivores of Neotropical rain forest trees. Is this true?
Hubert Herz and colleagues on Barro Colorado Island
(BCI) quantified the impact of leaf- cutters (species Atta
colombica) in old secondary forest in a study of 49 leaf-
cutter colonies over a 15- month period. The researchers
calculated the refuse deposition rate by measuring
the refuse leaf fragments deposited monthly at the ant
colonies compared with the harvesting rate. The results
showed that leaf- cutter ants were important herbivores.
The study revealed that Atta colombica harvested 13.2
tons of biomass and 13.1 ha (32.4 ac) of leaf area, and
deposited 9.4 tons of refuse material per year. At the
ecosystem level, the ants’ herbivory rates were 132 kg (291
lb) of biomass/ha/yr and 1,310 m^2 (14,100 ft^2 ) of foliage/
ha/yr. These figures represent 2.1% of the foliage area in
the forest, or 1.7% of the annual leaf- area production.
These figures, as impressive as they may seem, are
considerably lower than those generated in previous
published studies, which estimated that leaf- cutter
ants consumed as much as 12% of annual leaf area
and 17% of annual biomass. The impact of attines may
vary considerably from site to site. Consumption rates
among colonies varied six- fold, and thus these ants
may have different impacts in different forests, making
tropic dynamics ever more complex and generalization
that much more problematic.The study concluded that on BCI animals such as
sloths, deer, and howler monkeys have more impact on
leaf herbivory than leaf- cutter ants. Leaf- cutters appear
not to be the dominant herbivores, at least at BCI.It’s Not Always Coevolution: Army
Ants and Their (Mutualistic?)
AntbirdsLeaf- cutter ants, because they are so conspicuous, are
often confused with army ants, but the two are very
different kinds of ants. Army ants are carnivores and
represent a notable predatory force on small animals
inhabiting rain forest floor. These remarkable insects
may represent the most significant predator in some
rain forests, consuming more animal matter even than
cats or various snakes.
We can trace the history of army ants back to the
supercontinent Gondwana, approximately to 105 million
years ago in the Cretaceous. Dinosaurs presumably
stepped upon army ants. It was once believed that
Neotropical army ants (subfamily Ecitoninae) represent
a case of convergent evolution with Paleotropical driver
ants (subfamily Dorylinae), but molecular and other
analyses suggest a close relationship. These ants evolved
before the separation of Gondwana.
Two widely distributed army ant species, Eciton
burchelli and Labidus praedator, are prevalent in the
Neotropics. In all, there are five genera and about 150
species of Neotropical army ants. They likely represent
more biomass per unit area than all vertebrate predators
combined (such as cats, weasels, coatis, raccoons).
Eciton burchelli is one of the best- studied of
Neotropical army ants, and its natural history is the
focus of this section. Eciton varies in size and color. The
largest individuals are soldiers, with extremely large
and menacing mandibles, sharply hooked inward. The
smallest workers are only about one- fifth the size of
the soldiers. Color varies from orange and yellowish
to dark red, brown, or black depending on subspecies.
Eciton armies are immense, often containing in excess
of a million ants. Armies are nomadic, moving through
the forest, stopping at temporary bivouacs during
their reproductive cycles. They may bivouac for only a
night or for several weeks in the same area. When the
entire mass moves, it is usually a nocturnal migration.
Bivouacs are either underground or in hollow logs
or trees and consist of massive clusters of the antschapter 10 tropical intimacy: mutualism and coevolution 177