species) exposed by the ants. The relationship between
army ants and ant- swarm birds has been puzzling.
Scientists had reasoned that there were three possible
types of association in action: the ever- active birds
and ants may be mutualists, coevolved to help each
other flush prey; the pair may be commensal, with the
birds taking prey of no consequence to the ants; or the
birds may be parasitic, taking prey away from the ants.
A study performed at Soberania National Forest in
Panama (near Barro Colorado Island) by P. H. Wrege
and colleagues demonstrated that antbirds are actually
parasites of army ants.
The researchers, through a variety of creative
techniques (one involved spraying persistent antbirds
with a squirt gun to force them to flee), prevented ant-
following birds from attending swarms, while leaving
other swarms with their attendant birds. They censused
the arthropods and other animals captured by the ants,
and at swarms where birds were permitted to remain
they counted the prey items taken by birds. The results
revealed the following:- The birds are highly dependent on ants to make prey
available. Without the ants their foraging success
would be greatly diminished. - The average nomadic ant colony consumes about
22 g (0.75 oz) of leaf-litter arthropods daily, plus an
additional 22 g from social insects (other than the
army ants). - The larger the flock size, the higher the cost of lost
prey to the ants. - Each day antbirds take more than 200 prey items
that otherwise, presumably, would have gone to the
ants. This represents about 30% of the ants’ daily
leaf- litter arthropod intake or 15% of the entire daily
food requirement of a migrating ant colony.
The relationship between army ants and ant- following
birds is, at least if this study is typical, parasitic and not
in any way mutualistic (plate 10- 39).So What Is Coevolution?
The relationships among multitudes of species and
their various roles in seed dispersal, pollination,
and other interactions speak to the complexity and
interdependencies evident in rain forest ecosystems.
Biotic selection pressures prevail in the tropics. When
one species exhibits a trait that acts as a selection
pressure on another species, and the second species
in turn evolves a trait that acts as a counter- selection
pressure back upon the first, the evolutionary fates
of both species may eventually become permanently
interlocked. Should such interlocking occur, as we
have seen throughout this chapter, it is an example of
coevolution.
Coevolutionary interactions may evolve from parasitic
or predatory interactions, in which each species engages
in a reciprocal arms race, as when both predator and
prey evolve to be increasingly swifter. However, much
focus on coevolution involves mutualistic relationships,
when both species gain from the interaction. The
majority of mutualisms, such as most seed dispersal,
may be facultative, but others, such as fungus garden
ants, are obligatory.
But keep in mind, as is the case with the army ants
and the ant- following birds, what might appear to
be a mutualistic interaction may not be so in fact.
Mutualism is not always easy to demonstrate. Lots of
interactions remain antagonistic, as the next chapter
will demonstrate.chapter 10 tropical intimacy: mutualism and coevolution 179