A Tropical World of Interactions
With the profligate plant, animal, and microbial life
typical of rain forests, it should come as little surprise
to learn that the major resources of a tropical forest
are the organisms themselves. Species interact on a
daily basis in ecological time that, with generations,
becomes evolutionary time. The interdependencies
that result are often complex, to say nothing of
fascinating. Some species become increasingly
interlinked as predator and prey. Others are locked
into interdependent mutualistic associations through a
process that ecologists call coevolution. This is the real
fun of visiting the tropics, to witness firsthand some
of these amazingly elegant examples of interaction and
coevolution (plate 10- 1). This and the next chapter will
take you into that world.
Kinds of Interactions
An interaction between any two species may be
positive, negative, or neutral for either party. Both may
benefit from the interaction (+/+), one may benefit
while the other loses (+/−), both may lose (−/−),
though to varying degrees, or one may benefit while
the other experiences neither gain nor loss (+/0).
Commensalism (+/0) occurs when one species
benefits from the interaction and the other is not harmed
or significantly compromised. Epiphytic plant species
are generally commensal with their host plants (plate
10- 2). Normally epiphytes do not seriously reduce the
fitness of host plants, and epiphytes obviously benefit
from attachment on the host. It is true, however, that
epiphytes may, on occasion, weaken a branch by their
sheer combined weight, making it more susceptible to
breakage, or they may be sufficiently dense to interfere
with photosynthesis of the host tree. The relationship
in this case is more parasitic than commensal.
There are two manifestations of interactions in
which one species gains and one loses (+/−), predation
and parasitism. Predators and parasites gain while
their prey or host species obviously lose.
Competition occurs when two individuals both
require the same resource, and that resource happens
to be in limited supply. In competition among species
(discussed in chapter 9), one competitor obtains more
of the resource and benefits while the other suffers
some measure of loss. Competition may be viewed as
a lose- lose (−/−) type of interaction, but the minuses
are not necessarily of equal magnitude, and so the
cost is not the same to each competitor. This sort of
competition is likely common among species and
were it to continue indefinitely, one of the competitors
would eventually replace the other.
Perhaps the most fascinating of relationships is
mutualism (+/+), which occurs when two or more
species engage in an interaction from which both
species benefit. In evolutionary terms, the fitness of
each species is enhanced by the interaction. Animal-
mediated pollination of plants is an outstanding
example of a widespread form of mutualism.
Mutualistic relationships are common in tropical rain
forests and will form a focus of this chapter. Some are
relatively casual and some are entirely obligate.Seed Dispersal: The Unique
Importance of Fruits in the TropicsFruit is abundant and (relatively) constantly available
throughout the year in moist tropical forests and thus
an important resource. Fruiting trees attract numerous
animal species. Think of fruiting trees as the fast- food
restaurants of the tropical forest. Some animals feed on
fruit when it is abundant but not as a primary resource.Chapter 10. Tropical Intimacy: Mutualism and Coevolution
Plate 10- 2. Epiphytes on branch. Epiphytes are normally
commensal with their host plant, but sometimes epiphyte
load weakens the branch. Photo by Beatrix Boscardin.155