most Neotropical lowland forests. These two small cats
are similar in body size, though the Ocelot is a bit larger.
Both are essentially nocturnal, and it is not unreasonable
to assume that they feed on many of the same prey items:
rodents and other small mammals, birds, snakes, lizards.
The two differ somewhat in their foraging behaviors,
as the Ocelot is almost entirely terrestrial while the
Margay routinely climbs trees in the course of its
hunting behavior. Thus the Ocelot and Margay have
foraging niches that do not precisely overlap. Did
competition between these two small felines cause the
divergence of foraging niche, selecting for the smaller
cat to become more arboreal? There is no way to know.
It cannot be known whether resources were limited,
and even if they were, it cannot be known whether that
limitation was, indeed, an active selection pressure.
In other words, it cannot be known whether Margays
that climbed reproduced better than those that did not
because they climbed. Even if they did, such behavior
may have resulted from the availability of arboreal
resources presenting an opportunity for procuring
more calories and protein, not because Ocelots were
also foraging on the forest floor.
As the above example is meant to illustrate, evidence
for the interspecific competition hypothesis is mostly
circumstantial. Direct demonstrations of interspecific
competition are generally lacking in the tropics. Certain
patterns suggest, however, that competition among
species may be a component of tropical evolution and
diversification.
Varying bill shapes and gradations in body sizes
within many bird groups suggest that competition
may have influenced the evolutionary history of these
groups. Recall the six kingfisher species that cohabit
Neotropical rivers and backwaters, pictured earlier
in this chapter. The size range within the kingfisher
complex does suggest an evolutionary resource
partitioning, as each species has adapted to feeding
on an optimal size range of fish. That could be a result
of competition among the evolving populations, but
it could just as easily result from the range of prey
species available. Ringed and Amazon Kingfishers
routinely capture and devour prey that is larger than
an American Pygmy Kingfisher.
As this example suggests, different body sizes and
bill characteristics reflect specialization for capturing
differing food items. The very act of food capture per se
could and probably does select for such specializations,
though it is quite likely that the presence of similar species
with similar ecological needs could act as an additional
strong selection pressure in producing divergence
among species. Insectivorous bats, for example, feed
on different- size prey items and also forage at different
heights in the rain forest, a pattern possibly reflective of
avoidance competition among the bat species.
One way to support the possibility of interspecific
competition is to demonstrate that a species exhibits
a limited foraging niche, presumably because of the
presence of other competing species. Thomas Sherry
found that each of three flatbill flycatcher species found in
Costa Rica forages at a different height in the forest, and
certain flatbill species replace others in specific habitats—
for example, one species occurs in forest, while another
very similar species is only in disturbed brushy areas. This
pattern is suggestive of competitive relationships having
influenced the birds’ foraging choices.Plate 9- 23. Miconia plants produce fruits used by numerous
bird species, many of which, such as manakins, spread seeds.
Miconia is in the Melastomataceae family. Photo by Scott
Shumway.
Plate 9- 24. The Red- capped Manakin (Ceratopipra mentalis) is a
common disperser of Miconia seeds. Photo by James Adams.148 chapter 9 why are there so many species?