Andean bird communities also show indirect
evidence for interspecific competition. John Terborgh
and John Weske studied two Andean mountain peaks
in Peru, one of which was isolated, one of which was
not. Colonization by birds was less frequent on the
isolated peak, and thus this peak had reduced bird
species richness, missing an estimated 80– 82% of the
bird species that presumably would have occupied
the isolated peak had it been part of the main range
of the Andes. However, of the species that did occur,
71% had expanded their elevational range (compared
with populations on the other, more diverse mountain
peak) presumably because of the absence of similar
species that would have been competitors.
Interspecific competition for pollinators and seed
dispersers may have provided a selection pressure
resulting in staggered flowering patterns among some
plants. In Trinidad’s Arima Valley, 18 species of the
shrub genus Miconia (plate 9- 23) have flowering times
staggered in such a way that only a few species are
flowering in any given month. Is this, as suggested by
ornithologist David Snow, a possible evolutionary result
of competition among Miconia species for access to birds
that eat the fruit (plate 9- 24) and thus disperse the seeds?
Any Miconia species that flowered when most others did
not would be able to attract more birds to disperse its
seeds, thus it would have a selective advantage compared
with others of its own as well as other species. Over time,
the staggered flowering pattern emerged.
A similar pattern among plants that are bat pollinated
has been observed E. Raymond Heithaus in Costa
Rica. Of 25 commonly visited plant species, an average
of only 35.3% flowered in any given month.
The patterns described here for inferred interspecific
competition do not, in and of themselves, demonstrate
that niches are narrower in the tropics, and none
really demonstrate competition in a rigorous manner.
This is an ongoing weakness in attributing greater
species richness in the tropics to greater interspecific
competition. Nonetheless, patterns are compelling.
High species richness of rain forests may be partly
explained by competition among species, at least for
some species groups. It would be very helpful if robust
data replaced suggestive patterns.The Predation Hypothesis
Predators abound in the Neotropics. They represent
what ecologists term top- down ecological forces,meaning that in the food chain from sun to plant to
herbivore to various predators, predator influence may
be strong in potentially affecting species populations
lower on the food chain.
Suppose the caterpillars of four different species are
competing for the same plant. One species is increasing
at a much higher rate, causing the others to be driven
toward local extinction. What was a four- species system
seems destined to become a one- species system. But
suppose birds and lizards prey on the caterpillars (they
do!). Which of the four are the predators likely to take?
The most obvious and abundant species would seem the
likely choice. The result of continuous predation would
be to reduce the most rapidly growing population, the
presumptive “winning” species, allowing the various other
“losing” species to regain some control of the resources
and increase in population. This scenario shows how
predators may maintain species richness by switching
among prey based solely on prey abundances. The game
of competition is thus never permitted to play out.
The predation hypothesis posits that predators prevent
prey species from competing within their ranks to the
point of extinction. Predators are thought to switch their
attention to the most abundant prey, thus the rarer the
species, the safer it is from predators. This idea is a formPlate 9- 25. Predators such as this basilisk lizard abound in the
tropics and may collectively exert significant ecological effects
in structuring tropical ecosystems. Photo by James Adams.chapter 9 why are there so many species? 149