The New Neotropical Companion

two ways for this to happen. One is that a species’

realized niche (the actual resources it uses) is

forced by competition to become narrower than its

fundamental niche (the niche space it would occupy

in the absence of competing species). The second

way is that the actual fundamental niches may evolve

to be narrower.

• Intraspecific competition might drive each species
  to gradually expand its original foraging niche and
  utilize new resources, thus reducing the competition.
  This could be an ongoing evolutionary process and
  help explain why specialization is not necessarily
  more common in the tropics than elsewhere.
  It has long been argued that high species diversity
  in the tropics is related to levels of competition
  among species. Over time, interspecific competition
  has resulted in greater niche partitioning (a form of
  increased specialization). Each species evolves into
  somewhat of a specialist, focusing on a specific resource
  that it and it alone is best at procuring. This trend
  toward specialization due to interspecific competition
  leads to the packing of greater and greater numbers of
  species into tropical ecosystems while at the same time
  reducing the intensity of competition among species,

as each specializes to its exclusive pool of resources.

The interspecific competition hypothesis argues that

many if not most niches are narrower in the tropics than

in the temperate zone because competition inevitably

will compress them. The hypothesis is dependent on

the reality that greater specialization is possible in the

tropics because particular types of resources (such as

large lizards or long tubular flowers) are consistently

available throughout the year, showing seasonal

stability. In the temperate zone many specific types of

resources are available only during a brief season each

year, so species are forced to be generalists, particularly

if they do not migrate.

There are problems with this hypothesis. We have

already seen from the Askins study that woodpeckers

do not exhibit narrower niches in the tropics but rather

use a greater range of potential resources. Though

competition may have exerted a major influence in the

past, now that specialization and niche compression

have occurred, competition may be quite minimal or

even nonexistent. It should be obvious that this is a

very difficult hypothesis to test.

For example, both the Ocelot (Leopardus pardalis)

and the Margay (L. wiedii; plate 9- 22) occur throughout

Plate 9- 22. Margays routinely climb trees. See chapter 16 for more on Neotropical felines. Photo by James Adams.

chapter 9 why are there so many species? 147