yet another important resource on which some bird
species have specialized (plate 9- 20). For example, a visit
to a sandbar island along the Napo River in Ecuador
might, if you look carefully, turn up White- bellied
Spinetail (Synallaxis propinqua), Castelnau’s Antshrike
(Thamnophilus cryptoleucus), and Parker’s Spinetail
(Cranioleuca vulpecula). These three furtive and little-
known species are characterized as sandbar specialists,
nesting only among the dense vegetation that colonizes
riverine sandbars.
It has long been assumed that specialization is
widespread in tropical ecosystems, though that should
not be taken to mean that all species have narrow
foraging niches (see discussion below). Specialization
need not be the case for many if not most species if
numerous varied resources are available in tropical
lowland forests. In a study comparing the woodpecker
communities of Maryland, Minnesota, and Guatemala,
Robert Askins studied seven woodpecker species in
Guatemala, compared with only four in each of the
temperate study areas. Although there was almost
double the number of woodpecker species in the
tropical field site, these species were no more specialized
in their foraging behavior than their temperate relatives.
Instead, they utilized a wider range of resources. Some
of the tropical species fed on termites and ants, probing
into the excavations made by these insects, and the
tropical species fed more heavily on fruit than the
temperate woodpeckers. Two tropical woodpeckers in
particular, the Black- cheeked (Melanerpes pucherani;
plate 9- 21) and the Golden- olive (Colaptes rubiginosus),
utilized resources not available to temperate species. The
Black- cheeked frequently fed on fruit, and the Golden-
olive probed moss and bromeliads.
In contrast to woodpeckers, bats (order Chiroptera) do
represent a group that has specialized to various degrees in
tropical ecosystems. Recall the description of the amazing
adaptive radiation of Neotropical bats in chapter 8.
The Interspecific Competition Hypothesis
Suppose you are a budding tropical ecologist eager
to earn a PhD. You establish a tropical field site in
western Amazonia. You are determined to find out
what factors account for the high diversity of tree
frogs. You consider the possibility that their abundance
and distribution is caused by competition among the
various species driving them to specialize to exploit
different ecological niches. Now, how would you test
this idea? (Hint: it will not be easy.)
One of the most difficult measures to make in
ecological research is the degree to which competition
occurs between two or more species. One must be
able to demonstrate that two (or more) species are
seeking the same limited resource, and then measure
the degree to which each species negatively affects the
other(s) in contesting for the resource. It is essential to
identify the resource being contested and demonstrate
that it is, indeed, a limited resource. If it is not limited,
there will presumably be enough for both species, and
no competition would occur. This of course means
that if you observe two different species each using
the resource you cannot automatically conclude that
they are in competition, anymore than two people
ordering steaks at a restaurant are in competition. It
is also essential to show how the competition affects
each species. Another key component to interspecific
competition is that it has a higher cost to each species
than intraspecific competition. In other words, each
species has to— to some degree— negatively affect the
other. The degree to which they negatively affect one
another need not be equal. When species compete,
there are the following possible outcomes:- One species outcompetes the other, and the loser goes
extinct, a process termed competitive exclusion. But
this does not seem to be happening in tropical rain
forests, which are rich with species of numerous taxa. - The two species somehow subdivide the resource,
each specializing on a part of the resource spectrum.
This is called niche partitioning. When this happens
each species is said to have specialized. There are
Plate 9- 21. The Black- cheeked Woodpecker feeds heavily on
fruit, a resource not as constantly available to woodpeckers in
the temperate zone. Photo by John Kricher.146 chapter 9 why are there so many species?