Travel Reference
In-Depth Information
There are a few exceptions. The native people living around Bako Park near
Sarawak's capital of Kuching are now Muslim, and consider forest animals
unclean. As a result the animal and bird life in and around Bako is still abundant.
The Janzen-Connell ef ect is unlikely to be the only factor maintaining
ecosystem diversity. Recall that some tree species in tropical forests seem to
be uninfl uenced by this ef ect. There is another major theory about rainfor-
est diversity that may account for these species. It has been given the rather
clumsy name of the niche-complementarity theory, which we will shorten
quency-dependence.
The N-C theory assumes that the environment is divided up into a vari-
ety of niches defi ned by physical components such as sunshine, water, and
type of soil, and biological components such as dif erent kinds of prey. Trees,
of course, do not prey on other organisms, so a tree species is limited by
the availability of dif erent components of its physical environment. If a tree
species is rare in a particular part of the forest, it is at an advantage because
it basks in a plenitude of the environmental component that it depends on
the most. Trees of a species limited by a particular mineral in the soil, for
example, will thrive if they are not competing with other members of the
same species for that resource. But as trees of this species increase in num-
bers they must compete more and more amongst themselves for the limited
amounts of the mineral available in that part of the forest.
Tree species that are rare in the entire forest are more severely limited by
their environmental requirements than species that are common, but both
rare and common species are subject to frequency-dependent selection. Just
as with the Janzen-Connell hypothesis, the N-C hypothesis predicts that if a
species becomes locally more common in some part of the forest, it will have
trouble producing saplings there. And, just as with Janzen-Connell, N-C pre-
dicts that clusters of trees should be continually appearing and disappearing
in dif erent parts of the forest. Finally, like the Janzen-Connell hypothesis,
the N-C hypothesis predicts that many dif erent species will be able to coex-
ist simultaneously. These species will all oscillate back and forth in numbers,
because they are selected for when they are rare and selected against when they
are common.
