Geoscience Reference
In-Depth Information
drier at glacial maxima, with tropical forests being reduced
to 'islands' amidst more extensive savannas. Would not
such fragmentation actually promote greater biodiversity,
for speciation seems to be stimulated along ecotones?
MacDonald (2003) points out that whilst historical factors
may not explain the global biodiversity gradient, they
explain local differences. Lake Baikal in Russia has existed
for over 1 Ma and contains 580 species of benthic
invertebrates, whereas the similar Great Slave Lake in
Canada, formed in late glacial times just 10 ka years ago,
contains four deep-water invertebrate species.
The first is where large gradients of resources provide a
large number of habitats for a large number of different
species with different ranges of tolerance and niches. The
second scenario is where two habitats may be similar and
gradients of resources equal, but the distribution of a
species along a gradient is short because they have
specialized niches. In this case, the
competitive exclusion
principle
means that species with identical niches cannot
exist together, as one would eventually drive the other
to extinction. Therefore interspecific competition will
produce a large number of specialized species with small
niches. A third scenario is where resources are more
abundant in one area of high diversity than another;
more species exist either because greater resources allow
competitive pressure to be relaxed, allowing many general-
ist species to survive, or highly specialized niches permit
a greater number of species (
Figure 22.4
).
Equilibrium theories
Several theories stress present-day environmental
gradients and niches as the critical factors causing large
biodiversity in the tropics. There are three likely scenarios.
(a) Greater environmental variability
in high diversity area.
n = 3
n = 6
(b) Equal environmental variability,
but more specialised niches in
high diversity area.
n = 3
n = 6
(c) High diversity areas have either
n = 3
(i)
Specialised niches
n = 6
or
or
(ii) Less competition and more
niche overlap.
n = 6
Enviromental variability
Figure 22.4
Resource gradients (environmental variability) and niches in low-diversity and high-diversity areas.
Source: After MacDonald (2003)
