Geography Reference
In-Depth Information
Table 15.1
The values of biological diversity.
many marine ecosystems have extraordinarily high
levels of phylum diversity. As a consequence,
although they may be less species-rich than many
terrestrial ecosystems it is probable that they contain
considerably greater total genetic diversity.
All measures of diversity seem to be scale-
dependent. The total number of species can be
measured at a point in space (often referred to as
a-diversity). But community composition will
vary from point to point across a habitat. For
example, Newbery
et al
. (1992) found that in
thirty-two adjacent 0.25 ha sub-plots in a Borneo
rain forest, only 20 per cent of the species occurred
in more than half of the plots. The turnover in
species composition from point to point within a
habitat is usually referred to as ß-diversity and is
related to the size of species ranges. An a-diverse
community may, nevertheless, be composed of
widespread species. There will be a high degree of
compositional similarity from place to place, and
ß-diversity will be low. The Ugandan tropical rain
forest described by Hawthorne (1993) had high
a-and ß-diversity. After logging had driven some
forest species to local extinction and replaced
them with widespread weeds, a-diversity remained
high but ß-diversity almost certainly declined.
ECOLOGICAL PROCESSES GENERATING
AND MAINTAINING DIVERSITY
There are clear patterns in species diversity at a
global scale. Diversity tends to decrease with
altitude and latitude. Isolated or geographically
restricted areas have low diversity. However, some
patterns are unexpected. Diversity often peaks but
then declines along productivity and successional
gradients. Unfortunately, despite decades of
research activity there is still no widely accepted
view as to which processes are primarily
responsible for creating these patterns of diversity.
Without a sound understanding of how
