Untitled

2 . 5 THE GEOGRAPHIC DISTRIBUTION OF BIOLOGICAL DIVERSITY 27
and ecological input. Analogous indices are used in genetics: e.g. richness
(the number of alleles for the same locus), evenness (the relative frequency
of alleles), and heterozygosity, which associates the number of alleles
with their relative frequency. Another approach involves identifying the
diversity of ecosystems in a landscape, or habitats within an ecosystem.
It is possible to proceed as in taxonomy by identifying, naming and
classifying entities, comparing different situations, and then attempting
to generalize one’s observations. This typological approach has established
several categories of classification based on floral or faunal characteristics,
on assemblages of species (phytosociology), or on landscape
features (ecoregions, phenological structures, etc.). One example for a
typology of habitats is the classification system of European habitats
CORINE.
2 . 5 The Geographic Distribution of Biological
Diversity
Biological diversity is not evenly distributed over the surface of the
planet. Naturalists have attempted to determine large-scale tendencies
or patterns in the spatial distribution of biological diversity. Plotting
characteristics of climate against vegetation has long been used as a
means of identifying large biomes (Figure 2.2). Alternatively, assessing
the degrees of relationships between flora and fauna led to the delineation
of biogeographic areas. In each case, the typological process subscribes
to a hierarchical system, subdivided according to the degree of
accuracy required. Alternatively, scientists can try to identify distinct
areas that are particularly rich in endemic species.
Endemic species
A species is considered endemic to an area if occurs there and nowhere
else. Usually, endemicity is the consequence of the geographic isolation
of taxa that evolved independently. The area of endemism can either
be relatively large (three-quarters of the Malagasy mammalian species
are endemic), or else restricted to a well identified ecosystem, such as the
hundreds of cichlid fish species that inhabit the great lakes of east Africa
(Lake Victoria, Lake Malawi, Lake Tanganyika).