Environmental Engineering Reference
In-Depth Information
The diversity of any geographical area is made up of three components (Figure 23.2). The
within-habitat
diversity (the
alpha diversity
, α) is the diversity of a particular habitat
(e.g. a field or a woodland clearing). The
regional
diversity (the
gamma diversity
, γ)
covers a larger area and shows the sum of all alpha diversities. The
between-habitat
diversity (the
beta diversity
, β) is somewhat different as it measures the rate of change of
diversity between two habitats. It can be measured by setting up a transect line along an
environmental gradient such as a slope, a gradient of wetness or a catena of soil types,
and then recording the type and number of species at equidistant sample points. A
suitable community coefficient (see Chapter 20) is used to measure similarity in species
composition between any two sample points. A graph of similarity against distance is
constructed, so that the distance necessary to reduce similarity by 50 per cent can be
determined. The beta diversity is the reciprocal of this distance:
where
D
= distance required to reduce similarity by 50 per cent.
Figure 23.2
The three components of diversity within and
between communities in a landscape.
FACTORS INFLUENCING DIVERSITY
One of the basic patterns of species diversity on the globe is the inverse relationship with
latitude, i.e. species diversity increases as one travels from the poles to the equator. This
is a dramatic relationship, being shown in many plants and animals; twenty species of
tree in northern Canada increase to 600 species at the equator, and ten species of marine
crustaceans in the Arctic Ocean increase to 100 species in the Pacific. However, there are
exceptions, as, for example, in the number of breeding birds on the wetlands of Finland,
and the number of sawfly species in Eurasia, which increase from south to north (Figure
23.3). There are also, of course, areas of low species diversity in the tropics, as, for
example, in arid tropical deserts, where diversity is lower than in temperate forests.
However, this is due to dissimilar environments; when similar habitats are compared, the
latitudinal trend is quite common though not universal.
There have been many attempts to explain this trend. Many hypotheses have been put
forward, covering almost every environmental factor which changes with latitude.
However, explanations concentrate on four main possibilities - time, energy, stability and
geographical separation.
