Biodiversity in ecosystems - Fundamentals of the Physical Environment - page 539

In contrast, differentiation diversities measure changes
in species diversity along a gradient in the landscape. The
most commonly used is the between-habitat diversity,
the beta diversity ( ), an index of the rate of change of
diversity between two or more habitats or communities.
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 is used to meas-
ure 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:

= 1/ D

where D = distance required to reduce similarity by 50 per
cent. A second index of differentiation diversity is the
delta diversity ( ) , the change in species diversity between

landscapes along major climatic or physiographic
gradients.

LATITUDINAL GRADIENTS OF
BIODIVERSITY

Figure 22.3 Latitudinal gradient of bird diversity in Central
and North America.
Source: After Cook (1969)

The major global gradient of biodiversity is the relation-
ship with latitude, i.e. species diversity increasing as one
travels from equator to the poles ( Figure 22.3 ). This
dramatic relationship is evident on land and in the oceans,
and is shown in most taxa of 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. 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, these are clearly dissimilar environ-
ments; when similar habitats are compared, the latitudinal
trend is quite common though not universal.
There is an enormous ecological literature hypothe-
sizing about explanations for this gradient, and Tudge
(2005) talks of hundreds of scientific theories which have
been put forward, covering almost every environmental
factor that changes with latitude. However, it is possible
to see two groups of explanations: historical theories

emphasizing the time factor, and the so-called equilibrium
theories , based on gradient and niche theory, stressing
present-day environmental conditions.

Historical theories

The time hypothesis is one of the oldest suggestions, and
is based on the length of time which has been available in
tropical forests for speciation. It is argued that the absence
of Pleistocene glaciations in tropical regions means that
they contain a larger legacy of species from pre-glacial
times, i.e. tropical climates have been stable for long
periods of time. This is the stability time hypothesis ; the
implication is that the number of species in mid and high
latitudes will catch up in time, hence their lower diversities
are temporary.
As a sole explanation, this is a doubtful hypothesis
for several reasons. The geological fossil record shows
that the diversity gradient existed before the Quaternary.
Also, palaeoecological studies indicate that tropics have
themselves experienced climatic change, being cooler and