|
Understanding
Biodiversity
What is biodiversity?
Biodiversity: its meaning and measurement
Loss of biodiversity
The causes of loss of biological diversity
Why conserve biological diversity?
Biodiversity and the biosphere
What is biodiversity?
The word `biodiversity' is a contraction of biological diversity.
Diversity is a concept which refers to the range of variation or
differences among some set of entities; biological diversity thus
refers to variety within the living world. The term `biodiversity'
is indeed commonly used to describe the number, variety and
variability of living organisms. This very broad usage, embracing
many different parameters, is essentially a synonym of `Life on
Earth'.
Management requires measurement, and measures of diversity only
become possible when some quantitative value can be ascribed to them
and these values can be compared. It is thus necessary to try and
disentangle some of the separate elements of which biodiversity is
composed.
It has become a widespread practice to define biodiversity in terms
of genes, species and ecosystems, corresponding to three fundamental
and hierarchically-related levels of biological organisation.
Biodiversity: its meaning and measurement
The differences between these conceptual perspectives on the
meaning of biodiversity, and the associated semantic problems, are
not trivial. Management intended to maintain one facet of
biodiversity will not necessarily maintain another. For example, a
timber extraction programme which is designed to conserve
biodiversity in the sense of site species richness may well reduce
biodiversity measured as genetic variation within the tree species
harvested. Clearly, the maintenance of different facets of
biodiversity will require different management strategies and
resources, and will meet different human needs.
Even if complete knowledge of particular areas could be assumed,
and standard definitions of diversity be derived, the ranking of
such areas in terms of their importance with respect to biological
diversity remains problematic. Much depends on the scale that is
being used. Thus, the question of what contribution a given area
makes to global biological diversity is very different from the
question of what contribution it makes to local, national or
regional biological diversity. This is because, even using a
relatively simplified measure, any given area contributes to
biological diversity in at least two different ways - through its
richness in numbers of species and through the endemism (or
geographical uniqueness) of these species. The relative importance
of these two factors will inevitably change at different
geographical scales, and sites of high regional importance may have
little significance at a global level. Neither of these factors
include any explicit assessment of genetic diversity.
Although the word biodiversity has already gained wide currency
in the absence of a clear and unique meaning, greater precision will
be required of its users in order that policy and programmes can be
more efficiently defined in the future.
Loss of biodiversity
The loss of biological diversity may take many forms but at its most
fundamental and irreversible it involves the extinction of species.
Over geological time, all species have a finite span of existence.
Species extinction is therefore a natural process which occurs
without the intervention of man. However, it is beyond question that
extinctions caused directly or indirectly by man are occurring at a
rate which far exceeds any reasonable estimates of background
extinction rates, and which, to the extent that it is correlated
with habitat peturbation, must be increasing.
Unfortunately, quantifying rates of species extinction, both at
present and historically, is difficult and predicting future rates
with precision is impossible.
Documenting definite species extinctions is only realistic under a
relatively limited set of circumstances, where a described species
is readily visible and has a well-defined range which can be
surveyed repeatedly. Unsurprisingly, most documented extinctions are
of species that are easy to record (e.g. land snails, birds) and
inhabit sites which can be relatively easily inventoried (e.g.
oceanic islands). The large number of extinct species on oceanic
islands is not solely an artefact of recording, because island
species are generally more prone to extinction as a result of human
actions.
Rather than being derived from observed extinctions, therefore,
quoted global extinction rates are derived from extrapolations of
measured and predicted rates of habitat loss, and estimates of
species richness in different habitats. These two estimates are
interpreted in the light of a principle derived from island
biogeography which states that the size of an area and of its
species complement tend to have a predictable relationship; fewer
species are able to persist in a number of small habitat fragments
than in the original unfragmented habitat, and this can result in
the extinction of species.
Even on best available present knowledge, these estimates involve
large degrees of uncertainty, and predictions of current and future
extinction rates should be interpreted with very considerable
caution. Pursuit of increased accuracy in the estimation of global
extinction rates, however, whilst of great concern, is not a crucial
activity; it is more important to recognise in general terms the
extent to which populations and species which are not monitored are
likely to be subject to fragmentation and extinction.
Loss of biodiversity in the form of crop varieties and livestock
breeds is of near zero significance in terms of overall global
diversity, but genetic erosion in these populations is of particular
human concern in so far as it has implications for food supply and
the sustainability of locally-adapted agricultural practices. For
domesticated populations, loss of wild relatives of crop or timber
plants is of special concern for the same reason. These genetic
resources may not only underlie the productivity of local
agricultural systems but also, when incorporated in breeding
programmes, provide the foundation of traits (disease resistance,
nutritional value, hardiness, etc.) of global importance in
intensive systems and which will assume even greater importance in
the context of future climate change.
Erosion of diversity in crop gene pools is difficult to demonstrate
quantitatively, but tends to be indirectly assessed in terms of the
increasing proportion of world cropland planted to high yielding,
but genetically uniform, varieties.
The causes of loss of biological
diversity
Species may be exterminated by man
through a series of effects and agencies. These may be divided into
two broad categories: direct (hunting, collection and persecution),
and indirect (habitat destruction and modification).
Overhunting is perhaps the most
obvious direct cause of extinction in animals, as it has affected
several large and well-known species. In terms of overall loss of
biodiversity, however, it is undoubtedly far less important than the
indirect causes of habitat modification and loss. Nevertheless, as
it self-evidently selectively affects species which are or have been
considered a harvestable resource, it has important implications for
the management of natural resources.
Genetic diversity, as represented by
genetic differences between discrete populations within wild
species, is liable to reduction as a result of the same factors
affecting species. The genetic diversity represented by populations
of crop plants or livestock is liable to reduction as a result of
mass production; the desired economies of scale demand high levels
of uniformity.
Virtually any form of sustained human
activity results in some modification of the natural environment.
This modification will affect the relative abundance of species and
in extreme cases may lead to extinction. This may result from the
habitat being made unsuitable for the species (for example,
clear-felling of forests or severe pollution of rivers), or through
the habitat becoming fragmented. The latter has the effect of
dividing previously contiguous populations of species into small
sub-populations. If these are sufficiently small, then chance
processes lead to raised probabilities of extinction within a
relatively short time.
A major, though at present largely
unpredictable, change in natural environments is likely to occur
within the next century as a result of large-scale changes in global
climate and weather patterns. There is a high probability that these
will cause greatly elevated extinction rates, although their exact
effects are at present unknown.
Why conserve biological diversity?
This question can be asked from a
number of different perspectives, all conditioned by a variety of
cultural and economic factors. The various answers given, arguing
for the maintenance of biological diversity, have tended to become
increasingly confused. Different goals have different implications
for the elements and
extent of biological diversity that
must be maintained. Among these goals are the following:
- the present and potential use of
elements of biodiversity as biological resources
- the maintenance of the biosphere
in a state supportive of human life
- the maintenance of biological
diversity per se, in particular of all presently living species.
Biodiversity and the biosphere
Human activities are affecting the
biosphere on a global scale. It is important in the present context
to establish the extent to which losses in biological diversity may
contribute to these changes in having an impact on man.
One of the most obvious of such
global changes is the perturbation of the carbon cycle, leading to a
steady increase in atmospheric CO2 levels. This will probably have
far-reaching, although at present unpredictable, effects on global
climate patterns which may in turn have serious consequences for
human welfare.
A significant part of this is
ascribable to industrial processes, especially the burning of fossil
hydrocarbon fuels for energy generation. However, it is believed
that alteration of existing natural or semi-natural ecosystems is
also important. In particular the large-scale destruction of
tropical moist forests is implicated, both in contributing to
atmospheric CO2 through burning and in decreasing the carbon-fixing
potential of the biosphere. The high risk of serious consequences
for humans of global climate changes is itself a strong argument for
decreasing rates of forest clearance. It must, however, be stressed
that this argument applies to tropical moist forest as `forest',
rather than as `a highly diverse ecosystem'. Diversity is important
only to the extent that it contributes to the system functioning as
a carbon sink and the argument applies equally to other systems with
a similarly high capacity for carbon fixation, such as tropical
freshwater swamps, although these are far less diverse than tropical
moist forest. In more general terms, there appears to be no direct
or obvious link between the importance of an ecosystem in
maintaining essential global ecological processes and its diversity,
although more research is required
|
