MEASURING BIODIVERSITY VALUE
Measures of biodiversity are needed to determine
the 'where' of in situ conservation action rather than the 'how',
particularly in deciding which combinations of available areas could
represent and help sustain the most biodiversity value for the future.
This raises many questions, including:
A note on grids and data
Diversity is a property of sets of organisms. These sets are usually
defined by the volume of space in which they occur, although for many
studies the third dimension can be ignored, so that sets can be defined
by an area of land or sea.
We follow a popular approach of dividing the surface area of the
world into grid cells or polygons. For purposes other than choosing
among land-management units for conservation, equal-area (ref
10) or nearly equal-area grid cells (ref
5) are often used because they reduce the species-area effects on
diversity and rarity measures. Above all, the studies described here are
assumed to be based on unbiased knowledge of the occurrence of
organisms, or at the very least, on the best available knowledge of
their distributions (ref
5).
Biodiversity value in theory:
identifying a fundamental currency of value to people
Biodiversity has been seen as the total (and irreducible) complexity
of all life, including not only the great variety of organisms but also
their varying behaviour and interactions. From this viewpoint, no single
objective measure of biodiversity is possible, only measures relating to
particular purposes or applications. So for conservationists, for
example, a measure of biodiversity should quantify a value
that is both broadly shared among the people for whom they are acting
and considered as being in need of protection.
One of the more broadly shared and economically defensible values for
conserving wholesale biodiversity (rather than just the few components
or 'biospecifics' with obvious high use value at present) may be seen to
lie in ensuring continued possibilities both for adaptation, and for
future use by people in a changing and uncertain world (this is not to
deny other possible values). Consequently, biologists have argued that
this value in biodiversity is likely to be associated with the variety
of different genes that can be expressed by organisms as
potentially useful phenotypic traits or characters
(different chemicals, morphological features, functional behaviour).
Because we do not know yet precisely which genes or characters will be
of value in the future, first they must all be treated as having equal
value, and second, the greatest value for conservation will come from
ensuring the persistence of as many different genes or characters as
possible, as a form of insurance (ref
2, ref
3).
For example, a dandelion and a giant redwood can be seen to represent
a richer collection of characters in total, and so greater diversity
value, than another pair of more similar species, a dandelion and a
daisy (ref
1). This shows how the phenotypic characters (or the genes that code
for them) could provide a 'currency' of value for
biodiversity. Pursuing this idea, we will then need to maximise richness
in the character currency within the conservationists'
'bank' of managed or protected areas.
A consequence of this approach to valuing biodiversity is that it
provides one possible unified view of the traditional three levels at
which biodiversity has been described. In effect it uses genetic
diversity as a basis for valuing both species
diversity (for their relative richness in different genes) and
ecosystem diversity (for the relative richness in the
different processes to which the genes ultimately contribute). This
provides additional justification for multi-level approaches and
biodiversity-surrogate methods.
A particular strength of the single currency approach is that it
avoids the problems in compound measures of trying to trade-off measures
for different properties that really cannot be compared or
inter-converted (such as species richness and the relative abundances
among species that are combined in the diversity measures used in
community ecology), which otherwise can lead to confusion and a loss of
accountability. This advantage of accountability in terms of a single
currency of value becomes particularly important when faced with the
problem of choosing areas for biodiversity conservation, when many other
factors may be involved (see the sequential
approach to accommodating multiple factors).
The difficulty with this single currency approach is that the numbers
of genes or characters cannot usually be counted directly, so the
problem is how best to estimate them?
