Practical estimates of
biodiversity value:
using higher taxa as surrogates
Despite the great sampling effort
for the five plant families in the Flora Neotropica project, these
plants still represent less than 1% of the plant species and less than
2% of the plant families recorded within the study region. Consequently
some less direct method is needed to survey the broader variety of plants.
Higher taxon richness (using
genera or families) has been suggested to be useful as a surrogate for
species richness, and ultimately as a more remote
surrogate than species for gene or character richness
(ref 6). The particular attraction of
this approach is that it may prove suitable for use in more cost-effective
practical surveys, if it allows the taxonomic coverage to be broadened
without increasing costs. In comparison with the method of using small
indicator groups of species, it should have an advantage of precision for
predictions if it permits a broader coverage of the groups of organisms
surveyed, thereby avoiding an enormous extrapolation (mapping 1000 families
usually represents a much larger slice of biodiversity than mapping 1000
species). The choice of taxonomic rank to survey must be made with care
and there are potential pitfalls (ref 6).
Nonetheless, the approach shows promise.
Several studies now support the idea of a relationship between the numbers
of higher taxa, such as families, and the numbers of species among areas
(ref 10). Evidence at large spatial
scales is very difficult to collect. The largest comparable dataset available
for plants comes from the work of the late Alwyn Gentry, and this does
support a predictive relationship for patches among continents (log-transformed
on both axes, r² = 0.91) (ref
7) (below):
In the absence of direct counts of plant species richness, if this kind of
relationship can be assumed, then the example can use counts of the
numbers of seed plant families (from a total of 395) to represent relative
variation in the numbers of species (from a total of c. 300,000) expected
among equal-area grid cells (each cell c. 611,000 km2), with red for
high richness and blue for low richness (below):
Comparing
biological & environmental surrogate maps
Other uses of surrogacy for measuring biodiversity value include using
environmental factors that
are likely to govern the distribution of biodiversity, such as the net
primary productivity of the indigenous vegetation. One drawback of this
approach is that, unlike higher taxa, there is no information from productivity
estimates on the spatial turnover of groups of organisms among areas.
Another problem is that the governing relationships may not always be
conveniently linear. However, the geographic distribution of deviations
from the relationship between plant family richness and net primary
productivity (taken here from the published values modelled by Uchijima
& Seino) can be investigated by overlaying
estimates for the two in two separate colours (ref
14). The example uses green for plant family richness and blue for
the productivity scores, so that black grid cells on the map show low
values for both, white shows high values for both, grey shows similar
values for both (these scores lie on the diagonal of the colour key,
to the upper right), green shows an excess of plant families over productivity,
and blue an excess of productivity over plant families (Spearman correlation
coefficient rho= 0.85) (below):
(31 Kb image)
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Link to image showing overlay comparison between plant
family richness and primary productivity. |
