Geography Reference
In-Depth Information
ecosystem processes, including the regulation of
other species. There may be an argument for
reintroductions of these keystone species when
it is anticipated that they will have wider
conservation value. There is, for example,
considerable interest in the reintroduction of the
European beaver
(Castor fiber)
to Britain
(Macdonald
et al
. 1995). Where beavers dam
stream channels they may create habitat for
aquatic species that inhabit pools and stagnant
water.
Reintroductions may also be important where
a population of a species has declined to the point
where it is no longer considered viable. Bringing
in more individuals from a larger population
elsewhere may reinforce the recipient population.
Although this may often prevent the deleterious
effects of inbreeding depression in an isolated
population by bringing in new genotypes it may
also result in the dilution of important local
varieties. Some imported genotypes may be poorly
adapted to the local environmental conditions and
do little to enhance the viability of the recipient
population. There is also a risk of introducing new
diseases when infected individuals are used to
supplement a local population. One of the most
important considerations, however, in any species
reintroduction is the biogeography of the resultant
population. Many species, particularly where
conditions are marginal for population survival,
behave as metapopulations. There may be
considerable cost but little conservation value in
supplementing a local sink population. A good
understanding of the spatial dynamics of a regional
population may be crucial to ensuring the success
of a reintroduction programme (Hodder and
Bullock 1997).
Zoos and botanical gardens may serve a very
important education function that far exceeds
their somewhat doubtful value for reintroductions.
They provide an opportunity for society to marvel
at the diversity and beauty of organisms and can
alert the public to the threats of biodiversity loss.
In situ
conservation measures attempt to halt
the decline of species populations within their
natural range. Determining the status of a species
population is therefore of fundamental
importance. Lack of reliable information on
population levels can often confound conservation
efforts. Changes in the abundance of an organism
can only be assessed against knowledge of prior
levels. Unfortunately, there are very few
communities or even taxa for which accurate data
have been recorded over time. A further problem
is that where records have been kept they show
that the abundance of many organisms fluctuates
naturally from year to year. Long-term trends are
difficult to detect against this highly variable
background. For example, the abundance of larvae
of the larch bud moth
(Zeiraphera diniana)
was
monitored for over thirty years at a site in
Switzerland (Baltensweiler 1984). The population
density of moths varied by five orders of
magnitude during the period of the study (Figure
15.5). High population variance and strong auto-
correlation in population size from year to year
makes detection of long-term trends very difficult.
Even when long-term patterns are detectable,
it is difficult to extrapolate from data collected at a
single site to a more ecologically meaningful scale.
Populations of a species fluctuate not only in time
but also in space. An alternative and perhaps more
sensitive approach to monitoring species
populations ignores the size of a population but
looks instead at its geographical distribution.
Shaffer
et al
. (1998) have proposed that museums,
herbaria and other historic archives offer a
valuable source of information on geographical
distributions of organisms over time. Historical
records of the presence or absence of species at a
particular site are often more reliable than data on
their abundance. The latter are very dependent on
the sampling effort and method used. If records or
collections are treated as random samples from the
entire range of an organism, then changes over
geographically relevant scales can be assessed and
population fluctuations at individual locations
ignored. In this way, geographical research may
reveal more about patterns and changes in
biodiversity than detailed ecological studies of
population dynamics.
Although campaigns to save highly endangered
species stimulate considerable popular support, in
reality very few have proved successful. They also
