Geography Reference
In-Depth Information
Box 15.1
Determining the species status of the green frog
Globally, there has been a recent and alarming decline
in populations of many amphibians. Populations may
show high natural variation, and therefore there has
been considerable debate over the extent to which
widespread decline reflects a serious conservation
problem or just a stochastic fluctuation. The only way
that a satisfactory answer can be found is to mount
long-term monitoring of species numbers. Many
amphibians live in isolated habitat patches that may
interact as part of a large metapopulation. Hecnar and
M'Closkey (1997) monitored the distribution and
abundance of a common green frog
(Rana clamitans
mellanota)
in 160 ponds in southwestern Ontario,
Canada, at a variety of scales in order to discover how
the population was changing.
The geen frog is capable of inhabiting a wide range
of permamanent ponds. Subadults leave the pond where
they are born and disperse up to five km to a new site.
Turnover in the population is high, with very few adults
living longer than five years. Wetlands in southern
Ontario covered over 60 per cent of the land area up to
100 years ago. Extensive drainage for agriculture means
that wetlands now cover less than 10 percent.
In this survey, the number of green frogs at each pond
in three regions was counted. At a geographic scale,
there was very little change in the number of ponds
occupied by frogs during the three-year survey period.
However, there was considerable variation from region
to region, with frogs occupying under half the ponds
surveyed in one region and all of the ponds in another. At
sub-regional and local scales, it was found that in some
areas the number of ponds occupied by frogs was
declining, while it was increasing in others. The number
of adult green frogs remained stable at a geographic
scale but was increasing in one region, declining in
another and stable in the third. At the sub-regional and
local scale, trends in the abundance of frogs were
extremely variable. Abundance of frogs was stable at 20
per cent of ponds, increasing at 18 per cent, declining in
14 per cent and showing no consistent trend in the
remainder.
This study illustrates how the status of a species is
highly scale-dependent. Local-scale studies may come
to highly misleading conclusions about trends in the
distribution and abundance of the green frog if
extrapolated to a geographic scale. Variance in both
occupancy of ponds and adult numbers increased as the
spatial scale of the survey decreased. This study
confirmed that the smaller a local population the more
likely it is to become extinct. Extinctions occurred only
where there were fewer than ten adults in a pond. Frog
populations were spatially dynamic, with common
extinctions and recolonisations. The implications for
conservation biology are clear. First, it is not adequate to
assess the status of a species at a restricted spatial
scale. It also emphasises the importance of facilitating
species dispersal in order to ensure that a local
population can re-extablish itself after stochastic
extinction. Small, high-quality habitat patches may make
an important contribution to increasing species dispersal,
even though they may not be able to sustain a local
population over a long time period. Conservation needs
to understand the spatial dynamics of a species in order
to prevent decline at a geographic scale.
MANAGEMENT FOR BIODIVERSITY
Protection of endangered species
It is almost a truism to point out that the best
method for maintaining biodiversity is to control
the most important causes of decline. These are
habitat destruction and fragmentation, primarily as
a consequence of the expansion of cultivated and
pastoral areas. Habitat degradation also contributes
to loss of species. Pollution, the introduction of
invasive exotic species and overexploitation of
natural ecosystems are some of the most important
causes of degradation. Unfortunately, governments
have been reluctant to take action, especially where
controls would have significant impacts on people's
livelihoods. As a consequence, strategies for
biodiversity conservation are frequently limited to
attempts to exclude such influences from protected
areas and attempts to rescue highly endangered
species from extinction.
Ex situ
conservation strategies attempt to
preserve representatives of highly endangered
species outside their native ranges, usually in a
zoological or botanical garden. In principle,
individuals can then be reintroduced to their
natural environment. Controversy continues to
rage in the conservation world about the ethics
of reintroducing a species to sites where they are
known to have existed in the past but have
recently gone extinct. This may be desirable
where a habitat has been restored to the point
where it may, once again, adequately support the
species, or where some systemic extinction
pressure has been removed. It may be necessary if
the species in question has a poor colonising
ability and is unlikely to arrive unaided. Some
species may have a significant influence on
