Environmental Engineering Reference
In-Depth Information
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The sales of other commodities, such as dogs and buckets, were increasing,
and this may have played a part in the move to longer journeys through
increased profits.
Even though intuition may suggest that the trade is unsustainable and wiping the
pigs out in a wave of depletion from the market outwards, these other factors need
to be considered before a firm conclusion can be drawn. The question then is
where will the dealers find it unprofitable to travel further—and will this be
before the end of the wild pig habitat?
Source
: Clayton
et al
. (1997); Milner-Gulland and Clayton (2002).
It's also important to remember that, just as declines in population size over time
may not be unsustainable, but may instead culminate in an equilibrium population
size, changes in the spatial extent of harvesting can also
equilibrate
at a sustainable
level. It is to be expected that, if harvesters are based in a central location, the area
around that location will be depleted, and the areas further away will have higher
densities of hunted species. This is not necessarily an indicator of unsustainability.
In fact, spatial heterogeneity in hunting pressure may lead to a
source-sink
situ-
ation, in which animals are protected in unhunted locations and disperse out into
the hunted areas (Novaro
et al
. 2005). We need to recognise that spatial structure in
hunting is important for sustainability (Ling and Milner-Gulland in press), but the
relationship between lack of sustainability and the spatial extent of hunting is not
simple.
Simple regressions of trends in a variable over time are rarely adequate. Many factors
may cause an observed trend in CPUE, for example. The prey may simply have
become less detectable, perhaps due to the exploitation, leading to biased population
estimates. Other aspects of the environment may be changing. This may be associ-
ated with the exploitation (for example, logging of tropical forests not only alters the
habitat but opens it up to hunters), but may be entirely unrelated (for example, large-
scale climatic change can cause populations to fluctuate or decline). Finally, on a
shorter timescale, mobile species may simply move out of an area (Boyer
et al
. 2001).
The appropriate response to each of these factors would be different.
We might be content simply demonstrating that the observed offtake is sufficient
to cause the observed decline. This is not proof of causation, but it invokes the
principle of parsimony
—if one factor is enough to cause an outcome, this suggests
that other factors need not be involved. But this is potentially misleading, because
interacting factors may cancel each other out and have a minor net effect. It is
important to have an understanding of what is actually going on, or conservation
interventions aiming to alter the supposed causative factor will not be successful.
