Environmental Engineering Reference
In-Depth Information
longer or shorter period of time might be translated into a trend in abundance over
time for the area as a whole. The typical example of this in ecology is assuming that
succession in time can be investigated by looking at vegetation changes in space
(see Begon
et al
. 2005 for a discussion of succession). However, space and time are
confounded—for example, people may choose to hunt first in less costly or more
productive locations, moving onto lower quality habitats or more distant locations
as the earlier ones are exhausted.
It is remarkable how different the language describing regression techniques is
between biology and economics.
Econometrics
, the branch of economics dealing
with analyses of trends in variables, generally uses different software and different
tests of statistical validity to those in common use in biology. If you wish to
analyse trends in economic variables, such as prices, over time, it is advisable to
think about the particular issues with these types of data, consult an econometri-
cian, and possibly use an econometrics software package such as Stata or Microfit
(see Resources section).
4.3.1.1 Population size
The most direct approach to assessing biological sustainability for a single species
is simply to estimate the size of the population on a regular basis and attempt to
ensure that it does not consistently decline, and more specifically, that it does not
decline below a reference point such
N
MSY
(Box 4.1). However, it is important to
remember that a declining population is not necessarily a clear indicator of unsus-
tainable use, nor does a stable population necessarily indicate sustainability. Most
obviously, an overexploited population may be stable, but small and therefore at
risk. Conversely, when a previously untouched population is first exploited, it
declines, but this does not inevitably lead to overexploitation. If the exploitation
effort remains constant at an ultimately sustainable level, a new equilibrium will be
reached. However, this rebalancing can take a long time—perhaps 10 years or
more in species with low intrinsic growth rates. In this case, action to curb offtake
may be unnecessary. Given the likely social and economic costs of implementing
effort reduction, it would be important to avoid falsely concluding unsustainabil-
ity in this case. Uncertainty in population estimates can be reduced by looking for
trends over a longer time period, but this is likely to be costly and time consuming,
with the risk that the population is already overexploited before sufficient data are
available to prove it.
Often it is not possible to monitor population size directly. It may only be feas-
ible to monitor
one component of the population
, or to use trends in
relative
abundance
as proxies (see Chapter 2). As an example of the former, seals spend a
large proportion of their adult lives at sea, and one of the times when it is feasible
to count them is as pups (SMRU 2004). However, there are potentially big prob-
lems with only monitoring one life stage and assuming that this is a reliable index
