Environmental Engineering Reference
In-Depth Information
of total population size (Shea
et al
. 2006; Katzner
et al
. 2007). Taking the example
of grey seals in the UK, a change in the number of pups counted could be due to
reduced female fecundity or due to increased mortality among juveniles. There is
a two-fold difference in population size between these two scenarios; not only that,
but because the seal is long-lived it can take years for changes in survival or fecund-
ity to feed through into changes in the population size (SCOS 2006). Any future
debate about reopening seal culls to reduce damage to fisheries is likely to hinge on
getting an accurate estimate for seal population size.
While it is important to keep these potential problems in mind,
monitoring
population size
is probably the most effective method of assessment because it
measures directly the variable of interest from a conservation point of view. The
more indirect approaches described below provide a means to use a diversity of
different information when reliable population abundance information is unavail-
able. However, in general, the less direct the approach to understanding the cur-
rent status of a population is, the more assumptions are required, and the more risk
arises of being incorrect in the sustainability assessment.
4.3.1.2 Population structure
Sometimes, but not always, exploitation alters the
age structure
of populations. If
the harvest is age-selective, the selected ages will become less well represented in the
population. If harvest is unselective, the population may or may not become
skewed towards juveniles, depending on the nature of density dependence. The
extent to which harvest is expected to change age structure, if at all, is thus impos-
sible to generalise. It is particularly hard to define reference points for age structures
characteristic of overexploitation. Simple inferences about sustainability from
monitoring age structure are therefore rarely possible.
Sex structure can be heavily biased by
sex-selective harvesting
. In the saiga
antelope, the ratio of adult males to females went from about 1:5 to 1:100 due to
hunting targeting the male horns (Figure 4.1, Milner-Gulland
et al
. 2003). This
caused failure to conceive, which was, however, swiftly reversed when the sex
ratio rose above about 1:50. There has been concern about the influence of size-
selectivity on sex-changing fish; in these species, individuals change sex as they get
bigger, and so harvesting large animals may very strongly bias the sex ratio of the
population, with potentially serious consequences for sustainability (Platten
et al
.
2002; Molloy
et al
. 2007). However, sex ratios can be naturally highly skewed
without affecting population viability, and sex-bias in harvesting has only rarely
been shown to have affected population growth. The sex structure of a harvest is a
meaningful indicator of sustainability only when the natural sex structure is
known, and when the consequences of skewed sex structure for population viabil-
ity can be assessed.
Trends in size structure are quite widely used as an indicator of sustainability in
species such as fish, invertebrates and trees, where both fecundity and profitability
are strongly size-related. A large female fish can be many times more fecund than a
