observed growth rate is positive. (2) A trend may be due to transient dynamics.
For short periods, populations that are ultimately decreasing may show transient
increases due, for instance, to shifts in age-structure. (3) A declining trend is
ambiguous. Population size might decrease because the harvest is not sustain-
able; or it might decrease because a sustainable level of harvest is causing the
population size to shift to a lower equilibrium point. Total harvest might decrease
because the harvest rate is decreasing, or because the population is rapidly declining.
(4) Estimates of trends can be spurious. For example, a trend in population size
could be due to a change in survey effort, a shift in bird distribution relative to
survey strata, or a change in the ability to detect the birds because of a shift in, say,
vegetation structure. The methods described below provide an alternative to the
use of trend information for assessing the effects of exploitation.
13.2 Theoretical basis for sustainable exploitation
The challenge in the management of exploitation is to find the right balance
between allowing removal of animals for purposes sanctioned by society and pre-
venting population declines, loss of biodiversity, and extinction; that is, to deter-
mine what level of exploitation is sustainable. There is no precise definition of
sustainable. In the World Conservation Strategy (IUCN/UNEP/WWF 1980),
use of a natural resource is considered sustainable when the effect on the wild
population is not significant. But, exploitation can significantly affect the wild
population (in particular, by reducing the population size) yet still be sustain-
able, if the removal does not exceed the net production (Bennett and Robinson
2000). This condition can be met, however, at many different combinations of
population size and harvest rate, so ultimately, sustainability needs to be deter-
mined by the range of economic, cultural, and ecological values operating in a
particular setting. Nevertheless, there is a formal theoretical basis for the ecologi-
calsustainability of exploitation, a basis derived from equilibrium population
dynamics, and optimum sustained yield. For a more detailed treatment of this
subject, see Reynolds et al. (2001).
13.2.1Logistic growth model with perfect information
Consider the theoretical case where population size could be measured exactly,
where there were no stochastic dynamics, where harvest rates could be precisely
controlled, and where the population grew according to a logistic growth model.
The population size at each time step is governed by:
Nt 1 NtrmaxNt(1Nt/K)htNt (13.1)Theoretical basis for sustainable exploitation| 305