population size is an important element in the assessment of exploitation. Of
course, the size of a bird population is rarely known with much precision. To
guard against overexploitation due to this uncertainty, minimum population
estimates can be used. Thus, abundance estimates based on the number of
individuals actually counted or the lower end of a confidence interval around a
population estimate might be used in computations of allowable harvest.
13.1.3Estimates of harvest levels
To assess whether the current level of exploitation is sustainable, some measure
of the harvest is needed. Harvest can be measured on an absolute scale, as the
total number of animals removed, or on a relative scale, as a harvest rate relative
to the population size. These two approaches are appropriate under different
circumstances, but either can provide critical information for the assessment.
13.1.4Population models and associated parameters
To assess whether a particular harvest level is sustainable, given a current popula-
tion size estimate, an understanding of the underlying population dynamics is
needed. Ideally, this would include age-, sex-, or size-specific estimates of survival
and reproductive rates, a complete model of the life-history dynamics, and meas-
ures of the links between these life-history parameters, harvest rates, and envir-
onmental driving variables. A more minimal understanding of the population
dynamics, however, can serve as a starting point that provides an initial, conser-
vative assessment that can be revised as more information is gathered. Two para-
meters are valuable at the initial stage: the maximum potential growth rate for
the population, rmax, and the carrying capacity, K(see Section 13.6 for details on
estimating these quantities). As more information is gathered, it is useful to
understand the density-dependent processes that regulate the population, and
the nature of the density-independent forces that can affect it.
13.1.5The use of trends
Is information about the trends in population size or harvest levels useful in
assessing the impact of exploitation? For example, is an increasing trend in popu-
lation size satisfactory evidence that the harvest is sustainable? It is tempting to
conclude that it is, but there are several reasons why trends need to be interpreted
with caution. (1) Harvest may be sustainable but still be too large. For example,
in a species of conservation concern, it may be unacceptable to allocate a large
portion of the net productivity to harvest and so delay recovery substantially.
In such a case, the relevant comparison is between the observed growth rate and
the growth rate expected in the absence of harvest, not simply whether the
304 |Exploitation