13 Exploitation Michael C. Runge, William L. Kendall, and James D. Nichols 13.1 Introduction: assessment of exploitation 13.1.1 Taking a conservative approach Many bird populations are exploited for human purposes, through subsistence or commercial harvest, or live collection, for food, recreation, medicine, orna- ments, pets, or to reduce crop damage, or predation on game animals. While the motivations and methods are varied, they share one consequence—removal of birds from wild populations. Often, this removal and the consequences to the population are not quantified, but the outcome can be dire. Overkill is one of an “Evil Quartet” of causes of recent extinctions (Diamond 1989). The conservation biologist or wildlife manager wishing to prevent loss of bio- diversity and extinction due to exploitation is often faced with uncertainty regarding the status of the population, the level of harvest, and the dynamics of the population in question. How should one proceed? There are two kinds of error to guard against: doing nothing when exploitation is having a negative impact; and implementing unnecessary restrictions when exploitation is already sustainable. The focus of this chapter is to describe an assessment approach and monitoring tools to guard against the former error; that is, to take a conservative approach in the face of uncertainty. Exploitation of bird and mammal populations has tremendous economic and cultural importance, and managing exploitation requires careful consideration of those forces (Bennett and Robinson 2000). The conservation biologist needs to bring robust data and defensible analyses to the discussion. The three general types of information required are: minimum estimates of population size, estim- ates of harvest levels, and an understanding of population dynamics. 13.1.2 Minimum estimates of population size The effect of harvest on a population depends in large part on the magnitude of the harvest relative to the population size. In addition, the effect of harvest can be mediated by density-dependent dynamics. For these reasons, an estimate of the 304 | Exploitation 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. 13.1.3 Estimates 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. 13.1.4 Population 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 Theoretical basis for sustainable exploitation | 305 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- cal sustainability 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). 13.2.1 Logistic 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: ϭ ϩ Ϫ Ϫ ϩ Nt 1 Nt rmax Nt(1 Nt /K ) ht Nt (13.1) 306 | Exploitation where Nt is the population size at time t, ht is the harvest rate for the same time period, rmax is the maximum growth rate, and K is the carrying capacity. This model underlies the results of Caughley (1977:178–181) and is a simplified ver- sion (with ␪ ϭ 1) of the generalized logistic model used by Taylor and DeMaster (1993), Wade (1998), and Taylor et al. (2000). ϭ Imagine harvesting at a fixed rate, ht h (where the rate is relative to the popu- lation size), for an indefinite period of time. The population size will converge to and maintain an equilibrium value, Neq(h) that is a function of the fixed harvest rate (provided rmax is not too large, in which case cyclical or chaotic results can be obtained (May 1976)). The equilibrium population size is given by: Ϫ ϭ rmax h Neq(h) K ΂ ΃ (13.2) rmax (see Runge and Johnson 2002 for calculation methods); that is, the equilibrium ϭ Ն population size decreases linearly from K (when h 0) to 0 (when h rmax) (Figure 13.1a). (a)K (b) N K/2 Equilibrium Annual harvest 0 0 rmax /2 rmax 0 rmax /2 rmax Harvest rate Harvest rate Fig. 13.1 Maximum sustained harvest from a logistic model. (a) Equilibrium population size as a function of a fixed harvest rate. (b) Annual sustained harvest as a function of a fixed harvest rate. The maximum sustained yield is achieved by harvest at a rate of rmax/2, which achieves an equilibrium population size of K/2. Theoretical basis for sustainable exploitation | 307 Once the equilibrium population size (for a particular fixed harvest rate) is reached, the annual harvest is also constant and is given by Ϫ 2 ϭ ϭ rmax Kh Kh Heq(h) hNeq , (13.3) rmax which is parabolic with respect to h (Figure 13.1b). It is important to note that any fixed value of harvest rate less than rmax will produce an equilibrium population size and annual harvest that are both greater than zero. In this sense, any harvest rate less than rmax is sustainable. We can find the maximum sustainable harvest rate by differentiating equation 13.3 with respect to h, setting the result equal to zero, and solving for h (Runge and Johnson 2002). For the logistic growth model, the maximum sustainable harvest ϭ ϭ rate is h* rmax/2, which produces an equilibrium population size N * K/2, ϭ and an annual harvest of H * rmaxK/4 (Caughley 1977).