There is growing concern regarding potential demographic side effects and evolutionary consequences of selective harvesting on wildlife populations (Harris et al. 2002
; Coltman et al. 2003
). Perturbations of a population's demographic structure (Mysterud et al. 2002
; Milner et al. 2006
) and short- and long-term changes of morphological traits or life-history strategies due to artificial selective pressures (Festa-Bianchet & Apollonio 2003
) are some of the processes through which selective hunting may affect populations beyond more obvious, direct effects on population size and growth rate through the removal of individuals. In search of management strategies that minimize the demographic side effects and are ‘evolutionarily enlightened’ (Gordon et al. 2004
), it has been suggested that harvesting regimes should mimic natural mortality patterns (e.g. Milner et al. 2006
; Loehr et al. 2007
; Bergeron et al. 2008
). Surprisingly, the general applicability of this recommendation has received little theoretical or empirical evaluation. Recently, Proaktor et al. (2007)
presented model-based evidence that selection for lighter weight at first reproduction in ungulates could be a consequence of harvest and that harvest pressure is more important in driving this adaptive response than the degree of harvest selectivity. It seems plausible that this would apply to other situations in which the benefits of more and earlier reproduction eventually outweigh its costs (e.g. lower quality offspring), possibly due to higher overall mortality and consequently a greater chance of not reproducing later. To our knowledge, this is the only strong argument thus far in support of the statement that harvest selectivity patterns should mimic natural mortality, because a harvest biased towards younger (and lighter) individuals could minimize the aforementioned adaptive response. Even in such a case, simply targeting small (i.e. young) individuals may lead to further decreases in the size at, and time to, maturation as recent literature on fisheries-induced evolution suggests (Kuparinen & Merilä 2007
and references therein).
In this article, we are specifically concerned about the lack of scrutiny of the statement with regard to the immediate disruption caused due to demographic or other changes as a result of biased harvest. To avoid ambiguity, we identify a clear objective for harvest management with respect to selectivity, namely that harvesting and natural mortality acting on a population should result in a post-mortality population structure (or biological trait distribution) that is identical or at least very similar to the structure that would be expected in the absence of harvest (see also Harris et al. 2002
). With this objective in mind, we ask the question: should hunting mortality mimic natural mortality in order to limit the potential for disruptions caused by demographic or trait-distribution changes?
The effects of selection on trait distributions are now relatively well understood (e.g. Lynch & Walsh 1998
). Particularly relevant to our work is a paper by Vaupel et al. (1979)
, which explores the effects of viability selection on the distribution of a trait over time and age cohorts. The authors termed this trait ‘frailty’ to highlight the fact that it is related to an individual's risk of mortality, and assumed that the probability density function of frailty follows a gamma distribution. Vaupel et al. (1979)
further assumed that the force of mortality (a measure of an individual's risk) is a function of time, age and frailty. Although our basic approach is similar, we develop a slightly different model and explore the outcome through the simulations. We also extend Vaupel et al. (1979)
by discriminating between two mortality causes and by investigating how altering the shape of the viability selection function affects the post-mortality trait distribution.