Density‐Dependent Harvest Modeling for the Eastern Wild Turkey

Abstract

Many current wild turkey (Meleagris gallopavo) harvest models assume density‐independent population dynamics. We developed an alternative model incorporating both nonlinear density‐dependence and stochastic density‐independent effects on wild turkey populations. We examined model sensitivity to parameter changes in 5% increments and determined mean spring and fall harvests and their variability in the short term (3 yr) and long term (10 yr) from proportional harvesting under these conditions. In the long term, population growth rates were most sensitive to poult:female ratios and the form of density dependence. The nonlinear density‐dependent effect produced a population that maximized yield at 40% carrying capacity. The model indicated that a spring or fall proportional harvest could be maximized for fall harvest rates between 0% and 13% of the population, assuming a 15% spring male harvest and 5% spring illegal female kill. Combined spring and fall harvests could be maximized at a 9% fall harvest, under the same assumptions. Variability in population growth and harvest rates increased uncertainty in spring and fall harvests and the probability of overharvesting annual yield, with growth rate variation having the strongest effect. Model simulations suggested fall harvest rates should be conservative (≤9%) for most management strategies.

Variation in spring harvest rates of male wild turkeys in New York, Ohio, and Pennsylvania

“…Understanding the harvest rate and proportion of adult males in the population can provide useful information for making management decisions to best meet hunter desires and maximize hunter satisfaction (Healy and Powell 1999). Most mortality in male turkeys occurs during the spring (Godwin et al 1991, Thogmartin and Schaeffer 2000, Holdstock et al 2006) and spring hunting mortality is generally considered to be additive to other sources of mortality (Alpizar‐Jara et al 2001, McGhee et al 2008, Moore et al 2008). Consequently, spring hunting‐related mortality rates (legal harvest, illegal kills, and crippling loss) of more than 30–35% of the male population are thought to adversely affect hunter satisfaction because the proportion of adults in the population and harvest are predicted to decline (Vangilder and Kurzejeski 1995).…”

“…For male wild turkeys ( Meleagris gallapavo ), spring harvest is the single greatest mortality factor (Godwin et al 1991, Paisley et al 1996, Wright and Vangilder 2005) and is thought to be additive to other sources of mortality because most natural mortality occurs during the spring breeding season (Thogmartin and Schaeffer 2000, Holdstock et al 2006, Moore et al 2008). Although fall harvest is thought to have the greatest influence on population growth (Vangilder and Kurzejeski 1995, Alpizar‐Jara et al 2001, McGhee et al 2008), spring harvest can influence the number and proportion of adult gobblers in the population (Vangilder and Kurzejeski 1995). For example, Vangilder and Kurzejeski (1995) modeled a wild turkey population and by increasing overall spring harvest rates from 25% to 50%, the proportion of the population composed of adults declined from 72% to 56%.…”

Identifying target reference points for harvesting assessment‐limited wildlife populations: a case study

“…Given that multiple hunting seasons occur annually, it is intuitive that larger spring harvests should leave less room for large fall harvests. Thus, when our results are considered in light of earlier modeling studies that often used optimistic parameter combinations (i.e., high productivity, equal fall harvest vulnerability, and low spring harvest; Alpizar-Jara et al 2001, McGhee et al 2008 relative to the broader range of values we considered for structurally uncertain parameters, more conservative harvest recommendations are not a surprise. In our case, the effect of additional structural uncertainty was exacerbated by the high sensitivity of optimal fall harvest rates to the values of model parameters (optimal target harvest rates ranged from 0% to >10%, Fig.…”

“…It is therefore difficult to construct detailed, life-history-based models that directly portray elements of a Turkey life-cycle on an annual basis (sensu Vangilder and Kurzejeski 1995) that also include limitation of population growth at large abundances. This model was fit previously to Turkey population indices from 11 states by McGhee and Berkson (2007) to estimate the strength and form of the relationship between declines to population growth and abundance (h), and used by McGhee et al (2008) to identify maximumsustainable harvests for a unique set of model input parameters. This model was fit previously to Turkey population indices from 11 states by McGhee and Berkson (2007) to estimate the strength and form of the relationship between declines to population growth and abundance (h), and used by McGhee et al (2008) to identify maximumsustainable harvests for a unique set of model input parameters.…”

“…Annual process variation in population growth associated with environmental conditions (e p;t ) was drawn from a normal distribution using parameter values consistent with earlier studies (e p;t $ Normalðl ¼ 0; r p ¼ 0:15Þ; McGhee et al 2008, Stevens et al 2017b. In this model, sex-specific instantaneous growth rate (r i,t ) is a function of additional parameters representing non-hunting survival and population productivity (i.e., the number of female recruits per female in the population, after spring losses).…”