A Vital Rate Sensitivity Analysis for Nonstable Age Distributions and Short-Term Planning

There are multiple approaches to sensitivity analysis used to identify the age(or stage-) specific rate of reproduction or survival that most affects population growth—all of which involve evaluating effects on k, the dominant eigenvalue of the Leslie matrix. Management recommendations generated by these approaches can be difficult to determine, in part because results are valid only for populations with stable age distributions (i.e., the age structure is constant across years). Although these analytical approaches can identify the quickest way to increase population size to carrying capacity, they cannot identify best management options for short-term goals such as increasing population size above some critical value. We present a perturbation analysis—Vital Rate Sensitivity Analysis (VRSA)—to identify the vital rate that most limits population growth over any specified period by determining which vital rate has the greatest effect on population growth. The VRSA is effective for stable and nonstable age distributions, differentiates optimal shortand long-term management options, can incorporate stochasticity in vital rates, and can differentiate efficacy of alternative management strategies. In addition to being able to compare effects of incremental or specific changes in vital rates, VRSA can be used to compare specific management alternatives that affect single or multiple vital rates. We illustrate use of VRSA for a variety of management alternatives and hypothetical age-structures using demographic data from the literature for greater prairiechickens (Tympanuchus cupido) and red-cockaded woodpeckers (Picoides borealis). We also present a method, using Monte Carlo simulation, for measuring robustness of VRSA results. Under all of our management scenarios for greater prairie-chickens, we found juvenile survival to be the most sensitive vital rate for the specific comparisons made. For red-cockaded woodpeckers, the most sensitive vital rate differed by management scenario and confidence was higher for larger populations. Results for both species might change given other conditions and/or age structures. The VRSA should be an effective tool for wildlife managers who want to compare alternative management strategies to increase population size. A downloadable Excel spread sheet for calculating VRSA for a life-table is available at http://ase.tufts.edu/biology/faculty/reed/ software.html. (JOURNAL OF WILDLIFE MANAGEMENT 70(3):649–656; 2006)