Vital rate sensitivity analysis as a tool for assessing management actions for the desert tortoise

Abstract

Sensitivity analyses of population growth in desert tortoise ( Gopherus agassizii) have shown no consensus on the limiting vital rate. More importantly, the most sensitive vital rate might not be the most readily manipulated by management, so it begs the question of what actions would be most effective. We compared 13 management alternatives using a vital rate sensitivity analysis that is valid regardless of age structure, and is sensitive to initial population size and time frame, to determine the efforts required for equivalent population growth. We evaluated three time frames, each with five initial population sizes and three initial age distributions. To achieve equivalent population growth, mortality of older females needed to be reduced less than did mortality of other age classes. Similarly, fewer adults needed to be introduced to a population to have the same effect as releasing juveniles, but differences among adult age classes were trivial. A single release (headstarting) required fewer total individuals than did annual releases to achieve the same population growth. Also, the same population growth was more easily achieved when the initial age structure was deficient of young animals. Interestingly, because small tortoises are difficult to survey, some management alternatives could result in increased population size but decreased numbers of countable individuals over short to intermediate (25 years) time frames. Our paper demonstrates an approach to determine what constitutes equivalent management actions for population growth, thus allowing managers to more directly compare expected gains toward population recovery achieved by their resource-allocation decisions.