Alternatives to Robinson and Redford's method of assessing overharvest from incomplete demographic data

Abstract

Conservation biologists often must make decisions about the sustainability of harvest rates based on minimal demographic information. To assist them Robinson anti Redford (1991) formulated a method to estimate maximum rates of production which could be used to detect overharvesting based on only age at first reproduction, fecundity, and maximum longevity. By assuming constant adult survival we reduced the Euler equation to a simple form that allows calculation of population growth from the same minimal demographic data, brit that can incorporate empirical prereproductive and adult survival rates if available. With this formula, we computed growth rates rising various explicit survival schedules, and we compared these rates and those from Robinson and Redford's (1991) method to rates calculated from 19 relatively complete mammalian life tables gleaned from the literature. When we applied our method (assuming 1% survival to maximum longevity) and that of Robinson and Redford (1991) to the same minimal demographic data, we found that our growth rates were closer to those from complete life tables. We therefore reexamined the data of Fa et al (1995) and Fitzgibbon et al. (1995), who analyzed overharvesting of several populations of commercially exploited African mammals based on Robinson and Redford's (1991) methods Our reanalysis indicates that several additional populations may be overharvested. Our analysis also suggests that data on survival to age at first reproduction improves estimates of population growth rates more than data on age-specific adult survival. Regardless of the method, approximate growth rates based on incomplete life tables can be used to detect when populations are overharvested, brit one should not conclude that harvest rates are sustainable when they are less than approximate production rates because simplifying assumptions often lend to overestimates.