ALTERNATIVES TO ROBINSON AND REDFORDS METHOD OF ASSESSING OVERHARVESTFROM INCOMPLETE DEMOGRAPHIC-DATA

Conservation biologists often must make decisions about the sustainabi lity of harvest rates based on minimal demographic information. To ass ist them Robinson anti Redford (1991) formulated a method to estimate maximum rates of production which could be used to detect overharvesti ng based on only age at first reproduction, fecundity, and maximum lon gevity. By assuming constant adult survival we reduced the Euler equat ion to a simple form that allows calculation of population growth from the same minimal demographic data, brit that can incorporate empirica l prereproductive and adult survival rates if available. With this for mula, we computed growth rates rising various explicit survival schedu les, and we compared these rates and those from Robinson and Redford's (1991) method to rates calculated from 19 relatively complete mammali an 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 there fore reexamined the data of Fa et al (1995) and Fitzgibbon et al. (199 5), who analyzed overharvesting of several populations of commercially exploited African mammals based on Robinson and Redford's (1991) meth ods Our reanalysis indicates that several additional populations may b e overharvested. Our analysis also suggests that data on survival to a ge at first reproduction improves estimates of population growth rates more than data on age-specific adult survival. Regardless of the meth od, approximate growth rates based on incomplete life tables can be us ed to detect when populations are overharvested, brit one should not c onclude that harvest rates are sustainable when they are less than app roximate production rates because simplifying assumptions often lend t o overestimates.