AN AGE-STRUCTURED DEMOGRAPHIC-MODEL FOR THE ENDANGERED STEPHENS KANGAROO RAT

Effective conservation of endangered species often is hampered by inad equate knowledge of demography. We extracted information on survival a nd fecundity from an 18-month, live-trapping study of Dipodomys stephe nsi, and from this we developed an age-structured demographic model to assess population viability. Adult Stephens' kangaroo rats persisted longer than juveniles, and adult females persisted longer than adult m ales. Disappearance rates were high in the first months after initial capture. Thereafter, the fraction of animals persisting decreased slow ly and in an approximately linear fashion on a semilogarithmic scale, suggesting age-independent mortality factors such as predation. Juveni le persistence did not differ substantially between two years of strik ingly different rainfall. Onset of breeding followed the start of wint er rains. Length of the breeding season, average number of litters per female, and the fraction of first-year females breeding were much gre ater in the year of higher rainfall. We propose a birth-pulse demograp hic model for D. stephensi that distinguishes juvenile and adult age c lasses. Temporal environmental variation can be modeled adequately wit h a constant survivorship schedule and variable fecundity determined b y yearly precipitation. Several issues should be resolved, however, be fore conservation decisions are based on the model. Better estimates o f juvenile survivorship are critical, the quantitative relationship be tween precipitation and fecundity must be determined, and the potentia l for density dependence and source-sink population dynamics must be e valuated.