Population Density and Survival Estimates of the African Viperid, Bitis schneideri

Abstract

Although estimates of survival and population density are available for several snake populations, most of these are for populations in northern temperate environments. Here we present the results of a 3-yr mark–recapture study for an African species, the Namaqua Dwarf Adder (Bitis schneideri). We estimated survival, recapture probability, and population density by marking 279 adders (121 males, 108 females, 50 juveniles) on two study sites along the Namaqualand coast of South Africa. We recaptured 54 individuals (27 males, 23 females, 4 juveniles), yielding absolute recapture rates of 20.4% and 17.9% at each site respectively. The adult sex ratio of our sample did not differ significantly from equality. We fitted eight models that included both individual-dependent and time-dependent covariates to our recapture data, and compared models using Akaike information criterion corrected for small samples. The best models included snout–vent length and search effort covariates. Mean (±SD) estimated monthly apparent survival was low (0.86 ± 0.11 and 0.86 ± 0.12), as were mean recapture likelihoods (0.06 ± 0.04 and 0.06 ± 0.01). Population density estimates were similar for the two sites (7.52 ± 3.62 ha−1 and 8.31 ± 7.38 ha−1). Juvenile snakes exhibited higher survival than adult females, which, in turn, had higher survival than adult males. Juveniles had a lower recapture probability than adult males or females. The likelihood of an individual leaving the study area was 6.8% and 9.4% per month for the larger and smaller sites, respectively. On the basis of our measures of apparent survival and emigration, we estimate annual survival rates of 39% and 56% for the two sites respectively. Population characteristics of B. schneideri differ substantially from those of other viperids, highlighting the need for additional population studies of African snakes.