Effects of age and sex ratios on offspring recruitment rates in translocated black rhinoceros

Population parameters, performance and insights into factors influencing the reproduction of the black rhinoceros Diceros bicornis in Namibia

Estimating the population parameters, performance and factors that influence reproduction from long-term, individual-based monitoring data is the gold standard for effective wildlife management and conservation. Yet this information is often difficult and costly to collect or inaccessible to managers. We synthesized a 20-year set of individual-based monitoring data from a subset of black rhinoceros Diceros bicornis subpopulations across a range of environmental conditions in Namibia. Our findings demonstrate that despite the relatively arid landscape in Namibia, the black rhinoceros metapopulation is performing well, measured by age at first reproduction, inter-birth interval, population growth and survivorship. Information-theoretic modelling revealed that a univariate model including normalized differential vegetative index had a greater influence upon age at first reproduction than population density. The inter-birth interval model set identified cumulative rainfall during the 15 months prior to the birth month as the top model, although the mean normalized differential vegetative index during the inter-birth interval was comparable. There was little evidence for density-dependence effects on reproduction. These findings suggest that although browse quality could have a greater impact on parameters spanning multiple years, shorter-term parameters could be more influenced by rainfall. Our analysis also revealed a synchronous pattern of conceptions occurring in the rainy season. Our study provides a set of population parameter estimates for Namibian black rhinoceros subpopulations and preliminary insights on factors driving their reproduction. These expand our collective knowledge of global black rhinoceros population dynamics and improve our confidence and capability to adaptively manage the black rhinoceros metapopulation of Namibia.

Female reproductive skew exacerbates the extinction risk from poaching in the eastern black rhino

Variation in individual demographic rates can have large consequences for populations. Female reproductive skew is an example of structured demographic heterogeneity where females have intrinsic qualities that make them more or less likely to breed. The consequences of reproductive skew for population dynamics are poorly understood in non-cooperatively breeding mammals, especially when coupled with other drivers such as poaching. We address this knowledge gap with population viability analyses using an age-specific, female-only, individual-based, stochastic population model built with long-term data for three Kenyan populations of the Critically Endangered eastern black rhino (Diceros bicornis michaeli). There was substantial reproductive skew, with a high proportion of females not breeding or doing so at very low rates. This had a large impact on the projected population growth rate for the smaller population on Ol Jogi. Moreover, including female reproductive skew exacerbates the effects of poaching, increasing the probability of extinction by approximately 70% under a simulated poaching pressure of 5% offtake per year. Tackling the effects of reproductive skew depends on whether it is mediated by habitat or social factors, with potential strategies including habitat and biological management respectively. Investigating and tackling reproductive skew in other species requires long-term, individual-level data collection.

Black bear translocations in response to nuisance behaviour indicate increased effectiveness by translocation distance and landscape context

Abstract ContextTranslocation is a widely used non-lethal tool to mitigate human–wildlife conflicts, particularly for carnivores. Multiple intrinsic and extrinsic factors may influence translocation success, yet the influence of release-site landscape context on the success of translocations of wildlife involved in nuisance behaviour is poorly understood. Moreover, few studies of translocated wildlife involved in nuisance behaviour have provided estimates of translocation success under different scenarios. AimsWe evaluated the role of intrinsic (age, sex) and extrinsic (translocation distance, landscape composition) features on translocation success of American black bears (Ursus americanus) involved in nuisance behaviour and provide spatially explicit predictions of success under different scenarios. MethodsWe analysed data from 1462 translocations of 1293 bears in Wisconsin, USA, from 1979 to 2016 and evaluated two measures of translocation success: repeated nuisance behaviour and probability of returning to a previous capture location. Key resultsTranslocation distances ranged from 2 to 235km (mean=57km). Repeated nuisance behaviour was recorded following 13.2% of translocation events (192 of 1457) and was not significantly affected by translocation distance. Bears repeated nuisance behaviour and were recaptured at their previous captures site (i.e. returned) after 64% of translocation events (114 of 178). Return probability decreased with an increasing translocation distance, and yearling bears were less likely to return than were adults. The proportions of agriculture and forest within 75km and 100km respectively, of the release site had positive and negative effects on return probability. ConclusionsMangers can use bear characteristics and landscape context to improve translocation success. For example, achieving a 10% predicted probability of return would require translocation distances of 49–60km for yearlings in low-agriculture and high-forest landscapes. In contrast, estimated return probability for adults was ≥38% across all translocation distances (0–124km) and almost all landscape contexts. ImplicationsOur results emphasise the importance of considering the effects of landscape conditions for developing spatially explicit guidelines for maximising translocation success.