Testing ‘Proportion of Females Calving’ as an indicator for population-level reproductive performance for black rhinoceros (Diceros bicornis)

Abstract

The proportion of females calving (PFC) each year has been employed as an indicator of population reproductive performance in ungulates, especially for species that breed annually, because it requires less detailed population data than inter-birthing intervals and age at first reproduction. For asynchronous breeders with inter-birthing intervals longer than a year such as megaherbivores, however, it is unclear how much annual variation in PFC is expected and whether false signals of density feedback or environmental influence might result from analyzing PFC data. We used census data from a well studied, closed, expanding population of black rhinoceros (Diceros bicornis) to study annual variation in PFC over 22 years. Our analysis of PFC data yielded no false signals of density feedback but weak evidence for an unexpected influence of rainfall. The PFC data exhibited considerable variation, which we attribute to autocorrelation in the time series of PFC data, ‘demographic-founding effects’, changes in stage structure, and demographic stochasticity, some of which the modelling of PFC appears to confuse with an influence of rainfall. We expect such variation to be common in introduced populations and to persist for some years, complicating the interpretation of PFC, though moving averages of PFC can help if employed cautiously. While our analysis does not undermine the possible utility of PFC, the analysis and interpretation of PFC values require care.

Population parameters, performance and insights into factors influencing the reproduction of the black rhinocerosDiceros bicornisin Namibia

“…Heeding earlier calls for a shift towards more management-relevant research (Linklater, 2003), a number of applied studies of rhinoceros demographics have helped to advance our understanding of critical population dynamic issues for rhinoceros populations in situ. These include factors affecting rhinoceros sex ratios (Berkeley & Linklater, 2010), age at sexual maturity, inter-birth intervals and fecundity (Nhleko et al, 2017; Gedir et al, 2018), population performance indicators (Ferreira et al, 2011; Law et al, 2013; Law & Fike, 2018) as well as mortality rates in rhinoceros populations. Some studies have gone further by conducting population viability assessments (Thuo et al, 2015; Subedi et al, 2017) and/or building more complex and sophisticated population models (Brodie et al, 2011; Soka et al, 2014).…”