Dynamics of an expanding black rhinoceros (Diceros bicornis minor) population

Abstract

Understanding population dynamics is critical for meta-population management, especially of endangered species, and also for megaherbivore ecology. We employed complete individual life records to construct census data for a reintroduced black rhinoceros population over 22 years since its founding and investigated its dynamics. Akaike's information criterion applied to scalar models of population growth based on the generalized logistic unambiguously selected an exponential growth model (r=0.102±0.017), indicating a highly successful reintroduction. No evidence of density dependence was detected, and thus, we could not confirm the threshold model of density dependence that has influenced black rhinoceros meta-population management. Our analysis supported previous work contending that the generalized logistic is unreliable when fitted to data that do not sample the entire range of population sizes. A stage-based matrix model of the exponential population dynamics exhibited mild transient behaviour. We found no evidence of environmental stochasticity, consistent with our previous studies of this population that found no influence of rainfall on demographic parameters. Demographic stochasticity was present, principally reflected in annual sex-specific recruitment numbers that differed from deterministic predictions of the matrix model. Demographically driven process noise should be assumed to be a component of megaherbivore population dynamics, as these populations are typically relatively small, and should be accounted for in managed removals and introductions. Increase in age at first reproduction with increasing population size, as manifested in the study population, may provide a warning of possible density feedback prior to detectable slowing of population growth rate for megaherbivores.

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

“…But variation in both inter-birthing intervals and birth sex continued in our study population through 2008 even as the population stage structure closely approached a stable distribution. This variation was attributed to demographic stochasticity in Law, Fike & Lent (2015) and must be a component of the fluctuations in PFC for our study population. Consistent with our analyses and expectations of no environmental influence and only a subtle density feedback, there is no apparent trend in PFC values or a sustained reduction to lower values at any point.…”

“…However, the global model left unexplained 54% of variation in transPFC. The unexplained variation left in age at first reproduction and inter-birthing intervals in Law, Fike & Lent (2013) and birth sex in Law, Fike & Lent (2014) , was traced to the variation in birth numbers and sexes unexplained by the demographic models in Law, Fike & Lent (2015) and linked there to demographic stochasticity. We therefore propose that the variation in PFC unexplained by the AR modelling here also reflects demographic stochasticity.…”

“…The study site is considered excellent black rhinoceros habitat ( Van Lieverloo et al, 2009 ; Fike, 2011 ). Further details may be found in Fike (2011) and Law, Fike & Lent (2013) , Law, Fike & Lent (2014) , Law, Fike & Lent (2015) . All rhinoceroses were individually identifiable and we possessed complete individual histories 1986–2008 from which we computed exact PFC values 1987–2008 as follows.…”

“…Our aim in this paper is to investigate the performance of PFC as a measure of population-level reproductive performance by analyzing PFC for a population of black rhinoceros ( Diceros bicornis minor ) for which complete individual histories were known ( Fike, 2011 ). Previous study of the demography of this population based on the complete individual histories ( Law, Fike & Lent, 2013 ; Law, Fike & Lent, 2014 ; Law, Fike & Lent, 2015 ) permitted us to frame expectations of PFC if it is to provide a reliable measure of population-level reproductive performance and to test those expectations. We studied mortality, age at first reproduction, and inter-birthing intervals in Law, Fike & Lent (2013) , and birth sex in Law, Fike & Lent (2014) .…”

“…On the basis of these results, we conjectured that variation was due, at least in part, to demographic stochasticity. In Law, Fike & Lent (2015) , we fitted semi-annual population census counts to various models of population dynamics expected to be suitable for megaherbivores ( Owen-Smith, 2010 ). A model of exponential growth with intrinsic annual rate of growth estimated as 0.102 ± 0.017 was unambiguously the best model fit to the data.…”