Dynamics of an Expanding Black Rhinoceros (Diceros Bicornis Minor) Population

Abstract

Population dynamics is a central component of demography and critical for meta-population management, especially of endangered species. We employed complete individual life records to construct census data for a reintroduced black rhinoceros population over 22 years from its founding and investigated that population's dynamics to inform black rhinoceros meta-population management practice and, more generally, megaherbivore ecology. 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, but yielding no evidence of density dependence. This result is consistent with, but does not confirm, the threshold model of density dependence that has influenced black rhinoceros meta-population management. Our analysis did support previous work contending that the generalized logistic is unreliable when fit to data that do not sample the entire range of possible 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. Process noise derived from demographic stochasticity, 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 considered in managed removals and introductions. We suggest that an extended period of exponential growth is common for megaherbivore populations growing from small size and that an increase in age at first reproduction with increasing population size, manifest in the study population, may provide a warning of density feedback prior to detectable slowing of population growth rate for megaherbivores .

Intriguing density dependence but problematic birth rates in Greaver et al.’s study of a black-rhinoceros population

“…Normally, r1 would be interpreted as the intrinsic rate of growth for a population growing according to the Ricker model. Estimates of the intrinsic rate of growth of black rhinoceros populations reported in the literature are typically about 0.1 or smaller (Law et al 2015). The value 0.955 is therefore unrealistically high for the intrinsic rate of growth, indicating that the regression does not extrapolate to smaller population sizes beyond the range of data on which it was based.…”

“…In other words, regression does no better than the mean. Indeed, amongst scalar models of population growth based on the generalized logistic, AICc unambiguously selected a model of exponential growth for SKKR (Law et al 2015), with no evidence of density dependence in growth rates. Figure 3.…”

“…Figure 3. Values of r for the SKKR population of Law et al (2013Law et al ( , 2014Law et al ( , 2015. Normally, r1 would be interpreted as the intrinsic rate of growth for a population growing according to the Ricker model.…”

“…It is only recently that studies of black-rhinoceros demography have progressed much beyond natural history. Notable examples include Cromsigt et al (2002), Hrabar and du Toit (2005), Okita-Ouma et al (2009), Brodie et al (2011), Ferreira et al (2011, and Law et al (2013Law et al ( , 2014Law et al ( , 2015.…”

“…We will also take this opportunity to make some comparisons between the Ithala black-rhinoceros population and another black rhinoceros population in the Great Fish River Reserve, South Africa, the demographics of which were studied in Law et al (2013Law et al ( , 2014Law et al ( , 2015. As in those publications, we will refer to this population as the SKKR population.…”