Biology subjects

Peter R Law

Publications and source records attributed to Peter R Law.

3 recordsLinked to original sources

bioRxiv2016

Intriguing desnity dependence but problematic birth rates in greaver et al.'s study of a black rhinoeros population

Greaver et al. (2014) presented evidence for density dependence in the Ithala population of black rhinoceros. Finding that they did not place their regression-based evidence in a modelling context, we recast their result as an example of the ramp model of density dependence that underlies black rhinoceros meta-population management. Greaver et al. concluded that the Ithala population did not reach carrying capacity, a conclusion we consider unwarranted since they did not conduct any analyses of trends in demographic parameters with population density. Our interpretation implies that the Ithala population did indeed reach carrying capacity. Where relevant, we compared their results for the Ithala population with those for another southern African black-rhinoceros population in order to provide a broader basis for evaluating black rhinoceros demography. We detail inconsistent presentation of data in their paper that plagued our efforts to understand their results and also draw attention to possible errors in some analyses. In particular, we argue that the results on birth rates reported by Greaver et al. appear dubious. Greaver et al. have presented important evidence for density dependence in a population of black rhinoceros but we suggest they have underutilized their data in interpreting this density dependence while misanalysing birth data.

Ecology↗

bioRxiv2015

DYNAMICS OF AN EXPANDING BLACK RHINOCEROS (Diceros bicornis minor) POPULATION

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 populations dynamics to inform black rhinoceros meta-population management practice and, more generally, megaherbivore ecology. Akaikes 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.

Ecology↗

bioRxiv2013

Black rhinoceros demography should be stage, not age, based.

Biologically meaningful and standardized definitions of life stages are essential for demographic studies, especially for endangered and intensively managed species such as rhinoceros. Focusing on the black rhinoceros Diceros bicornis, we argue that standardized biological definitions of calf, subadult, and adult, rather than age classes, provide the appropriate basis for black rhinoceros demography. Age classes do not correlate well with the risks of mortality nor characterize, at least for females, the attainment of reproductive maturity. Black rhinoceros demography based on age classes, typical of existing literature, obscures the significance of studies of mortality and fecundity. Comparison of studies also requires standardized definitions. We propose biologically meaningful definitions of life stages appropriate for the demography of the black rhinoceros and encourage the community of rhinoceros researchers to reach consensus on standardized, demographically appropriate definitions for adoption for each rhinoceros species.

Ecology↗