Population densities of eastern black rhinoceros : unravelling the controls

Key words: black rhinoceros, browser, corticosterone, diet, density dependence, minerals, moisture, physiological stress, savanna, soil nutrients, woody cover. Understanding the forces that cause variability in population sizes is a central theme in ecology. The limiting factor in populations of large mammals which are not controlled top‐down by predation is food, i.e., such populations are controlled by bottom‐up processes. However, there is little evidence of density dependence in large‐ to mega-herbivores. Yet, conservationists have managed Critically Endangered mega‐herbivores like rhinoceros as if their population growth were density dependent, i.e., following a logistic growth curve, focusing on large growth at population densities presumed to be at half‐carrying capacity (K/2). This would enable them to translocate animals at presumed half‐carrying capacity to retain local population densities and to create new populations in areas of suitable habitat, where animals are considered safe against poaching. This study focused on one such mega‐herbivore, the eastern black rhinoceros (Diceros bicornis michaeli) to re‐consider the density dependence population regulation in a mega‐herbivore and uses the findings to contribute to possible solutions towards conservation challenges facing this species. The expectations were that increases in population density would result in a decrease in reproductive performance, and that physiological stress levels in animals in populations of high density would be higher than in animals in populations of low densities. Nine populations of black rhinoceros across Kenya were studied, with variation in their respective densities, Plant Available Moisture (‘PAM’ i.e., ‘soil moisture’) and Plant Available Nutrients (‘PAN’ i.e., ‘soil fertility’). Data from available records (1993‐2010) were used to assess reproductive performance. Dietary quality and levels of corticosterone were estimated through faecal analysis from animals sampled in the field and from data on feeding trials of black rhinoceroses in zoos (dietary analysis only). Woody cover estimates were used to assess available browse for black rhinoceros. Two measures of density were used, i.e., absolute density (animals/km2) and relative density, i.e., absolute density as a ratio to the estimated maximum stocking density or ‘carrying capacity’. The effects of PAM and PAN, and subsets of PAM (rainfall and temperature) were incorporated and controlled in testing expectations. No evidence for density dependence was found. Reproductive performances tended to be better where PAM was high, PAN was low and woody cover was sparse. PAM was found to be directly correlated with quality of dietary browse. Black rhinoceros populations appeared controlled more by bottom‐up processes through key resources, even though their densities were perhaps too low to fully support this alternative view. It was apparent that the density dependence concept still requires more investigation. Deliberate efforts should be made to secure high PAM – low PAN – sparse woody cover areas for conservation of black rhinoceros. Conservation managers are advised to consider set percentage translocations, as opposed to the current translocation of black rhinoceros on the basis of an imaginary ‘carrying capacity’ and density dependence.

Bibliographic Details

Main Author:	Ouma, B.O.
Other Authors:	Prins, Herbert
Format:	Doctoral thesis biblioteca
Language:	English
Published:	Wageningen University
Subjects:	browsing, kenya, nutrient availability, population density, population ecology, rain, rhinoceros, savannas, temperature, afgrazen, populatie-ecologie, populatiedichtheid, regen, savannen, temperatuur, voedingsstoffenbeschikbaarheid,
Online Access:	https://research.wur.nl/en/publications/population-densities-of-eastern-black-rhinoceros-unravelling-the-
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