Effects of elephant carcasses on vegetation cover, herbivore behaviour, and potential Anthrax transmission in central Etosha National Park
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Date
2021
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University of Namibia
Abstract
Anthrax is primarily a disease of herbivores caused by a soil-borne, spore-forming and Grampositive bacterium, Bacillus anthracis. This study investigated the role large-bodied animal carcasses may play in anthrax transmission dynamics in an African savanna. Specifically, I examined how African elephant (Loxodonta africana) carcasses affected vegetation cover, soil pathogen concentrations, and herbivore behaviour at carcass sites over time in central Etosha National Park, Namibia. Carcass site soil and vegetation sampling was conducted three times over
a growing season (January, April and July), while motion-triggered camera traps monitored activity at elephant carcass sites over a three-year period in central Etosha National Park. Elephant carcasses were placed into age classes for soil and vegetation sampling based on the time of death as follows: recent (0-2 years old), old (2-5 years old), and very old (>5 years old). This study i) measured the area of soil disturbance around the elephant carcass, ii) measured for vegetation
cover at elephant carcass, iii) determined the concentration of B. anthracis spores in soils at zebra and elephant carcass sites, and iv) monitored animals activity at elephant carcass sites using motion sensing video camera. The area of soil disturbance at elephant carcass sites, while substantial in the year of death (up to 31.55m2), showed a sharp decline after the first year (to lessthan 1m2). Vegetation cover was generally highest near the centre of carcass sites and declined
with increasing distance away, except for some of the younger sites (<2 years old) with more soil disturbance at the closest sampling distance. Among site ages, the 2-5 years old carcass sites had higher vegetation cover than younger or older sites. There was seasonal variation in vegetation cover that varied with site age, with cover at older carcass sites dropping off considerably in the dry season. Although B. anthracis spore concentrations showed a declining trend with site age for
elephant and zebra carcass sites, these trends were not statistically significant. Over 35 trap months, the video cameras recorded a total of 31,068 videos of which 12,728 were mammals and the rest by wind, rain and birds. Video recordings at elephant carcass sites showed that 21 animal species (ungulate herbivores, carnivores and small mammals) visited these sites. Behaviours displayed by mammals and vultures were recorded, and ranked according to risk of anthrax exposure in descending order as; foraging, bone contact with mouth, smelling, kicking/touching, resting, investigating and walking by. The behaviours displayed, and which were most common, varied among animal species. These findings were compared to previous work on carcass sites from plains zebra (Equus quagga) carcass sites. Both species had significant effects on soil and vegetation at carcass sites, although vegetation responses at elephant carcass sites appear to occur on a longer time lag, in the 2-5 year window, as compared to vegetation at zebra carcass sites peaking the year after death. Anthrax cases of both species showed persistence of B. anthracis spores over time at sites, with no significant difference in spore concentrations between the two species. The biggest difference between the two carcass types was in which species visit these sites and how they interacted with potentially infectious material at the carcass sites. Elephant carcasses have the potential to expose a different suite of host species to anthrax, than observed at plains zebra carcass sites. While zebra are the main host species in Etosha, and most cases in the system are detected in grazing herbivores, large-bodied, wide-ranging animals like elephants that use different habitats than grazing hosts could serve as a bridging host, bringing anthrax into different habitats, and exposing a wider variety of host species.
Description
A thesis submitted in fulfillment of the requirements for the Degree of Master of Science (Biology)
Keywords
Anthrax transmission, Bacillus anthracis, Behavioural activities, Camera-trapping, Elephant carcasses, Herbivorous mammals, Loxodonta africana