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University of Nevada, Reno (1999)

Acacia woodland ecology and elephants in northern Botswana

Barnes, Myra Evelyn

Titre : Acacia woodland ecology and elephants in northern Botswana

Auteur : Barnes, Myra Evelyn

Etablissement de soutenance : University of Nevada, Reno

Grade : Doctor of Philosophy in Ecology, Evolution and Conservation Biology 1999

The reduced number of mature Acacia erioloba trees in riparian areas where wildlife concentrate during the dry season in northern Botswana has been attributed to elephants. I used field data to determine the biotic and abiotic factors limiting seed germination, seedling establishment, and growth of A. erioloba to reproductive maturity from 1994–1997. Changes in species composition and conservation concerns in wildlife areas with A. erioloba mortality were investigated. Predators of Acacia seeds include insects, primates, elephants Loxodonta africana, ungulates and rodents. Seeds dispersed in elephant dung germinated more quickly than seeds from trees. No new seedlings established in drought or average rainfall years. More seedlings emerged in wet years but 85% of seedlings died from dessication within two months and none survived a year. Established A. erioloba seedlings were reversed to ground level by fire or browsing elephants but most survived, producing coppice growth from the roots. Small ungulate browsing suppressed growth, keeping seedlings vulnerable to fire and delaying growth to reproductive maturity. Mortality and severe browsing damage by elephants intensified in the Savuti area of Chobe National Park after the installation of two additional artificial water points in 1995. Elephants pushed over or broke main stems on A. erioloba trees <15 cm in diameter. Giraffe browsing suppressed mean height increase of trees <5.5 m and influenced tree shape. The number of mature A. erioloba trees decreased at study areas with surface water during the dry season. As canopy trees decreased, smaller and shrubby multi-stemmed species filled the gaps between canopy trees, with no apparent loss in main-stem coverage per hectare. However, most of these species are leafless during the dry season, providing forage only until the mid dry season and little shade. During the dry season, elephant densities of 7–12/km2 have been recorded along the Chobe and Linyanti Rivers. However, annual population increases of 6%, even during drought years, suggest elephants are not nutritionally stressed. Before elephants reach an ecological carrying capacity in northern Botswana, vegetation changes may alter the aesthetic appearance of riparian woodlands and influence the number and abundance of other species.

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Page publiée le 21 novembre 2017, mise à jour le 16 décembre 2019