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This study determined the abundance, density and population structure of the marula tree, Sclerocarya birrea, in three game reserves in South Africa, and assessed patterns and amounts of new and cumulative impact of elephants. Elephant feeding was very patchy so several attributes of individual trees, sampled transects and communities that might influence elephant herbivory were investigated. The incidence and type of elephant impact (bark, branch or stem breakage) were significantly related to tree diameter, but not to fruiting nor proximity to roads. At the transect level, elephant impact was influenced by density of marula trees, but was not influenced by proximity to roads, nor proportion of marula trees bearing fruits in the vicinity. At the community level, elephant impact was higher on reserves with higher total marula densities. Fourfold differences in elephant densities (0.08–0.30 elephants km−2) did not explain marula consumption: the percentage of trees with branch damage was similar across reserves and bark damage was inversely proportional to elephant density. Variation across reserves may reflect local and landscape-level marula tree abundance, differences in alternative food plants and individual feeding habits. The recorded levels of impact appeared to be sustainable because mortality rates were low, affected trees often recovered, and small trees were not preferentially preyed upon. Resume Cette etude determine l'abondance, la densite et la structure de la population de l'arbre a Marula, Sclerocarya birrea, dans trois reserves de faune sud-africaines et evalue le schema et la totalite des impacts nouveaux et successifs des elephants. Les elephants se nourrissaient ca et la, de sorte que l'on a etudie divers attributs des arbres pris individuellement, des transects echantillons et des communautes, qui pouvaient influencer le caractere herbivore des elephants. L'incidence et le type d'impact (ecorces, branches ou jeunes pousses) etaient significativement lies au diametre de l'arbre, mais non a la fructification, ni a la proximite des routes. Au niveau du transect, l'impact des elephants etait influence par la densite des arbres a Marula, mais pas par la proximite des routes, ni par la proportion d'arbres a Marula en fruits dans le voisinage. Au niveau de la communaute, l'impact des elephants etait plus eleve dans les reserves qui comptaient la plus forte densite totale d'arbres a Marula. Une difference de 1 a 4 dans la densite des elephants (0.08-0.30/km2) n'explique pas la consommation de marula : le pourcentage d'arbres presentant des branches endommagees etait similaire dans toutes les reserves, et les dommages causes aux ecorces etaient inversement proportionnels a la densite des elephants. La variation observee entre les reserves pourrait refleter l'abondance des arbres a Marula tant locale que liee au paysage, des differences dans l'abondance de nourriture vegetale alternative et les habitudes alimentaires individuelles. L'importance de l'impact releve semblait etre soutenable parce que le taux de mortaliteetait faible, que les arbres touches recuperaient souvent et que les petits arbres n'avaient pas souvent la preference des elephants. |
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