Levels of aloe mortality with and without elephants in the Thicket Biome of South Africa

Abstract

Studies concerning the influence of African elephants (Loxodonta africana) on vegetation have produced contradictory results; some show minimal or no effect while others report significant elephant-induced effects. Elephants are generalist megaherbivores but will selectively feed from preferred plant species. We investigated the mortality of aloe plants (highly preferred food items for elephants) at five sites with elephants (treatment) and five paired sites without elephants (control) in the Eastern Cape Province of South Africa. A significantly higher proportion of aloes were dead at treatment sites and significantly more aloes that had lost their crown (headless) were found at treatment sites compared with controls. We conclude that although the proportions of dead aloes at treatment sites were significantly higher, it remains unclear whether there is a need to be concerned with the potential small-scale extinction of aloes from parts of the Eastern Cape Province. The observed mortality may merely be an artefact of the loss of large herbivores through disease (e.g. rinderpest) and hunting in the past. RésuméDes études de l'influence des éléphants africains (Loxodonta africana) sur la végétation ont produit des résultats contradictoires: certaines montrent un effet minimal, voire nul, alors que d'autres signalent des effets significatifs. Les éléphants sont des grands herbivores généralistes, mais ils se nourrissent sélectivement de certaines parties des plantes. Nous avons étudié la mortalité de plants d'aloès (nourriture fortement privilégiée par les éléphants) à cinq sites avec éléphants (traitement) et à cinq autres sites sans éléphant (témoins), dans la Province du Cap oriental, en Afrique du Sud. Il y avait une proportion significativement plus grande d'aloès morts dans les sites du traitement, et ils étaient significativement plus nombreux à avoir perdu leur couronne (étêtés) que dans les sites témoins. Nous concluons que, bien que la proportion d'aloès morts soit significativement plus grande dans les sites du traitement, il n'est pas évident de savoir s'il faut s'inquiéter de cette possible extinction à petite échelle des aloès dans certaines parties de la Province du Cap oriental. La mortalité observée peut bien être simplement un artefact de la perte des grands herbivores en raison de maladies (ex. la peste bovine) et de la chasse dans le passé.

African aloe ecology: A review

“…Studies on the impact of elephant browsing on tall single-stemmed aloes have shown that there is generally a higher proportion of dead aloes at sites where elephants are present than control sites where they are absent Bernard, 2008, 2009;Penzhorn et al, 1974). Elephants either break off the aloe's crown, or push the whole plant over to access the succulent apical meristem, both of which usually result in plant death (Parker and Bernard, 2009). Furthermore, Breebart et al (2002) showed that goat browsing has a similar impact on A. ferox, with only 2% of individuals surviving after utilization by Boer goats.…”

“…Aloes can be important indicator species in particular vegetation types, such as Aloe africana and A. ferox in the Thicket Biome of the Eastern Cape (Parker and Bernard, 2009). When thicket vegetation becomes heavily browsed by livestock or elephants and/or other herbivores, aloes are exposed to further exploitation by herbivores, as well as physical factors such as strong winds that could increase their mortality (D.M.…”

“…Elephants have a predilection for consuming aloes and will selectively feed on them if available Bernard, 2008, 2009). Greater kudu are also known to browse aloe leaves; baboons feed on the leaves and flowers, while porcupine and black rhinoceros are known to uproot entire plants (Parker and Bernard, 2009).…”

“…davyana and A. dichotoma is also thought to be a source of moisture and carbohydrates for honeybees during winter (Human and Nicolson, 2008;Tribe and Johannsmeier, 1996). The dry inflorescences of aloes are used by a range of hymenopteran insects from four families (Anthophoridae, Chrysididae, Megachilidae and Sphecidae) as nest cavities (Parker and Bernard, 2009). Some mammals, particularly elephants, utilize aloe leaves as a food source.…”

Impending local extinction of Aloe ferox Mill. populations in the absence of elephants and black rhinos?

“…The physiognomy of aloes makes them highly susceptible to herbivory (Breebaart, Bhikraj & O'Connor, ; O'Connor, Goodman & Clegg, ; Shackleton & Gambiza, ; Parker & Bernard, , ). For example, indigenous herbivores such as bushbuck, warthogs, vervet monkeys, black rhino, eland, greater kudu, porcupines and chacma baboons have been described to have negative effects on aloe populations (Breebaart, Bhikraj & O'Connor, ; Shackleton & Gambiza, ; Parker & Bernard, , ). Elephants in particular may cause a great deal of damage, and if present at high densities, they can be a major contributor to large declines in aloe populations (Penzhorn, Robbertse & Olivier, ; Barratt & Hall‐Martin, ).…”