Impacts of foraging behavior [sic] by Cape porcupines and their effects on nutrient cycling in mesic savannas.
Kraai, Unathi Masiobi.
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Through feeding and associated activities, herbivores play a major role in determining the structure of savannas. The Cape porcupine (Hystrix africaeaustralis) is a semi-fossorial, large (ca. 12 kg) herbivorous rodent with a generalist foraging strategy that feeds on plant parts occurring above- and below ground. Subterranean foraging by porcupine may influence biotic and abiotic processes in that area. The extent of soil and vegetation perturbation may be pervasive on the landscape so that these animals may be considered as ecosystem engineers. The digging activities of ecosystem engineers are significant as they influence soil properties (e.g. nutrient cycling) including germination of trapped seeds and establishment of seedlings. These changes may occur at small and large scale on a landscape. The utilisation of woody vegetation and ecosystem engineering by such animals, particularly by shy and nocturnal species, is understudied in African savannas. The study was aimed at: (1) quantifying the extent of herbivory by the porcupines on target trees during the wet and dry season in three mesic savanna sites, and (2) evaluating the effects of Cape porcupines’ digging on nutrient cycling (total carbon and total nitrogen) and quantify establishment of vegetation on the mounds. Sampling was undertaken at three mesic savanna sites in South Africa: (i) Roodeplaat Farm in Gauteng Province; (ii) Goss Game Farm; and (iii) Bisley Valley Nature Reserve, both in KwaZulu-Natal Province. I used 30 m × 30 m plots to quantify porcupine foraging holes and bark damage on adult trees at Roodeplaat and Goss while 10 m × 10 m plots were used at Bisley where porcupines foraged on seedlings and saplings of woody plants. I also collected porcupine dung samples over the dry and wet season for micromorphological examination of porcupine diet. I collected soil samples from the mound soil of foraging holes and from adjacent locations within 0.5 m of the hole for analysis of amounts of total carbon and total nitrogen. Measurements of foraging holes comprised of two perpendicular diameters on the soil surface and the maximum depth. Porcupines utilised different tree species of various sizes at the three sites while targeting specific parts of these trees. At Roodeplaat, porcupines targeted Vachellia robusta on which they consumed the trunk part immediately below ground, whereas at Bisley, roots and the lower trunk of V. nilotica seedlings and saplings were utilised, also through digging holes while the bark of the lower trunk (up to 0.7 m) of Spirostachys africana trees was stripped off at Goss. I found that 70% of young V. nilotica trees in or adjacent to holes in Bisley were scarred or destroyed as a result of porcupine feeding on them, while 16% of S. africana trees were wounded at Goss. Only 7% of V. robusta trees were damaged at Roodeplaat. In Bisley, I found that grasses and forbs established faster on the mound than on the surrounds, i.e. seedlings germinated first on the mound than the adjacent not disturbed soil. I also found that foraging holes provide shelter to other animals especially those from the arthropod group e.g. spiders. Amounts of total carbon and total nitrogen were similar between the mounds and undug soil. These findings are discussed in terms of nutrient cycling through digging, breaking down of plant parts and herbivore-induced mortality of main tree species. I argue that tree thinning from ringbarking by porcupine through their foraging activities ameliorates woody plant encroachment in mesic savannas.