Woody Plant-Herbivore Interactions in Semi-Arid Savanna Ecosystems

Abstract

Savannas cover more than ten percent of the world’s land surface and more than fifty percent of Africa, providing browse to millions of mammalian herbivores (Scogings & Mopipi, 2008). Although herbivory is a major driver of ecosystem functioning in semi-arid African savannas plant-herbivore interactions are poorly understood (Skarpe, 1992; Scholes, 1997; Scogings, 2003). African savannas and large herbivores coevolved, with woody plants developing defences against herbivory (Du Toit, 2003). The herbivores have in turn evolved counter measures against the plant defences. Large herbivores counteract the effects of plant defence by selective foraging, fragmentation of intact plant tissues, microbial fermentation and expanded guts for microbial breakdown, whereas plants protect themselves through morphological, structural and chemical adaptations (Borchard et al., 2011). African savanna ecosystems under heavy browsing have few hardy woody species that are resistant to or are defended against defoliation. Cornell and Hawkins (2003) suggested that plants acquire better defences with time which herbivores in turn learn to partly or fully overcome. Hartley & Jones (1997) found woody plants to be able to live in environments where herbivores were common because of their ability to resist or recover from intense herbivore pressure. The varying defences that plants exhibit is a reflection of the diversity of herbivores and abiotic conditions. Plant defences exert selective pressure on mammalian herbivores, with the result that many have developed mouthparts and digestive systems that facilitate the use of particular plant types. The chemical defences of terrestrial plants reflects in part the biochemical evolution of early land plants and the problems those plants encountered.