Mesobrowser Abundance and Effects on Woody Plants in Savannas

Abstract

Savannas, as defined in Chapter 1, are any formation with a C4 grass understory and a woody canopy layer that has not reached a closed canopy state (Ratnam et al. 2011). Climate, soils, fire, and herbivory are widely recognized as four key drivers of the structure, composition, and functioning of savanna ecosystems. The relative role of these factors, and how they interact at varying spatial and temporal scales, has been the subject of an immense body of ecological literature. Continental‐scale syntheses focused on these drivers show that climate and soils are primary determinants of woody vegetation abundance and species composition in savannas (Sankaran et al. 2008; Lehmann et al. 2014). However, at local spatial scales, fire and herbivory can regulate woody plant abundance below climatically determined maxima and alter plant growth rates, size class distributions, and community composition (e.g. Sankaran et al. 2013; Scholtz et al. 2014). Fire is well known to shape woody vegetation structure and composition in savannas worldwide (Lehmann et al. 2014), while less is known about the role of herbivores and the complex suite of factors that can regulate herbivore abundance and plant defenses (Chapter 15). Given that herbivore management strategies and land use patterns often strongly influence herbivores at local scales, an understanding of the consequences for savanna vegetation is needed to guide management and land use planning policies. The most dramatic and visible effects of herbivores on woody plants occur in African savannas with elephants (Loxodonta africana), particularly where elephants have increased in density following movement restrictions or where iconic stands of large trees are experiencing elephant‐induced mortality (e.g. Skarpe et al. 2004; Chapter 17 this volume). However, savannas support a guild of smaller browsing and mixed‐feeding mammalian herbivores that vary widely in abundance, diversity, feeding niches, and Mesobrowser Abundance and Effects on Woody Plants in Savannas David J. Augustine, Peter Frank Scogings, and Mahesh Sankaran