African savannas support a large diversity and abundance of wild and domestic herbivores (Smithers 1983; Coppock et al. 1986; Scholte et al. 2007;Homewood 2008; Fynn and Bonyongo 2011). African savannas are characterized by patchy rainfall, frequent droughts and extended dry seasons culminating in a stressful hot late dry season (Ellis and Swift 1988;Fynn et al. 2014) with grasses drying out and protein and energy concentrations falling well below maintenance levels (Ellis and Swift 1988; Owen-Smith 2008; Hopcraft et al. 2010). The ability of herbivore populations to adapt to this spatial and temporal variability in the quantity and quality of forage depends upon their ability to move spatially and access green areas associated with patchy rainfall events (Fryxell et al. 2005), as well as migrate between optimal wet- and dry-season habitats, which determines their population productivity and stability (Scoones 1995; Illius and O’Connor 2000; Owen- Smith 2002, 2004; Fynn 2012).
Functional heterogeneity of wet- and dry-season habitats in African savannas is distributed as a continuum of different plant communities on soil moisture-driven productivity gradients from low productivity but high-quality habitats in moisture-limited regions to high productivity but low quality habitats in the wettest regions of the gradient (Maddock 1979; Hopcraft et al. 2010; Fynn and Bonyongo 2011; Fynn et al. 2014). Soil moisture gradients may vary in length from landscape-scale topo-edaphic graphic gradients (e.g. Bell 1970) to regional-scale rainfall gradients (Maddock 1979; Fynn and Bonyongo 2011) or regional-scale contrasts between drylands and extensive wetlands (e.g. Fryxell and Sinclair 1988; Howell et al. 1988; Scholte et al. 2007; Fynn et al. 2014). In contrast to temperate environments (North America and Europe) where plant growth and greenness is limited by temperature during the winter, plant growth and greenness in African savannas is generally limited by soil moisture over the dry season (Knapp et al. 2006). Grass growth and green leaf production over the dry season is not possible in dryland habitats but is characteristic of wetland habitats (floodplains, swamps and dambos), owing to their shallow water table (Vesey-FitzGerald 1960; Roberts 1988; Pamo 1998; Fynn et al. 2014). Thus productive wetland habitats can be critical sources of forage for herbivores over the dry season.
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