1096 hotspots such as abandoned kraals structurally open by maintaining a high local soil fertility, which, in turn, con- strains woody recruitment in a negative feedback loop. The maintenance of nutrient hotspots such as abandoned kraals by herbivores contributes to the structural heterogeneity of nutrient-poor savanna vegetation. Keywords competition Bush encroachment Tree–grass Nutrient hotspot Seedling Predation Introduction An important theme in savanna ecology is how resources, such as water and nutrients, and disturbances, such as ﬁre and herbivory, determine the structure of the vegetation, i.e., the balance between woody and herbaceous biomass (Scholes and Archer 1997; Sankaran et al. 2004, 2005, 2008). Variation in the tree–grass balance largely deter- mines savanna functioning, because it affects animal pro- duction and shapes animal assemblages (Scholes and Archer 1997; Sankaran et al. 2004, 2005). Direct effects of large herbivores (hereafter herbivores) on savanna structure (e.g., browsers decreasing woody cover) are well docu- mented (Scholes and Archer 1997; van Langevelde et al. 2003). Indirect effects of herbivores, such as herbivores reducing grass fuel loads which in turn suppresses the impact of ﬁre on the tree layer (van Langevelde et al. 2003; Holdo et al. 2007; but see Hanan et al. 2008), are less well studied in spite of their potential to inﬂuence savanna structure (Waldram et al. 2008). Similarly, herbivores may change the availability of soil nutrients, which, in turn, alter the competitive balance between functionally differ- ent growth forms (Wilson and Tilman 1993; Bokdam 2001). Such an indirect effect has to our knowledge not yet been demonstrated for trees and grasses in savanna-grass- land systems. This is in spite of reports of strong herbivore effects on local soil nutrient availability (Augustine et al. 2003) and signiﬁcant correlations of woody cover with soil nutrient availability (Sankaran et al. 2008). In this paper, we address the question whether the effects of large her- bivores on soil nutrient availability affect the tree–grass balance in a semi-arid savanna. Large herbivores alter soil nutrient availability for plants through changes in soil nutrient cycling rates and spatial redistribution of soil nutrients (Ritchie et al. 1998; Bardgett and Wardle 2003). In a comprehensive review, Bardgett and Wardle (2003) concluded that differences in soil fer- tility determine the direction of nutrient-mediated herbi- vore effects on nutrient cycling rates and plant productivity; positive net effects occur under fertile soil conditions and negative net effects under infertile soil conditions. Enhanced local nutrient cycling mediated by 123 Oecologia (2011) 165:1095–1107 large herbivores has been invoked to explain the formation and maintenance of grazing lawns in African savannas (McNaughton et al. 1997). In addition, animals spatially redistribute nutrients if they take in nutrients in one area but excrete nutrients in resting or sleeping areas (Singer and Schoenecker 2003; Moe and Wegge 2008). Substantial amounts of nutrients can be redistributed by large herbi- vores in this way (Jewell et al. 2007). In a modeling study, de Mazancourt et al. (1998) showed that, although enhanced nutrient cycling contributes to increased nutrient availability in the short-term, a net import of nutrients is crucial for the long-term maintenance of nutrient-rich, productive grazing areas. In Laikipia, Kenya, Augustine et al. (2003) found that impala were responsible for a net inﬂux of nutrients into abandoned livestock holding-pen areas, as they deposited more nutrients than were con- sumed and exported. Augustine (2004) suggested that this was the result of anti-predator behaviour of impala that preferred the treeless abandoned sites. These patches offered improved visibility in the generally wooded study area (Augustine 2004) and probably improved vigilance against predators (Riginos and Grace 2008). There are various hypotheses regarding the effects of nutrient availability on woody recruitment in arid savanna and grassland systems. For example, Schlesinger et al. (1996) claim that tree recruitment is enhanced in so-called ‘islands of fertility’. In support, Gillson and Ekblom (2009) suggest that an increase in nitrogen availability resulted in an increase in woody vegetation in the Kruger National Park, South Africa. Heisler et al. (2004) did not ﬁnd any effect of nitrogen availability on shrub expansion, while Sankaran et al. (2008) found in a meta-analysis that tree cover was negatively correlated with soil N availability in African savannas. As a mechanism explaining the latter relationship, it has been proposed that co-existing grasses increasingly outcompete woody seedlings under fertile conditions, which feed back to a more open vegetation structure in fertile areas (Sankaran et al. 2008; Augustine and McNaughton 2004). There is experimental evidence supporting this hypothesis (Kraaij and Ward 2006; van der Waal et al. 2009). Whichever is correct, herbivore-medi- ated changes in soil nutrients may inﬂuence the structure of savanna vegetation, which, if tree recruitment is con- strained in fertile areas, may attract further nutrient imports by herbivores preferring open patches in a positive feed- back loop. To our knowledge, a link between savanna vegetation structure and nutrient redistribution by wild herbivores has not yet been demonstrated, but may be an important mechanism maintaining spatial heterogeneity in savanna systems (Blackmore et al. 1990; Augustine and McNaughton 2004). To address these issues, we studied a ‘natural’ experi- ment in a semi-arid savanna, where commercial livestock