1106 contributes to the structural heterogeneity of the study area. In nutrient-poor savannas, fertile patches, e.g., beneath large trees, termite mounds, sodic patches and abandoned livestock management areas, provide important sources of scarce nutrients to herbivores (Augustine et al. 2003; Grant and Scholes 2006; Mourik et al. 2007; Treydte et al. 2007; Ludwig et al. 2008; Treydte et al. 2008). In addition, if the anti-predator behavior of large herbivores indeed causes the maintenance of abandoned kraal sites as structurally open, nutrient hotspot areas (Fig. 3), then this is an example of a top–down cascading effect in which predators affect consumers, which in turn feed back to primary producers via control over a limiting soil resource. Such an ecological cascade may not be limited to abandoned kraal sites in nutrient-poor savannas but may also operate in other systems, upholding resource heterogeneity. Acknowledgments We thank the management of the APNR for allowing us to work in the reserves and the Dr. Marie Luttig Trust for providing accommodation. We particularly thank Colin Rowles, Mrs. S. Blatherwick, Carl van den Berg, and Marius Fuls for logistical assistance during the project. Moagi Keretetse, Wim Braakhekke, Jan Bokdam and Claudius van de Vijver are thanked for useful scientiﬁc input. Many thanks to Reis Manhique, Floris van Beest and Lemson Monareng for help in the ﬁeld. We also thank Shell–SA for a fuel donation and Omnia for donating fertilizer. The research was funded by NWO–WOTRO (grant no. W01.65.216, C.v.d.W.) and the DELTA fund (A.K.). A.K. is also indebt to Peter Kool and Carin van Drunen for ﬁnancial support. Financial support to S.M. was provided by a grant from the Stichting Dr. Hendrik Muller’s Vaderlandsch Fonds. Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which per- mits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited. References Anderson et al (2010) Ecology’s cruel dilemma, phylogenetic trait evolution and the assembly of Serengeti plant communities. J Ecol (in press) Anonymous (1990) Handbook of standard soil testing methods for advisory purposes. Soil Science Society of South Africa, Pretoria Augustine DJ (2003) Long-term, livestock-mediated redistribution of nitrogen and phosphorus in an East African savanna. J Appl Ecol 40:137–149 Augustine DJ (2004) Inﬂuence of cattle management on habitat selection by impala on central Kenyan rangeland. J Wildl Manage 68:916–923 Augustine DJ, McNaughton SJ (2004) Regulation of shrub dynamics by native browsing ungulates on East African rangeland. J Appl Ecol 41:45–58 Augustine DJ, McNaughton SJ (2006) Interactive effects of ungulate herbivores, soil fertility, and variable rainfall on ecosystem processes in a semi-arid savanna. Ecosystems 9:1242–1256 Augustine DJ, McNaughton SJ, Frank DA (2003) Feedbacks between soil nutrients and large herbivores in a managed savanna ecosystem. Ecol Appl 13:1325–1337 123 Oecologia (2011) 165:1095–1107 Bardgett RD, Wardle DA (2003) Herbivore-mediated linkages between aboveground and belowground communities. Ecology 84:2258–2268 Belsky AJ, Amundson RG (1986) Sixty years of successional history behind a moving sand dune near Olduvai Gorge, Tanzania. Biotropica 18:231–235 Bigalke RC (2000) Functional relationships between protected and agricultural areas in South Africa and Namibia. In: Prins HHT, Groothuis JG, Dolan TT (eds) Wildlife conservation by sustain- able use. Kluwer, Boston, pp 169–202 Blackmore AC, Mentis MT, Scholes RJ (1990) The origin and extent of nutrient-enriched patches within a nutrient-poor savanna in South-Africa. J Biogeogr 17:463–470 Bokdam J (2001) Effects of browsing and grazing on cyclic succession in nutrient-limited ecosystems. J Veg Sci 12:875–886 Bransby DI, Tainton NM (1977) The disc pasture meter: possible applications in grazing management. Afr J Range For Sci 12:115–118 Coates Palgrave K, Coates Palgrave M, Drummond RB, Moll EJ (2002) Trees of Southern Africa. Struik, Cape Town Coetsee C, February EC, Bond WJ (2008) Nitrogen availability is not affected by frequent ﬁre in a South African savanna. J Trop Ecol 24:647–654 Cohn EJ, van Auken OW, Bush JK (1989) Competitive interactions between Cynodon dactylon and Acacia smallii seedlings at different nutrient levels. Am Midl Nat 121:265–272 Davis MA, Wrage KJ, Reich PB (1998) Competition between tree seedlings and herbaceous vegetation: support for a theory of resource supply and demand. J Ecol 86:652–661 De Mazancourt C, Loreau M, Abbadie L (1998) Grazing optimization and nutrient cycling: when do herbivores enhance plant produc- tion? Ecology 79:2242–2252 Eckhardt H, van Wilgen BW, Biggs HC (2000) Trends in woody vegetation cover in the Kruger National Park, South Africa, between 1940 and 1998. Afr J Ecol 38:108–115 Estes RD (1992) The behavior guide to African mammals. University of California Press, Los Angeles Fornara DA, Du Toit JT (2008) Responses of woody saplings mammalian herbivory in an exposed to chronic African savanna. Ecoscience 15:129–135 Gibbs Russell GE, Watson L, Koekemoer M, Smook L, Barker NP, Anderson HM, Dallwitz MJ (1990) Grasses of southern Africa. Mem Bot Surv S Afr 58:1–437 Gillson L, Ekblom A (2009) Resilience and thresholds in savannas: nitrogen and ﬁre as drivers and responders of vegetation transition. Ecosystems 12:1189–1203 Goheen JR, Keesing F, Allan BF, Ogada DL, Ostfeld RS (2004) Net effects of large mammals on Acacia seedling survival in an African savanna. Ecology 85:1555–1561 Grant CC, Scholes MC (2006) The importance of nutrient hot- spots in the conservation and management of large wild mammalian herbivores in semi-arid savannas. Biol Conserv 130:426–437 Hanan NP, Sea WB, Dangelmayr G, Govender N (2008) Do ﬁres in savannas consume woody biomass? A comment on approaches to modeling savanna dynamics. Am Nat 171:851–856 Heisler JL, Briggs JM, Knapp AK, Blair JM, Seery A (2004) Direct and indirect effects of ﬁre on shrub density and aboveground productivity in a mesic grassland. Ecology 85:2245–2257 Hejcman M, Klaudisova M, Stursa J, Pavlu V, Schellberg J, Hejcmanova P, Hakl J, Rauch O, Vacek S (2007) Revisiting a 37 years abandoned fertilizer experiment on Nardus grassland in the Czech Republic. Agric Ecosyst Environ 118:231–236 Holdo RM, Holt RD, Coughenour MB, Ritchie ME (2007) Plant productivity and soil nitrogen as a function of grazing, migration and ﬁre in an African savanna. J Ecol 95:115–128