Oecologia (2011) 165:1095–1107 DOI 10.1007/s00442-010-1899-3 E C O S Y S T E M E C O L O G Y - O R I G I N A L P A P E R Large herbivores may alter vegetation structure of semi-arid savannas through soil nutrient mediation Cornelis van der Waal • Ada Kool • Seline S. Meijer • Edward Kohi • Ignas M. A. Heitko¨nig • Willem F. de Boer • Frank van Langevelde • Rina C. Grant • Mike J. S. Peel • Rob Slotow • Henrik J. de Knegt • Herbert H. T. Prins • Hans de Kroon Received: 5 May 2010 / Accepted: 23 December 2010 / Published online: 12 January 2011 The Author(s) 2011. This article is published with open access at Springerlink.com Abstract In savannas, the tree–grass balance is governed by water, nutrients, ﬁre and herbivory, and their interac- tions. We studied the hypothesis that herbivores indirectly affect vegetation structure by changing the availability of soil nutrients, which, in turn, alters the competition between trees and grasses. Nine abandoned livestock holding-pen areas (kraals), enriched by dung and urine, were contrasted with nearby control sites in a semi-arid savanna. About 40 years after abandonment, kraal sites still showed high soil concentrations of inorganic N, Communicated by Jason Kaye. In cooperation with: N. M. Knox, A. K. Skidmore, E. Mwakiwa, Y. Pretorius, S. E. van Wieren. C. van der Waal, A. Kool, and S. S. Meijer authors contributed equally. C. van der Waal A. Kool S. S. Meijer E. Kohi I. M. A. Heitko¨nig W. F. de Boer F. van Langevelde H. J. de Knegt H. H. T. Prins Resource Ecology Group, Wageningen University, Droevendaalsesteeg 3a, 6708 PB Wageningen, The Netherlands S. S. Meijer UCD Forestry, Agriculture & Food Science Centre, University College Dublin, Belﬁeld, Dublin 4, Ireland S. S. Meijer World Agroforestry Centre, Nairobi, Kenya R. C. Grant Scientiﬁc Services, Kruger National Park, Private Bag X402, Skukuza 1350, South Africa M. J. S. Peel Agricultural Research Council-Range and Forage Institute, PO Box 13054, Nelspruit 1200, South Africa extractable P, K, Ca and Mg compared to controls. Kraals also had a high plant production potential and offered high quality forage. The intense grazing and high herbivore dung and urine deposition rates in kraals ﬁt the accelerated nutrient cycling model described for fertile systems else- where. Data of a concurrent experiment also showed that bush-cleared patches resulted in an increase in impala dung deposition, probably because impala preferred open sites to avoid predation. Kraal sites had very low tree densities compared to control sites, thus the high impala dung deposition rates here may be in part driven by the open structure of kraal sites, which may explain the persistence of nutrients in kraals. Experiments indicated that tree seedlings were increasingly constrained when competing with grasses under fertile conditions, which might explain the low tree recruitment observed in kraals. In conclusion, large herbivores may indirectly keep existing nutrient R. Slotow Biological and Conservation Sciences, Westville Campus, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South Africa H. de Kroon Institute for Water and Wetland Research, Department of Experimental Plant Ecology, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands C. van der Waal (&) Agri-Ecological Services, PO Box 28, Omaruru 9000, Namibia e-mail: corwaal@gmail.com 123