Droughts Decouple African Savanna Grazers from Their Preferred Forage with Consequences for Grassland Productivity
- 3 Downloads
Grazing lawn and flammable-tussock grass communities are contrasting resource pools for mammalian grazers in terms of forage quantity and quality. Drought events fundamentally alter forage availability within these communities and therefore should alter herbivore use with repercussions for the recovery and functioning of ecosystems after drought. During and after an intense El Niño drought (2014–2017) in Kruger National Park, South Africa, we addressed two questions: (1) how does herbivore use of different grass types change during a drought and (2) how do these changes affect grass productivity post-drought? We monitored grazer use of three different grass communities (lawn, tussock and burned-tussock) at a landscape scale and measured primary productivity monthly during and post-drought. For the first drought year, grazer numbers were highest on grazing lawn communities. This pattern continued into the second dry growing season, until herbivores finally left the study area. Both lawns and tussock grasslands recovered rapidly after the first good rainfall (productivity > 150 g m−2 per month). However, grazers did not return to feed on the same patches they had frequented pre-drought resulting in grazing lawn grasses self-shading and senescing. Longer droughts have the potential to decouple grazers and grazing lawns with negative impacts on lawn productivity and persistence that could drive the loss of lawns in savanna landscapes and impact mesoherbivore populations. It is clear from our results that grazer effects need to be incorporated into drought frameworks to understand the consequences of droughts for grassland function.
KeywordsAfrican savannas drought fire ecology grazer ecology grazing grazing lawns productivity
We thank Khensani Mkhonto for collecting data in the field. We are grateful to SANParks and Working on Fire for their continued support, particularly Navashni Govender and Tercia Strydom. This work was funded by the USAID/NAS program “Partnerships for Enhanced Engagement in Research” (Sub-Grant 2000004946, Cycle 3) and the Andrew Mellon Foundation. JED was funded by the National Research Foundation Freestanding, Innovation and Scarce Skills Development Fund (Grant No. 94250).
- Anderson TM, Hopcraft JGC, Eby SL, Ritchie M, Grace JB, Olff H. 2010. Landscape-scale analyses suggest both nutrient and antipredator advantages to Serengeti herbivore hotspots. Ecology 91:1519–29. http://www.ncbi.nlm.nih.gov/pubmed/20503883. Accessed 27 Jan 2015.
- Bates D, Mächler M, Bolker BM, Walker SC. 2015. Fitting linear mixed-effects models using lme4. J Stat Softw 325:883–5. http://www.sciencemag.org/cgi/doi/10.1126/science.1176170. Accessed 17 Oct 2017.
- Bond WJ, Keeley JE. 2005. Fire as a global ‘herbivore’: the ecology and evolution of flammable ecosystems. Trends Ecol Evol 20:387–94. http://www.ncbi.nlm.nih.gov/pubmed/16701401. Last accessed 15/06/2011.
- Bonnet O, Fritz H, Gignoux J, Meuret M. 2010. Challenges of foraging on a high-quality but unpredictable food source: the dynamics of grass production and consumption in savanna grazing lawns. J Ecol 98:908–16. http://doi.wiley.com/10.1111/j.1365-2745.2010.01663.x. Last accessed 24/02/2012.
- Codron D, Codron J, Lee-Thorp JA, Sponheimer M, de Ruiter D, Sealy J, Grant R, Fourie N. 2007. Diets of savanna ungulates from stable carbon isotope composition of faeces. J Zool 273:21–9. http://doi.wiley.com/10.1111/j.1469-7998.2007.00292.x. Last accessed 28/06/2011.
- Crawley MJ, Ed. 2012. The R book. West Sussex: Wiley.Google Scholar
- Cromsigt JPGM, Rensburg SJ, Etienne RS, Olff H. 2009. Monitoring large herbivore diversity at different scales: comparing direct and indirect methods. Biodivers Conserv 18:1219–31. http://www.springerlink.com/index/10.1007/s10531-008-9506-1. Last accessed 28/08/2011.
- Hempson GP, Archibald S, Bond WJ, Ellis RP, Grant CC, Kruger FJ, Kruger LM, Moxley C, Owen-Smith N, Peel MJS, Smit IPJ, Vickers KJ. 2015. Ecology of grazing lawns in Africa. Biol Rev 90:979–94. http://rstb.royalsocietypublishing.org/lookup/doi/10.1098/rstb.2015.0309. Accessed 20 Sept 2016.
- Knapp AK, Hoover DL, Blair JM, Buis GM, Burkepile DE, Chamberlain A, Collins SL, Fynn RWS, Kirkman KP, Smith MD, Blake D, Govender N, O’Neal P, Schreck T, Zinn A. 2012. A test of two mechanisms proposed to optimize grassland aboveground primary productivity in response to grazing. J Plant Ecol 5:357–65.CrossRefGoogle Scholar
- Macandza VA, Owen-Smith N, Cross PC. 2004. Forage selection by African buffalo in the late dry season in two landscapes. S Afr J Wildl Res 34:113–21.Google Scholar
- Mc Naughton SJ. 1984. Grazing lawns: animals in herds, plant form, and coevolution. Am Nat 124:863–86. http://www.jstor.org/stable/2461305. Accessed 14 Apr 2011.
- Mc Naughton SJ. 1985. Ecology of a grazing ecosystem: the Serengeti. Ecol Monogr 55:259–94. http://www.jstor.org/stable/1942578. Accessed 19 Jan 2011.
- Mucina L, Rutherford MC, Eds. 2006. The vegetation of South Africa, Lesotho and Swaziland. Pretoria: South African National Biodiversity Institute.Google Scholar
- R Development Core Team. 2016. R: A language and environment for statistical computing, reference index. Version 3.3.1. Vienna, Austria: R Foundation for Statistical Computing. Retrieved from http://ww-w.R-project.org.
- Ratnam JV, Behera SK, Masumoto Y, Yamagata T. 2014. Remote effects of El Nino and Modoki events on the austral summer precipitation of southern Africa. Am Meteorol Soc 27:3802–16.Google Scholar
- Ribeiro PJ, Diggle PJ. 2016. Package ‘geoR’.Google Scholar
- Tainton NM. 1999. Veld management in South Africa. Pietermaritzberg: Natal University Press.Google Scholar
- Venter FJ, Scholes RJ, Eckhardt HC. 2003. The abiotic template and its associated vegetation pattern. In: Du Toit JT, Rogers KH, Biggs HC, Eds. The Kruger experience: ecology and management of savanna heterogeneity. Washington: Island Press. p 83–129.Google Scholar
- Whyte IJ, Joubert SCJ. 1988. Blue wildebeest population trends in the Kruger National Park and the effects of fencing. S Afr J Wildl Res 18:78–87.Google Scholar