Herbivory and drought interact to enhance spatial patterning and diversity in a savanna understory
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The combination of abiotic stress and consumer stress can have complex impacts on plant community structure. Effective conservation and management of semi-arid ecosystems requires an understanding of how different stresses interact to structure plant communities. We explored the separate and combined impacts of episodic drought, livestock grazing, and wild ungulate herbivory on species co-occurrence and diversity patterns in a relatively productive, semi-arid Acacia savanna. Specifically, we analyzed 9 years of biannual plant community data from the Kenya long-term exclosure experiment, a broad-scale manipulative experiment that has excluded different combinations of large mammalian herbivores from 18 4-ha plots since 1995. During droughts, we observed low species diversity and random species co-occurrence patterns. However, when rain followed a major drought, areas exposed to moderate cattle grazing displayed high species diversity and evidence of significant species aggregation. These patterns were not apparent in the absence of cattle, even if other large herbivores were present. To explore possible mechanisms, we examined patterns separately for common and rare species. We found that aggregation patterns were likely driven by rare species responding similarly to the availability of open micro-sites. Our results indicate that in a productive, fire-suppressed savanna, the combination of periodic drought and moderate cattle grazing can enhance plant biodiversity and fine-scale spatial heterogeneity by opening up space for species that are otherwise rare or cryptic. Our findings also emphasize that domestic herbivores can have significantly stronger impacts on plant community dynamics than wild herbivores, even in an ecosystem with a long history of grazing.
KeywordsAbiotic stress Consumer stress Plant–herbivore interactions Species coexistence Competition-colonization tradeoff
For the collection of these data, we are grateful to Frederick Erii, John Lochukuya, Jackson Ekadeli, Mathew Namoni, Patrick Etelej and David Kinyua. We also thank the Mpala Research Centre and its staff for their logistical support. Neil Willits provided valuable statistical advice. We received helpful comments on the manuscript from V. Eviner, Young Lab 2010–2011 and Eviner Lab 2011. The KLEE plots were built and maintained by grants from the James Smithson Fund of the Smithsonian Institution (to A.P. Smith), the National Geographic Society (4691-91), the National Science Foundation (LTREB BSR-97-07477, 03-16402, and 08-16453), and the African Elephant Program of the US Fish and Wildlife Service (98210-0-G563) (to T.P. Young). Additional financial support was provided by an NSF GRF (to L.M. Porensky), a Smithsonian Postdoctoral Fellowship (to S.E. Wittman), and a Princeton University Council on Science and Technology Postdoctoral Fellowship (to C. Riginos). This research complies with the current laws of the country (Kenya) in which the research was performed.
- Gotelli NJ, Entsminger GL (2006) EcoSim: null models software for ecology. Ver. 7.0. Acquired Intelligence Inc. and Kesey-Bear. http://www.garyentsminger.com/ecosim/index.htm
- Gotelli NJ, Graves GR (1996) Null models in ecology. Smithsonian Institution Press, WashingtonGoogle Scholar
- IPCC (2012) Managing the risks of extreme events and disasters to advance climate change adaptation. In: Field CB, Barros V, Stocker TF, Qin D, Dokken DJ, Ebi KL, Mastrandrea MD, Mach KJ, Plattner G.-K., Allen SK, Tignor M, Midgley PM (eds) A special report of working groups I and II of the intergovernmental panel on climate change. Cambridge University Press, CambridgeGoogle Scholar
- Osnas EE, Ankney CD (2003) Null models of North American prairie duck communities: local habitat conditions and temporal scale influence community patterns. Evol Ecol Res 5:913–932Google Scholar
- Sala OE, Chapin FS, Armesto JJ, Berlow E, Bloomfield J, Dirzo R, Huber-Sanwald E, Huenneke LF, Jackson RB, Kinzig A, Leemans R, Lodge DM, Mooney HA, Oesterheld M, Poff NL, Sykes MT, Walker BH, Walker M, Wall DH (2000) Global biodiversity scenarios for the year 2100. Science 287:1770–1774PubMedCrossRefGoogle Scholar
- Silvertown J, Smith B (1989) Mapping the microenvironment for seed-germination in the field. Ann Bot 63:163–167Google Scholar