Does grazer identity matter? Examining the effects of three guilds of herbivores on savanna herbaceous primary productivityGrace K. Charles1,2, Lauren M. Porensky2,3, Corinna Riginos2,4, Kari E. Veblen2,5 and Truman P. Young1,21Department of Plant Sciences, University of California, Davis, CA 95616; 2Mpala Research Centre, PO Box 555, Nanyuki 10400 Kenya; 3USDA-ARS Rangeland Resources Research Unit, 1701 Centre Ave, Fort Collins, CO 80526; 4 Department of Zoology and Physiology, University of Wyoming, Laramie, WY, 82071; 5Department of Wildland Resources and Ecology Center, Utah State University, Logan, UT

OverviewGrazing intensity (driven by cattle

presence) stimulates mean herbaceous productivity

• The herbaceous community compensated for increasing herbivore pressure by increasing ANPP.

• Compensation is hypothesized to have evolved as a strategy to limit reductions in plant fitness.

• Additional compensation mechanisms include increased nutrient cycling or plant release from self-shading.

• Wildlife and cattle can have unique, additive, and interactive effects on productivity

• Mean productivity, measured both via productivity cages and NDVI, generally increased in response to increasing herbivore biomass-density – driven primarily by cattle presence

• Variability in ANPP and NDVI were mostly driven by specific types of herbivores, with wild herbivores constraining variability more than cattle

• Replacing wildlife with cattle at similar biomass-densities could lead to similarly productive, but more heterogeneous rangelands

AcknowledgementsWe thank the KLEE crew (F. Erii, J. Lochukuya, J. Ekadeli and M. Namoni) for their excellent field assistance. The KLEE exclosure plots were built and maintained by grants from the James Smithson Fund of the Smithsonian Institution (to A.P. Smith), The National Geographic Society, The National Science Foundation and the African Elephant Program of the U.S. Fish and Wildlife Service (to T.P. Young, C. Riginos, and K.E. Veblen). This project was partially funded by NSF graduate research fellowships (to L.M.P and G.K.C.) and a NSF DDIG grant (to L.M.P).

The Kenya Long-term Exclosure Experiment (KLEE)

Wild herbivores and livestock share the majority of rangelands worldwide, yet few controlled experiments have addressed their individual, additive, and interactive impacts on ecosystem function. While ungulate herbivores generally reduce standing biomass, their effects on aboveground net primary production (ANPP) can vary by spatial and temporal context, intensity of herbivory, and herbivore identity and species richness. We used a long-term exclosure experiment to examine the effects of three groups of wild and domestic ungulate herbivores (megaherbivores, mesoherbivore wildlife, and cattle) on herbaceous productivity in an African savanna. Using both field measurements (productivity cages) and satellite imagery (NDVI), we measured the effects of different herbivore guilds – separately and in different combinations – on herbaceous productivity across both space and time.

Mesoherbivores and megaherbivores reduce variation in productivity

...but ANPP is highest when standing biomass is low, suggesting that the

herbaceous community can compensate for increasing herbivore pressure by

increasing ANPP

Herbivore treatments create a strong herbivory gradient

East African rangelands are home to diverse herbivore assemblages

Results

Conclusions

Six herbivore treatments: letters indicate which herbivore guilds are allowed: C: CattleW: Mesoherbivore wildlifeM: Megaherbivore wildlife (elephants and giraffes)

200m

C WC MWC

O W MW

Three replicate blocks

Increasing herbivore pressure

Herbaceous productivity measurements1) Satellite imagery – NDVI 2) Moveable productivity cages

-2009, 2011, & 2013 images-Calculated NDVI:

Methods

-10 Surveys from Feb. 2010 – Sept. 2012-Measurements across all KLEE plots-Measured both caged and uncaged biomass from three randomly selected cages per surveyHerbivores reduce standing biomass…