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Does grazer identity matter? Examining the effects of three guilds of
herbivores on savanna herbaceous primary productivit...
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Impacts of different large herbivores on ecosystem function: cattle increase mean productivity, and wild herbivores reduce variability around the mean

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Poster presented at ESA 2016 Fort Lauderdale.

Background/Question/Methods

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 herbivore identity and species richness, intensity of herbivory, and spatial and temporal context. Some evidence indicates that moderate levels of herbivory can stimulate aboveground productivity, but few studies have explicitly tested the relationships among herbivore identity, grazing intensity, and ANPP. Here, we used a long-term exclosure experiment to examine the effects of three groups of wild and domestic ungulate herbivores (megaherbivores, meso-herbivore wildlife, and cattle) on productivity in an African savanna. Using both field measurements (productivity cages) and satellite imagery, we measured the effects of different herbivore guilds – separately and in different combinations – on productivity across both space and time.

Results/Conclusions

Results from both productivity cage measurements and satellite NDVI demonstrated a positive relationship between mean productivity and total ungulate herbivore pressure, driven in particular by the presence of cattle. In contrast, we found that variation in productivity across space and time was driven by herbivore type; wild herbivores (primarily meso-herbivore wildlife) significantly reduced heterogeneity in ANPP across both space and time. Our results indicate that replacing wildlife with cattle (at moderate densities) could lead to similarly productive, but more heterogeneous herbaceous rangelands.

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Impacts of different large herbivores on ecosystem function: cattle increase mean productivity, and wild herbivores reduce variability around the mean

  1. 1. Does grazer identity matter? Examining the effects of three guilds of herbivores on savanna herbaceous primary productivity Grace K. Charles1,2, Lauren M. Porensky2,3, Corinna Riginos2,4, Kari E. Veblen2,5 and Truman P. Young1,2 1Department 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 Overview Grazing 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 rangelandsAcknowledgements We 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 Herbivore Treatment O W MW C WC MWC StandarddeviationofNDVI 0.00 0.01 0.02 0.03 0.04 Meso-wildlife: ** Megaherbivores: ** Herbivore Treatment O W MW C WC MWC StandardizedmeanNDVI 0.90 0.92 0.94 0.96 0.98 1.00 1.02 1.04 Cattle: *** Cattle x Meso-wildlife: ** ...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: Cattle W: Mesoherbivore wildlife M: Megaherbivore wildlife (elephants and giraffes) 200m C WC MWC O W MW Three replicate blocks Increasing herbivore pressure Herbivore biomass estimate (kg km -2 ) 0 1000 2000 3000 4000 5000 6000 Biomass(gm -2 ) 300 400 500 600 700 MWC C WC MW WO Biomass (g m -2 ) 300 400 500 600 700 ANPP(gmonth -1 m -2 ) -10 0 10 20 30 40 O W MW C WC MWC Herbaceous productivity measurements 1) Satellite imagery – NDVI 2) Moveable productivity cages -2009, 2011, & 2013 images -Calculated NDVI: 𝑁𝐼𝑅 − 𝑅 𝑁𝐼𝑅 + 𝑅 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…

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