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…