Large Herbivores and Process Dynamics in a Semi-arid Grazing Ecosystem
David J. Augustine1, Mahesh Sankaran2, and Samuel J. McNaughton1
of Biology, Syracuse University, Syracuse NY, USA, and 2NERC Centre for Population Biology, Imperial College, UK & NERL, Colorado State University, CO, USA
Ungulate Consumption
Introduction:
Herbivores can exert both positive and negative effects on aboveground
net primary production (ANPP) and rates of nutrient cycling in
ecosystems. We examined whether herbivore effects on ecosystem
dynamics are contingent on soil fertility by quantifying the interactions
among soils, grasses and grazers (see central diagram) on nutrient-rich and
nutrient-poor soils (Table 1) within a climatically homogenous savanna in
Laikipia, Kenya (the Mpala Ranch and Research Centre, MRC).
Figure 4. Effects of grazers
and soil fertility on nitrogen
in microbial biomass (MBN).
Across all communities,
grazers
significantly
reduced MBN (P < 0.05).
Fig. 3
Fig. 2
103.0
Grazed
Fenced
ctrl
83.0
63.0
excl
43.0
Plant Community
23.0
3.0
0.05
0.10
0.15
0.20
0.25
Soil N (%)
Fig. 1
Grazers increase inorganic N pool size:
Figure 5. Extractable inorganic N
concentrations in relation to grazing
and community type during the 2001
growing
season.
Grazers
significantly increased inorganic N
pool size in both communities in
months 3, 4, 5, and 7 (P < 0.05)
Fig. 4
Inorganic
Nutrients
Figs. 5-7
Decomposers
Grazed Bushland
35
Ungrazed Bushland
mg inorganic N / kg dry soil
Methods:
ANPP and ungulate consumption rates were measured with the moveable
grazing cages. Grazer effects on ANPP and N cycling were measured with
permanent herbivore exclosures constructed in 1999 and monitored in
2001. Inorganic N pool size was measured monthly with KCl extracts of
soils, and N-mineralization rates were measured with monthly in situ soil
incubations in PVC tubes. Nylon resin bags filled with ion-exchange resin
(Dowex MR-3) were incubated monthly in the soil to measure adsorption
rates of NO3- and NH4+ from the soil solution, which provides an
integrated measure of soil N availability over time.
123.0
MBN
1Dept.
30
Grazed Glades
Ungrazed Glades
25
20
15
10
5
0
3
4
5
6
7
8
Month
2.5
mg N mineralized kg dry soil -1 month-1
Ungrazed
Glade
Grazed
Bushland
Table 1. Soil properties (mean,
2
1.5
Grazed
Fenced
Figure 6.
Effect of grazers on Nmineralization rates during the first month
of the 2001 growing season. Grazers
significantly decreased mineralization
rates in both communities (P < 0.05).
1
0.5
0
-0.5
-1
-1.5
1 SE in parentheses) of
Moveable grazing cage
Glades
Bushland
Total soil P (%)
Total soil C (%)
% Sand
Bushland
0.19 (0.02)
0.12 (0.01)
0.13 (0.04)
1.76 (0.20)
75.3 (2.2)
Figure 2. Relationship between ANPP and
large herbivore consumption rates at MRC.
The solid line shows the regression for
MRC (r2 = 0.94, Consumption = 0.8091 
ANPP – 30.68, F1,16 = 115.43, P < 0.0001)
and the dashed line shows the reported
relationship for grasslands in Yellowstone
and Serengeti National Parks.
0.015 (0.002)
1.09 (0.10)
74.3 (2.9)
% Clay
10.0 (0.7)
14.8 (1.0)
Soil pH
6.64 (0.15)
5.72 (0.04)
500
Consumption (g/m2)
Total soil N (%)
Glades
Figure 7.
Effect of
grazers on rates of
inorganic N adsorption to
ion-exchange resin bags
summed over the first 4
months of the growing
season. Grazer effects
on N availability were
contingent
on
community type and
mirrored effects on
ANPP (Fig. 3).
600
400
300
200
100
0
0
100
200
500
400
500
350
Figure 3. Effects of large herbivores on
herbaceous ANPP in glade and bushland
communities during a 5-month growing
season in 2001. Error bars show +1 SE
based on among-block variation;
statistical tests of differences between
grazing treatments control for amongblock variation. Herbivore effects were
positive in N-rich glades and negative
in N-poor bushland sites.
bushland
glades
300
250
200
150
100
50
0
1.00
P = 0.024
400
500
300
400
200
300
200
100
100
0
0
Grazed
Fenced
Grazed
Fenced
600
1.50
2.00
Log10(mm rainfall)
2.50
3.00
Conclusions:
2001
400
ANPP (g m -2 5 months-1)
400
600
P < 0.001
ANPP (g/m2)
450
ANPP (g/m 2)
Figure 1.
Herbaceous
ANPP at nutrientrich (glades) and
nutrient-poor
(bushland) study
sites as a function
of rainfall.
300
mg inogrnic N kg resin-1 4 months-1
glades and bushland study sites at the Mpala Research Centre.
P=0.013
350
300
Grazed
Fenced
Grazer effects on N cycling and ANPP are closely coupled in their direction,
magnitude and spatial extent
•Positive effects on inorganic N availability and ANPP in N-rich glades
250
200
•Negative effects on inorganic N availability and ANPP in N-poor bushland
P=0.028
150
Grazer effects on microbial biomass coupled to effects on N mineralization
rates, but NOT inorganic N availability
100
50
0
Bushland
Glades
•Consistent negative grazer effects on microbial biomass and N mineralization, but
N availability influenced by both mineralization and inorganic N pool size