Modification of the Savanna Functioning by Herbivores

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10
Modification of the Savanna Functioning
by Herbivores
Xavier Le Roux, Luc Abbadie, Herv´e Fritz, and H´el`ene Leriche
10.1 Introduction
Beyond consumption of a given amount of the net primary production (NPP),
herbivores may have major effects on ecosystem structure, functioning and
dynamics (e.g., [35]). Many authors have represented plant-herbivore interac-
tions by predation-like relationships [8] assuming that herbivory has a purely
negative impact on plant growth. It is now recognized that grazing may be
not detrimental, and even favorable for plants. In particular, herbivory can
promote grassland soil nitrogen cycling [43, 35, 29] which strongly influences
plant responses to grazing [9]. Furthermore, herbivores can largely influence
the temporal changes in tree/grass balance, directly through the reduction in
competition intensity or indirectly through the reduction in fire frequency and
intensity (e.g., [25, 10, 38]). In Lamto, the large herbivore biomass has often
been regarded as low as for other West African savannas [2, 11]. The scarcity
of large herbivores in Lamto was assumed to be the result of man’s actions
that have, for instance, virtually exterminated elephants and hippopotami.
This historical pattern is consistent with the results of several comparative
studies showing that apparent low herbivore biomass in West African savan-
nas were due to the loss of large mammal species, i.e., truncated herbivore
communities, primarily in relation with human demography and development
in the course of the last century [15, 7]. Studies on herbivory in Lamto sa-
vannas have mainly addressed two issues to date: (1) quantifying large and
small herbivore densities, biomasses and green grass consumption rate and
(2) testing the effect of grazing on the functioning and primary production of
the grass layer.
10.2 Herbivore densities, biomasses, and green grass
consumption rate in Lamto savannas
10.2.1 Invertebrate herbivores
The density and biomass of invertebrate herbivores have been accurately
determined in Lamto savannas, mainly during the 1970s. Acridids (mainly

186 Xavier Le Roux et al.
Anablepia granulata,Catantopsilus taeniolatus,Orthochtha brachycnemis and
Rhabdoplea munda) represent around 16,000 individuals ha−1, i.e., 554 g ha−1
dry mass [18]. Acridid biomass as high as 1,060 g ha−1has been reported in
December. Grass consumption rate in the field were estimated from daily
consumption rate of different species in the laboratory and field estimates
of biomass. Annual consumption was around 69.6 kg ha−1[18], as compared
to the total primary production values ranging from 30 to 60 t ha−1(see
Sect. 7.4). Extrapolation of these results to other herbivore arthropods living
in the aboveground grass layer (which biomass was estimated at Lamto) leads
to annual grass consumption estimates around 240 kg ha−1(excluding ants;
Fig. 10.1) [27].
In addition, consumption by termites ranges from 30 to 50 kg ha−1[26].
However, termites consume dead rather than green material. As a whole, the
biomass of animal consumers excluding large mammals (around 80 kg ha−1)
is quite low at Lamto [27] (Fig. 10.1). In contrast to detritivorous organisms
such as termites and earthworms that have a huge impact on the savanna
functioning (Fig. 10.1) (see Chaps. 13 and 16), such animal herbivores thus
do not play an important role for the savanna functioning.
Fig. 10.1. Simple representation of utilization of primary production (P) by herbi-
vores (Ih) and detritivores (Isg ). Losses by fire (F), respiration by herbivores (Rh)
and respiration by detritivores (Rsg) are indicated (after [28]).

10 Modification of the Savanna Functioning by Herbivores 187
10.2.2 Large mammal herbivores
The number of large grazers has increased recently due to hunting prohibition
on the reserve area: antelope (e.g., 0.025 Kobus kob kob ha−1) and buffalo
(0.0204 Syncerus caffer nanus ha−1; Fig. 10.2) densities both fall within the
range of values found in protected areas in Western Africa (Table 10.1).
The moist savanna grasslands of Lamto however correspond to selected
habitat for the kob, which can reach densities as high as 0.7 individuals ha−1
in preferred habitats (e.g., [42], in the Comoe National Park). The other un-
gulates encountered at Lamto are the bushbuck (Tragelaphus scriptus)and
Fig. 10.2. The Lamto buffaloes (photography by L. Abbadie).
Table 10.1. Densities of kob antelope (Kobus kob, average population body mass
50 kg) and buffalo (Syncerus caffer nanus, average population body mass 450 kg)
in Lamto savannas and other protected savanna areas of West Africa (after [19]).
Protected area Kob (km−2)Buffalo(km
−2)
Lamto (Cˆote d’Ivoire) 2.49 2.40
Arly (Burkina faso) 0.65 10.90
B´enou´e(Cˆote d’Ivoire) 1.14 1.55
Como´e(Cˆote d’Ivoire) 2.15 4.35
Deux-Bale (Burkina Faso) 0.07 0.18
Kainji (Nigeria) 0.03 1.18
Manovo-Gouda (R´ep. Centrafricaine) 1.80 2.60
Niokolokoba (S´en´egal) 0.33 0.55
Penjari (B´enin) 1.27 14.70

188 Xavier Le Roux et al.
Maxwell’s duiker (Cephalophus maxwellii). The red-flanked duiker (Cephalo-
phus rufilatus), black-fronted duiker (Cephalophus nigrifrons) and bushpig
(Potamochoerus porcus) may still exist in very low numbers. Recently, spoors
of hartebeest (Alcephalus busephalus major) were observed in the reserve. The
density values of large grazers obtained for the Lamto savanna are low com-
pared to high densities of large herbivores reported in some East and Southern
African savannas. However, the roles of soil nutrient and associated plant nu-
trient concentration are crucial in patterns of ungulate community abundance
[2, 16]. Thus, herbivore biomass in West African savannas, i.e., nutrient poor
ecosystems (see Sect. 4.3), should be compared with herbivore biomass in
other savanna ecosystems with similar low soil nutrient availability. Fritz [15]
compared herbivore biomass in different savanna ecosystems accounting for
their soil nutrient richness and found that medium-size ungulates are more
abundant in West Africa than their counterparts in nutrient-poor East and
Southern African savannas for a given level of primary production. Although
large herbivore biomass is correlated to aboveground net primary production
in both West African and East and Southern African savannas (Fig. 10.3),
the major differences in ungulate biomass between subregions are not strictly
related to differences in primary production. Missing large predators (such as
lion and hyena) and the quasi-absence of very large herbivores (megaherbi-
vores, such as elephants [39]), i.e., of key competitors [17], probably explain
this pattern of higher medium-size ungulate biomass in nutrient-poor West
Fig. 10.3. Relationship between aboveground net primary production (ANPP) and
large herbivore biomass (LHB) of truncated herbivore communities (i.e., without
megaherbivores and buffalo, for (squares) West African savannas, east and southern
African savannas with low soil nutrient availability and east and southern African
savannas with high soil nutrient availability (after [15], with permission of Blackwell
Publishing).

10 Modification of the Savanna Functioning by Herbivores 189
African savannas compared to that of nutrient-poor East and Southern sa-
vannas. Thus, in contrast to views prevailing during the 1970s and 1980s, the
potential for high large herbivore biomass is now recognized for such nutrient-
poor ecosystems. Furthermore, megaherbivores probably represented a large
fraction of the primary consumer trophic level in the past at Lamto, as in
most ecosystems with high rainfall and low soil nutrient status [2, 17]. Their
density could still increase if more drastic protection rules are applied. In the
meantime, an increase in protection would certainly benefit the medium-size
ungulates. This has spurred on development of field experiments and modeling
approaches to better understand the potential role of mammal herbivores (i.e.,
mainly grazers) on the functioning of the grass layer in the Lamto savanna.
10.3 Field studies of grazing effect on the
savanna functioning
10.3.1 Response of grass production to grazing
Grazing, as a removal of living tissue, has first been considered as detrimental
to plants. However, experimental results (e.g., [32, 33, 22, 6]) showed that net
primary production, NPP, can be maintained (compensatory growth) or stim-
ulated (overcompensatory growth) in response to grazing (i.e., the herbivory
optimization hypothesis HOH). Some authors [32, 23, 22] suggested that an
optimal plant removal level should occur beyond which production is reduced.
The ecological significance and generality of these findings were jeopardized
by critical appraisals of published data [3, 4, 5].
The HOH was tested in the Lamto savanna by analyzing the growth of
grasses in response to a clipping (3 levels) ×fertilization (2 levels) trial in
the field [30]. During 3 months, the effects of clipping and fertilization on the
dry matter and nitrogen yields to producers (i.e., mass or nitrogen amount
of residual phytomass at the end of the experiment) and to grazers (i.e.,
mass or nitrogen amount of clipped-off tissues during the experiment) were
surveyed. Total phytomass yield and yield to grazers were maintained under
moderate clipping frequency and fertilization as compared to control condi-
tions (Fig. 10.4). Both clipping frequency decreased total phytomass yield
to producers as compared to control plots. Clipping frequency significantly
increased nitrogen concentrations in the total yield, in the yield to produc-
ers and in the yield to grazers [30]. Total nitrogen yield and nitrogen yield
to grazers were 65% and 91%, respectively, higher on the plots experiencing
moderate clipping frequency with fertilization as compared to control plots
(Fig. 10.5). Root phytomass was not influenced by clipping frequency and
fertilization [30]. These results provide evidence that the HOH is realistic for
the Lamto grass layer in the short term. The observed changes in the pattern
of mass- and nitrogen-yield distribution concurrently to the improved grass

190 Xavier Le Roux et al.
Clipping frequency
(per month)
Total yield to producers (g/m2)Total yield (g/m2) Total yield to grazers (g/m2)
Fig. 10.4. Dry matter yield to producers (below 0-10 cm height), yield to grazers
(over 10 cm height) and total yield as a function of clipping frequency (per month)
with (dotted line) and without (solid line) fertilization (after [30], with permission
of the Ecological Society of America).

10 Modification of the Savanna Functioning by Herbivores 191
Clipping frequency
(per month)
Total nitrogen yield (gN/m2)Nitrogen yield to grazers
(gN/m2)
Nitrogen yield to producers
(gN/m2)
Fig. 10.5. Nitrogen yield to producers (below 0-10 cm height), yield to grazers (over
10 cm height) and total yield as a function of clipping frequency (per month) with
(dotted line) and without (solid line) fertilization (after [30], with permission of the
Ecological Society of America).

192 Xavier Le Roux et al.
quality observed (i.e., higher nitrogen concentration) show that grazers can
modify ecosystem processes in such a way as to alleviate nutritional deficien-
cies in the Lamto savanna. Longer-term experiments testing the sustainability
of such a response of the grass layer to grazing are under progress at Lamto.
10.3.2 Response of soil microbial activities following a
grazing event
Herbivory has important effects on soil nitrogen cycling [43, 35], which
strongly influences plant responses to grazing. Grazer-induced decreases in
microbial immobilization and increases in N mineralization have generally
been observed [44, 24, 45, 46, 12, 47, 36, 29]. Such changes generally result in
improved N availability to plants in intensively grazed sites [24, 34, 21]. How-
ever, few studies have quantified the effects of grazing on two key processes
involved in soil N cycling, i.e., nitrification and denitrification [13, 14, 40, 20].
These two microbially mediated processes largely control the balance between
the forms of soil mineral nitrogen (NO−
3versus NH+
4) available to plants and
nitrogen conservation at the ecosystem level. The short-term response of ni-
trification and denitrification to a clipping event was tested in a Hyparrhenia
grassland at Lamto [1]. The grass layer was clipped once on 0.5 m2plots
during the early rainy season in April, and potential nitrification and denitri-
fication were surveyed before and after clipping. Soil was sampled in the 0-10
cm layer below clipped Hyparrhenia tufts at the center of clipped plots and
below control, unclipped Hyparrhenia tufts. The main results of this study
were (1) clipping did not influence the water regime in the upper soil layer
(Fig. 10.6) and (2) denitrification was significantly higher below clipped Hy-
parrhenia tufts than below unclipped tufts 6 days after the clipping event
(Fig. 10.6). This study showed the potential of one effect of grazing, i.e., de-
foliation, on soil processes driving N cycling. Enhancement of root exudation
by clipped plants (e.g., [41]) could partly explain the observed increased den-
itrification activity because denitrifiers are heterotrophs for C. Experiments
for quantifying the long-term effect of grazing on key soil microbial activities
and the diversity of associated microbial functional groups are under progress
at Lamto. This will greatly improve our understanding of the effect of grazing
on savanna functioning.
10.4 Modeling approaches for understanding
grazing effect on the savanna functioning
Many functional processes controlling NPP are affected by grazing, such as
modification of light availability, reduction of water stress, accelerated nutri-
ent recycling, changed allocation of assimilates within the plant and enhanced
photosynthetic rates [33, 37]. Modeling is thus a useful tool to unravel the roles

10 Modification of the Savanna Functioning by Herbivores 193
Soil moisture (%)
PDA (mg N h-1 g-1)
Time after clipping (d)
Fig. 10.6. Changes in soil moisture and potential denitrifying activity (PDA) fol-
lowing a clipping event. 50 ×50 cm2areas centered on a Hyparrhenia individual
tuft were clipped at the beginning of the experiment. Soil moisture and microbial
activities were surveyed for the 0-10 cm soil layer below the Hyparrhenia individuals
(7 replicates per treatments) (M. Bardy and X. Le Roux, unpublished).
of all these processes in grass response to grazing. Leriche et al. [31] used a
modeling approach to better understand the impact of grazing on grass pro-
duction in Lamto by simulating the response of grass NPP to plant biomass
removal. The process-based PEPSEE-grass model (see Sect. 9.2) parameter-
ized for Lamto grasslands was used (1) to quantify the relative importance of

194 Xavier Le Roux et al.
key functional processes (i.e., changes in light absorption efficiency, reduction
of water stress, improved canopy nitrogen status and ensuing productivity
rate, changes in the pattern of root/shoot allocation) in the response of NPP
to grazing and particularly those that can lead to compensatory growth and
(2) to test the grazing optimization hypothesis under different functional hy-
potheses at the canopy and annual scales for the Lamto savanna. Simulations
were performed using a constant or resource-driven root/shoot allocation coef-
ficient and assuming a dependence or independence of conversion efficiency of
absorbed light into dry matter on nitrogen availability [31]. Main results were
as follows: (i) the response of NPP to grazing intensity emerged as a complex
result of both positive and negative, direct and indirect, effects of biomass
removal on light absorption efficiency, soil water availability, grass nitrogen
status and productivity, and root/shoot allocation pattern; (ii) overcompensa-
tion was observed for aboveground NPP when assuming a nitrogen-dependent
conversion efficiency and a resource-driven root/shoot allocation (Fig. 10.7);
(iii) the response of NPP to grazing was mainly controlled by the effect of
plant nitrogen status on conversion efficiency and by the root/shoot alloca-
tion pattern (Fig. 10.7), while the effects of improved water status and reduced
light absorption were secondary. The originality of this work was to provide a
comprehensive representation of the functional response of grasslands to graz-
ing. Given the assumptions made in the model, this study does not provide
evidence for or against the grazing optimization hypothesis in West African
humid grasslands. However, the changes in plant nutrient status and produc-
tivity and the response of the root/shoot allocation pattern were identified as
the two key interacting processes controlling the response of Lamto grassland
NPP to increasing grazing intensity. Thus, predicting the response of Lamto
grass NPP to increasing grazing intensity requires one to couple a model sim-
ulating the savanna functioning (as TREEGRASS, see Sect. 9.3) to a model
simulating the nitrogen dynamics in the soil-plant system (as SOMCO, see
Sect. 12.5). Such an approach should accurately represent the interactions be-
tween plant functional processes (N uptake, litter or exsudate inputs to the
soil, dependence of grass productivity on nutrient availability and root/shoot
allocation pattern), soil microbial activities (soil organic matter dynamics and
soil nutrient availability, mineralization from urine and feces, nitrification and
denitrification) and soil water balance (which controls both soil and plant
functioning).
10.5 Conclusion
The density, biomass and consumption rate of animal herbivores are presently
low in the Lamto savanna, but the density of mammal herbivores is higher
than that reported in other nutrient-poor savannas. According to Lamotte
[27], “earthworms, termites and decomposing microorganisms occupy the
role played elsewhere [in eastern and southern African savannas] by large
herbivorous mammals” because they “discreetly insure, together with fire,
the mineralization of organic matter produced every year by green plants.”

Fig. 10.7. Simulated response of grass biomass, aboveground net primary produc-
tion (ANPP), total net primary production (TNPP), and grass root/shoot ratio to
grazing intensity. Four versions of the PEPSEE model were used (crosses: constant
conversion efficiency and constant root/shoot ratio; squares: constant conversion ef-
ficiency and resource-driven root/shoot ratio; solid circles: nitrogen-dependent con-
version efficiency and constant root/shoot ratio; empty circles: nitrogen-dependent
conversion efficiency and resource-driven root/shoot ratio). All values are normalized
to values simulated without herbivory (after [31], with kind permission of Springer
Science and Business media).

196 Xavier Le Roux et al.
However, the density of mammal herbivores could still increase if more drastic
protection rules are applied, which could have major influences on the savanna
functioning and dynamics, because the way large herbivores consume and
process plant matter is very different from that of decomposers. Studies of
the impact of large grazers on vegetation functioning and dynamics will thus
develop at Lamto. In particular, a better understanding of processes driving
the response of savanna functioning to grazing on the long term (years to
decades) is needed. Studies on the role of grazers on the tree/grass equilibrium
should also be initiated. This will help a better understanding of the role
of large grazers in the past in West African humid savannas. This will also
help better management of the Lamto savanna if conservation priorities allow
substantial increase in the density/diversity of large herbivores at Lamto in
the future.
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