Did predation cause prey population declines on pro-wildlife ranches in Laikipia District?
Nicholas Georgiadis, Festus Ihwagi, and J. G. Nasser Olwero
Mpala Research Centre
Acknowledgements This study was supported by Mpala Wildlife Foundation. We thank G. Grant, J. Ruggieri and P. Valentine, O. de
Beaumont, and E. Parfett for permission to work on their ranches, and C. Mortensen for contributing such sound ecological
observations. G. Powys, T. Dyer, M. Littlewood, J. Wreford-Smith, and J. Evans shared their valuable recollections of changing
wildlife and livestock management in Laikipia over recent decades. Research assistance from F. Lomojo was greatly appreciated.
RATIONALE
THE PRINCIPAL CAUSE OF INCREASING PREDATION
• Conservation in African drylands will entail the persistence of wildlife in human-occupied landscapes, and
management intervention that is guided by an understanding of how anthropogenic factors influence large-scale
ecological processes.
The principal factor causing predators to increase in Laikipia was a change in land use from cattle ranching to wildlife
conservation and eco-tourism that began in the early 1990’s. Under ranching, plains zebras and predators were severely
suppressed. The switch to eco-tourism allowed a 5-fold increase in plains zebra abundance, and created a demand for living
predators. Although not limited by predators, plains zebras supported a substantial predator community. We infer that
increasing predator pressure caused predation-susceptible prey species to decline.
• In Laikipia District, a dry savanna region in central Kenya, wild ungulates share the landscape with humans and
livestock.
• We examined why five of nine abundant wild ungulate species (waterbuck, Thomson’s gazelle, buffalo, eland and
hartebeest) declined in abundance for more than a decade on properties where wildlife was actively conserved and
highly valued (the ‘pro-wildlife properties featured in the previous poster).
• Declines were substantial, amounting to 37% of total wild herbivore biomass (excluding elephants) between 1990
and 2005. Declines were unintended, unwelcome, and not understood.
• Of additional interest was why other species declined only slightly or not at all (plains zebra, Grant’s gazelle,
impala, and giraffe).
Among ungulate prey species in
Laikipia, plains zebras and giraffes, the
dominant grazers and browsers, are
not limited by predators, but by rainfall
and density-dependence. By contrast,
eland, hartebeest, and waterbuck
seem to be limited by predation.
THE APPROACH
Focusing on hartebeest where appropriate (the species that declined the most), but generalizing to other species,
we addressed twelve possible causes of decline for which data permitted a direct test, or available information
permitted a reasoned evaluation. The first five are human-induced: displacement by livestock, habitation, or
cultivation, and over-consumption, either due to illegal hunting or sanctioned harvesting. The remaining seven are
ostensibly ‘natural’: exceptional rainfall patterns, inter- and intra-specific competition, parasitism, low birth rates, low
survival rates, and predation.
SYNTHESIS AND APPLICATIONS
RESULTS
Fig. 3. Ranked contributions to total biomass by
dominant prey species in Serengeti, and in Laikipia
before (centre) and after (right) the restoration of
predators. Where known, species are coloured
according to whether they are limited by rainfall
(blue), density (green), or predation (red). Changes
in prey community structure in Laikipia reflect the
switch from ‘bottom-up’ to ‘top-down’ control.
1.0
Sub-Adults
a
a
Adult Males
a
a
Zebra
LAIKIPIA 1990
60%
Zebra
40%
Giraffe
20%
Hartebeest
0%
0%
60%
Zebra
40%
Giraffe
Eland
20%
Hartebeest
0%
1 2 3 4 5 6 7 8 9
Rank
1 2 3 4 5 6 7 8 9
Rank
LAIKIPIA 2005
80%
Percent Biomass
Wildebeest
40%
20%
80%
Percent Biomass
60%
SERENGETI NP
1 2 3 4 5 6 7 8 9
Rank
Adult Females
b
0.8
DO CATTLE AFFECT PREDATOR-PREY DYNAMICS VIA COMPETITION WITH ZEBRAS?
0.6
Adding cattle to the landscape may affect predator-prey dynamics in both direct and indirect ways, depending on rainfall:
0.4
b
0.2
a
b
a
a
a
a
a
a
a
gM
od
ro
el 2
Gr
ow
th
Mo
del
Mu
gie
el 1
Ze
2500
2000
1500
1000
500
0
1990
As a consequence, removal of cattle to favor wildlife may actually depress predator-susceptible prey species (Fig. 4).
nin
3500
3000
-500
1985
 in extreme years (floods or drought), episodic die-offs of livestock directly subsidize predators;
 in below-average rainfall years, competition between cattle and wild grazers (primarily zebras in Laikipia) may indirectly limit
predator carrying capacity.
cli
De
4000
No. of Hartebeest
Fig. 2. ‘Bottom-up’ to ‘Top-down’ control: Hartebeest population dynamics in
Laikipia District, 1985-2005 (circles), divided into an initial phase during which the
population was rainfall-dependent (filled circles), and a latter phase when the
population declined steadily (open circles). The first phase was modeled by Δ =
483.4R–438.4 (r=0.83, n=6, P<0.05), were Δ is the change in hartebeest numbers
on pro-wildlife properties, and R is the Rainfall Deficit Index from one census to the
next. The solid black line depicting the rainfall-dependent model is projected
beyond 1997 as a dotted black line, showing how the observed population
trajectory deviated radically from expectations based on rainfall. The declining
phase was modeled by a best-fit simulation (solid grey line), in which the number
of calves and sub-adults reaching adulthood was only 5-10% per year.
gM
od
lio
De
cli
nin
So
a
ram
El
Ka
re
ora
Ng
Mp
ger
a
ala
0.0
Se
Fig. 1. Among hartebeest, there were striking
differences in age structure (shown at right)
between Solio ranch, where predators have been
absent from for several decades, and four core
ranches (Mpala, Segera, Ngorare and El Karama),
where predators are abundant (map shown at left).
A further predator removal comparison was
afforded when hartebeest were enclosed within a
predator-proof sanctuary on Mugie ranch in 2003.
Their numbers doubled within two years, and all
calves survived.
Proportion of Population .
Calves
80%
Percent Biomass
Of a dozen factors potentially contributing to observed herbivore declines on pro-wildlife ranches in Laikipia District,
the following could not be ruled out: illegal hunting of eland when they move off pro-wildlife properties; short-term
effects of disease associated with an exceptionally wet El Niño event in 1997-1998; short-term effects of drought
associated with an exceptionally dry La Niña event in 1999-2000, possibly interacting with lungworm in hartebeest;
low survival in hartebeest, especially among juveniles and adult males, and possibly low birth rate; and predation.
Each may have contributed, but only predation is consistent with the timing, synchrony, duration and species
composition of the observed declines (Figs. 1 and 2).
Herbivore dynamics in Laikipia shared features with previously reported responses by herbivores to predator manipulation in
Kruger and Serengeti National Parks. All revealed profound impacts of predators on only a subset of prey species that
typically are not apparent under unperturbed conditions. And all featured one or a few numerically dominant herbivore
species, which were primarily limited by rainfall or density, supporting a predator community that in turn limited the abundance
of other prey species (Fig. 3).
1995
Years
2000
2005
Fig. 4. This hypothesis is expounded as follows:
Plains zebras and cattle are rainfall-limited in Laikipia (1), and both
are moderate-sized grazers. These are necessary and may be
sufficient conditions for them to be competing, at least in dry years
(2). Zebras form the majority of the prey biomass, and support a
large predator community, but are not limited by predators (3). Other
wild herbivores, however, are predator-limited (4). Removal of cattle
may therefore result not only in an increase in zebras, but also in
predators, causing further declines in other prey species. The
principal elements of this hypothesis require confirmation,
particularly the relative extent to which humans in the landscape
affect predators and prey (5 and 6).
5
-
HUMANS
4
6
-
-
OTHER
WILD HERBIVORES
PREDATORS
+
3
2
ZEBRAS
+
CATTLE
+
1
RAINFALL