Journal of Environmental Protection, 2011, 2, 855-866
doi:10.4236/jep.2011.26097 Published Online August 2011 (http://www.SciRP.org/journal/jep)
855
Patterns of Variation of Herbivore Assemblages at
Nairobi National Park, Kenya, 1990-2008
Alfred O. Owino1, Moses Lekishon Kenana2, Paul Webala3, Samuel Andanje1, Patrick O. Omondi1
1Biodiversity Research & Monitoring Division, Kenya Wildlife Service, Nairobi, Kenya; 2Nairobi National Park, Nairobi, Kenya;
3Kenya Wildlife Service Training Institute, Naivasha, Kenya.
Received April 5th, 2011; revised May 24th, 2011; accepted July 6th, 2011.
ABSTRACT
Wildlife, especially mammals populations dynamics in many conservation areas are influenced by ecosystem processes
and increasingly by climate change. Generally, cyclic population dynamics is relatively common among small mam-
mals, especially in high latitudes but is not yet established among many African savanna ungulates. Habitat fragmenta-
tion and loss propagated by anthropogenic activities are responsible for the decline in populations of many wildlife
species leading to the confinement many wildlife species particularly herbivores within parks and reserves as a con-
servation measure. We assessed the patterns of variation in abundance of eight herbivore species (African Buffalo,
Eland, Burchell’s Zebra, Wildebeest, Giraffe, Grant’s Gazelle, Thomson’s Gazelle and Impala) at Kenya’s Nairobi
National Park using population counts data over the period 1990-2008. Overall, the eight herbivores abundances de-
clined within the Park with significant declines in Wildebeest (R2 = 0.54), Grant’s Gazelle (R2 = 0.72) and Impala (R2
= 0.80). Seasonality had effects on herbivore numbers and assemblages at the Park with the numbers of individual spe-
cies increasing within the Park during dry seasons compared to wet seasons (t-test, t = 4.45, p = 0.03). Land use
changes and urban development, especially in the dispersal areas and the accompanying effects of climate change of
reduced rainfall and longer periods of drought had significant negative impacts on herbivore assemblages at the Park.
We discuss the significance of the population fluctuations of the eight species at the Park, the potential impacts of the
changes on Park ecosystem processes and the expected long-term population dynamics of the species if the conditions
remain as witnessed over the past two decades.
Keywords: Nairobi National Park, Herbivores, Habitat Fragmentation, Climate Change
1. Introduction
The monitoring of populations of wildlife species is an
established management practice [1]. This implies site
monitoring so that changes in the populations can be as-
sessed against a standard level [2]. To define such stan-
dards, natural variability must first be examined through
surveillance—a repeated set of surveys conducted in a
standardized manner over longer periods. Wildlife popu-
lation trend analyses obtained through regular long-term
censuses datasets have been of continual interest to eco-
logists and wildlife management authorities [1]. Exten-
sive research, especially on small mammal in the north-
ern latitudes has implicated a predator-prey interaction as
the most pervasive cause of population cycles [3]. In the
search for general patterns of wildlife population trends,
the influence of the past is largely ignored, despite sub-
stantial evidence of historical constraints that may range
from major events to short-term disturbances [4]. How-
ever, there has been a general appreciation of the influ-
ence of rainfall, food availability and periodicity in the
dynamics, especially of large herbivore populations [5].
In addition, there has been a growing interest on the ef-
fects of anthropogenic activities, invasive/alien species,
and most recently climate change as key drivers of wild-
life population trends, especially in the tropics [1,6-9].
In Africa, populations of many wildlife species have
declined substantially inside and outside the protected
areas [1,6,10-13]. Contributory causes include recurrent
droughts [6,14,15], land-use changes [6,16], growing
human settlements [17], illicit hunting [18 ] and livestock
incursions into protected areas [6]. In East Africa wildlife
population dynamics within many conservation areas are
influenced by many factors including trophic interactions
with resources [19], autocorrelated exogenous factors
[20], climate change effects [5] and land use changes/

Patterns of Variation of Herbivore Assemblages at Nairobi National Park, Kenya, 1990-2008 856
reduction in ecological ranges [7,21].
Given that the delineation of many conservation areas,
especially in East Africa did not align conservation areas
with ecosystem boundaries, many conservation areas do
not encompass whole ecosystems [22]. Consequently,
variations in wildlife numbers, especially herbivores ob-
served within confines of parks and reserves are common
because their natural ranges do or do not extend well
beyond the boundaries of the protected areas particularly
for the for the fenced protected areas. For example, mi-
grations of wildebeest in the Mara-Serengeti ecosystem
occur between a conservation area and adjoining disper-
sal areas, clearly showing that the Mara-Serengeti eco-
system are not adequate for the protection and viability,
especially for the migratory wildebeest [23]. For the
fenced protected areas such as Lake Nakuru National
Park, herbivores are confined within smaller areas with
limited access to the surrounding areas. In parks such as
Amboseli in southern Kenya, the land use changes in the
surrounding areas are increasingly confining African
Elephant Loxodonta africana within the park leading to
significant habitat alterations because of herbivory inten-
sity. Because wildlife in nature are neither distributed
uniformly or at random, but instead form spatial patterns
[24], the type of spatial arrangement present may suggest
certain interactions within and between species, such as
competition, predation, and reproduction [25]. On the
other hand, certain spatial patterns may also rule out
specific ecological theories previously thought to be true
[26].
Nairobi National Park is a peri-urban protected area
that represents a small portion of the larger Nairobi—
Athi Kapiti ecosystem in southern Kenya. It has in the
past been operationally defined by the range of the mi-
gratory wildlife species such as wildebeest as exempli-
fied in the Mara-Serengeti ecosystem [8,27-29]. The Park
currently experiences serious anthropogenic effects and
the impacts of expansion of Nairobi City, and has in re-
cent times become increasingly susceptible to a multitude
of external pressures likely of influence wildlife popula-
tion dynamics. Given the susceptibility of the Park to
influences from external pressures, Kenya Wildlife Ser-
vice—the wildlife management authority in Kenya has
been conducting regular and systematic wildlife popula-
tion counts in the Park. This is especially important be-
cause the Park is one of the remaining major concentra-
tion areas for plains game species in southern Kenya, and
the knowledge of pattern s of variations in the n umbers of
common wildlife species is critical for conservation and
management decisions.
In this paper, we present results from long term wild-
life population monitoring data (1990-2008) at Nairobi
National Park. We use index numbers of wildlife popula-
tions to investigate patterns of variations of eight com-
mon herbivore species recorded within the Park, and as-
sess the seasonal variability across wet and dry seasons.
We discuss the relationships of the population dynamics
with the ecological characteristics of the Park, and the
expected future changes in the populations if the condi-
tions witnessed in the p ast two decades persist.
2. Study Area and Methods
2.1. Study Area
Nairobi National Park (01˚17'S, 36˚49'E) is an area of
natural landscape at grassland-forest boundary, only 7
km from the centre of Kenya’s capital city, Nairobi [30].
The Park was gazetted in 1947, and covers an area of
about 117 km2 (Figure 1). Various habitat types include-
ing open rolling grass plains, riverine woodlands, valley
thicket and bush, man-made dams and ponds, rocky
gorges and up land dry forests o ccur within the Park [31].
It is fenced along three sides, where it is adjacent to ur-
ban housing, industry, roads and airports. Ecologically,
the Park is intimately linked to Kitengela and Athi-Kipiti
plains which adjoin it to the south, forming a single eco-
logical unit [32]. Being clo se to Nairobi City, and with a
variety of wildlife species, it is a popular destination for
tourists, but faces obvious additional problems from the
expanding urban area and human population growth
Figure 1. Nairobi National Park and locations of the census
blocks.

Patterns of Variation of Herbivore Assemblages at Nairobi National Park, Kenya, 1990-2008857
around it [33]. Despite all these challenges, it still serves
as a dry season concentration area for major wildlife spe-
cies particularly Wildebeest Connochaetes taurinus,
Burchell’s Zebra Equus burchelli and Eland Taurotragus
oryx that make up over 50% of the total wildlife biomass
of the park [32]. During wet seasons, most of these major
plains game species disperse to the south outside the
protected area boundary where they spend significant
time of their annual seasonal cycles on private or com-
munal lands. However, the migratory movement of wild-
life has increasingly become constrained by sprawling
settlements within the dispersal area.
2.2. Animal Counts and Data Analysis
Regular wildlife counts were conducted in the months of
February, April, June, August, October and December
coinciding with the wet and dry seasons as part of the
on-going long-term wildlife population monitoring by
the wildlife management authority—the Kenya Wildlife
Service. Fifteen (15) blocks (Figure 1) were assigned to
teams of volunteers using vehicles on specified roads
within the blocks. All plains game species on either side
of the road up to an estimated distance of 250 m on both
sides were counted using binoculars and numbers re-
corded in designed datasheets over a single morning
from 0600 hrs to mid-day using a combination of the
Distance and the Strip census techniques [34]. The dis-
tance to which animal or groups of animals were sighted
was estimated and recorded at right angles to the vehicle.
This estimation of distance allowed for application of
species correction factors [34]. The count data for each
plains game wildlife species recorded were summarized
for each block and for the entire Park over the census
period, 1990-2008. In this paper, we focused on the eight
common herbivores counted over the period, i.e. African
Buffalo Syncerus caffer; Eland Taurotragus oryx;
Burchell’s Zebra Equus quagga burchelli; Wildebeest
Connochaetus taurinus; Giraffe Giraffa camelopardalis
tippelkirchi; Grant’s Gazelle Gazella granti; Thomson’s
Gazellle Gazella rufifrons and Impala Aepyceros
melampus. When examining overall variability in num-
bers of these species, we analyzed data for each month
separately and combined per year and only significant
trends shown [35]. The logarithmic indices of relative
abundance for each species were computed as follows:
Population index (γ) = log10 ((λ/
 )*100)/2
where (λ) = Observed abundance – from each annual
count; (
 ) = Mean abundance of count for each species.
The abundance for one year (λ) is scaled as a percentage
of mean abundance (
 ) over the nu mber of annu al counts
(including the year in question, 1990- 2008). The log (we
have used base ten) of this is taken to avoid inconsisten-
cies caused by different scale factors [36]; this is then
halved so that the mean abundance corresponds to an
index of 1.0. The sample variances of these indices were
then taken as a measure of relative variation for each
species. Pearson correlation co-efficient were computed
from the indices to determine if patterns of variation
were related. The population fluctuations across seasons
and years was established using autocorrelation autocor-
relelograms based on approximate 95% tolerance limits
under the assumption that the underlying autocorrelations
is zero at all lags [5,37-39], and autocorrelations used to
document the existence of patterns of abundance for the
eight herbivores.
3. Results
3.1. Overall Variations in Abundances
The annual abundance indices for the eight common
plains game counted at Nairobi National Park are shown
in Table 1. Wildebeest (1788), Zebra (1310) and Impala
(420) had the highest mean abundances over the period
1990-2008. Giraffe (81) and Grant’s Gazelle (89) had the
lowest records. Overall, wildebeest was the most variable
over the period (variance = 0.19). Generally higher re-
cords for all plains game species were made in the peri-
ods between 1990 and 2000 with numbers of all the spe-
cies showing consistent declines thereafter. The overall
pattern of abund ance showed that the numbers of Wilde-
beest (R2 = 0.54, P = 0.01), Grant’s Gazelle (R2 = 0.72,
P = 0.002) and Impala (R2 = 0.80, P = 0.0001) declined
significantly (Figures 2(a)-(c)). Other species showed
annual fluctuations of different strengths but these were
not significant although the general pattern was of a de-
cline.
3.2. Seasonal and Annual Variations
The eight species showed seasonal variations of different
strengths across the census months of February, April,
June, August, October and December over the two deca-
des at the Park coinciding with the wet seasons (i.e. April,
October and December) and dry seasons (February, June,
August). A pair-wise comparison of overall annual abun-
dance pooled by seasons sh owed significant d ifference in
numbers of the eight species within the Park (t = 4.45, P
= 0.03, n = 57) with a general pattern of lower numbers
for all the eight during the wet season months of April
and October within the Park. The pair-wise assessments
of abundance for individual species during dry and wet
seasons showed variations of different levels but most
were not significant, except for Burchell’s Zebra (P =
0.04) and eland (P = 0.03). Giraffe were generally fewer
in the Park during wet seasons but numbers increased
during drier months although the increases were not sig-

Patterns of Variation of Herbivore Assemblages at Nairobi National Park, Kenya, 1990-2008 858
Table 1. Annual abundance indices of eight herbivore species at Nairobi National Park, 1990-2008.
Herbivore species
Year African
Buffalo Eland Burchell’s Zebra Wildebeest Giraffe Grant’s Gazelle Thomson’s Gazellle Impala
1990 0.98 1.09 1.08 1.09 1.09 1.09 1.09 1.13
1991 1.00 1.08 0.99 0.95 1.05 1.05 1.00 1.09
1992 1.05 0.97 0.99 1.06 1.01 1.06 0.91 1.08
1993 1.05 1.00 1.03 1.16 1.03 1.06 0.96 1.08
1994 1.01 0.97 1.02 1.16 1.03 1.08 0.97 1.08
1995 1.02 0.98 1.01 1.16 1.03 1.07 0.97 1.08
1996 1.02 0.98 1.09 1.23 1.01 1.04 1.09 1.06
1997 0.99 0.90 1.05 1.11 0.94 0.99 0.99 0.98
1998 1.01 0.97 1.08 1.07 0.99 0.99 1.06 1.01
1999 0.88 0.94 1.02 1.01 0.94 0.97 1.06 0.97
2000 0.90 0.95 1.12 1.09 0.97 1.02 1.11 1.02
2001 0.87 0.82 0.72 0.01 0.93 0.96 1.01 0.91
2002 0.99 0.96 0.77 0.04 0.95 0.89 1.02 0.88
2003 0.96 1.03 0.70 0.06 0.95 0.86 0.90 0.81
2004 0.91 1.04 0.91 0.39 0.95 0.92 0.91 0.81
2005 1.07 1.07 1.04 0.57 0.98 1.01 0.98 0.90
2006 1.05 1.05 0.97 0.56 1.01 0.94 0.94 0.88
2007 0.93 0.90 0.92 0.29 0.99 0.85 0.91 0.82
2008 1.12 1.09 1.00 0.43 1.06 0.93 0.92 0.97
Mean of (  ) 198 125 1310 1788 81 89 101 420
Variance of (γ) 0.005 0.005 0.014 0.192 0.002 0.006 0.005 0.011
nificant. Autocorrelations (Figure 3(a)-(h)) of the counts
with the previous year with half cycle periods showed
different patterns for all the eight herbivores, with the
wildebeest numbers showing least autocorrelations after
the year 2000.
3.3. Correlations in Abundance of the Eight
Herbivores
There were substantial direct correlations in indices of
abundance of the eight species over the period. Of the
seventeen (17) direct correlations (with uncorrected P <
0.05), seven were significant. Such correlations would in
any case be expected to arise by chance in this set of 36
paired analyses. In particular, significant correlations
were noted between numbers of zebra and wildebeest
(Pearson, R = 0.71), Thomson’s gazelle (R= 0.63) and
impala (R = 0.62). Other significant correlations were
noted in the numbers of Grant’s gazelle and wildebeest
(R = 0.77), impala and Grant’s gazelle (R = 0.93), giraff e
and eland (R = 0.61). However, based on behavioral
ecology of the species considered these correlations could
have some ecological meanings. On the other hand, the
spatial distribution patterns of zebra in the Park were
particularly striking with records made in most census
blocks, and their numbers were directly correlated with
most plains game species in the Park.
4. Discussion
Given its spectacular diversity of wildlife and increasing
human population, Kenya continues to experience sig-
nificant challenges in wildlife conservation, and areas
such as Masai Mara Reserve have witnessed significant
declines in wildlife populations [7]. Our analysis of pat-
terns of variation in numbers of eight common herbi-

Patterns of Variation of Herbivore Assemblages at Nairobi National Park, Kenya, 1990-2008859
R
2
= 0.54
0
1000
2000
3000
4000
5000
6000
1985 1990 1995 2000 2005 2010
Years
Mean No. counted
(a)
R
2
= 0.72
0
50
100
150
200
1985 19901995 20002005 2010
Year s
Me an No. co u nted
(b)
R
2
= 0. 80
0
200
400
600
800
1985 19901995 20002005 2010
Years
Mean No. counted
(c)
Figure 2. (a) Regression analysis of the numbers of wilde-
beest at Nairobi National Park, Kenya; (b) Regression
analysis of the numbers of Grant’s gazelle at Nairobi Na-
tional Park, Kenya; (c) Regression analysis of the numbers
of impala at Nairobi National Park, Kenya.
vores in the partially fenced Nairobi National Park sug-
gests that many species of herbivore s within the Park ar e
experiencing declines of different levels depending on
their ecology and habitat requirements. This is not par-
ticularly surprising given the anthropogenic pressures
that the Park continue s to experien ce. Th e dis pers al ar eas
for wildlife are increasingly encroached resulting in few
dispersal areas. The wildlife species that find their ways
out of the Park rarely return because of continued human
encroachment and settlement in the dispersal area. Our
field observations showed that in the long-term land use
changes within the dispersal areas could have significant
impacts on the future status of the Park as one of the re-
maining refuges for wildlife in southern Kenya. In addi-
tion, fluctuations in rainfall and increased forage compe-
tition from cattle within the dispersal area may further
contribute to the decline for most species, especially in
the Athi-Kipiti plains.
4.1. Significance of Population Changes
The eight species showed substantial variability in num-
bers at the Park over the period, which is unsurprising
given their migratory patterns in and out of the Park
through the southern sections. However, the overall de-
cline in numbers shown by the species in the Park could
act as an early warning on the need for urgent conserva-
tion actions for the long-term viability of the Park and
the dispersal areas within the Athi-Kapiti areas as most
plains game species are known to traverse this area and
beyond feeding and calving [40]. The potential impacts
of climate change resulting in temporal rainfall variabi-
lity would further underpin the dynamics of wildlife
habitat at the Park since there is a well established rela-
tionship between rainfall and primary production of grass
in semi-arid tropics [5,14]. This coupled with the an-
thropogenic factors would further exacerbate the decline
patterns for herbivores and other wildlife species [29,33,
41,42].
The increase in numbers of the plains game species
within the Park during dry season indicated that the Park
has still maintained its reputation as a concentration area
for wildlife species, although the several man-made dams
providing water for wildlife within the Park are increase-
ingly affected by prolonged seasons of drought leading to
reduced water volumes. However, this pattern could be
unpredictable as observed in other similar ecosystems in
Kenya such as the Mara-Serengeti that has been charac-
terized by alternating periods of predominantly d ry years
followed wet years lasting for long periods [14]. The
need for water reservation structure to store more water
for use during dry seaso ns s hould be given priority.
During wet seasons, several season wetlands emerge
beyond the Park boundaries, especially in the southern
sections. There is always a tendency of most species
spreading out governed by water availability and lush
vegetation in the entire plain including the Park as
documented by previous studies [43]. Wildebeest in par-
ticular spend the wet seasons outside the Park in the
southern sections where the grass growth is more pro-
ductive and rich in nutrients, and use the opportunity to
breed before moving back to the Park. However, their
movements back to the Park together with other plains

Patterns of Variation of Herbivore Assemblages at Nairobi National Park, Kenya, 1990-2008 860
(a)
(b)

Patterns of Variation of Herbivore Assemblages at Nairobi National Park, Kenya, 1990-2008861
(c)
(d)

Patterns of Variation of Herbivore Assemblages at Nairobi National Park, Kenya, 1990-2008 862
(e)
(f)

Patterns of Variation of Herbivore Assemblages at Nairobi National Park, Kenya, 1990-2008863
(g)
(h)
Figure 3. Autocorrelations with 95% tolerance limits in numbers of eight herbivores ((a) African Buffalo’s; (b) Eland; (c)
Burchell’s Zebra; (d) Wildebeest; (e) Giraffe; (f) Grant’s Gazelle; (g) Thomson’s Gazelle and (h) Impala) species at Nairobi
National Park, 1990-2008.

Patterns of Variation of Herbivore Assemblages at Nairobi National Park, Kenya, 1990-2008 864
cing challenges due to dimi-
bility of Indication
how correlated
further evident that patterns
of
ation for long-term
he
5. Acknowledgements
nitoring programme is con-
monitoring and assessment
[1] C. Stoner, T. Cgwa, G. Sabuni, M.
Borner and C Large Herbivore
ame appear to experieng
nishing space. This could explain their patterns as shown
by autocorrelations after year 2000. Human settlements
and livestock grazing within the dispersal area increased
after the year 2000, and this has limited the movements
of wildebeest and other plains game species to and from
the Park.
4.2. Relia
Wildlife species with similar ecology do s
patterns at specific sites, although not with complete
consistency but the reliability of population estimates
derived from censuses can be affected by counting errors
and biases as animal numbers may be underestimated
due to vegetation cover, especially for ground censuses
[35,44]. However, our results still confirmed and ex-
tended patterns as documented from analysis of a subsec-
tion of the K ruger data [5].
From our analyses, it was
Pop
variations for browsing species such as giraffes and
elands were partly attributed to their ecology. These spe-
cies feed mainly in browse, and fluctuations in their
numbers within the Park were inter-correlated. These
correlations make sens e, but it is difficult to interp ret the
links between them, except in terms of overarching vari-
ables such as browse quality and availability. The ob-
served correlation patterns for Burchell’s Zebra with the
abundances of other herbivores reaffirms the role of ze-
bra in the vegetation succession within the Park. These
ultimately influence abundance and distribution of other
herbivores [45]. The repercussions resulting from indi-
rect effects of rearrangement of wildlife communities and
changes in the patterns of inter-specific interactions [45]
will probably transfor m the character of wildlife species’
interactions and fundamental ecosystem processes in
unforeseen ways in the Park [14,45].
Our analyses provide useful inform
rbivore populations’ management at Nairobi National
Park as they show long-term patterns of variations in
numbers under a changing habitat condition. This would
be important for the long-term conservation and mana-
gement of wildlife in the Park. The continuation of
monitoring of wildlife population through regular census
would be essential as has been done over the past two
decades. However, there is still need for collection of
additional and complimentary information of biotic and
abiotic information to allow for more focused analyses.
Specific research topics linking wildlife population fluc-
tuation patterns with the potential impacts of climate
change and anthropogenic effects would be useful. In
addition, land use planning and management of wildlife
corridors and dispersal areas require considerable atten-
tion from all wildlife management stakeholders.
The wildlife census and mo
ducted as part of biodiversity
by Kenya Wildlife Service for all conservation areas in
the country. We thank Nairobi National Park manage-
ment and volunteers who have over the years participated
in these counts since the programme was initiated. We
further thank anonymous reviewers who provided com-
ments and suggestions on this manuscript.
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