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Austral Ecology (2010) ••, ••–••
Disturbance, species loss and compensation: Wildfire and
grazing effects on the avian community and its food supply
in the Serengeti Ecosystem, Tanzania
aec_2167
1..10
A. K. NKWABI,1 A. R E. SINCLAIR,1,2* K. L. METZGER,1,2 AND S. A. R. MDUMA1
Serengeti Biodiversity Program, Tanzania Wildlife Research Institute, Arusha, Tanzania; and 2Centre for
Biodiversity Research, University of British Columbia,Vancouver, British Columbia, v6t 1z4, Canada
(Email: [email protected])
1
Abstract An important question in biodiversity studies is whether disturbances in ecosystems will cause a net loss
of species or whether such losses can be compensated by replacement of other species. We use two natural
disturbances, fire and grazing, to examine the response of bird and arthropod communities in grasslands of
Serengeti, Tanzania. Both burning and grazing by migrant ungulates take place at the end of the rains in June–July.
We documented the communities before disturbance, then 1, 4 and 20 weeks after disturbance on three replicate
plots and compared them with three undisturbed plots. Birds were recorded by observation, arthropods from
pitfall, tray trap and sweepnet samples. We expected that as the grass biomass was reduced by either disturbance,
bird communities would change with concomitant change in arthropod food abundance. Alternatively, bird
communities would change not with the absolute amount of food but with the greater accessibility of food as the
grass structure changed from long to short grass. Results showed first that both bird species richness and abundance
increased after both types of disturbance, but burnt sites showed a greater increase than that for grazed sites.
Second, there was a change in bird species composition with disturbance. The functionally equivalent athi
short-toed lark (Calandrella athensis) was replaced by the red-capped lark (Calandrella cinerea). Third, the abundance of most groups of arthropods was lower on disturbed sites than those on undisturbed sites, and the reduction
of arthropod numbers was greatest on burnt sites. These results imply that bird abundance did not occur through
an increase in arthropod abundance but rather through a change in the grass structure making food more
accessible; and the higher predation could have caused the lower arthropod abundance. In addition, some bird
species replaced others thus functionally compensating for their loss.
Key words: arthropods, compensation, ecosystem function, grasslands, Serengeti birds.
INTRODUCTION
An important question in biodiversity studies is
whether disturbances in ecosystems will cause a net
loss of species or whether such losses can be compensated by replacement of other species. In this paper we
document the impacts of two natural disturbances, fire
and grazing, on the diversity and abundance of birds
and arthropods in the Serengeti grassland ecosystem.
The Serengeti ecosystem supports some 640 species of
birds, of which the greatest number are insectivores.
Grass fires have dominated African savanna landscapes for hundreds of millennia (Bird & Cali 1998)
and plants are fire tolerant. The savanna plant community has been shaped by the frequency of burns
(Frost & Robertson 1987). Hence, the vegetation in
savanna Africa has become adapted and shaped by fire
*Corresponding author.
Accepted for publication June 2010.
© 2010 The Authors
Journal compilation © 2010 Ecological Society of Australia
so that it can be considered as a natural disturbance
(van Langevelde et al. 2003), and this applies in the
Serengeti ecosystem (Norton-Griffiths 1979; Sinclair
& Norton-Griffiths 1979; Stronach 1988; Sinclair
et al. 2007). Fire is frequent on the Serengeti long
grass plains but not on the short grass plains where
migratory grazers concentrate in the wet season, preventing the accumulation of combustible fuel loads
(Norton-griffiths 1979; McNaughton 1985).
The Serengeti supports the largest herds of migrating ungulates in the world. Estimates put wildebeest
(Connochaetes taurinus) around 1.3 million, zebra
(Equus burchelli) at 250 000 and Thomson’s gazelle
(Gazella thomsoni) at 440 000 (Mduma & Hopcraft
2008). During November–May the migrant grazers
occupy the short grass plains. Due to the rainfall gradient these short grass areas dry out first, and as they
do so in May–June the herds move rapidly westwards
through the long grass areas, grazing down the tall
grass, before moving into the woodlands. They remain
in the long grass areas for a few days only and so create
doi:10.1111/j.1442-9993.2010.02167.x
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A . K . N K WA B I ET AL.
a rapid change in grass structure (Sinclair 1977).They
do not return until the next wet season 4–6 months
later.
Both burning and grazing disturbances have their
major effect by altering the grass structure from long
grass (80–100 cm) to shorter grass (5–25 cm), and
both occur at about the same time of year, the beginning of the dry season. Fire removes effectively all the
biomass, which must then re-grow as short grass.
Because fires occur at the start of the dry season (the
majority occur in July) subsequent rainfall is low and
re-growth is in the form of leaves less than 15 cm high
with no flowerheads. Grass then remains in this structure until the rains begin in November. Grazing
removes some 50% of the biomass leaving a stubble
some 10–25 cm high (Sinclair 1977; Sinclair et al.
1985). Thus, fire is a more extreme disturbance than
grazing in altering the grass structure. In addition, fire,
unlike grazing, can potentially kill arthropods living in
the long grass (Evans 1984). Time is required both to
re-grow green vegetation and to recolonize the habitat
with arthropods.
We consider to what extent the community of birds
using the Serengeti long grass plains is affected by
these two natural disturbances. We examine three
aspects: First, how does the richness of bird species
change with burning and grazing and to what extent is
this affected by disturbances to their arthropod food
supply? Second, how does the abundance of birds that
use the grass layer change with disturbance? We
hypothesized that the richness and abundance of bird
species should be altered because the arthropod food
supply in the grass layer should decline when the
structure is changed from tall grass to short grass.
Alternatively, bird species richness and abundance
should be altered because the change in grass structure
could affect their foraging mode even if food abundance did not change. Third, are there changes in the
types of birds in the different grass structure? Richness
alone does not indicate whether species replace each
other within functional groups. Thus, we examine
changes in individual species.
MATERIALS AND METHODS
Study area
The Serengeti-Mara ecosystem (Fig. 1) is an area of some
25 000 km2 on the border of Tanzania and Kenya, East Africa
(34° to 36°E, 1° to 3°30′S). The extent of the ecosystem is
defined by the movements of the migratory wildebeest.There
are two major habitat types, a treeless grassland in the southeast of the system, and savanna dominated by Acacia trees in
the west and north. This study is confined to the plains
Fig. 1. Map showing the Serengeti National Park, and the long and short grass plains. The sites where replicate treatments
(burned, grazed and undisturbed plots) are located are indicated by arrows.
doi:10.1111/j.1442-9993.2010.02167.x
© 2010 The Authors
Journal compilation © 2010 Ecological Society of Australia
D I S T U R B A N C E O N AV I A N A N D A RT H R O P O D C O M M U N I T Y
habitat, which is divided into two parts, the long grass and
the short grass plains (Fig. 1).
Soils on the eastern plains are highly saline, alkaline and
shallow as a result of their recent volcanic origin. Consequently, the grasslands here are only some 10–15 cm high.
The soils become progressively deeper and less alkaline
towards the northwest plains and into the woodlands.
Rain typically falls in a bimodal pattern, with the long rains
during March–May and the short rains in November–
December (Norton-Griffiths et al. 1975). There is a rainfall
gradient from the dry southeast plains (<500 mm per year) to
the wet northwest long grass plains (800 mm) and continuing on to the Kenya border (1200 mm per year) (Sinclair &
Norton-Griffiths 1979).
The bird fauna
Descriptions of the avifauna in the Serengeti ecosystem are
given in Sinclair (1978), Folse (1982), Sinclair et al. (2002)
and Mwangomo et al. (2007). Although the whole Serengeti
ecosystem supports some 640 species of birds (A. Sinclair
unpubl. data, 2009), we consider in this paper only those
inhabiting the treeless grassland habitat often referred to as
the plains. These grasslands, as described above, differ in
both structure and plant composition between the southeast
short grass swards and the northwest long grass stands. Associated with these plant communities are different associations
of bird species. They support four major groups of insectivorous birds: cisticolas, larks, pipits and plovers, in two feeding
guilds; (i) those, like some cisticolas (small warbler-like
forms), that in tall grass feed among the grass stems above
ground; and (ii) those that feed on the ground, which include
species from all four groups, in both long and short grass.
There are also two main families of seed eating birds, the
weavers and estrildines. In all there are 27 families with 78
species documented in Appendix S1 for the different grassland types.
Sites and treatment plots
The long grass plains comprise a highly uniform grassland
community over large areas (about 1500 km-2) dominated by
tall (100 cm) grasses such as Themeda triandra, Pennisetum
mezianum and Digitaria macroblephora. The migratory herds
of wildebeest move through these long grass plains each year
at the end of May and their routes and timing have been
highly predictable for the past 40 years. Three sites were
selected for the treatment plots after the wildebeest had
arrived in the long grasslands and after burning commenced,
the two treatments starting effectively simultaneously. The
selection of sites had to await these events in order to place
the treatments. Within a site there were three replicates for
grazing and three for burning with paired undisturbed plots
(ungrazed and unburnt). Prior to the treatments we set up six
pretreament plots in the general vicinity of the sites. The
exact locations of the grazed plots were chosen once the
herds had moved through, which normally takes a few days.
Burns were set by park rangers during the time that the
wildebeest migration moved through, and located at our
© 2010 The Authors
Journal compilation © 2010 Ecological Society of Australia
3
request. The burnt treatment plots were located within these
burned areas. The matched undisturbed plots were then
placed nearby where neither grazing nor burning had taken
place. Each replicate plot was 70 m ¥ 70 m in size (0.5 ha),
located using GPS, and marked by flagging tape at the
corners of the site. The demarcation defined the boundaries
for counting birds.
Censusing birds
Six plots were surveyed per day (that is all burn and undisturbed or grazed and undisturbed were counted on 1 day).
These counts were repeated for 3 days and the numbers for
each plot averaged over the 3 days. The number of days was
determined from a pilot study where plots were counted for
up to 6 days. Since all species present on a plot had been
recorded after 3 days this number of days was chosen. A
20-min count period was used in each plot, and the time of
day between 0630 and 1030 h in the morning was rotated
among the six plots to even out this possible bias. Birds were
counted by walking slowly across the plot and back again.
Birds were identified by both sight and call, and numbers
recorded (Pomeroy & Tengecho 1986; Pomeroy 1992; Bibby
et al. 2000).
Birds were counted in four time periods. Period 1, pretreatment: Counts were conducted in long grass areas prior
to burning and grazing to act as the baseline before treatments were applied. Period 2: Counts were conducted
shortly (<1 week) after burning or grazing affected the study
plots. Period 3: Plots were monitored 4 weeks after burning
or grazing, this time being in the middle of the dry season but
after green re-growth had taken place. Period 4: Counts were
conducted at the beginning of the subsequent rains when
grass began rapid growth (20 weeks after disturbances). This
was to examine whether there was a delayed effect of disturbance on use by birds.
As part of a long-term study we conducted a vehicle
transect through both the long grass (30 km) and short grass
plains (25 km) (Fig. 1) in the years 1997–2008. Birds were
counted within 50 m of the vehicle twice a year. This provided an index of abundance of species within the communities of the two habitats.
Arthropod sampling
Methods for the collection of arthropods followed those recommended by New (1998). Ground surface dwelling arthropods – ants, spiders and Coleoptera – were sampled with
pitfall traps (8.7 cm diameter). Tray traps (28 cm diameter),
painted yellow because they are seen as green by arthropods,
attracted flies and wasps. Traps were left for 3 days. Sweep
net samples (standard 30 sweeps per sample) captured
arthropods in the herb layer, largely Orthoptera and
Hemiptera. Two samples from each method were pooled
from each plot. Captured arthropods were collected and
preserved in alcohol. Arthropod samples were sorted into
major groups, counted and voucher specimens were
mounted for later identification.
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A . K . N K WA B I ET AL.
Data Analysis
In this analysis we included all birds that fed on arthropods
or seeds in the herb and ground layers. We excluded vertebrate feeders, and aerial insectivores because their activities
generally covered a much larger scale than that of our
experiments.
Species richness was estimated using rarefaction (Krebs
2001). Rarefaction is a statistical method for estimating the
number of species in a given sample of individuals. It allows
two samples of different sizes, representing different sampling efforts, to be compared. There are two restrictions on
the use of this method (Krebs 2001). First, the groups of
species must be taxonomically similar. Second, the two
groups must be estimated by similar methods. Both of these
restrictions were met because similar groups of birds were
compared in the analyses and the data were collected by the
same methods.
The abundances of ground feeding birds in the different
treatments and time periods were averaged across the three
replicates for each treatment and time, and standard errors
computed. The abundances of above ground arthropods
were averaged across the replicates for different methods of
capture, treatments and time periods, and standard errors
computed.
RESULTS
Bird species richness
Rarefaction estimates for ground feeding birds were
used to calculate the probable number of species seen
in burned versus undisturbed and grazed versus undisturbed sites on the long grass plains for the three time
periods, namely 1, 4 and 20 weeks, following the start
of the perturbations (Fig. 2). The rarefaction curves
show that immediately after either disturbance there
was an increase in bird diversity relative to the undisturbed sites and this persisted throughout the subsequent dry season and early growing season (20 weeks).
However, with burning this increase in diversity
became less pronounced as time proceeded after the
disturbance (Fig. 2a,b,c), while the increase in diversity after grazing became more pronounced with time
(Fig. 2d,e,f)
burned and undisturbed plots was accounted for by
the burning effect where mean abundance of 139 birds
per plot on burns was much higher than the 45 on
undisturbed plots (t = 5.12, n = 18, P = 0.0001).
Although there was a grazing effect (mean abundance
grazed 140, undisturbed 98 per site) it was not significant (t = 1.4, n = 18, P = 0.17).
Inspection of the data on individual bird species
showed that the differences in abundance with treatment were due largely to changes in the larks, cisticolas
and plovers (Table 1). Thus, of the top 10 most frequent species in undisturbed plots, only two, rufousnaped lark (Mirafra africana), African quail finch
(Ortygospiza atricollis) appear in the top 10 burned plot
species, and then at much lower frequency. In burned
and grazed plots there were four abundant species that
were not recorded at all in undisturbed plots [redcapped lark (Calandrella cinerea), Richard’s pipit
(Anthus novaeseelandiae), black-winged plover (Vanellus
melanopterus), chestnut-banded sandgrouse (Pterocles
exustus)]. We have chosen six species to illustrate the
different responses to treatments (Fig. 4). Both the
athi short-toed lark (Calandrella somalica) (ground
living) and croaking cisticola (Cisticola natalensis)
(grass dwelling) had higher abundances in undisturbed plots throughout the period of study. The Fischer’s sparrow-lark (Eremopterix leucopareia) was
abundant in undisturbed plots when they were still
green (pretreatment) but they moved into both burnt
and grazed plots when these appeared while their
abundance in undisturbed plots declined progressively
through the dry season. Richard’s pipit, crowned
plover (Vanellus coronatus) and red-capped lark were
not present in pretreatment plots but appeared in
treated plots in large numbers when these disturbances
occurred while being largely absent in the equivalent
undisturbed plots.
Some species appeared to replace others when the
treatments were imposed. The athi short-toed lark,
initially abundant in pretreated sites, dropped in
number as soon as burning and grazing occurred (Fig.
5), and only regained abundance on those plots once
growth took place at 20 weeks. At the same time the
red-capped lark, which was absent before the treatments, increased to high numbers in burned and
grazed plots replacing the athi short-toed lark.
Abundance of grassland birds
Transects in long grass and short grass plains
Generally, in the undisturbed plots abundances of
birds dropped from time period 1 (pretreatment) to
period 2 and subsequently as conditions became drier
(Fig. 3). In contrast, bird abundance remained higher
on the combined burnt and grazed treatments than the
equivalent undisturbed plots, and in all time periods
following application of the disturbances (Fig. 3)
(t = 3.8, n = 36, P = 0.0006). This difference between
doi:10.1111/j.1442-9993.2010.02167.x
Data from the twice yearly vehicle transects showed
the frequency distribution of the insectivorous and
granivorous ground and herb layer birds for the two
habitats (Appendix S1). This is our best estimate of
these bird communities in the two habitats. We compared the frequency distributions of birds in the undisturbed and treated (burnt, grazed) plots with those
© 2010 The Authors
Journal compilation © 2010 Ecological Society of Australia
D I S T U R B A N C E O N AV I A N A N D A RT H R O P O D C O M M U N I T Y
a
unburnt
burnt
Burnt 1 week
d
Grazed 1 week
5
ungrazed
grazed
30
25
25
20
Species
Species
30
20
15
10
15
10
5
5
0
0
0
b
200
400
Individuals
600
0
800
e
Burnt 4 weeks
25
200
400
Individuals
600
Grazed 4 weeks
16
14
20
Species
Species
12
15
10
10
8
6
4
5
2
0
0
0
c
200
400
Individuals
600
0
f
Burnt 20 weeks
100
200
300
Individuals
400
Grazed 20 weeks
18
25
16
14
Species
Species
20
15
10
12
10
8
6
4
5
2
0
0
0
200
400
600
Individuals
0
50
100
Individuals
Fig. 2. Rarefaction curves for total number of ground feeding bird species on the long grass plains observed at different times
in weeks following application of burning and grazing (solid circles) in comparison to undisturbed (unburnt and ungrazed) plots
(open triangle). Vertical lines, one S.D.
frequencies obtained from the standard vehicle
transects through the long grass and short grass plains
(Table 2).The Bray-Curtis similarity coefficients show
that the untreated long grass communities were most
similar to the long grass plains community, and least
similar to the short grass plains community. The similarity value between untreated and treated communities falls between these former two values. Moreover,
the treated (burnt plus grazed) bird communities were
most similar to the short grass plains community.
© 2010 The Authors
Journal compilation © 2010 Ecological Society of Australia
Abundance of arthropods
In general, there was greater abundance of arthropod
groups in the undisturbed grassland compared with
that in both burnt and grazed plots (Appendix S2),
and this difference is illustrated in Figure 6 for the
combined catch by each method. The only exceptions
were that wasps were marginally more abundant in
grazed and burnt plots than in undisturbed plots, and
beetles were more abundant in grazed plots (Fig. 6,
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A . K . N K WA B I ET AL.
Longgrass burnt
Longgrass unburnt
a
Longgrass grazed
b
Longgrass ungrazed
200
200
160
Number
Number
160
120
80
40
120
80
40
0
0
Pre
1
4
20
Pre
Weeks
1
4
20
Weeks
Fig. 3. Mean abundance of birds per sites of all ground feeding birds in plots at different time periods following burning (a)
or grazing (b) compared with abundances on sites prior to these perturbations or at contemporaneous undisturbed sites. Open
bar, unburnt or ungrazed; solid bar, burnt or grazed; vertical lines, one S.E.
Table 1. Top: The number and proportion of birds recorded for the top 10 species in the combined undisturbed sites,
compared with that of the same species in the combined burnt and grazed plots (disturbed). Bottom:The number and proportion
of birds recorded for the top 10 species in the disturbed plots compared with that of the same species in the undisturbed plots
Top 10 in undisturbed plots
Species
Zitting cisticola
Athi short-toed lark
Rufous-naped lark
Croaking cisticola
African quail finch
Fan-tailed widowbird
White-winged widowbird
Rosy-breasted longclaw
Senegal bustard
White-tailed bushlark
Number
undisturbed
Proportion undisturbed
Number
disturbed
Proportion
disturbed
749
632
271
133
98
79
61
50
46
45
0.308
0.260
0.111
0.055
0.040
0.032
0.025
0.021
0.019
0.018
182
77
182
0
43
0
0
0
9
29
0.067
0.029
0.067
0
0.016
0
0
0
0.003
0.011
Top 10 in treated plots
Species
Red-capped lark
Crowned plover
Caspian plover
Fischer’s sparrowlark
Zitting cisticola
Rufous-naped lark
Richard’s pipit
Black-winged plover
Superb starling
Capped wheatear
Number
disturbed
704
301
226
209
182
182
130
125
92
87
Appendix S2). Time trends in catch were not consistent across treatments or trapping method. In both
treatments catches were usually higher after 20 weeks,
when re-growth following the start of the rains was
occurring, compared with the 1- and 4-week
samples.
doi:10.1111/j.1442-9993.2010.02167.x
Proportion
disturbed
0.261
0.112
0.084
0.077
0.067
0.067
0.048
0.046
0.034
0.032
Number
undisturbed
0
1
0
33
749
271
7
0
19
0
Proportion
undisturbed
0
0
0
0.014
0.308
0.111
0.003
0
0.008
0
DISCUSSION
We proposed that bird richness and abundance should
change with burning or grazing due to a concomitant
change in food supply. The results indicated that on
the long grass plains there was an increase in bird
© 2010 The Authors
Journal compilation © 2010 Ecological Society of Australia
D I S T U R B A N C E O N AV I A N A N D A RT H R O P O D C O M M U N I T Y
Burnt
Grazed
Control
ASTL
25
7
CRCI
3.5
3
20
15
Mean
Mean
2.5
10
2
1.5
1
5
0.5
0
0
Pre
1
4
20
Pre
1
4
Period
20
Period
RIPI
FSLA
9
7
8
6
7
5
Mean
Mean
6
5
4
3
4
3
2
2
1
1
0
0
Pre
1
4
Pre
20
1
4
20
Period
Period
CRPL
RCLA
9
35
8
30
7
25
Mean
Mean
6
5
4
3
20
15
10
2
5
1
0
0
Pre
1
4
20
Period
Pre
1
4
20
Period
Fig. 4. Mean abundance per site of individual bird species observed at different time periods in long grass plains. Open bar,
undisturbed; solid bar, burnt, and hatched bar, grazed. Vertical lines, one S.E. ASTL, athi short-toed lark; CRCI, croaking
cisticola; FSLA, Fischer’s sparrow-lark; RIPI, Richard’s pipit; CRPL, crowned plover; RCLA, red-capped lark.
species richness on both burned and grazed areas
compared with those on the undisturbed plots. Insectivorous birds formed the majority of ground-living
birds in these Serengeti habitats.Thus, arthropod food
abundance was taken as the measure of food supply for
these birds. In general, most groups of arthropods
declined in abundance in both burning and grazing
treatments in contrast to the increase in bird richness
and abundance, a result contrary to our expectations.
Thus, these higher abundances of birds were not due
to higher absolute food resources. The lower food
resources could have resulted either from the damaging effects of the disturbances or from the higher depredations of the more abundant insectivorous birds.
An alternative explanation is that bird species richness and abundance changed with these disturbances
© 2010 The Authors
Journal compilation © 2010 Ecological Society of Australia
due to changes in grass structure that affected foraging mode, but not necessarily food abundance.
Because our results showed that food abundance
declined as both bird richness and abundance
increased they imply that birds were taking advantage
of the change in grass structure, were able to access
food easier and so reduce arthropod abundance. In
essence, this result implies that there was a change in
ecosystem function by increasing the top-down predation effect on arthropods due to a change in
habitat structure.
Two other factors could contribute to both higher
bird species richness and higher abundance in burned
and grazed areas on the long grass plains compared
with those on the undisturbed plots. First, although
grass height was lower in burned and grazed areas,
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A . K . N K WA B I ET AL.
35
8
30
7
6
5
20
4
15
3
10
Mean ASTL
Mean RCL A
25
2
5
1
0
0
0
1
2
3
4
5
Period
Fig. 5. Mean abundance per plot of red-capped lark
(RCLA solid circles) and athi short-toed lark (ASTL open
triangles) on burned areas of the long grass plains. Time 1,
pre-treatment; time 2, 1 week; time 3, 4 weeks; time 4,
20 weeks after burning.
Table 2. The Bray-Curtis coefficients of similarity for the
bird communities on the long grass and short grass plains
recorded on the vehicle transects, and on the long grass
undisturbed plots, and the burnt and grazed plots
Bray-Curtis coefficients
Undisturbed versus long grass transect
Undisturbed versus short grass transect
Undisturbed versus burnt + grazed
Burnt + grazed versus long grass transect
Burnt + grazed versus short grass transect
Coefficient
0.384
0.092
0.243
0.329
0.546
habitat heterogeneity through a mosaic of different
heights may actually have been greater. Second, many
bird species in these areas are nomadic and this habit
enables them to locate recently burnt or grazed
patches. Influxes of species, such as red-capped larks,
black-winged plovers and Richard’s pipits appeared on
burned sites, while these and Caspian plovers
appeared on grazed sites. Their nearest locations were
on the short grass plains, which occur some 60 km to
the southeast.
It is possible that the change in grass structure
from tall to short grasslands could be mimicking the
creation of patches of short grass plains within the
long grass habitat. Such patches may be attracting
the avifauna of the short grass plains. Although we
did not conduct site counts on the short grass plains,
we have the 11 years of data from transect counts
conducted in both the long and short grass plains.
These showed that the bird communities of the disturbed sites were more similar to that on the short
grass plains than that on the undisturbed long grass
plains. Thus, the perturbations of burning and
grazing changed the bird community towards that
seen on the short grass plains.
doi:10.1111/j.1442-9993.2010.02167.x
We also examined whether individual species might
replace each other ecologically, showing opposite
trends in abundance with treatment. The results
showed that at least in the long grass habitats some lark
species with similar ground living insectivorous food
habits replaced others after treatments were imposed
(Fig. 5). Therefore, these results show that species can
replace each other functionally following disturbances
such as burning and grazing.
Few other studies have examined the effects of
burning and grazing on bird populations in Africa
(Parr & Chown 2003). Ogada et al. (2008) reported
that the removal of large herbivores from African
savanna plots increased bird diversity. The effects of
the large mammals were through the removal of tree
canopy, which reduced the number of bird species.
There were little effect of the mammals on the grass
layer. O’Reilly et al. (2006) have documented the
effects of experimental fires on African savanna birds
in Kenya. They recorded that burning increased bird
diversity relative to controls but there were no significant effects on either richness or total abundance.
Thus, numbers were distributed more evenly across
the same species, unlike our study that found a clear
increase in both richness and abundance with both
burning and grazing. Jansen et al. (1999) found that
the richness of grassland birds and the density of the
red-winged francolin (Francolinus levaillantii) were
negatively correlated with grazing intensity and annual
burning, whereas grey-winged francolin (Francolinus
africanus) density was positively correlated with
grazing intensity in South Africa. A comparison of
grazed and ungrazed sites in southeast Arizona showed
an increase in some bird species on grazed lands that
were habitat generalists, but it also demonstrated
declines in some grassland sparrow species like the
Botteri’s sparrow and Cassin’s sparrows in the same
area (Bock & Bock 1993). Woinarski (1990) found in
northern Australia that short-term effects of fire
involved a substantial increase in both diversity and
density of birds. Thus, our results are consistent with
some other studies, namely increases in richness and
abundance with burning and grazing, but not all
results are consistent with this conclusion. It is likely
that higher frequencies of burning and grazing may
reverse the effects we found, and so variation in the
severity of disturbance needs further investigation.
Our study also throws light on the possible mechanisms producing the changes in the bird communities.
CONCLUSIONS
This study investigated how the richness and abundance of bird species changed with two types of disturbance to the grass layer, burning and grazing, and
examined whether there was a concomitant change in
© 2010 The Authors
Journal compilation © 2010 Ecological Society of Australia
D I S T U R B A N C E O N AV I A N A N D A RT H R O P O D C O M M U N I T Y
40
45
40
35
35
30
30
Number
Number
Grazed
Ungrazed
Burnt
Pit grazed
Unburnt
Pit burnt
25
20
15
25
20
15
10
10
5
5
0
0
Pre
1
4
Pre
20
1
Weeks
20
Tray trap grazed
70
180
60
160
140
Number
50
Number
4
Weeks
Tray trap burnt
40
30
120
100
80
60
20
40
10
20
0
0
Pre
1
4
20
Pre
1
Weeks
4
20
Weeks
Sweep burnt
Sweep grazed
45
45
40
40
35
35
30
30
Number
Number
9
25
20
25
20
15
15
10
10
5
5
0
0
Pre
1
4
20
Weeks
Pre
1
4
20
Weeks
Fig. 6. Mean abundance per sample for arthropods caught by pitfall traps (top, ants, spiders and Coleoptera), tray traps
(middle, Diptera, wasps and bees) and sweepnets (bottom, Hemiptera and Orthoptera) in plots at different time periods
following burning (left three figures) or grazing (right three figures) compared with abundances on plots prior to these
perturbations (Pre) or at contemporaneous undisturbed sites. Open bar, undisturbed; and solid bar, burnt or grazed. Vertical
lines, one S.E.
the arthropod food for the largely insectivorous bird
community. We found that both bird diversity and
abundance increased with both disturbances. In contrast, the abundance of arthropods declined with both
disturbances.Thus, the bird communities appear to be
changing in response to changes in the structure of
their habitat providing different conditions for new
species to enter, and enabling them to access arthropod food and lowering its abundance. This means
there may be a change in ecosystem function through
a change in the top-down predation cascade. The disturbances created a grassland structure that resembled
the short grass plains, and it was the bird species
normally residing in the short grass plains that
appeared on the disturbed sites.
© 2010 The Authors
Journal compilation © 2010 Ecological Society of Australia
Species that prefer long grass declined in number
with the onset of disturbance. However, some were
replaced by functional equivalents. Thus, the redcapped lark and the athi short-toed lark, which are
functionally equivalent, replaced each other when the
grassland changed from long to short grass in both
treatments.This replacement compensated for the loss
of some species due to the disturbance and contributed to maintaining diversity.
ACKNOWLEDGEMENTS
We thank the directors of Tanzania National Parks and
the Tanzania Wildlife Research Institute for permission
doi:10.1111/j.1442-9993.2010.02167.x
10
A . K . N K WA B I ET AL.
to work in the park.We thank especially John Mchetto,
Stephen Makacha and Joseph Masoy for their assistance in the fieldwork.This work was supported by the
Canadian Natural Sciences and Engineering Research
Council grant to ARES. All research contained in this
study complied with the laws of Tanzania.
REFERENCES
Bibby C. J., Burgess N. D., Hill D. A. & Mustoe S. (2000) Bird
Census Techniques. Academic Press, London.
Bird M. I. & Cali J. A. (1998) A million-year record of fire in
sub-Saharan Africa. Nature 394, 767–9.
Bock C. E. & Bock J. H. (1993) Cover of perennial grasses in
southeastern Arizona in relation to livestock grazing.
Conserv. Biol. 7, 371–7.
Evans E. (1984) Fire as a natural disturbance to grasshopper
assemblages of tallgrass prairie. Oikos 43, 9–16.
Folse L. J. (1982) An analysis of avifauna-resource relationships
on the Serengeti Plains. Ecol. Monogr. 52, 111–27.
Frost P. G. H. & Robertson F. (1987) The ecological effects of
fire in savannas. In: Determinants of Tropical Savannas (ed. B.
H. Walker) pp. 93–140. ICSU Press, Miami.
Jansen R., Little R. M., Crowe T. M. & Ikanda D. (1999)
Implications of grazing and burning of grasslands on the
sustainable use of francolins (Francolinus spp.) and on
overall bird conservation in the highlands of Mpumalanga
province, South Africa. Biodivers. Conserv. 8, 587–602.
Krebs C. J. (2001) Ecological Methodology. Harper and Row, New
York.
van Langevelde F., van de Vijver C. A. D. M., Kumar L. et al.
(2003) Effects of fire and herbivory on the stability of
savanna ecosystems. Ecology. 84, 337–50.
McNaughton S. J. (1985) Ecology of a grazing system: The
Serengeti. Ecol. Monogr. 55, 259–94.
Mduma S. A. R. & Hopcraft J. G. C. (2008) The main herbivorous mammals and crocodile in the Greater Serengeti
Ecosystem – Appendix. In: Serengeti III: Human Impacts on
Ecosystem Dynamics (eds A. R. E. Sinclair, C. Packer, S. A.
R. Mduma & J. M. Fryxell) Chicago University Press,
Chicago.
Mwangomo E. A., Hardesty L. H., Sinclair A. R. E., Mduma S.
A. R. & Metzger K. (2007) The association of two endemic
birds in the Serengeti ecosystem: Habitat selection, diet and
group formation of the rufous-tailed weaver, Fischer’s lovebird and associated species. Afr. J. Ecol. 46, 267–75.
New T. R. (1998) Invertebrate Surveys for Conservation. Oxford
University Press, Oxford.
Norton-Griffiths M. (1979) The influence of grazing, browsing,
and fire on vegetation dynamics of the Serengeti. In:
Serengeti: Dynamics of an Ecosystem (eds A. R. E. Sinclair &
M. Norton-Griffiths) University of Chicago Press,
Chicago.
Norton-Griffiths M., Herlocker D. & Pennycuick L. (1975) The
patterns of rainfall in the Serengeti ecosystem, Tanzania.
East Afr.Wildl. J. 13, 347–74.
doi:10.1111/j.1442-9993.2010.02167.x
O’Reilly L., Ogada D., Palmer T. M. & Keesing F. (2006) Effects
of fire on bird diversity and abundance in an East African
savanna. Afr. J. Ecol. 44, 165–70.
Ogada D. L., Gadd M. E., Ostfeld R. S., Young T. P. & Keesing
F. (2008) Impacts of large herbivorous mammals on bird
diversity and abundance in an African savanna. Oecologia
156, 387–97.
Parr C. L. & Chown S. L. (2003) Burning issues for conservation: A critique of faunal fire research in Southern Africa.
Austral Ecol. 28, 384–95.
Pomeroy D. E. (1992) Counting Birds. African Wildlife Foundation, Nairobi.
Pomeroy D. E. & Tengecho B. (1986) A method of analysing
bird distributions. Afr. J. Ecol. 24, 243–53.
Sinclair A. R. E. (1977) The African Buffalo. University of
Chicago Press, Chicago.
Sinclair A. R. E. (1978) Factors affecting the food supply and
breeding season of resident birds and movements of palaearctic migrants in a tropical African savannah. Ibis 120,
480–97.
Sinclair A. R. E., Dublin H. & Borner M. (1985) Population
regulation of Serengeti Wildebeest: A test of the food
hypothesis. Oecologia 65, 266–8.
Sinclair A. R. E., Mduma S. A. R. & Arcese P. (2002) Protected
areas as biodiversity benchmarks for human impacts: Agriculture and the Serengeti avifauna. Proc. of the Roy. Soc. of
London 269, 2401–5.
Sinclair A. R. E., Mduma S. A. R., Hopcraft J. G. C., Fryxell J.
M., Hilborn R. & Thirgood S. (2007) Long-term ecosystem
dynamics in the Serengeti: Lessons for conservation.
Conserv. Biol. 21, 580–90.
Sinclair A. R. E. & Norton-Griffiths M. (1979) Serengeti – Dynamics of An Ecosystem. University of Chicago Press, Chicago.
Stronach N. R. H. (1988) Grass Fires in Serengeti National Park,
Tanzania: Characteristics, Behavior and Some Effects on Young
Trees. University of Cambridge, Cambridge.
Woinarski J. C. Z. (1990) Effects of fire on the bird communities
of tropical woodlands and open forests in northern
Australia. Aust. J. Ecol. 15, 1–22.
SUPPORTING INFORMATION
Additional Supporting Information may be found in
the online version of this article:
Appendix S1. Family, Latin name and common
name of bird species identified in long grass
(transects), short grass (transects) and undisturbed
(this study) habitats.
Appendix S2. The abundance per sample and standard errors (in parenthesis) of the main arthropod
groups collected at different time periods in the burnt
and grazed plots compared to those collected in the
undisturbed plots (unburnt and ungrazed). Pre =
pretreatment, time 1 = 1 week, time 4 = 4 weeks, time
20 = 20 weeks after the treatments.
© 2010 The Authors
Journal compilation © 2010 Ecological Society of Australia