Disturbance, Species Loss and Compensation: Wildfire and Grazing

Austral Ecology (2010) ••, ••–••

Disturbance, species loss and compensation: Wildﬁre and grazing effects on the avian community and its food supply

in the Serengeti Ecosystem, Tanzaniaaec_2167 1..10

A. K. NKWABI,1 A. R E. SINCLAIR,1,2* K. L. METZGER,1,2 AND S. A. R. MDUMA1 1Serengeti Biodiversity Program,TanzaniaWildlife Research Institute, Arusha,Tanzania; and 2Centre for Biodiversity Research, University of British Columbia,Vancouver, British Columbia, v6t 1z4, Canada (Email: [email protected])

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 so that it can be considered as a natural disturbance (van Langevelde et al. 2003), and this applies in the An important question in biodiversity studies is Serengeti ecosystem (Norton-Griffiths 1979; Sinclair whether disturbances in ecosystems will cause a net & Norton-Griffiths 1979; Stronach 1988; Sinclair loss of species or whether such losses can be compen- et al. 2007). Fire is frequent on the Serengeti long sated by replacement of other species. In this paper we grass plains but not on the short grass plains where document the impacts of two natural disturbances, fire migratory grazers concentrate in the wet season, pre- and grazing, on the diversity and abundance of birds venting the accumulation of combustible fuel loads and arthropods in the Serengeti grassland ecosystem. (Norton-griffiths 1979; McNaughton 1985). The Serengeti ecosystem supports some 640 species of The Serengeti supports the largest herds of migrat- birds, of which the greatest number are insectivores. ing ungulates in the world. Estimates put wildebeest Grass fires have dominated African savanna land- (Connochaetes taurinus) around 1.3 million, zebra scapes for hundreds of millennia (Bird & Cali 1998) (Equus burchelli) at 250 000 and Thomson’s gazelle and plants are fire tolerant. The savanna plant com- (Gazella thomsoni) at 440 000 (Mduma & Hopcraft munity has been shaped by the frequency of burns 2008). During November–May the migrant grazers (Frost & Robertson 1987). Hence, the vegetation in occupy the short grass plains. Due to the rainfall gra- savanna Africa has become adapted and shaped by fire dient 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 *Corresponding author. grass, before moving into the woodlands.They remain Accepted for publication June 2010. in the long grass areas for a few days only and so create

© 2010 The Authors doi:10.1111/j.1442-9993.2010.02167.x Journal compilation © 2010 Ecological Society of Australia 2 A. K. NKWABI ET AL. a rapid change in grass structure (Sinclair 1977).They supply? Second, how does the abundance of birds that do not return until the next wet season 4–6 months use the grass layer change with disturbance? We later. hypothesized that the richness and abundance of bird Both burning and grazing disturbances have their species should be altered because the arthropod food major effect by altering the grass structure from long supply in the grass layer should decline when the grass (80–100 cm) to shorter grass (5–25 cm), and structure is changed from tall grass to short grass. both occur at about the same time of year, the begin- Alternatively, bird species richness and abundance ning of the dry season. Fire removes effectively all the should be altered because the change in grass structure biomass, which must then re-grow as short grass. could affect their foraging mode even if food abun- Because fires occur at the start of the dry season (the dance did not change. Third, are there changes in the majority occur in July) subsequent rainfall is low and types of birds in the different grass structure? Richness re-growth is in the form of leaves less than 15 cm high alone does not indicate whether species replace each with no flowerheads. Grass then remains in this struc- other within functional groups. Thus, we examine ture until the rains begin in November. Grazing changes in individual species. 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, MATERIALS AND METHODS 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 Study area with arthropods. The Serengeti-Mara ecosystem (Fig. 1) is an area of some We consider to what extent the community of birds 25 000 km2 on the border ofTanzania and Kenya, East Africa using the Serengeti long grass plains is affected by (34° to 36°E, 1° to 3°30′S). The extent of the ecosystem is these two natural disturbances. We examine three defined by the movements of the migratory wildebeest.There aspects: First, how does the richness of bird species are two major habitat types, a treeless grassland in the south- change with burning and grazing and to what extent is east of the system, and savanna dominated by Acacia trees in this affected by disturbances to their arthropod food 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 DISTURBANCE ON AVIAN AND ARTHROPOD COMMUNITY 3

habitat, which is divided into two parts, the long grass and request.The burnt treatment plots were located within these the short grass plains (Fig. 1). burned areas. The matched undisturbed plots were then Soils on the eastern plains are highly saline, alkaline and placed nearby where neither grazing nor burning had taken shallow as a result of their recent volcanic origin. Conse- place. Each replicate plot was 70 m ¥ 70 m in size (0.5 ha), quently, the grasslands here are only some 10–15 cm high. located using GPS, and marked by flagging tape at the The soils become progressively deeper and less alkaline corners of the site. The demarcation defined the boundaries towards the northwest plains and into the woodlands. for counting birds. 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 Censusing birds the wet northwest long grass plains (800 mm) and continu- ing on to the Kenya border (1200 mm per year) (Sinclair & Six plots were surveyed per day (that is all burn and undis- Norton-Griffiths 1979). turbed 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 The bird fauna up to 6 days. Since all species present on a plot had been recorded after 3 days this number of days was chosen. A Descriptions of the avifauna in the Serengeti ecosystem are 20-min count period was used in each plot, and the time of given in Sinclair (1978), Folse (1982), Sinclair et al. (2002) day between 0630 and 1030 h in the morning was rotated and Mwangomo et al. (2007). Although the whole Serengeti among the six plots to even out this possible bias. Birds were ecosystem supports some 640 species of birds (A. Sinclair counted by walking slowly across the plot and back again. unpubl. data, 2009), we consider in this paper only those Birds were identified by both sight and call, and numbers inhabiting the treeless grassland habitat often referred to as recorded (Pomeroy & Tengecho 1986; Pomeroy 1992; Bibby the plains. These grasslands, as described above, differ in et al. 2000). both structure and plant composition between the southeast Birds were counted in four time periods. Period 1, pre- short grass swards and the northwest long grass stands. Asso- treatment: Counts were conducted in long grass areas prior ciated with these plant communities are different associations to burning and grazing to act as the baseline before treat- of bird species.They support four major groups of insectivo- ments were applied. Period 2: Counts were conducted rous birds: cisticolas, larks, pipits and plovers, in two feeding shortly (<1 week) after burning or grazing affected the study guilds; (i) those, like some cisticolas (small warbler-like plots. Period 3: Plots were monitored 4 weeks after burning forms), that in tall grass feed among the grass stems above or grazing, this time being in the middle of the dry season but ground; and (ii) those that feed on the ground, which include after green re-growth had taken place. Period 4: Counts were species from all four groups, in both long and short grass. conducted at the beginning of the subsequent rains when There are also two main families of seed eating birds, the grass began rapid growth (20 weeks after disturbances).This weavers and estrildines. In all there are 27 families with 78 was to examine whether there was a delayed effect of distur- species documented in Appendix S1 for the different grass- bance on use by birds. land types. 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 Sites and treatment plots counted within 50 m of the vehicle twice a year. This pro- vided an index of abundance of species within the commu- The long grass plains comprise a highly uniform grassland nities of the two habitats. 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 Arthropod sampling at the end of May and their routes and timing have been highly predictable for the past 40 years. Three sites were Methods for the collection of arthropods followed those rec- selected for the treatment plots after the wildebeest had ommended by New (1998). Ground surface dwelling arthro- arrived in the long grasslands and after burning commenced, pods – ants, spiders and Coleoptera – were sampled with the two treatments starting effectively simultaneously. The pitfall traps (8.7 cm diameter). Tray traps (28 cm diameter), selection of sites had to await these events in order to place painted yellow because they are seen as green by arthropods, the treatments. Within a site there were three replicates for attracted flies and wasps. Traps were left for 3 days. Sweep grazing and three for burning with paired undisturbed plots net samples (standard 30 sweeps per sample) captured (ungrazed and unburnt). Prior to the treatments we set up six arthropods in the herb layer, largely Orthoptera and pretreament plots in the general vicinity of the sites. The Hemiptera. Two samples from each method were pooled exact locations of the grazed plots were chosen once the from each plot. Captured arthropods were collected and herds had moved through, which normally takes a few days. preserved in alcohol. Arthropod samples were sorted into Burns were set by park rangers during the time that the major groups, counted and voucher specimens were wildebeest migration moved through, and located at our mounted for later identification.

Data Analysis burned and undisturbed plots was accounted for by the burning effect where mean abundance of 139 birds In this analysis we included all birds that fed on arthropods per plot on burns was much higher than the 45 on or seeds in the herb and ground layers. We excluded verte- undisturbed plots (t = 5.12, n = 18, P = 0.0001). brate feeders, and aerial insectivores because their activities Although there was a grazing effect (mean abundance generally covered a much larger scale than that of our grazed 140, undisturbed 98 per site) it was not signiﬁ- experiments. cant (t 1.4, n 18, P 0.17). Species richness was estimated using rarefaction (Krebs = = = 2001). Rarefaction is a statistical method for estimating the Inspection of the data on individual bird species number of species in a given sample of individuals. It allows showed that the differences in abundance with treat- two samples of different sizes, representing different sam- ment were due largely to changes in the larks, cisticolas pling efforts, to be compared. There are two restrictions on and plovers (Table 1). Thus, of the top 10 most fre- the use of this method (Krebs 2001). First, the groups of quent species in undisturbed plots, only two, rufous- species must be taxonomically similar. Second, the two naped lark (Mirafra africana), African quail ﬁnch groups must be estimated by similar methods. Both of these (Ortygospiza atricollis) appear in the top 10 burned plot restrictions were met because similar groups of birds were species, and then at much lower frequency. In burned compared in the analyses and the data were collected by the and grazed plots there were four abundant species that same methods. were not recorded at all in undisturbed plots [red- The abundances of ground feeding birds in the different treatments and time periods were averaged across the three capped lark (Calandrella cinerea), Richard’s pipit replicates for each treatment and time, and standard errors (Anthus novaeseelandiae), black-winged plover (Vanellus computed. The abundances of above ground arthropods melanopterus), chestnut-banded sandgrouse (Pterocles were averaged across the replicates for different methods of exustus)]. We have chosen six species to illustrate the capture, treatments and time periods, and standard errors different responses to treatments (Fig. 4). Both the computed. athi short-toed lark (Calandrella somalica) (ground living) and croaking cisticola (Cisticola natalensis) (grass dwelling) had higher abundances in undis- RESULTS turbed plots throughout the period of study. The Fis- cher’s sparrow-lark (Eremopterix leucopareia)was abundant in undisturbed plots when they were still Bird species richness green (pretreatment) but they moved into both burnt and grazed plots when these appeared while their Rarefaction estimates for ground feeding birds were abundance in undisturbed plots declined progressively used to calculate the probable number of species seen through the dry season. Richard’s pipit, crowned in burned versus undisturbed and grazed versus undis- plover (Vanellus coronatus) and red-capped lark were turbed sites on the long grass plains for the three time not present in pretreatment plots but appeared in periods, namely 1, 4 and 20 weeks, following the start treated plots in large numbers when these disturbances of the perturbations (Fig. 2). The rarefaction curves occurred while being largely absent in the equivalent show that immediately after either disturbance there undisturbed plots. was an increase in bird diversity relative to the undis- Some species appeared to replace others when the turbed sites and this persisted throughout the subse- treatments were imposed. The athi short-toed lark, quent dry season and early growing season (20 weeks). initially abundant in pretreated sites, dropped in However, with burning this increase in diversity number as soon as burning and grazing occurred (Fig. became less pronounced as time proceeded after the 5), and only regained abundance on those plots once disturbance (Fig. 2a,b,c), while the increase in diver- growth took place at 20 weeks. At the same time the sity after grazing became more pronounced with time red-capped lark, which was absent before the treat- (Fig. 2d,e,f) ments, 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 Data from the twice yearly vehicle transects showed period 2 and subsequently as conditions became drier the frequency distribution of the insectivorous and (Fig. 3). In contrast, bird abundance remained higher granivorous ground and herb layer birds for the two on the combined burnt and grazed treatments than the habitats (Appendix S1). This is our best estimate of equivalent undisturbed plots, and in all time periods these bird communities in the two habitats. We com- following application of the disturbances (Fig. 3) pared the frequency distributions of birds in the undis- (t = 3.8, n = 36, P = 0.0006). This difference between turbed and treated (burnt, grazed) plots with those doi:10.1111/j.1442-9993.2010.02167.x © 2010 The Authors Journal compilation © 2010 Ecological Society of Australia DISTURBANCE ON AVIAN AND ARTHROPOD COMMUNITY 5

unburnt ungrazed a Burnt 1 week burnt d Grazed 1 week grazed 30

25 30 20 25 15 20

Species 15 10

Species 10 5 5 0 0 0200400600800 0 200 400 600 Individuals Individuals

b Burnt 4 weeks e Grazed 4 weeks 25 16

20 14 12 15 10 8

Species 10 Species 6

5 4 2 0 0 0200400600 0 100 200 300 400 Individuals Individuals

c Burnt 20 weeks f Grazed 20 weeks 25 18 16 20 14 12 15 10 8 Species Species 10 6 5 4 2 0 0 0200400600 050100 Individuals 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 Abundance of arthropods transects through the long grass and short grass plains (Table 2).The Bray-Curtis similarity coefﬁcients show In general, there was greater abundance of arthropod that the untreated long grass communities were most groups in the undisturbed grassland compared with similar to the long grass plains community, and least that in both burnt and grazed plots (Appendix S2), similar to the short grass plains community.The simi- and this difference is illustrated in Figure 6 for the larity value between untreated and treated communi- combined catch by each method. The only exceptions ties falls between these former two values. Moreover, were that wasps were marginally more abundant in the treated (burnt plus grazed) bird communities were grazed and burnt plots than in undisturbed plots, and most similar to the short grass plains community. beetles were more abundant in grazed plots (Fig. 6,

a Longgrass burnt b Longgrass grazed Longgrass unburnt 200 Longgrass ungrazed 200 160 160 r 120 120

80 80 Numbe Number 40 40

0 0 Pre 1 4 20 Pre 1 4 20 Weeks 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

Number Number Proportion Species undisturbed Proportion undisturbed disturbed disturbed

Zitting cisticola 749 0.308 182 0.067 Athi short-toed lark 632 0.260 77 0.029 Rufous-naped lark 271 0.111 182 0.067 Croaking cisticola 133 0.055 0 0 African quail ﬁnch 98 0.040 43 0.016 Fan-tailed widowbird 79 0.032 0 0 White-winged widowbird 61 0.025 0 0 Rosy-breasted longclaw 50 0.021 0 0 Senegal bustard 46 0.019 9 0.003 White-tailed bushlark 45 0.018 29 0.011

Top 10 in treated plots

Number Proportion Number Proportion Species disturbed disturbed undisturbed undisturbed

Red-capped lark 704 0.261 0 0 Crowned plover 301 0.112 1 0 Caspian plover 226 0.084 0 0 Fischer’s sparrowlark 209 0.077 33 0.014 Zitting cisticola 182 0.067 749 0.308 Rufous-naped lark 182 0.067 271 0.111 Richard’s pipit 130 0.048 7 0.003 Black-winged plover 125 0.046 0 0 Superb starling 92 0.034 19 0.008 Capped wheatear 87 0.032 0 0

Appendix S2). Time trends in catch were not consis- DISCUSSION tent across treatments or trapping method. In both treatments catches were usually higher after 20 weeks, We proposed that bird richness and abundance should when re-growth following the start of the rains was change with burning or grazing due to a concomitant occurring, compared with the 1- and 4-week change in food supply. The results indicated that on samples. the long grass plains there was an increase in bird doi:10.1111/j.1442-9993.2010.02167.x © 2010 The Authors Journal compilation © 2010 Ecological Society of Australia DISTURBANCE ON AVIAN AND ARTHROPOD COMMUNITY 7

ASTL Burnt CRCI 25 Grazed 3.5 Control 3 20 2.5

15 2 Mean Mean 1.5 10 1 5 0.5

0 0 Pre 1 4 20 Pre 1 4 20 Period Period FSLA RIPI

9 7

8 6 7 5 6 5 4 Mean Mean 4 3 3 2 2 1 1 0 0 Pre 1 4 20 Pre 1 4 20 Period Period CRPL RCLA

9 35 8 30 7 25 6 5 20 Mean 4 Mean 15 3 10 2 5 1 0 0 Pre 1 4 20 Pre 1 4 20 Period 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 due to changes in grass structure that affected forag- compared with those on the undisturbed plots. Insec- ing mode, but not necessarily food abundance. tivorous birds formed the majority of ground-living Because our results showed that food abundance birds in these Serengeti habitats.Thus, arthropod food declined as both bird richness and abundance abundance was taken as the measure of food supply for increased they imply that birds were taking advantage these birds. In general, most groups of arthropods of the change in grass structure, were able to access declined in abundance in both burning and grazing food easier and so reduce arthropod abundance. In treatments in contrast to the increase in bird richness essence, this result implies that there was a change in and abundance, a result contrary to our expectations. ecosystem function by increasing the top-down pre- Thus, these higher abundances of birds were not due dation effect on arthropods due to a change in to higher absolute food resources. The lower food habitat structure. resources could have resulted either from the damag- Two other factors could contribute to both higher ing effects of the disturbances or from the higher dep- bird species richness and higher abundance in burned redations of the more abundant insectivorous birds. and grazed areas on the long grass plains compared An alternative explanation is that bird species rich- with those on the undisturbed plots. First, although ness and abundance changed with these disturbances grass height was lower in burned and grazed areas,

35 8 We also examined whether individual species might replace each other ecologically, showing opposite 30 7 trends in abundance with treatment. The results 6 25 showed that at least in the long grass habitats some lark M A 5 ean ASTL species with similar ground living insectivorous food 20 4 habits replaced others after treatments were imposed 15 (Fig. 5).Therefore, these results show that species can 3 Mean RCL replace each other functionally following disturbances 10 2 such as burning and grazing.

5 1 Few other studies have examined the effects of burning and grazing on bird populations in Africa 0 0 (Parr & Chown 2003). Ogada et al. (2008) reported 012345 Period that the removal of large herbivores from African savanna plots increased bird diversity. The effects of Fig. 5. Mean abundance per plot of red-capped lark the large mammals were through the removal of tree (RCLA solid circles) and athi short-toed lark (ASTL open canopy, which reduced the number of bird species. triangles) on burned areas of the long grass plains. Time 1, There were little effect of the mammals on the grass pre-treatment; time 2, 1 week; time 3, 4 weeks; time 4, 20 weeks after burning. 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 Table 2. The Bray-Curtis coefficients of similarity for the diversity relative to controls but there were no signifi- bird communities on the long grass and short grass plains cant effects on either richness or total abundance. recorded on the vehicle transects, and on the long grass Thus, numbers were distributed more evenly across undisturbed plots, and the burnt and grazed plots the same species, unlike our study that found a clear Bray-Curtis coefficients Coefficient increase in both richness and abundance with both burning and grazing. Jansen et al. (1999) found that Undisturbed versus long grass transect 0.384 the richness of grassland birds and the density of the Undisturbed versus short grass transect 0.092 red-winged francolin (Francolinus levaillantii) were Undisturbed versus burnt + grazed 0.243 negatively correlated with grazing intensity and annual Burnt + grazed versus long grass transect 0.329 Burnt + grazed versus short grass transect 0.546 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 habitat heterogeneity through a mosaic of different an increase in some bird species on grazed lands that heights may actually have been greater. Second, many were habitat generalists, but it also demonstrated bird species in these areas are nomadic and this habit declines in some grassland sparrow species like the enables them to locate recently burnt or grazed Botteri’s sparrow and Cassin’s sparrows in the same patches. Influxes of species, such as red-capped larks, area (Bock & Bock 1993). Woinarski (1990) found in black-winged plovers and Richard’s pipits appeared on northern Australia that short-term effects of fire burned sites, while these and Caspian plovers involved a substantial increase in both diversity and appeared on grazed sites. Their nearest locations were density of birds. Thus, our results are consistent with on the short grass plains, which occur some 60 km to some other studies, namely increases in richness and the southeast. abundance with burning and grazing, but not all It is possible that the change in grass structure results are consistent with this conclusion. It is likely from tall to short grasslands could be mimicking the that higher frequencies of burning and grazing may creation of patches of short grass plains within the reverse the effects we found, and so variation in the long grass habitat. Such patches may be attracting severity of disturbance needs further investigation. the avifauna of the short grass plains. Although we Our study also throws light on the possible mecha- did not conduct site counts on the short grass plains, nisms producing the changes in the bird communities. 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 dis- CONCLUSIONS turbed sites were more similar to that on the short grass plains than that on the undisturbed long grass This study investigated how the richness and abun- plains. Thus, the perturbations of burning and dance of bird species changed with two types of dis- grazing changed the bird community towards that turbance to the grass layer, burning and grazing, and seen on the short grass plains. examined whether there was a concomitant change in doi:10.1111/j.1442-9993.2010.02167.x © 2010 The Authors Journal compilation © 2010 Ecological Society of Australia DISTURBANCE ON AVIAN AND ARTHROPOD COMMUNITY 9

Burnt Grazed Pit burnt Pit grazed Unburnt Ungrazed 45 40 40 35 35 30 30 25 25 20 20 Number Number 15 15 10 10 5 5 0 0 Pre 1 4 20 Pre 1 4 20 Weeks Weeks Tray trap burnt Tray trap grazed

70 180

60 160 140 50 120 40 100 30 80 Number Number 60 20 40 10 20 0 0 Pre 1 4 20 Pre 1 4 20 Weeks Weeks Sweep burnt Sweep grazed

45 45 40 40 35 35 30 30 r 25 25 20 20 Number Numbe 15 15 10 10 5 5 0 0 Pre 1 4 20 Pre 1 4 20 Weeks 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 Species that prefer long grass declined in number community. We found that both bird diversity and with the onset of disturbance. However, some were abundance increased with both disturbances. In con- replaced by functional equivalents. Thus, the red- trast, the abundance of arthropods declined with both capped lark and the athi short-toed lark, which are disturbances.Thus, the bird communities appear to be functionally equivalent, replaced each other when the changing in response to changes in the structure of grassland changed from long to short grass in both their habitat providing different conditions for new treatments.This replacement compensated for the loss species to enter, and enabling them to access arthro- of some species due to the disturbance and contrib- pod food and lowering its abundance. This means uted to maintaining diversity. 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 ACKNOWLEDGEMENTS the short grass plains, and it was the bird species normally residing in the short grass plains that We thank the directors of Tanzania National Parks and appeared on the disturbed sites. theTanzaniaWildlife Research Institute for permission

© 2010 The Authors doi:10.1111/j.1442-9993.2010.02167.x Journal compilation © 2010 Ecological Society of Australia 10 A. K. NKWABI ET AL. to work in the park.We thank especially John Mchetto, O’Reilly L., Ogada D., Palmer T. M. & Keesing F. (2006) Effects Stephen Makacha and Joseph Masoy for their assis- of fire on bird diversity and abundance in an East African tance in the fieldwork.This work was supported by the savanna. Afr. J. Ecol. 44, 165–70. Ogada D. L., Gadd M. E., Ostfeld R. S., Young T. P. & Keesing Canadian Natural Sciences and Engineering Research F. (2008) Impacts of large herbivorous mammals on bird Council grant to ARES. All research contained in this diversity and abundance in an African savanna. Oecologia study complied with the laws of Tanzania. 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. REFERENCES Pomeroy D. E. (1992) Counting Birds. 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