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Ostrich 2008, 79(2): 133–140 Printed in South Africa — All rights reserved
OSTRICH ISSN 0030–6525 EISSN 1727–947X doi: 10.2989/OSTRICH.2008.79.2.2.576
Effects of habitat structure and shrub encroachment on bird species diversity in arid savanna in Northern Cape province, South Africa Timo Kaphengst1 and David Ward2* University of Greifswald, Institute of Botany and Landscape Ecology, Grimmer Straße 88, 17487 Greifswald, Germany 2 Department of Conservation Ecology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa; current address: School of Biological and Conservation Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, South Africa * Corresponding author, e-mail:
[email protected] 1
Bird community diversity was assessed in semi-arid savanna in the Northern Cape province of South Africa and related to vegetation diversity and foliage height diversity. As this is an area in which shrub encroachment is common, the effects of shrub or bush encroachment on bird species diversity were specifically addressed. An experiment in which the dominant encroaching species, Acacia mellifera, was removed from ten 1 ha plots was used and compared with ten controls. A strong positive association with foliage height diversity was shown. No effect of experimental removal of the dominant encroacher, A. mellifera, on bird species diversity or species richness was found. However, four species increased in relative abundance in cut plots and two species decreased in relative abundance in these plots.
Introduction Many studies have shown that high faunistic species diversity is positively correlated with high structural diversity of habitats (e.g. MacArthur and MacArthur 1961, Cody 1975, 1983, Meik et al. 2002, Blaum et al. 2007). Early studies showed that high avian diversity is associated more strongly with highly structured patterns of habitats rather than with floral composition or prey diversity (MacArthur and MacArthur 1961, Cody 1983). Because species diversity has become an important indicator of habitat integrity and conservation status, much emphasis has been placed on assessing the major factors controlling such diversity (see e.g. Weller 1999, Canterbury et al. 2000, Scott and Helfman 2001). In the savannas of the world, there is alarm over largescale encroachment by shrubs and bushes, which cause substantial structural changes in habitat diversity (see e.g. Ward 2002, 2005, Wiegand et al. 2005, 2006). Shrub encroachment is defined as an increase in woody plant density and cover in grassland or savanna ecosystems with an accompanying change in herbaceous cover and composition of the natural vegetation (O’Connor and Crow 2000, Hoffman and Ashwell 2001, Ward 2005). This phenomenon has mostly been of concern to ranchers because shrub encroachment results in the suppression of palatable grasses and increases in woody species unpalatable to domestic livestock (Ward 2002, 2005). Although the factors responsible for shrub encroachment have not been convincingly elucidated (Teague and Smit 1992, Smit et al. 1996, Ward 2002, 2005), it is likely that there is a combination of several factors such as changes in rainfall, occurrence of fire, soil composition, grazing intensity, patch dynamics and, possibly, increased carbon dioxide concentration that affect the onset of shrub encroachment (Archer 1995, Ward 2002, 2005, Wiegand et al. 2005, 2006, Kraaij and Ward 2006).
Despite the fact that impoverishment of both plant and animal species diversity as a consequence of shrub encroachment has been demonstrated at several sites (reviewed by Ward 2006), few studies have investigated its consequences for bird species diversity in semi-arid savannas. In this study we investigated the effects of vegetation structural diversity in a semi-arid savanna on bird species diversity near Kimberley, Northern Cape province, South Africa, with special emphasis on the effects of shrub encroachment on bird diversity. Because bird species diversity is usually most closely related to structural diversity of habitats (MacArthur and MacArthur 1961), we predict that habitats with high structural diversity (i.e. with a mixture of open grassy areas and scattered trees [open savanna]), and those with a high number of shrub and tree strata will have the highest species diversity. Furthermore, reductions in structural diversity caused by encroachment will result in reductions in bird species diversity. These changes are most likely to be found in species typical of open savannas, such as larks and bee-eaters. To assess the effects of changes in habitat structural diversity on bird species diversity, we used a combination of observational and experimental approaches. That is, we compared bird species diversity in sites differing in structural diversity and then assessed bird diversity in paired sites where encroaching shrubs have either been removed (treatment) or remain undisturbed (control). Materials and methods Study site The study was carried out on Pniel Estates, situated 30 km north of Kimberley in the Northern Cape province of South Africa. Pniel Estates, which is 25 000 ha in size,
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lies within the so-called Kimberley conservation triangle, which is bounded by the following coordinates: 28°27′04″ S, 24°42′03″ E; and 28°44′05″ S, 24°41′00″E. Bordered by the Vaal River to the north and west, the area consists of semi-arid savanna (mean annual rainfall is 360 mm; CV = 39%) and is classified by Mucina and Rutherford (2006) as Kimberley Thornveld, which is part of the transition zone between the Nama-Karoo biome of the South African interior, the Desert biome in the north-west and the Savanna biome in the east. Due to the variety of soil types, the study site consists of a high variety of savanna types (woodland, open and closed savanna and ephemeral pans—seasonally inundated shallow depressions with low shrubs and grasses) and is used as a wildlife preserve as well as a cattle ranch. On deep sandy soils, woodlands are mainly dominated by Acacia erioloba trees (5–10 m height). On more clayey soils, the shorter (3–6 m) Acacia tortilis is dominant. On rocky slopes near the Vaal River, shrubs such as Acacia mellifera, Grewia flava and Tarchonanthus camphoratus outcompete the tree species and occur in different compositions. Most of the area is dominated by shrubs, such that the woodlands occur patchily within the shrubland. Because of this patchiness, the area provides a high diversity of different bird habitats at reasonable distances from one another. Given that the altitude of the area varies little (altitude range between 1 051 m and 1 160 m above sea level), the different habitats differ only in soil composition and vegetation structure in a comparably small geographical scale (Smet and Ward 2005a, 2005b, Britz and Ward 2007a, 2007b). Field work was undertaken in January 2004 including bird-species censuses and vegetation analysis. In total, 12 habitat types were identified due to their vegetation structure, using the Braun-Blanquet method (MuellerDombois and Ellenberg 1974). Bird censuses In each of the habitats, within 500 m × 500 m sections, which were chosen visually to represent the habitat at large, all individuals were censused visually and auditorily. Furthermore, all birds were recorded for 1 h from the centre of the above-mentioned area. These were all repeated on two occasions. This combination of censuses provided the quantitative assessment of birds within a representative area of 25 ha in each habitat and assessments of qualitative aspects such as bird activity. To determine the particular habitat use of each bird species, every individual’s preferred stratum (tree, shrub and herbaceous layer) was noted. Observed breeding activities such as carrying nest material, territorial defence or mating, as well as foraging habits, were recorded. In those cases where a bird (or a flock of birds) flew over the area, it was only recorded if clearly associated with the habitat. Vegetation data In every 25 ha habitat, the vegetation composition and structure was characterised using three vegetation plots for which the Braun-Blanquet method was applied (MuellerDombois and Ellenberg 1974). In the woodlands, an area of 2 500 m2 (50 m × 50 m) was used for assessment, while in the shrubs and in the pans a smaller area of 400 m2
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(20 m × 20 m) was regarded as a sufficient size (owing to greater vegetation uniformity) for the vegetation measurements. Where the habitat showed a high degree of heterogeneity in its vegetation composition this was considered in the selection of the three Braun-Blanquet plots. We calculated average values representing the whole habitat for all parameters of each of the three plots. In our study area, the habitats are mostly open areas with a high percentage cover of herbaceous plants. Thus, a measure of foliage density is not suitable. Instead, the mean percentage cover of the differentiated strata (trees, shrubs and herbaceous layer), their mean heights and litter and rock cover (relational scale from 0 to 3; 0 = no litter or rock, 3 = much litter or rock [>75%]) were estimated. Rather than determining every herbaceous plant species, only the dominant grasses and herbs (more than 5% cover) were recorded. Trees and shrubs were identified to species level. Because birds are not only attracted by the structural variety of vegetation within an area, plant species that flowered or seeded during the study period were recorded, considering them as a source of food that might attract birds. Statistical analyses We first carried out detrended correspondence analysis using the MVSP package (Kovach 1999) to correlate the bird and plant data sets to distinguish structural parameters significantly associated with the occurrence of bird species and their community distributions. Furthermore, the analysis demonstrated the differences among habitats based on their share of the differentiated strata, plant species dominance and additional parameters such as percentage cover of litter and rocks. We compared the bird species diversity of every habitat using the Shannon-Wiener Index (Shannon and Weaver 1949). Combining species richness (number of species per habitat) and distribution of individuals per species, the Shannon index has been criticised by numerous scientists (see e.g. Hurlbert 1971) as a limited measure of species diversity. Use of the index is justified if the relationship between species richness and the distribution of the individuals within the species (species evenness) varied little among habitats (Hurlbert 1971), which was the case in our study. After the diversity index was calculated, a standard bootstrap method for estimating upper and lower 95% confidence intervals of the species diversity index was undertaken. A randomisation test for significant differences in diversity between all pairs of samples was undertaken (Solow 1993). This test resamples 10 000 times from a distribution of species abundances produced by a summation of the two samples. We used simple linear regression analysis to assess the relationships between bird species richness per habitat and vegetation variables: mean tree height, maximum vegetation height and coefficient of variation in vegetation height; the last mentioned was used as a measure of structural diversity. The DECORANA axis (DCA) values (see above) of the vegetation parameters was used as a second measure of structural diversity and were also compared by simple linear regression to bird species richness. We used Bonferroni adjustments to control for comparison-wise error.
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When different habitats are compared according to their species diversity along a gradient the effect of species turnover has to be taken into account. The rate of species turnover between habitat types is expressed by β diversity, which measures two attributes: the number of distinct habitats within a region and the replacement of species by another between parts of the same habitat (Willson and Shmida 1984). We used the Whittaker (1975) index for β diversity, viz.:
βW = S/α – 1 where S is the total number of species recorded and α the mean sample diversity (diversity is measured as species richness) (Whittaker 1960). Geographic distances between all pairs of habitats were measured using a Global Positioning System (GPS). We used a Mantel test (Mantel 1967) to determine whether Whittaker’s measure of β diversity is correlated with geographic distance rather than, or in addition to, changes in vegetation structural diversity. This possibility has to be excluded to establish that differences in species diversity are not related to distances between habitats rather than to habitat variety. Bird distribution in habitats The bird species were divided into trophic guilds. The occurrence of guilds in a habitat reveals information about sources of food. Birds as consumers in the trophic pyramid are directly dependent on producers (plants) or, if they feed on other animals, on organisms at a lower trophic scale. Therefore, the number of different trophic guilds in a habitat can be regarded as an indicator of structural diversity represented by various plants with different heights and strata providing structural and trophic niches for organisms. In our case, we used number of guilds as an additional comparative measure between encroached and unencroached habitats. We followed Cody’s (1983) classification of forest bird guilds and included some additional guilds that occurred in savanna habitats only. Preferred strata in habitats For every habitat, the percentage value of the preferred strata of all bird species was measured. As this percentage depends on the percent availability of trees, shrubs, grasses and ground (either bare or vegetated), a measure of preference for each of these strata in terms of relative availability is needed. For this we used Ivlev’s (1961) electivity index. This index is: Ei = (ri – ni)/(ri + ni) where ri is the percentage of birds in the particular stratum and ni is the percentage of that stratum available. The proportion of available habitat strata was determined by the Braun-Blanquet method (Mueller-Dombois and Ellenberg 1974). The vegetation parameters consisted of mean percentage cover of the habitats averaged over all three vegetation plots of each habitat. Because herbaceous plants partly grow under shrubs and trees and shrubs are often situated underneath trees, the sum of tree, shrub and herbaceous layers occasionally exceeds 100%.
Small-scale habitat comparison To focus more closely on the effects of shrub encroachment on bird species diversity, an experiment was carried out within two heavily encroached habitats (Rocky area III and IV). Twenty plots, each 1 ha in size, were compared in terms of their bird species richness. On 10 plots, all Acacia mellifera (the dominant species) were removed so that these ‘cut’ plots represent a more open and therefore more structured vegetation than the heavily encroached and undisturbed control plots. We noted that two shrub/tree species remained in these plots: Tarchonanthus camphoratus and Acacia tortilis. In all of the plots, bird individuals were recorded within a 15 min time span by the census point method as noted above. Because these data on bird species richness and diversity did not conform to the assumptions of parametric tests, we used Resampling Stats (Simon 1995) to compare the bird species richness of these plots. Results Over the study period, 102 bird species were detected on Pniel Estates, of which 78 were recorded in the mentioned habitats. The most common species were Kalahari Scrub-Robin Cercotrichas paena, Chestnut-vented Tit-Babbler Parisoma subcaeruleum and Neddicky Cisticola fulvicapilla, which had the highest values in terms of relative abundance and occurred in almost all habitats (Table 1). Table 2 describes the 12 differentiated habitats in terms of their structural features and their bird species diversity. Acacia erioloba woodland grows considerably higher than the Acacia tortilis woodland (see Table 1; mean height of A. erioloba woodland was 7.0 ± 1.14 m compared to 4.0 ± 0.97 m for A. tortilis woodland). No trees were found in the pans or in the shrub savanna. The shrub layer was found in the woodlands and in the shrub savannas, although a higher percentage shrub cover was obtained in the encroached areas and in the Open savanna III where hardly any trees occur. The herbaceous plants play a significant role in the woodlands except for Woodland I, where they are replaced by a comparatively higher percentage cover of shrubs. Grasses cover almost half of the area in the encroached Rocky area III. With regard to vegetation composition, the pans were an exception among the habitats. They mainly consisted of short-growing plants separated by occasional patches of shrubby vegetation (e.g. Leucosphaera bainesii, Salsola rabiena and Pentzia calcarea). In general, percentage of litter and rock increased with the absence of trees in the habitats. The results of the bird species richness and diversity measures are shown in Table 2. Species richness and bird diversity have their largest values in the Acacia erioloba woodlands, particularly with regard to species richness. There were very few differences in bird species evenness among the habitats. There was no significant correlation between species diversity and evenness (r = 0.06, P = 0.854). In the rocky areas, where the mean vegetation height was the lowest, species diversity and richness were low. The values of the encroached areas were not as low in bird species richness and diversity as one might expect (see Rocky Areas II and IV, for example, in Table 2). The
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Table 1: Most common bird species recorded in Pniel Estates. For further descriptions of guilds, see Cody (1983)
Species
Individuals
Kalahari Scrub-Robin
Habitats
Individuals habitat–1 4.45
Relative abundance 0.18
49
11
46
11
4.18
0.17
35
11
3.18
0.13
33 25
10 8
3.30 3.13
0.13 0.13
Barn Swallow
Cercotrichas paena Parisoma subcaeruleum Cisticola fulvicapillus Prinia flavicans Calendulauda africanoides Hirundu rustica
22
11
2.00
0.08
Lesser Grey Shrike
Lanius minor
16
5
3.20
0.13
Cape Turtle-Dove
15
4
3.75
0.15
European Bee-eater
Streptopelia apicola Merops apiaster
14
4
3.50
0.14
Cape Clapper Lark
Mirafra apiata
14
9
1.56
0.06
Scaly-feathered Finch
Sporopipes squamifrons Tricholaema leucomelas Streptopelia senegalensis Parus cinerascens
14
3
4.67
0.19
14
9
1.56
0.06
12
5
2.40
0.10
11
6
1.83
0.07
Chestnut-vented Titbabbler Neddicky Black-chested Prinia Fawn-coloured Lark
Acacia Pied Barbet Laughing Dove Ashy Tit
Guild Insectivore Slow-searching omnivore Insectivore Insectivore Slow-searching omnivore Flying insectivore Slow-searching omnivore Ground-searching omnivore Flying insectivore Slow-searching omnivore Granivore Slow-searching omnivore Ground-searching granivore Insectivore
Table 2: Vegetation, structural parameters and bird species richness among habitats
Habitat Woodland Ia Woodland IIb Woodland IIIc Open savanna Id Open savanna IIe Open savanna IIIf Shrub savannag Rocky area Ih Rocky area IIi Rocky area IIIj Rocky area IVk Pansl
Total 51.66 53.33 48.33 53.33 48.33 51.66 42.50 40.00 38.33 61.66 56.66 70.00
Structural parameters Cover (%) Tree Shrub Herbs 8.33 25.00 23.33 5.00 20.00 35.00 6.66 14.66 35.00 8.33 16.66 31.66 8.33 5.66 38.33 1.66 35.00 23.33 0.00 27.50 17.50 6.00 20.00 22.50 3.33 18.33 16.66 1.66 35.00 46.66 1.33 46.66 23.33 0.00 10.00 60.00
Scale (0–3) Litter Rock 0.33 0 0 0 0 0 0 0 1.33 0 1.00 1.00 2.00 0.50 1.00 2.00 0.33 2.00 1.00 2.33 2.33 2.33 1.00 1.00
Bird species richness
ShannonWiener index
Variance H
31 29 33 22 18 18 15 11 18 11 16 22
3.1819 3.0478 3.3155 2.8755 2.6985 2.6992 2.5895 2.2503 2.6855 2.1622 2.5781 2.8119
0.010 0.009 0.009 0.012 0.015 0.011 0.017 0.026 0.013 0.198 0.023 0.015
Acacia erioloba woodland, domesticated livestock Acacia erioloba woodland, domesticated livestock c Acacia erioloba woodland, game reserve d Acacia tortilis woodland, fairly open, domesticated livestock e Acacia tortilis woodland, very open, domesticated livestock f Acacia tortilis woodland, dominated by shrubs (Grewia flava, Tarchonanthus camphoratus), game reserve g Grewia flava, Tarchonanthus camphoratus, game reserve h Shrubs (Acacia mellifera, Tarchonanthus camphoratus), single trees (Acacia tortilis), domesticated livestock i Shrubs (Acacia mellifera, Tarchonanthus camphoratus), partly encroached, domesticated livestock j Shrubs (Acacia mellifera, Tarchonanthus camphoratus), rocky slopes, heavily encroached, domesticated livestock k Shrubs (Acacia mellifera, Tarchonanthus camphoratus), rocky slopes, heavily encroached, game reserve l Very low vegetation partly interrupted by shrubs (Grewia flava, Tarchonanthus camphoratus), game reserve a b
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SPECIES RICHNESS
30
y = 2.9469x + 2.5004 r = 0.83 p < 0.001
25 20 15 10 5 2 4 6 8 10 MAXIMUM VEGETATION HEIGHT (m)
Figure 1: Correlation of bird species richness to maximum vegetation height of all habitats (excluding pans)
30 SPECIES RICHNESS
randomisation test showed that significant differences in species diversity compared to all other habitats occur mostly in Woodland III and Rocky area III (Table 3). Both habitats also represent the highest (Woodland III) and the lowest (Rocky area II) values in species diversity and species richness. The biggest differences in bird species diversity were between Woodland III and Rocky area III as well as between Open Savanna III and Woodland I and Rocky Area III. The pans were only significantly different from Woodland III and Rocky area III, although different bird species occurred in the pans relative to other habitats. There was no significant correlation between the Whittaker β diversity values and geographic distance (Mantel test: r = 0.07, P = 0.335). Thus, geographic separation does not explain differences in diversity. The linear regression analysis of the different structural vegetation parameters and bird species richness showed that there was no correlation between bird species richness and DCA axes (r² = 0.16, P = 0.201 (n = 12 for the first axis); r² = 0.001, P = 0.909 for the second axis). There was also no significant correlation with the coefficient of variation in vegetation height (r² = 0.63, P = 0.193). However, a significant correlation was found between the number of bird species and the mean height of the trees in these habitats (Figure 1) as well as the maximum height of vegetation (r2 = 0.83, P < 0.001) (Figure 2). Thus, according to the linear regression analysis, the bird species richness depends on the vegetation height and the presence of trees rather than on the structural diversity. In these correlations, we excluded the pans from the analysis because their vegetation heights diverged significantly from those of all other habitats. Correlation values were much higher when the pans were excluded, although both produced significant (p < 0.05) results. The spectrum of bird guilds differed considerably among habitats. In all, nine trophic guilds were differentiated (see Figure 3). There were only two guilds that occurred in all habitats: the slow-searching omnivores and the insectivores. The ground-searching omnivores, the granivores and the flying insectivores were recorded in all habitats except one. Birds-of-prey (Southern Pale Chanting Goshawk Melierax canorus, Gabar Goshawk Melierax gabar, Lanner Falcon Falco biarmicus, Lesser Kestrel Falco naumanni, Rock Kestrel Falco rupicolus and Eurasian Hobby Falco subbuteo) and gleaners were only found in open savanna habitats while frugivores/nectarivores (White-backed Mousebird Colius colius and Red-faced Mousebird Urocolius indicus) and sallying flycatchers were also found mainly in these habitats. There were granivores such as Buntings (Lark-like Bunting Emberiza impetuani, Goldenbreasted Bunting Emberiza flaviventris), which occurred in the woodlands only, as did insectivores such as Common Fiscal Lanius collaris and Brubru Nilaus afer, and sallyers such as Fork-tailed Drongo Dicrurus adsimilis. In contrast, there were few species that occurred only in the rocky areas, except for the Red-backed Shrike Lanius collurio. Ground-searching insectivores, granivores and omnivores were mostly represented in the pans. Five of them were only observed there (Crowned Lapwing Vanellus coronatus, Namaqua Sandgrouse Pterocles namaqua, Orange River Francolin Scleroptila levaillantoides, Red-capped Lark
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y = 2.4837x + 11.195 r = 0.70 p < 0.001
25 20 15 10 5 2
4 6 MEAN TREE HEIGHT (m)
Figure 2: Correlation of bird species richness to mean tree height of all habitats (excluding pans)
Calandrella cinerea and Cape Sparrow Passer melanurus). Two weaver species (White-browed Sparrow-Weaver Plocepasser mahali and Southern Masked-Weaver Ploceus velatus) were found in both the woodlands and in the pans. There was a significant positive correlation between the number of species and the presence of guilds within the habitats (r = 0.89, P < 0.001; n = 12). This can be viewed as another indicator of bird species diversity that is higher in richly structured habitats than in monotonous shrubby areas with low stratum differentiation. Surprisingly, there was no correlation between the percentage of litter and/or rocks and the presence of guilds such as slow-searching omnivores or granivores. Birds in savanna habitats preferred to stay on trees (see Table 4). Except for three habitats, the tree layer had the highest values. However, we note that observations on the ground were rarely made because the birds are frequently well hidden within grass or behind shrubs. Nevertheless, the indices indicate that birds fly to trees in most of the habitats where trees occur. In habitats where only single trees interrupted the dominance of shrubby vegetation, higher activity of avifauna around the trees was noted. Although
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Slow-searching omnivores Insectivores
Sallying flycatchers
Granivores
Ground-searching omnivores
Birds of prey
Flying insectivores
Nectarivores
Gleaners
NUMBER OF SPECIES
35 30 20 15 10
Pa ns
W oo dl an d
I W oo dl an d II W oo dl an d O III pe n sa va nn a O I pe n sa va nn O a pe II n sa va nn a III Sh ru b sa va nn a R oc ky ar ea I R oc ky ar ea II R oc ky ar ea III R oc ky ar ea IV
5
HABITATS
Figure 3: Distribution of bird guilds within habitats Table 3: Significance values for similarities for bird species composition between all habitats (randomisation test). Bold numbers represent significant values (p < 0.05) for pairs of habitats. See text for further details
Habitat Woodland I Woodland II Woodland III Open savanna I Open savanna II Open savanna III Shrub savanna Rocky area I Rocky area II Rocky area III Rocky area IV
Open savanna I 0.1037 0.46 0.0133
Open savanna II 0.081 0.2741 0.0063 0.373
Open savanna III 0.013 0.1518 0.0008 0.3016 0.9962
Shrub savanna 0.1851 0.376 0.0354 0.4129 0.6563 0.708
the percentage cover of the shrub stratum also exceeds that of trees within the open savanna habitats, birds were commonly observed in the tree crowns. Experimental plots The removal of the main encroaching species, Acacia mellifera, from the experimental plots did not have any significant effect on bird species richness or diversity. There was no significant difference in bird species richness between ‘cut’ and ‘uncut’ plots (t = 0.906, P = 0.382,
Rocky area I 0.0273 0.1382 0.0078 0.1073 0.1956 0.2446 0.2561
Rocky area II 0.0435 0.1774 0.0048 0.3361 0.9491 0.9455 0.7279 0.2915
Rocky area III 0.0001 0.0053 0.0001 0.0105 0.0333 0.0372 0.0897 0.7717 0.0392
Rocky area IV 0.1404 0.3334 0.0212 0.3618 0.6029 0.6953 0.9599 0.3112 0.6923 0.1093
Pans 0.0933 0.4269 0.0046 0.6687 0.5998 0.4627 0.6431 0.303 0.4719 0.0053 0.5535
df = 18). Mean ± SD species richness on the cut plots was 6.9 ± 2.84 and on the uncut plots was 6.0 ± 1.33. There was also no significant difference in the numbers of individuals among cut and uncut plots (t = 0.817, P = 0.427). Using DECORANA, we found no significant differences in bird species composition between the ‘cut’ and the ‘uncut’ plots (P = 0.29, n = 1000 resamples; Figure 4). However, a number of species changed in relative abundance in these experimental plots. We arbitrarily chose a doubling of species relative abundance as our threshold. Four species,
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Table 4: Preferred strata for birds within habitats. Calculated using Ivlev’s electivity index (see text for further details). Ground = vegetated or bare ground. Positive values indicate preference for a habitat stratum and negative values indicate avoidance
Habitat Woodland I Woodland II Woodland III Open savanna I Open savanna II Open savanna III Shrub savanna Rocky area I Rocky area II Rocky area III Rocky area IV Pans
DECORANA AXIS 2
5
Tree 0.78 0.88 0.86 0.75 0.73 0.79 1.00 0.54 0.51 0.85 0.86 1.00
Preferred stratum Shrub 0.01 0.02 -0.20 0.42 0.77 0.41 0.53 0.60 0.64 0.36 0.28 0.36
Ground -0.54 -1.00 -0.91 -1.00 -0.87 -0.84 -0.43 -1.00 -0.41 -0.81 -1.00 0.13
Uncut Cut
4 3 2 1 1
2 3 4 DECORANA AXIS 1
5
Figure 4: Distribution of ‘cut’ and ‘uncut’ plots using DECORANA (for more details, see Materials and methods). Differences are due to differences in their bird species’ composition
namely Fawn-coloured Lark Calendulauda africanoides, Kalahari Scrub-Robin C. paena, Neddicky C. fulvicapilla and Pririt Batis Batis pririt, increased in relative abundance in the experimental (‘cut’) plots. All of these species are known to prefer more open habitats (with the exception of the Kalahari Scrub-Robin, which tends to prefer shrubs on the edges of open areas; TK and DW pers. obs.). In contrast, the White-browed Sparrow-Weaver P. mahali and Barn Swallow Hirundo rustica were more common in the control (‘uncut’) plots. Discussion The distribution of bird guilds within the different habitats indicates the greater variety of food sources and the better capture conditions for birds in woodlands compared to open savannas. Sallying flycatchers, for example, were only detected in woodlands where they found perches for their sallies at a greater range of different heights. An investigation in southern Spain on the European Wheatear Oenanthe
oenanthe and European Stonechat Saxicola rubicola showed that birds benefit from high perches because they gain a better overview of the area (Moreno 1984). In that study, a positive correlation between perch height and sally distance showed that prey were more easily detected from high perches (Moreno 1984). Consequently, trees extend the capture possibilities of guilds such as sallying flycatchers and other insectivores to a greater distance. This is also supported by the high preference of birds for trees in areas where trees occur. The trees function as a lookout post and, occasionally, as a food source (see e.g. MacArthur and MacArthur 1961, Cody 1983). In this study, the presence of trees within the habitats seemed to be more significant for the occurrence of bird species than the composition of the different vegetation strata (see also MacArthur and MacArthur 1961). In other words, savanna habitats with dense, shrubby vegetation and single trees are likely to have higher bird species diversity than habitats with moderately spaced shrubs but without trees. The significant ecological function of trees, especially of Acacia erioloba, in arid and semi-arid regions was noted by Milton and Dean (1995). They showed for the Kalahari desert, a similar ecosystem to ours, that Acacia erioloba trees increase environmental heterogeneity and therefore species diversity for birds and other organisms by providing resources and services (e.g. for birds: shelter, fruit, nests, perch and roost sites). These services, which can also be provided by adult Acacia tortilis trees, are seldom provided by shrubs or saplings (Milton and Dean 1995). The importance of trees for birds is also indicated by the results of this study, namely significant positive correlations between bird species richness and mean height of the trees and maximum height of vegetation. The effect of shrub encroachment on bird species diversity was not entirely clear in our study. While the differences between woodland and open savanna habitats and rocky areas are obvious, bird species diversity differs little between encroached and unencroached rocky areas. On a small scale (as shown in the experimental plots), only a few bird species frequent more open vegetation structure when the area is affected by dense vegetation due to shrub encroachment. We suggest that the lack of difference in species richness may be due to a fixed availability of food in the habitat, as also elucidated by Cody (1975, 1983, 1993), MacArthur et al. (1972), Silva et al. (1997) and Mönkönnen et al. (2006), among others. Acknowledgements — We thank Errol and Barbara Tegg for their hospitality and Cara Nieuwoudt for her assistance. We thank the National Research Foundation of South Africa for funding.
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Received July 2007, accepted April 2008 Editor: MD Anderson
