Raptors and land-use systems in southern Africa

Afr. J . Ecol. 1985, Volume 23, pages 11-20

Raptors and land-use systems in southern Africa R O L A N D B R A N D L * , H A N S U T S C H I C K t a n d K L A U S SCHMIDT KES *Lehrstuhl Tierokologie, Universitat Bayreuth, A m Birkengut. 0-8580 Bayreuth, TLehrstuhl fu Landschaftstechnik, Universitat Miinchen, Winzererstrasse 45, 0-8QOO Miinchen 40, and SUntere Hagenstrasse 34, 0-8562 ffersbruck, West Germany Summary Data about the relative densities of raptors are analysed in comparison to landscape patterns in southern Africa. The impact of man on the environment reduces the diversity of raptor communities. Different raptor groups are in- fluenced differently by man, with habitat destruction as the outstanding negative factor, especially on eagles. Black-shouldered kite (Elanus caeruleus) and black kite (Milvus migrans) are enhanced by human activities, the former by habitat, the latter by diet. A comparison of data from the literature indicates a decline of raptors (black-shouldered kite excluded) during the last 30 years in Transvaal and Orange Free State.

Resume Des donnees concernant les densites relatives de certains rapaces sont analyses en fonction des types d’habitats en Afrique australe. L’impact de l’homme sur I’environnient reduit la diversite des communautes de rapaces. Des groupes differents de rapaces sont influences differemment par I’homme, la destruction de l’habitat etant le facteur negatif majeur, specialement chez les aigles. L’Clanion blanc (Elanus caeruleus) et le milan noir (Milvus migrans) sont favorises par les activites humaines, le premier par l’habitat, le second par la nourriture. Une comparaison des donnees de la litteratwe indique un decIin des rapaces (excepte l’elanion blanc) durant les 30 dernieres annCes au Transvaal et dans 1’Etat Libre d’Orange.

Introduction Raptors (order: Falconiformes) occupy top positions in terrestrial and aquatic food webs. This group offers itself as an indicator of the worldwide pollution by pesticides (Newton, 1979) and serves as a measure of man’s impact on landscape patterns, even in remote areas (Reichholf, 1974). The usage of organisms as information systems for underlying landscape patterns is a typical biogeographical task (Muller, 1981).

Two basic approaches can be adopted in analysing animal populations in natural environments (Price, 1971): (i) one can trace a population within one area over an extensive period of time, or (ii) one can gather data over a short period of time but on a large geographical scale. For long-living animals the first approach is very time-consuming and the second one seems far more appropriate. In ou r study we analyse raptor densities in the southern part of Africa with respect

*To whom correspondence should be addressed.

12 R. Brandl, H . Utschick and K. Schmidtke

to different land use systems by man. Southern Africa is especially suitable for such an investigation, because all transitions exist between nearly uninfluenced and heavily managed areas (Puzo, 1978).

Materials and methods During a journey through South Africa, Botswana and Zimbabwe in August/ September 1983 we performed roadside counts in territories ranging from intensively managed farmland to National Parks. For consistency we decided to exclude our reserve counts, because the speed of the car was much lower, with a higher probability of detecting a bird. We made fifteen counts along roads (322 raptors on 4960 km) and eight counts in reserves (238 raptors). The most extensive roadside counts are available for South Africa (especially Cape Province: Rowan, 1964; Siegfried, 1966a, b, 1968a, b; and for other parts of South Africa: Cade, 1969). Some data for East Africa are given in Brown (1972) and Thiollay (1978).

From the data we derived the number of species seen in a particular area. There was no correlation between numbers of species and the distance along roads (from 170 to 520 km; r=0.38; df=13; P>O.I) nor with the time travelled (between 2 and 9 hours; r=0.40; df=13; P>O.l), so no need exists for any correction. Sometimes it was difficult to determine an individual to species level, and we grouped unknown raptors into broader taxa. We counted such an individual as a new species only when it was evident that it was not from a group already recorded in the area. Our estimate of the number of species is therefore a conserva- tive measure and not comparable to figures derived from faunal lists over periods of several years (Bezel, 1982).

Besides the density of all raptors we used certain raptor species and operation- ally defined groups in separate analyses, if more than fifteen individuals were counted. Raptor groups meeting this criterion are vultures (Aegypiidae), falcons (Falco spp.), eagles (especially including the genera Aquilu, Polemaetus and Cir- caetus) and hawks (Accipiter spp. and Melierux spp.). We excluded that bateleur (Terathopius ecaudatus) from the eagles, because this species provides enough material for a separate analysis.

In the present paper we analyse only a limited amount of material, and to avoid the risk of a statistical error of type I1 (to strengthen the power of our tests; Cohen, 1977), we relaxed the significance criterion to 10%.

For a description of landscape patterns, we estimated the percentage area occupied by trees, bushes, grassland, intensively managed farmland and the amount of land use for grazing cattle. Furthermore, we estimated the impact of man on the landscape pattern on a scale from 1 to 5 (1 =intensive impact; 5 =no impact). Table 1 compares our more subjectively derived measure of land-use patterns (L) with a system provided by Puzo (l978), and Table 2 presents the correlation coefficients between L and landscape parameters. Table 2 reveals a negative correlation between L and the proportion of intensively managed farmland and a marginally significant negative correlation with the proportion of grasslands, whereas the proportion of bushes is positively correlated with L. The proportion of area covered by trees includes plantations, as well as natural wooded areas, and therefore shows an intermediate correlation coefficient. These results are in good agreement with Table 1 and we feel that our subjectively defined L can serve as a rough measure for the impact by man.

Raptors and land use 13

Table 1 . Comparison of our subjective measure of man's impact ( L ) with a system provided by Puzo (1978)

L Systems according to Puzo (1978)

MODERN COMMERCIAL LAND-USE SYSTEMS I 2 non-intensive livestock

3 cultivation; crops and livestock; 4 pastoralism; 5

intensive 'mixed'; intensive crop emphasis

TRADITIONAL NON-INTENSIVE LAND-USE SYSTEMS

little or no agro-pastoralism; reserves and Parks

Table 2. Correlation coefficients of L (impact scale) with several estimated landscape parameters (arcsine-transformed; df= 13)

L correlated with P

r (two-tailed test)

YO intensive farmland -0.83 < 0.00 I YO grassland - 0.45 <O. I O/o non-intensive cattle land -0.13 >0.1 Yo trees 0.28 > O . I % bushes 0.52 < 0.05

Results Figure I provides a summary of the localities surveyed, the estimated raptor densities and the number of species involved. Of the fifteen counts only fourteen appear in Fig. 1, because one road was surveyed twice and the mean is shown in Fig. 1. A qualitative interpretation of the map suggests higher raptor densities and species' numbers in northern Botswana and southern Zimbabwe and low densities in parts of South Africa.

The overall raptor density is about 4.8 individuals 100 km-', a value comparable to estimates in Cade (1969) for counts outside reserves in South Africa. Raptor counts within reserves yield densities far above our estimates. Thiollay (1978) in Uganda found densities of 120-270 individuals 100 km-' in National Parks.

The interpretation of data in the context of human land-use patterns becomes complicated if one realizes that the distribution of vegetation types (as given in Acocks, 1975; or Werger & Coetzee, 1978) may have produced observed trends. In a first approach, we derived from the distribution maps in McLachlan and Liversidge ( 1 978) approximate numbers of raptor species on quadrats of 2 degrees in areas studied in the present paper (Fig. 2). We found no indication to expect lower numbers of raptor species in those areas, where low values appeared in our field counts.

14 R . Brandl, H . Utschick and K . Schmidtke

8 Harare

ZIMBABWE

W Fig. 1 . Map showing the approximate localities of our transects, measured raptor densities and numbers of species. Numbers beside the circles indicate the impact of man ( L ) according to Table I .

Table 3 presents the correlation coefficients between different estimated landscape parameters and density, as well as raptor diversity. Numbers of species show a positive correlation with L (higher number of raptor species in undisturbed areas) and the inclusion of counts within reserves retains the pattern (Fig. 3). Density of all raptors is only marginally positively correlated with L, apparently due to different reactions of particular raptor groups in respect to our land-use system (L; Table 3). Most groups are at least marginally positively correlated with L, but one species, the black-shouldered kite (Elanus caeruleus), reveals a negative


3 9 L 3 L6

LO 1L L651

1 9 51 _ _ _ - 'go

I I

I I

2 3' 3 3 O Fig. 2 . Estimated species numbers of raptors, as expected by the distribution maps in McLachlan & Liversidge (1978). in 2-degrees quadrat areas visited in the present study.

Table 3. Correlation coefficients between densities of raptor groups and species numbers with landscape parameters

No. of Y O V O Y O Y O O h individuals bushes trees cattle land grassland farmland L

Density No. of species

Density of raptor groups: Vultures Fa I cons Eagles Hawks

Greater kest re1

Black kite Bateleur Black-

Density of species:

shouldered kite

322 0.30 -0.24 -0.01 - 0.49* 0.1 I - 0.09

I I I 0.36 -0.26 ~ 0.08 24 0.37 -0.47* 0.09 17 0 5 4 t 0.36 0.15 21 0.30 0.38 0.10

17 0.44* -0.37 -0.13 13 -0.27 0.14 - 0.04 32 0.00 0.64$ 0.03

14 -0.10 -0.42 0.10

0.06 0.33

0.03 0.27

-0.62t - 0.4 I

0.12 0.16

- 0.46*

0.17

-0.33 0.45* - O m 0.63t

-0.35 0.51* -0.27 0.51* -0.59t 0.72$ -0.54T 0.60t

-0.32 0.57t -0.05 -0.10 -0.43 0.46*

Percentages arcsine-transformed; df= 13: *P<O.l; tP<0.05; $P<O.OI: two-tailed test.

relationship. The density of this species is enhanced by a more intensive impact of man on the landscape (Skead, 1974). The black-shouldered kite seems flexible enough to use highly managed areas, even though telegraph poles serve as the only perching possibilities (for the feeding ecology of this species see Mendelsohn, 1982). The black kite (Milvus migrans) shows no significant correlation with L, probably caused by the more opportunistic feeding behaviour of this species, showing high densities around villages, where it acts as a scavenger (Galushin, 1971; Pomeroy, 1975). If one inspects Fig. 4 scattergram of L against the density

16 R. Brandl, H . Utschick and K . Schmidtke

u1 Q u Q

.-

3 10-

f

L 0 L

n

z

5 -

a

a . a a a

a .. a . . a

a

0

I 1

1 2 3 L 5

Decreosing impact of mon Fig. 3. Relationship of the number of raptor species and the land-use system ( L ) in southern Africa. For a definition of L see Table I .

0

0

0

_ - . m e 4

0 0 - 1 2 3 L 5

Decreasing impoct of man Fig. 4. Scattergram of black kite densities against land-use system (L) .

of the black kite an optimum L value is suggested between 1.5 and 3 and a simple linear correlation analysis is inappropriate.

Additional information can be derived from a closer inspection of the correlation coefficients between the densities of different raptor groups and landscape patterns (Table 3): 1 The vultures show no significant correlation with any estimated landscape parameter. This is probably due to the foraging strategy of this group, which enables this guild to localize carrion over large distances (Houston, 1979), if the vegetation is not too dense. The correlation with L stems from the fact that in agriculturally used areas no carrion is available due to effective hygiene by farmers. We can rule out biogeographic effects in this correlation. Parts of South


Table 4. Different impact types of man on raptors (raptors and raptor groups are ordered according to their vulnerability)

Positive impact Negative impact

Influence Via habitat* Black-shouldered kite Eagles

Hawks Greater kestrel

Falcons Bateleur

Via diet Black kite Vultures

*We are aware that habitat destruction also affects prey density of raptors. Our distinction is only made for species where man directly provides or reduces food supply.

Table 5. Raptor densities (individuals 100 km- '; without vultures) derived from roadside counts in Transvaal and the Orange Free State in the course of nearly 3 decades

Distance 1955; 1965t 1983$

(7023 km) (2750 km) (I560 km)

Black-shouldered kite I .3 I .6 2.4 Other raptors 2. I 1.5 0.8

*Data from Rudebeck. taken from Table 26.4 in Cade (1969):

tData from Cade (1969) Table 26.5: winter densities. $Present study: winter densities.

summer densities, residents only.

Africa are known to be depleted of their original rich community of large mammals (Eltringham, 1979). Therefore gaps in the distribution of vultures in some parts of the Orange Free State and Transvaal, as revealed by maps in Steyn (l982), are secondary and due to the disappearance of large wild mammals, and accordingly of carrion. 2 The falcons show a marginal significant negative correlation with the proportion of trees. Sixty-five percent of all recorded falcons in our samples were greater kestrels (Fako rupicoloides), a species known to prefer drier, open areas with scattered acacia trees (McLachlan & Liversidge, 1978). In a separate analysis of the greater kestrel the correlation is in the same direction, but not significant (Table 3). Steyn (1982) comments that the greater kestrel may have expanded its range, especially in areas where woodland has been cleared. But the analysis of our density data in comparison to L does not conform to the proposition, that the greater kestrel profits from the impact of man on landscape patterns. 3 The eagles (bateleur excluded) demonstrate a very diverse pattern, which parallels the relationships of L with landscape elements (Table 2). Eagles need habitats with taller vegetation elements for perching places and nesting sites.


1 ' L '10

- N E I" x - P b 10000

'oooi 04 . O 5 o6 ~7 . t9 ,

01 0

1 0 20 30 LO Individuals km - 2

Fig. 5. Relationship between human density (km-') in ten countries of southern Africa against proportion of area permanently protected, and mean reserve size (km2). Data from Huntley (1978). Statistical tests one-tailed, with an arcsine-transformation of the percent protected area and a log- transformation of mean reserve size. 1. Botswana; 2, Angola; 3. Southwest Africa; 4. Zambia; 5. Mozambique; 6, Zimbabwe; 7, South Africa; 8, Swaziland; 9, Lesotho; 10, Malawi.

The transformation of the landscape into open pastures and farmland may be responsible for the decline of certain eagle species in parts of Africa. 4 The distribution of hawks can be explained by similar arguments. This species group needs dense vegetation types (Accipiter) or perching places (Melierax) for its hunting strategies. Intensive farmland (compare the negative correlation between hawk density and O/o farmland) provides no habitats for these tactics. 5 The bateleur exhibits above all a positive correlation with the proportion of trees, which indicates that the bateleur (like other eagles) needs denser and taller vegetation types and that the disappearance of this vegetation in farmland reduces the density of this species (Brown, Urban & Newman, 1982).

Discussion In spite of limitations, our approach yields some results, which allow predictions about the fate of raptor populations in Africa. In Table 4 we condensed the results into positive and negative effects of man on raptors. Using the correlation coefficient between density and L as a measure of vulnerability, one can assume groups with high positive values (Table 3) as extremely susceptible to impact by man. From Table 3 we would predict that eagle populations especially will


decline. Gargett (cited in Newton, 1979) notes a decline of black eagles (Aquila verreauxi) in heavily used tribal land of Zimbabwe. Groups with negative coefficients would be predicted to be enhanced by human activities, but it becomes clear from Table 4 that they are a minority. Altogether we expect raptor diversity to decline through human impact.

In the literature, some data are available for a comparison of our raptor densities with values from previous years (Cade. 1969). Cade provides data for Transvaal and the Orange Free State and Table 5 presents a comparison. During the course of nearly three decades the black-shouldered kite increased in density. The densities of the black-shouldered kite given in Table 5 are much lower than estimates by Skead (1974), ranging from 3.7 to 16.7 individuals (100 km)-l. Skead’s counts were performed in favourable habitats and during periods of a rodent plague, whereas values in Table 5 are densities calculated from a diverse set of habitats. In contrast, the data for the other raptors suggest a decline. The results in Table 5 provide only circumstantial evidence, because the densities for 1955 are summer densities and not directly comparable to winter counts. Beyond this, climatic conditions (like droughts) may be the cause of the pattern in Table 5 (Cade, 1969).

In Botswana about 18% of the whole country is set aside as permanently protected areas (Huntley, 1978). At first glance this seems very high. A further increase of human population (Brown, 1980; Delany & Happold, 1979) will produce pressure on the protected areas, with the consequence of fewer and smaller reserves. If one analyses data from countries of southern Africa in this respect, one finds the expected trends revealed (Fig. 5 ; one-tailed tests). Eagles and vultures need huge territories and only one pair breeds in areas of 30-190 km2 (in extremes up to 400 km2; Newton, 1979; Pennycuick, 1976). If one assumes that for long-term conservation a minimum effective population size of fifty is needed (SoulC, 1980; Shaffer, 1981), one can estimate that areas up to 10,000 km2 are needed for the conservation of raptors in reserves. From Fig. 5 we doubt whether such a policy is practicable under the pressure of a rapidly increasing human population. Conservation of raptors (as with other wildlife species) requires landscape management techniques, which allow usage of resources both by wildlife and man.

Acknowledgments H. Zimmermann, Professor H. Remmert, Dr D. Stechmann and G. Heusinger commented on early drafts of the paper. Professor H. Zwolfer kindly provided the opportunity to use the computer facilities of the Department of Animal Ecology in Bayreuth and the referees considerably improved and shortened our paper.

Dedicated to the memory of the late L. H. Brown.

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(Received 15 February 1984) '

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Raptors and land-use systems in southern Africa