Biological Conservation 67 (1994) 125-128

HABITAT SELECTION BY THE LITTLE BUSTARD Tetrax tetrax IN CULTIVATED AREAS OF CENTRAL SPAIN

Carmen Martinez Museo Nacional de Ciencias Naturales, CSIC, C/ Jos~ Guti~rrez Abascal 2, 28006 Madrid, Spain

(Received 26 September 1992; revised version received 18 March 1993; accepted 20 March 1993)

Abstract Habitat selection of the little bustard Tetrax tetrax during the breeding season was analysed in four cultivated areas of Central Spain. Little bustards preferred long-fallow land, legume crops and fallow land. Cereal crops, ploughed land and shrubland were avoided One of the most important habitat requirements was vegetation whose average height does not exceed 20 cm. The species shows some tolerance to the proximity of buildings, villages and roads. Conservation implications are discussed

Keywords: Central Spain, cultivated land, habitat selection, little bustard, Tetrax tetrax.

INTRODUCTION

The little bustard Tetrax tetrax is a grassland bird with a very fragmented current distribution after a severe population decline during this century. Its natural habitat is steppe grassland and although it has adapted to the cultivation which has replaced these areas (Glutz et al., 1973; Cramp & Simmons, 1980), increasing agricultural development threatens the species within most of its range (Schulz, 1985). An understanding of the habitat requirements of the little bustard in agricultural environments would allow predictions of the species' response to environmental changes derived from different agricultural practices and their implications for conservation.

Habitat selection studies of the little bustard are scarce and qualitative (Schulz, 1980, 1985; Moseykin, 1992). The purpose of this study is to determine quanti- tatively the habitat selection patterns of the little bustard within cultivated areas during the breeding season. To this end, I analyse the relationship between the spatial distribution of territorial males and habitat characteristics at four localities in Central Spain.

STUDY AREAS

Four areas representing the range of agricultural habi- tats of little bustards in Central Spain were selected: Valdetorres de Jarama (Madrid), Zarza de Tajo

Biological Conservation 0006-3207/94/$07.00 © 1994 Elsevier Science Ltd, England. Printed in Great Britain

125

(Cuenca), Ocafia y Oropesa (Toledo) (Table 1). The study areas are characterized by flat terrain and by a lack of arboreal vegetation. They are used for cereal cultivation, principally wheat Triticum sp. and barley Hordeum sp., with a physiognomy very similar to that of a steppe ecosystem. In Valdetorres de Jarama most of the land, approximately 95%, is used for cereal crops, the rest being pasture. In Zarza de Tajo 75% of the land is cultivated and 25% shrubland. In Oropesa 80% of the land is cultivated and 20% pasture. In Ocafia 75°,4, is used for cereal cultivation and 25% is vineyards, legumes and olive groves.

METHODS

Habitat selection was studied at two scales: (1) the re- lationship between the distribution of territorial males with an analysis of habitat characteristics; (2) selection of land-use types associated with the calling period.

Field procedure Field work was carried out in April and May 1992. Twenty-two variables were measured around locations of 67 territorial males (see Table 1 for distribution of territory units in the study areas). Variables were se- lected as the most characteristic physiognomic and floristic features of the environments studied, and on the basis of the biological relevance to the species (Noon, 1981). These variables were measured in an identical manner in 67 randomly located sample units, whose distribution in the study areas was the same as that of the territories (Table 1).

Each sample unit was a circle of 100 m radius. At 45 points, located on lines oriented in the four cardinal compass directions (5, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 m from the circle centre) the presence or ab- sence of the following types of land use was recorded: shrubland, pasture (lands with a natural cover of grass), long-fallow land (agricultural lands left fallow for more than one year), cereal crop, ploughed land, fallow (cultivated lands lying idle during the growing season), legume crop, vineyard, olive grove, track and stream. Height of the vegetation at these points was also measured. Average and maximum vegetation height and percentages of cover of the different types of land use were derived from the field data. Number of

126 C Martinez

Table 1. Densities of breeding males and number of territories used in the study

Site Coordinates Altitude (m) Area (ha) Density No. territories males/km 2 (sample units)

Valdetorres de Jarama 40°42'N, 3°29'W 675 1 200 Zarza de Tajo 40°YN, 3°10'W 600 900 Oropesa 39°57'N, 5 ° 1 I'W 350 500 Ocafia 39°55'N, 3°30'W 700 1 100

3.5 18 3.7 17 3 12 3.9 20

substrata, Shannon-Wiener index of diversity (H'= -2~ pi In pi) and index of habitat breadth (HB = e n') (Hill, 1973; Alatalo & Alatalo, 1977) were calculated from the cover data. Finally, the following habitat structure variables were measured at each sampling unit: number of holdings; irregularity of the terrain; number of trees or shrubs higher than 2 m; distance to nearest building, nearest village and nearest road.

Land-use selection was analysed by comparing habitat use during the calling period with habitat availability within territories and in the study area. Habitat use is defined as the percentage of males calling from each land-use type, and the indices of diversity (H') and habitat breadth (HB) were calculated from this percent- age. Habitat availability is defined as the percentage occupied by the different types of land use. Available habitat was quantified using the data obtained from the sampling units. The former indices were also calculated from availability values.

Data analysis Data were transformed before analysis using arcsine

~/x for cover variables and log x for the rest of the variables. One-way analyses of variance (ANOVA) were performed to compare habitat characteristics inside territories with those of random units. Stepwise discriminant analysis was used to identify the most important environmental variables in the spatial distri- bution of the little bustard, by discriminating between territory units and random units. Selection of different types of land use during the calling period was analysed by comparing the use data with the availability data by means of chi-square goodness-of-fit tests. These analyses were performed separately for each type of land use, so that each one was compared with the others taken together. Analyses were performed using the statistical package BMDP-90 (Dixon, 1990).

RESULTS

Long-fallow lands (E l ,13 2 = 12-13, p < 0.001) and legume crops (F1,132 TM 8.50, p<0-01) were preferred, whereas cereal crops (F1,132 TM 9.63, p < 0-01) and ploughed lands (FL132 = 6.52, p<0.05) were little used in relation to

Table 2. Summary statistics of habitat used by the territorial males of the little bustard and the available habitat in four localities of Central Spain. Sample size -- 134 units (territory units = 67, random units = 67)

Territory units Random units

Variable ~ SD Min. Max. X SD Min. Max.

Cover Shrubland Pasture Long-fallow land Cereals Ploughed land Fallow Legumes Vineyard Olive grove Track Stream

3.1 12.3 0 77-8 6.2 17.5 0 68.9 5.6 20-7 0 100 5.1 18.4 0 97.8

22.1 34-2 0 100 6 17.9 0 100 21.9 21.8 0 75-6 37.9 36.1 0 100 18.8 22-6 0 88-9 30.8 31.2 0 100 9.9 24.1 0 100 4.1 16.2 0 100

12.1 23.5 0 93.3 2.9 12-3 0 77.8 4.5 10.9 0 51.1 3.4 11.3 0 51.1 0.1 0.8 0 6.7 1.2 7.5 0 57.8 1.7 1.2 0 4.4 1.9 1.6 0 6.7 0.3 0-9 0 4.4 0.6 2.8 0 20

Diversity Number of substrata 3.6 Index of diversity 0.8 Index of habitat breadth 2.4

Environment structure Average vegetation height (cm) 18.9 Maximum vegetation height (cm) 59.6 Number of trees or shrubs (>2m) 0.7 Number of holdings 4.2 Distance to nearest building (m) 930 Distance to nearest village (m) 3 120 Distance to nearest road (m) 1 740

1.2 1 6 3 1.1 1 6 0.4 0 1.58 0.64 0-38 0 1.29 0.89 1 4.85 2.04 0-75 1 3.63

8.8 3.2 42.3 24.7 15.5 0 66.5 23-8 13 110 65.6 26.1 0 165

2-3 0 16 4.6 19.4 0 120 2 1 10 3.8 1-7 1 8

606 125 2 400 838 622 25 2 700 1 421 500 5 700 2 873 1 507 60 5 800 1 233 100 4 300 1 636 1 211 90 4 300

Little bustard habitat selection in Central Spain 127

Table 3. The frequency of three classes of average vegetation height in territory units and random units

Classes of average <20 cm 20-40 cm >40 cm vegetation height (%) (%) (%)

Territory units 70-1 28.4 1.5 Random units 42 40 18

their availability (Table 2). The number of substrata within the territories of breeding males was signifi- cantly higher than in the rest of the area (Fl.t32 = 7.63, p<0.01). The territories were also characterized by higher habitat diversity (El,13 2 = 5.11, p<0.05) and breadth (FIj32 = 6.46, p < 0-05). Another important component in habitat selection was vegetation height, which was significantly lower within territories (F1,13 2 •

7.09, p<0.01). The most frequent average vegetation height class within the territories (55.2%) was 10-20 cm. The frequencies of the three classes of average vegeta- tion height defined (Table 3) differed significantly both inside and outside the territories (A2 =- 15.5, d.f.=2, p<0-01).

Coverage of long-fallow land, legume crops and fallow land, and habitat breadth, were the best discrim- inant variables between territory units and random units, with 72.4% of the units correctly classified according to the jackknife classification.

Substrata used during the calling period did not differ significantly from substrata available inside the territories (Table 4). Long-fallow land 0(2=97.3, d.f.--1, p<0.001), legume crops (A2=43.8, d . f .= l , p<0.001) and fallow ( ~ = 20.9, d.f.-- 1, p< 0.001) were used for calling in a higher proportion than their availability in the study areas, whereas cereal crops (X 2= 22.3, d.f. = 1, p<0.001), ploughed land (A2 = 13.6, d . f .= l , p<0.001) and shrubland ~=4-7, d . f .= l , p<0.05) were less frequently used than would be expected from their availability. Diversity in the use of the different types of land use during calling was similar to their availability within territories, both being much higher than habitat diversity in the whole study area.

Table 4. Use frequency of the different types of land use during the calling period (n = 134 territorial males) in relation to their availability inside and outside the territories, in four localities

of Central Spain

Type of land use % use during % available % available calling period inside territories in study areas

Shrubland 1.5 3.1 6.2 Pasture 7.5 5.6 5.1 Long-fallow land 26.1 22.1 6 Cereals 17.9 21.9 37.9 Ploughed land 15.7 18.8 30-8 Fallow 11.2 9.9 4.1 Legumes 12.7 12-1 2.9 Vineyard 4.5 4.5 3.4 Olive grove 0 0.1 1.2 Track 3 1-7 1.9 Stream 0 0.3 0.6

Index of diversity 1.96 1.97 1.73 Index of habitat breadth 7-10 7.17 5.64

DISCUSSION

This study demonstrates that little bustard distribution in cultivated areas is largely determined by the diversity of substrata, with special preference for fallow land and for legume crops. The selection of long-fallow lands has also been recorded by other authors (Kirikov, 1966; Kostin, 1978; Cheylan et al., 1983). Among possible reasons for this preference is the higher floristic diversity of fallow lands (see Hidalgo de Trucios & Carranza, 1990; Hellmich, 1991), which are colonised by a herbaceous vegetation with a predominance of pioneer species producing abundant seeds (Fenner, 1985). This is especially relevant in cultivated areas that are characterized by their structural simplicity and by the poverty of spontaneous vegetation due to intensifi- cation of agriculture. Moreover, the higher floristic diversity of the plant communities is reflected in a higher specific richness of the arthropod fauna. This may be very important given the importance of the animal component in the diet of the little bustard during spring (Glutz et al., 1973; Cramp & Simmons, 1980; Schulz, 1985). The preference for legumes has been reported by other authors (Andr6, 1985; Asmod6, in press; Voisin et al., 1991), and is common to other members of the family Otididae such as the great bustard Otis tarda (Martinez, 1991a). This preference is probably explained by their high nutritive value and digestibility (Jarrige, 1981). The former is very impor- tant because the diet of herbivorous species is deter- mined by the energy obtained from the food as well by its digestibility (Pyke, 1984).

The results of this study confirm the importance of vegetation height to the little bustard (Kostin, 1978; Schulz, 1980; Moseykin, 1992). Vegetation height is a very important habitat component due to its decisive influence on two of the basic habitat requirements of this species: cover and visibility. On the one hand cereal crops provide cover against potential predators. On the other, the need for good visibility to watch the sur- roundings effectively, as well as for open land to locate display sites, requires the presence of substrata in the territories in which vegetation height does not exceed the bird's height, such as fallow, long-fallow lands and legume crops.

The unimportance of the irregularity of the terrain and the number of trees in the spatial distribution of the little bustard may be explained by the scale used. It is very likely that the relevance of these variables is not evident at a territorial scale (Rotenberry, 1986; Wiens et aL, 1987). Proximity to buildings, villages and roads does not influence the spatial distribution of the little bustard. The results of this study indicate that this species has a higher degree of tolerance to human disturbances than the great bustard (Martinez, 1991b).

The principal implication for conservation is the im- portance of the diversity of the landscape mosaic. The negative impact of intensified agriculture results from the reduction of habitat diversity, as well as in the area of preferred habitats, such as fallow lands and legume

128 C. Mart inez

crops, and the increase in area of unsuitable habitats, such as cereal crops and ploughed lands. Furthermore, another negative aspect of the process of intensifica- tion, which implies the creation of large patches and the uniformity of cultivation, is the loss of the gradient in vegetation height.

In conclusion, although the agricultural environ- ments seems to provide suitable habitats for the little bustard, the intensification of agriculture is an impor- tant threat to the species. Given that in Spain and Portugal, which hold the largest populations of little bustard, agricultural intensification is unavoidable for political and economic reasons, I suggest that land managers should be given an incentive to increase the agricultural mosaic in cultivated areas. This could be achieved by the temporary set-aside of cultivated land and by increased cultivation of legumes. All these suggestions concur with proposals for 'Environmentally Sensitive Areas', where farmers receive grants as an incentive to comply with certain management pre- scriptions which favour the conservation of the steppe habitat and its associated fauna (Tucker, 1991). In Central Spain, there is no ESA area, although there is a possibility that Valdetorres de Jarama will be proposed as such in the near future, since it is in the inventory of ' Important Bird Areas' (Grimmet & Jones, 1989) and has been designated a 'Special Protection Area'.

ACKNOWLEDGEMENTS

We thank J. Peral for his valuable help in collecting field data and R. Johnston for his comments on an earlier draft of the manuscript and for the revision of the English translation. B. Thomas also reviewed the English translation. This study has been carried out by the Sociedad Espafiola de Ornitologia. Financial support was provided by the Royal Society for the Protection of Birds.

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