Vol. 32 – 2006 - Ecologia Mediterranea

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MASSIMO TERZI, MARCO MARVULLI Figure 1 – Study area (1a); boundaries of the parks and steppe-grassland range (shaded areas) in the study area (1b); circles of 7.5 km radius around the nest sites (1c); priority zones in the south-eastern part of the Alta Murgia park (1d). 32 also show different behaviours in relation to local landscape structures (Franco & Sutherland 2004). The European population is estimated at 15000-20000 pairs. In decreasing order, the largest populations are those of Spain, Turkey, Greece and Italy (Biber 1996). In Italy, the Murge hills are the most important breeding site with about 2000 pairs, that is 70-80% of the total for Italy (Palumbo 2001). Here, the lesser kestrel nests mainly in the historical centre of towns in old buildings or single trees, while it feeds in the large open areas around the towns covered mainly by cereal crops and steppe-grasslands (Palumbo 2001). Methods The overall features of the Murge steppes are in some ways close to garrigue and pseudogarrigue; in others, especially at the edge of the range, the grasslands take on a savannahlike appearance. As the increasing cover of shrubs and trees can reduce steppe foraging value (Donzar et al. 1993; Biber 1996; Bustamante 1997; Tella et al. 1998; Franco & Sutherland 2004), according to Bianco (1962), we distinguished three physiognomic steppe types: (i) shrub steppe (S-steppe), with scattered shrubs; (ii) steppe woodland (Wsteppe), with scattered small trees (tree cover ecologia mediterranea – Vol. 32 – 2006
10%); and (iii) barren steppe (B-steppe), without shrubs and trees. Starting from aerial photographs taken in 1998, we digitized and georeferenced the range of the three types of steppe in the study area using GIS technology (Figure 1b). Adding the types, we obtained another three categories: barren plus shrub steppe (BSsteppe); shrub steppe plus steppe woodland (SW-steppe); and total steppe cover (Tsteppe). Given that the steppes harbour the main prey of the lesser kestrel and represent its most important trophic areas, we assumed their extents to be a useful surrogate for food resources. Moreover, we also considered the distances of foraging areas from the nesting sites as distance could play an important role in the balance between the energy gained from prey and the energy employed to find it. In order to determine whether steppe variables (extent and types, and at different distances from nest sites) were significant predictors of the colony sizes (dependent variables), a multiple linear regression analysis (MLR) was conducted. Palumbo (2001) estimated the sizes of the colonies in the Murge district in 1997 and 1998. We performed three sets of analyses using, separately, the estimated number of pairs (pre-reproductive census) of 1997, 1998 (Table 1) and their average. However, the results were not significantly different. Therefore, we report only the analysis performed using the average. In its pre-reproductive stage, the lesser kestrel travels 11-13 km to reach foraging areas (Liven-Schulman et al. 2004). Therefore, we considered the surfaces of the three steppe types within a 12.5 km of radius of every colony. These surfaces were then divided into five adjoining circular crowns (the first is a circle), each one 2.5 km in width. As the distances between colonies were not always farther than 25 km, we assumed that a given steppe area reachable by two or more subpopulations was used mainly but not exclusively by the nearest one. Every time two or more subpopulations potentially shared a steppe area, this was divided in percentages among them, using the inverse of the mean radius of every circular crown. In this way, we obtained the (potentially useful) steppe areas inside the five circular crowns around every nest site for the categories considered (Table 1). We performed a MLR between the logarithm of the census data (dependent variable) and ecologia mediterranea – Vol. 32 – 2006 Priority Zones for Mediterranean protected agro sylvo pastoral landscapes the logarithm of the steppe areas (explanatory variables) in the five crowns for each category. The logarithmic transformation of both variables was obtained by applying the Box- Cox (1964) method. For each steppe category, the explanatory variables were indicated as x 1 – x 5 in relation to the five circular crowns. Using a stepwise procedure, we tested all the possible models with the explanatory variables, from a simple linear regression for each variable to all their possible combinations. We selected the ones whose partial regression coefficients were significant. We did not consider the meaningless models from an ecological point of view, such as those relating to two or more non-consecutive circular crowns. The only variables with partial regression coefficients significantly different from zero were x 2 and x 3 of the categories including the barren steppes (B-, BS- and T-steppe). However, owing to collinearity, in the MLRs with only these two variables, their partial regression coefficients were not significantly different from zero. Therefore, we grouped the data of the circular crowns 2.5-5 and 5-7.5 km and obtained a new variable, indicated as x 23 . Results Results referring to shrub steppes and steppe woodlands both alone and in their reciprocal combinations (S-steppe, W-steppe and SWsteppe) were of no statistical importance in accounting for colony size. For the other categories, all including the barren steppes (Bsteppe, BS-steppe and T-steppe), only the partial regression coefficients of x 2 and x 3 were significant at the 0.05 level. The simple linear regressions using, separately, these two computed variables were always highly significant but the strength of the relationship was weaker for x 3 than for x 2 . The new explanatory variable (x 23 ), grouping together the steppe surfaces inside the crowns of 2.5- 7.5 km, was always correlated significantly (p < 0.01) with the dependent variable (Table 2). In all the cases, the determination coefficients were relatively high (for x 23 : r 2 > 0.71), despite the analysis being carried out without considering other likely important factors (e.g. factors related to nest sites). Thus, steppe distribution and extent explain variance in colony size to a considerable degree. The results show that the hypothesised relation- 33
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Vol. 32 – 2006 - Ecologia Mediterranea

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