Vol. 32 – 2006 - Ecologia Mediterranea

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-
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