Altitude and Rock Cover Explain the Distribution and Abundance of a Medite
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Altitude and Rock Cover Explain the Distribution and Abundance of a Mediterranean Alpine Lizard 1,2,3 1 2 CAMILA MONASTERIO, ALFREDO SALVADOR, AND JOSE´ A. DI´AZ 1Departamento de Ecologı´a Evolutiva, Museo Nacional de Ciencias Naturales, CSIC, Jose´ Gutie´rrez Abascal 2, E-28006 Madrid, Spain 2Departamento de Zoologı´a y Antropologı´aFı´sica (Vertebrados), Facultad de Biologı´a, Universidad Complutense, E-28040 Madrid, Spain Journal of Herpetology, Vol. 44, No. 1, pp. 158–163, 2010 Copyright 2010 Society for the Study of Amphibians and Reptiles Altitude and Rock Cover Explain the Distribution and Abundance of a Mediterranean Alpine Lizard 1,2,3 1 2 CAMILA MONASTERIO, ALFREDO SALVADOR, AND JOSE´ A. DI´AZ 1Departamento de Ecologı´a Evolutiva, Museo Nacional de Ciencias Naturales, CSIC, Jose´ Gutie´rrez Abascal 2, E-28006 Madrid, Spain 2Departamento de Zoologı´a y Antropologı´aFı´sica (Vertebrados), Facultad de Biologı´a, Universidad Complutense, E-28040 Madrid, Spain ABSTRACT.—West European Rock Lizards within the Iberolacerta group have a restricted distribution, with small, widely separated ranges in highland areas. The aim of this study was to identify possible habitat requirements (including habitat structure, type of vegetation, and refuge availability) and topographic factors (altitude and orientation) that may determine variations in the abundance of Iberolacerta cyreni on a 300-km2 mountain range and to discuss the implications of our results for the conservation of this endangered endemism. Both a stepwise regression and a best model selection approach showed that lizard abundance was positively correlated with only two predictors: altitude and cover of large rocks. Thus, the successful exploitation of alpine habitats by I. cyreni seemed to depend on the abundance of large rocks that may provide suitable basking substrates while minimizing predation risk. The positive association between altitude and lizard abundance predicts a fragmented distribution with isolated populations in the mountain peaks. Mediterranean mountain ranges are unique ecolog- declining population trend (Pe´rez-Mellado and Chey- ical systems, where complex topography and histor- lan, 2005; Amo et al., 2007). Habitat preferences of this ical climate changes have promoted high levels of species have been previously analyzed by Martı´n and diversity and endemicity (Taberlet et al., 1998; Hewitt Salvador (1997), but only at a particular site at 1,900-m 1999; Garcı´a-Barros et al., 2002). Moreover, their altitude. Therefore, it is of interest to explore the ecosystems are expected to be particularly sensitive variations of lizard abundance at a larger, regional to global change (Nogue´s-Bravo et al., 2007; Thuiller et scale, covering the whole area of one of the three al., 2005), which emphasizes the need to obtain mountain ranges in which the species is known to be reliable information on the altitudinal distribution of present. Moreover, the Sierra de Guadarrama, where the organisms that are confined to these environ- our study was conducted, has been proposed to be ments. For that purpose, a good starting point is to declared a National Park, which emphasizes the explore the correlations with both biotic and abiotic concern about the degradation of this relevant natural factors (Hoffmann and Blows, 1994; Holt, 2003; Fortin site. Therefore, the results of this study may give et al., 2005). This approach has been widely used in a helpful insights into management and conservation variety of species including lizards (Dı´az and Carras- strategies that should help to preserve the vulnerable cal, 1991; Dı´az, 1997; Fisher et al., 2002). populations of this species and the welfare of its The aim of this study was to identify possible alpine habitat. habitat requirements (including habitat structure, type of vegetation, and refuge availability) and topograph- MATERIALS AND METHODS ic factors (altitude and orientation) that may deter- Study Area.—The Sierra de Guadarrama is a mine the range limits of an alpine Rock Lizard in a 2 mountain range located in Central Spain that extends 300-km area in Central Spain. West European Rock over 80 km in a southwest to northeast direction, Lizards within the Iberolacerta group have a restricted reaching its highest peak (Pen˜ alara) at 2,428 m above distribution, with small, widely separated ranges in sea level (a.s.l.). Its continental climate is characterized highland areas, but the causes of their confinement by contrasting seasonal conditions, with cold wet remain largely unexplored. Iberolacerta cyreni is en- winters and marked summer droughts. Average demic to the Sistema Central mountains (Sierras de minimum (February) and maximum (July) monthly Guadarrama, Gredos, and Be´jar), Spain. It was 2 formerly considered a subspecies of Iberolacerta temperatures are 3.3uC and 20.8uC (Puerto de monticola but has recently achieved full species status Navacerrada meteorological station, 1,860 m a.s.l.), based on morphological and molecular evidence respectively. Average annual precipitation is 1,409 mm (Arribas, 1996; Carranza et al., 2004; Crochet et al., (Montero and Gonza´lez, 1984). Mountain bases 2004). This taxon is listed as Endangered in IUCN Red (1,200–1,700 m a.s.l.) are covered with monospecific Lists because of its reduced extent of occurrence and oakwoods of Quercus pyrenaica, which are progres- acute fragmentation, with habitat loss and habitat sively replaced by Scots Pine (Pinus sylvestris) forests degradation (e.g., development of infrastructure such at higher altitudes. These forested areas, that spread as roads and ski resorts) being the major causes of its from 1,500–2,100 m a.s.l., gradually become sparse patches, until vegetation is dominated by a mosaic of dense mixed-shrub formations (of perennial Juniperus 3 Corresponding Author. E-mail: camila@mncn. communis and Cytisus oromediterraneus) interspersed csic.es with small meadows (of Festuca and other grasses). SHORTER COMMUNICATIONS 159 Mountain tops are also characterized by extensive TABLE 1. Habitat variables (N 5 31) measured patches of large granite rocks and scree with scattered along the transects or obtained with a GPS (altitude) shrub formations (Rivas-Martı´nez et al., 1987; Costa et or from 1 : 25,000 maps (orientation). Cover values are al., 2005). given in percentages. Lizard Abundance and Habitat Characterization.—From May to July 2006, we carried out surveys to relate the Mean 6 SD abundance of I. cyreni with potentially important habitat variables in different sites of the Sierra de Leaf litter cover 14.4 6 13.3 Guadarrama. Study sites (N 5 31) were spread Grass cover 24.7 6 9.1 throughout the mountain range, across an altitudinal Small rock cover (,25 cm diameter) 6.4 6 6.9 gradient between 1,800 and 2,300 m a.s.l. The sampled Medium sized rock cover (25–100 cm surface covered an area of approximately 300 km2 diameter) 10.6 6 8.2 including all the major alpine and subalpine habitat Large rock cover (.100 cm diameter) 8.1 6 7.9 types. Thus, our range of habitats encompassed those Cytisus oromediterraneus shrub cover 22.6 6 15.1 in which this lizard is abundant (rock outcrops with Juniperus communis shrub cover 21.1 6 15.2 mixed-shrub formations), together with those in which Pinus sylvestris saplings cover 3.4 6 4.3 it is rare or practically absent (i.e., transition and pure Distance to nearest refuge (cm) 52 6 44 pinewoods, respectively). Our sample of sites allowed Altitude (m a.s.l.) 1978 6 118 different combinations of altitude, cardinal orienta- Orientation (negative cosine of tion, slope, and habitat structure. cardinal orientation) 0.157 6 0.682 At each study site, one of us (CM) walked a 200-m transect counting the number of lizards seen within a 5-m wide belt (2.5 m each side of the progression line). abundance. For that purpose, we combined the This census belt width allowed us to detect all active traditional null hypothesis testing approach with lizards, either directly by sight or by hearing their inferences based on model selection (Johnson and escape movement in vegetated areas before identify- Omland, 2004). We performed a preliminary Principal ing them. Censuses were done between 0900 and Component Analysis (PCA) to explore the structure of 1300 h (Mean European Time), only on sunny and the correlation matrix among the microhabitat vari- windless days. All transects were repeated twice, and ables involved (Table 1), and we noted which vari- the maximum number of lizards observed was used ables had the greatest loadings on each principal as a relative abundance index. This simple index does component. These variables, together with the topo- not provide a measure of the actual density of lizards graphic ones (i.e., altitude and orientation) and the but allows between-habitat comparison of abundanc- mean distance to the nearest refuge (which was es, facilitating the detection of underlying biogeo- excluded by design from the PCA to keep refuge graphical patterns (Dı´az and Carrascal, 1991; Fisher et availability as a separate variable), were introduced as al., 2002; Martı´n and Lo´pez, 2002). predictors in a Stepwise Multiple Regression analysis. Habitat structure, vegetation type, and distance to Normality of regression residuals was examined the nearest refuge (rock or shrub that could be a using normal probability plots. To corroborate the feasible shelter for lizards) were measured every 20 m adequacy of our predictive model, we used the along the transects (10 sampling points per transect). Akaike Information Criterion (AIC), which is the most Four 5-m lines were laid out radiating from the widely used computational approach to model selec- sampling location into the four cardinal directions. At tion. Prior to these analyses, variables were checked 1-m intervals along these lines, we registered the for normality using Shapiro-Wilk’s tests, and loga- presence or absence of small rocks (,25-cm diameter), rithmic transformations were applied when necessary.