Habitat selection by iberian lynx
Vegetation structure and the index of prey abundance in sites used by dispersing and resident lynx and sites that were probably not used by lynx were compared. The results are valid only if a high degree of confidence exists in the classification of these areas. However, GPS, with an expected mean error of 50 m, was used both to record lynx positions and to relocate points during the autumn surveys. So error could not be avoided. Surveys had to be carried out in autumn to standardize results, mainly for the index of rabbit abundance. Nevertheless, to control for any potential error associated with lynx positions, simultaneous surveys were also undertaken during the same day, or the following day after, a lynx was located. In these cases, the exact location of the animal was surveyed. Comparison of the results obtained on vegetation structure during the autumn and simultaneous surveys indicated that only tree and short shrub layer heights were significantly different, which suggests that, in general, results obtained during autumn sampling were also representative of the areas used by lynx. In addition, there was a distance of 200 m between the furthest 40-cm diameter subplots surveyed for rabbit pellets for each point, thus pellets were counted at actual locations of lynx. Differences in tree height between autumn and simultaneous surveys cannot be clearly accounted for. However, the taller shrub heights recorded during the simultaneous surveys may be a result of the timing of surveys during daylight when lynx are expected to be resting (Beltrán & Delibes 1994) and therefore favouring sites with slightly more protection and cover.
It is reasonable to assume that the sites considered as unused had been determined accurately as a large data set of lynx locations collected over 15 years exists for the study area. The GPS error should not affect results because the sites were at least 400 m from any known lynx location.
One clear fact emerged regarding the characteristics of the habitats used by lynx: sites used by resident lynx were very different from sites used during dispersal and from unused sites. However, there was little difference between sites used during dispersal and unused sites, even though unused sites were within scrubland and plantation habitats (i.e. habitats potentially suitable for lynx). Sites used by resident individuals had less tree cover, shorter trees, higher percentage of understorey cover, taller shrubs and higher rabbit abundance. A lower percentage of tree cover and higher of understorey vegetation cover were expected because lynx prefer areas of Mediterranean scrubland in their home ranges (Palomares et al. 2000). Trees were shorter because the species in resident areas were predominantly Quercus sp. and Oleaeuropaea L. as opposed to the taller pines and eucalyptus. As expected, more rabbit pellets were found in resident areas because rabbits thrive best in Mediterranean scrubland in Doñana (Palomares, Calzada & Revilla 1996a). Rabbits are the main prey of lynx (Delibes 1980) and, therefore, they are vital for the survival of resident lynx. Conversely, during dispersal lynx can cope with lower rabbit densities. Results also showed that rabbits were more evenly distributed in resident areas than in dispersal areas. Rabbit pellets were only absent in one resident plot, whereas about 10% of dispersal plots had no pellets. Although the number of plots without pellets was higher for unused sites, differences were not significant, indicating that, in general, rabbit abundance was not much higher in dispersal sites than in unused sites. Height of short shrub layer was the only other variable of vegetation structure that differed between dispersal and unused sites, suggesting that lynx look for sites that provide higher protection while dispersing.
Simultaneous surveys can provide good insights into the characteristics of the areas used by dispersing lynx during rest. On half of the occasions, lynx used small vegetation patches (one or two axes shorter than 300 m) within a varied habitat mosaic characterized by higher cover and taller shrubs than that recorded in unused sites. This result clearly suggested the necessity of patches of vegetation with high understorey cover for resting. This necessity is further influenced by that fact that dispersing lynx must move through habitats with a high human presence, carrying a high risk of mortality (Ferreras et al. 1992).
Dispersing lynx increased their use of sites with a higher percentage of shrub cover the longer dispersal lasted. This result suggests that, after an exploratory phase, lynx concentrate on the more suitable areas where they try to acquire territory.
Vegetation structure and prey abundance clearly affected range size. Sandell (1989) predicted that female range size should correlate negatively with food abundance in solitary carnivores. However, empirical data are scarce on the subject, and only in a few cases could the relationship be verified with relative accuracy (Ward & Krebs 1985; Litvaitis, Sherburne & Bissonette 1986; White & Garrott 1997). In Iberian lynx, territories with more rabbits and lower shrub cover were smaller. Both variables were a measure of range quality. However, it was not clear which variable best explained range size because a high negative correlation was found between shrub cover and the index of rabbit abundance. As stated, rabbits thrive best in habitats adjacent to patches of shrub and grassland, or on the edge of shrubland and grassland habitats (Rogers & Myers 1979; Rogers 1981; Soriguer & Rogers 1981; Beltrán 1991; Palomares, Calzada & Revilla 1996a; Palomares & Delibes 1997). Thus, one variable is clearly dependent on the other. However, it was interesting to find a positive correlation between tall shrub cover and the index of rabbit abundance, and a negative one with short shrub cover. Tall shrub cover could be favoured because it provides rabbits with more adequate protection from predators and better substratum for warrens. When tall shrubs such as Pistacialentiscus are present, more than 70% of warrens are built below the protection of these shrubs, and very few beneath short shrubs (F. Palomares, unpublished data). Conversely, if understorey cover provided by short shrubs is very dense, the area available for grass would decrease, and so would the carrying capacity of rabbits.
Medium and large carnivorous mammals are considered as key and umbrella species because of their role in shaping community structure, their need for large areas to survive, and their specific habitat requirements (Noss et al. 1996). Because of this, management plans focused on these species, or the design of reserves to protect them, may also help to preserve many other species and the overall health of natural ecosystems.
The Iberian lynx is the largest predator (with the exception of wolves Canislupus L. in a small part of its distribution area) of the Mediterranean ecosystem in south-western Europe (Delibes 1983). It is a keystone species that regulates the community structure of this ecosystem with strong direct and indirect effects on other species (Palomares et al. 1995, 1996b, 1998). Lynx are effective killers of smaller carnivores such as domestic cats Feliscatus L., red foxes Vulpesvulpes L., Egyptian mongooses Herpestesichneumon L. and European genets Genettagenetta L. In areas with high lynx densities, it has been shown that densities of mongooses and genets are between 10 and 20 times lower than in areas without lynx. This, in turn, has strong positive effects on species at lower trophic levels, such as rabbits, which probably also affects primary producers (McLaren & Peterson 1994). The positive effect of lynx on rabbits is particularly important for the conservation of the predator community because rabbits are preyed upon by more that 30 species in south-western Europe (Delibes & Hiraldo 1981; Jaksic & Soriguer 1981), many of them threatened or endangered species. In addition, rabbits are an important game species in the area (Rogers, Arthur & Soriguer 1994), therefore any positive effect on this prey species is likely to have a positive impact on both human and non-human communities.
Conservation of the Iberian lynx requires good-quality habitats where individuals can settle and breed, and adequate corridors connecting these areas, because lynx presently exhibit a metapopulation structure (Rodríguez & Delibes 1992; Gaona, Ferreras & Delibes 1998) that is unlikely to return to a continuous distribution area. General characteristics of habitats sustaining stable lynx populations should include isolated trees and, ideally, 40% cover of understorey vegetation (half of which should be tall shrubs). Rabbits should also be abundant in resident areas, with autumn mean number of rabbit pellets being around 80 pellets m−2. In areas with 114 pellets m−2 lynx density may be higher due to the decrease in range size. Estimations of absolute rabbit abundance in part of the study area indicate that an average of 114 pellets m−2 represents mean rabbit densities of about 4·6 rabbits ha−1[95% confidence interval (CI) = 3·85–5·91] in spring, and about 1·0 rabbits ha−1 (95% CI = 0·78–1·13) in autumn (Palomares, in press).
The landscape of the Doñana area has been historically modified by fire, which has degraded the climax vegetation of the area into a few species of shrubs (Granados-Corona, Martín-Vicente & García-Novo 1986). Rabbits seem to benefit from fires as they increase their activity on burned areas (Moreno & Villafuerte 1995). Thus, controlled fires have been recommended as a useful tool for the maintenance of good lynx habitat (Moreno & Villafuerte 1995), by decreasing understorey cover and so increasing rabbit density. Nevertheless, fire would also destroy tall shrubs that are beneficial to lynx and rabbits. Therefore, fire or other types of selective clearing focused on short shrubs should be applied to small areas avoiding tall shrubs.
Dispersing animals often use lower quality habitats from which they tend to select areas with higher understorey cover and rabbit abundance within patches of forested habitats. These results stress the importance of small patches of natural vegetation consisting of 50% understorey cover and between 2 and 2·5 times fewer rabbit pellets than resident areas. Plantations are very frequently used by dispersing lynx (Palomares et al. 2000). Understorey vegetation of plantations is frequently removed by humans, thus decreasing their suitability to dispersing lynx. If plantations are to be an adequate transition habitat for lynx, the preservation of small patches with enough understorey vegetation should be considered.
An interesting result from this study indicates that areas suitable as corridors had lower habitat quality than areas suitable for resident individuals. This result allows for some relaxation in the design of reserves connected by corridors. Corridors might also be moderately exploited by humans (pine seed harvesting and forestry, in the case of the Iberian lynx). A problem often arising with some corridor proposals is the enormous cost requirement for their implementation (Simberloff et al. 1992). Thus proposals with economically self-sustaining corridors with less critical design than core areas are desirable and recommended.