Role of isolation and habitat quality in shaping species abundance: a test with badgers (Meles meles L.) in a gradient of forest fragmentation

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Abstract
Aim: This paper describes badger (Meles meles L. 1758) abundance in remnant forests of the two Iberian plateaux. The relative role of factors related to habitat quality and to isolation were tested in order to explain variations in badger abundance according to forest‐scrubland cover at the landscape scale. It was predicted that isolation would be more important where there is less forest cover (greater fragmentation), while habitat quality would predominate in areas where forest cover is greater (less fragmentation).Location: Thirty‐six forests (> 90 ha) were sampled, 16 in the North and 20 in the South plateau. Twelve forests were sampled in each one of the following categories of regional forest‐scrubland cover in the landscape (10 × 10 km): < 20%, 20–30%, > 30%.Methods: In each forest, 4‐km transects were followed to look for badger setts. The number of badger setts km–1 was used as a surrogate of badger abundance. The transects were divided into 250 m segments in which a series of environmental variables related to habitat quality were estimated. Moreover, for each forest patch three variables related to isolation on a landscape or regional scale were measured on forest maps (1 : 400,000): the distance to riparian woodland, distance to the mountains of Central Spain and distance to other large forests on the plateau (> 10,000 ha). The prediction through Pearson correlations was tested and forward stepwise multiple regressions performed with the different subsets of data (different forest‐scrubland categories).Results: The results support the established prediction, in areas with under 20% forest cover, that isolation was the best predictor of badger abundance in forest fragments, while in areas with over 20% cover, the most important predictors were those associated with intraforest habitat quality (shrub cover and rock cover).Main conclusions: Data for badger abundance in fragmented forests indicate that factors determining species distribution and numbers in heterogeneous environments are highly dependent on landscape pattern. Isolation is the main factor explaining distribution in highly fragmented situations in which the pattern can be explained through metapopulation models. However, in less fragmented situations, distribution models are more easily described in terms of habitat suitability (niche models).