Influence of habitat quantity, quality and isolation on the distribution and abundance of two epiphytic lichens

1 We assessed the relative importance of habitat quantity, quality and isolation for the distribution and local abundance of two epiphytic lichens, Parmelina tiliacea, which is red-listed in Sweden, and the common Pleurosticta acetabulum. We predicted that habitat isolation should constrain the distribution of the mainly vegetatively dispersed P. tiliacea more than the sexually dispersed P. acetabulum. 2 All patches of habitat containing suitable host trees for the study species were mapped from aerial, infra-red photographs. Presence and abundance of the lichens were recorded for a total of 3237 trees in 94 patches. 3 Patch identity had strong influence on presence of both species on individual trees. Within patches, presence was positively correlated with tree size and was also influenced by tree species. 4 At patch-level, species presence was positively correlated with tree number and negatively correlated with the proportion of the boundary that bordered coniferous forest and with isolation. Abundance was mostly correlated with tree size or with the number of large trees at tree- and patch-levels, respectively. For P. acetabulum, within-patch abundance was also influenced by isolation from other patches. 5 Presence at patch-level was affected by isolation for both species, but this effect differed between the three isolation measures used. There was no clear evidence that the distribution of P. tiliacea was more constrained by isolation than that of P. acetabulum. 6 Whilst this study emphasizes the importance of habitat quantity for species occupancy and local abundance, it shows that explanations of species occurrence in fragmented landscapes must also involve variables describing habitat quality and spatial configuration. Moreover it illustrates the difficulties in predicting species' occupancy from general knowledge about their dispersal traits.