A hypervolume approach to niche specialism, tested for the old-growth indicator status of calicioids

Certain lichen epiphytes are restricted to old-growth forest stands with long ‘ecological continuity’, explained by i) niche specialism and their dependence on microhabitats associated with old stands including veteran or senescent trees, and/or ii) dispersal limitation with probabilities of colonization being relaxed over extended time periods. ‘Calicioid’ species are among the most important old-growth indicators, yet they reproduce sexually via small spores that appear widely dispersed at ecological scales. This suggests that they should have a high level of niche specialism compared to lichen epiphytes in general, explaining their role as old-growth indicators. However, comparisons of niche specialism are challenging, and this study uses epiphytic, corticolous calicioid species as an appropriate test case. Having measured 20 variables that constrain the lichen epiphyte niche, these were collapsed into a ‘hypervolume’ representing the sampled environmental space available for occupancy by lichens in Scotland as a study system. It was then possible to examine the occupancy of this hypervolume by individual lichens (niche breadth), with the proportion/percent occupied used to estimate a niche specialism score. Consequently, epiphyte calicioid species are confirmed to have a high degree of niche specialism compared to lichen epiphytes in general, and compared to other old-growth indicators, with their niche position directed towards drier climates including locally sheltered microhabitats associated with old-growth forest structure.