Community structure of epilithic lichens on the cliffs of the Niagara Escarpment, Ontario, Canada

The spatial variation in epilithic lichen community structure was investiga ted as part of a larger study of the vegetation and ecology of the tall lim estone cliffs within the Niagara Escarpment Biosphere Reserve in southern O ntario, Canada. The cover of all lichen taxa was visually estimated for a t otal of 199 samples taken from the top, middle, or bottom of the cliff face at five sites. Twelve environmental variables were also measured. Twenty-s even lichen taxa were identified on the samples. Multivariate ordinations o f species composition (DCA, CCA, PCCA) revealed variation in community stru cture on multiple scales, but no groupings of sites that would have suggest ed the presence of several distinct species assemblages. A gradient in spec ies composition from north to south, most clearly reflected in the decreasi ng cover of foliose and fruticose species, may reflect a gradient in human disturbance. There was also intermediate-scale patchiness in species compos ition in a horizontal plane across cliffs, but despite earlier claims made in the literature, no evidence of vertical zonation of the lichens on cliff s was found. Species composition also responded to small-scale factors poss ibly related to exposure, light, or moisture. Unlike community composition, the total cover of all lichens was homogeneous over large spatial scales a nd varied only on a small scale, illustrating that scale as well as resolut ion of a study may influence the ecological patterns seen. More than half o f the species found on the Niagara Escarpment are rare on rock substrates e lsewhere in southern Ontario, and two are new for North America (Candelarie lla heidelbergensis (Nyl.) Poelt and Lecanora perpruinosa Froberg). The res ult that cliffs support a distinct flora containing many rare species sugge sts that they are a reservoir for biodiversity not just for vascular plants , but also for lichens.