The Distributional Pattern of Crustose Lichens on Limestone Cliffs at Rattlesnake Point, Ontario

The patterns of distribution of Verrucaria nigrescens Pers., V. amylacea Mass., Caloplaca cirrochroa (Ach.) Th. Fr., and jointly of C. aurantiaca (Lightf.) Th. Fr., and C. citrina (Hoffm.) Th. Fr. on limestone cliffs at Rattlesnake Point, Ontario, are analyzed. The first three species show pattern at a scale of 10 inches (25 cm). A variation in abundance with vertical position on the cliff is established for all species except V. amylacea by means of the covariance of species frequency with position; it is attributed to an environmental gradient up the cliff face. Covariances between slope measurements and species frequencies demonstrate the influence of slope on the patterns of the species. Calculations of the relative deviations from expected frequency at different slopes illustrate a probable competition effect whereby C. cirrochroa excludes other species from surfaces at places near its optimum slope. Rattlesnake Point conservation area is situated at Nelson Township, Halton County, Ontario, (Lot 11, Concession 5, Lat. 79055'W, Long. 43028'N). The main feature of the area is the outcropping of the Niagara limestone which gives rise to cliffs about 40 feet (13 m) high. The cliffs in the conservation area are mostly southto southwest-facing and are approximately vertical. The jointing of the limestone gives it the appearance of piles of large blocks each 3-6 feet (1-2m) high. There is an accumulation of block scree at the foot of the cliffs, and in places the whole face of the cliff has slipped forward and down, forming more substantial piles of dibris. These areas support a varied bryophyte and lichen flora and are overgrown by Vitis sp. and Tilia americana. The flora of the cliff surfaces is much more restricted: the only abundant species are Verrucaria nigrescens Pers., V. amylacea Mass.,3 Caloplaca aurantiaca (Lightf.) Th. Fr., C. citrina (Hoffm.) Th. Fr., and C. cirrochroa (Ach.) Th. Fr., and they only grow on the lower parts of the cliff. The investigation sought to determine and to explain the patterns of distribution of these species 1 We thank Mr. P. Greig-Smith for his advice on the calculation of the correction for covariance with position and Mr. P. W. James for identifying the lichens. This work was supported by National Research Council Grant A-2910. 2 Department of Botany and Scarborough College, University of Toronto, Toronto. 3 The record at Rattlesnake Point is the first for this species in North America. Specimens have been deposited in the British Museum where the material was checked by Mr. P. W. James. The species has since been found at Upper Silver Creek, ten miles north of Rattlesnake Point, and material from that locality is also deposited in the British Museum. This content downloaded from 207.46.13.168 on Wed, 22 Feb 2017 18:12:08 UTC All use subject to http://about.jstor.org/terms 1966] YARRANTON & GREEN: CRUSTOSE LICHENS 451 on the cliffs. To our knowledge no previous studies of this type have been carried out on crustose lichens, indeed very little ecological study has been devoted to them at all.