Abstract
Competition between crustose and squamulose lichens was investigated at 2 sites in Wales [UK]. At one site, 2 competing species (Caloplaca heppiana and C. aurantia) were well matched as neither exclusively overgrew the other, while at the other site, Aspicilia calcarea competed with C. heppiana, the former usually overgrowing the latter at marginal contacts, but A. calcarea was itself overgrown at window contacts. The development of a crustose lichen community in Gwynedd, Wales, is briefly described. Four main factors control the development and dynamics of these communitiies: colonization rate and density; radial growth rate; type of contact between species; and rate of senescence.