Early recruitment equals long-term relative abundance in an alpine saxicolous lichen guild

The relative abundance within a guild of six species of the lichen-forming fungal genus Umbilicaria was studied during primary colonization of rock surfaces in a chronosequence of ca. 245 y in an alpine glacier foreland in southern Norway. Although the density of the guild grew from zero to more than 1000 thalli/m2 and the species differ substantially in life history traits such as initial growth rate, maximal size, maturation rate and propagule types, the relative abundance among the species remained almost unchanged through those years. The relative abundance of species is correlated with their life history parameters, such as initial growth rate and size-related maturation. The pattern of relative abundance was also similar in the saxicolous communities outside the foreland, which are potentially several thousand years old. Outside the foreland however the density of the guild is only 1/10 of that in the oldest parts of the foreland, due to soil formation and vegetation growth that have covered many of the low profile rock habitats. Thus the areas affected by the disturbance of glacier expansions and retreats provide temporary opportunities for large increases in the population sizes of the members of the saxicolous community. The observations support the view that pre-emption of habitat rather than competitive exclusion is common in saxicolous lichen communities and that "succession" consists in the addition but rarely the loss of species.