Competitive equivalence in a community of lichens on rock

Abstract

Lichens and mosses cover 70–100% of the rock surface in a forested Appalachian boulderfield, and competition for space is intense. This paper examines overgrowth ability and its morphological correlates in four common species of foliose lichen on rocks. Overgrowth requires one lichen thallus to overtop another at the point where they meet. Therefore, I quantified margin height for a number of thalli of each of four lichen species. Two “umbilicate” species attached to the rock only at the thallus center showed a positive relationship between thallus size and margin height: large thalli often reached considerable heights above the rock surface, yet most also had points along their margin that were quite low and flat. Two other “nonumbilicate” species were characteristically flatter and showed no dependence of margin height on thallus diameter. Differences among species, among thalli of the same species, and among different points on a single thallus accounted for approximately equal amounts of variance in margin height. To determine the success of species in overtopping each other, I then recorded 639 instances of apparent overgrowth (overtopping of one thallus by another) on several rocks in the boulderfield. Of the nine pairs of species that met often enough to permit statistical analysis, only four pairs showed a consistent winner. Species in the remaining five pairs were competitively equivalent, neither winning significantly more than half the encounters although each individual encounter had a clear winner. Overgrowth rates measured from sequential photographs were highly variable, but many species pairs showed no substantial differences between growth rates over another thallus and growth rates over bare rock; only one species appeared to be affected by overgrowing other thalli. Overgrown thalli, as well as thalli experimentally shaded by gluing an overhanging rock chip above their margin for a year grew very slowly or not at all in the region of overgrowth, and the overgrown region of the thallus was often markedly discolored or disintegrating. Thus, foliose lichens compete strongly for space at the study site, yet because competitive success is at least partly based on a morphological character (margin height) that is inherently quite variable even in a single thallus, many pairs of species appear to be competitively equivalent.