Habitat overlap/niche segregation in two Umbilicaria lichens: a possible mechanism

Summary Umbilicaria muhlenbergii and Umbilicaria papulosa are two similar lichens which show extensive habitat overlap. Current theory predicts that for those resources that are limited, and of critical importance to both species, segregation is demand for or use of those resources ought to be present. Controlled experiments showed that the upper and lower cortical surfaces of the two species functioned in very different ways so that U. muhlenbergii is wetted most rapidly by runoff and mist-like precipitation whereas U. papulosa is wetted most rapidly by large raindrops. An analysis of the meteorological conditions in the natural habitat for the two lichens shows that the four months providing significant productivity in U. papulosa are the same months with a very high frequency of thunderstorms (which produce large drops). Conversely, the months which produce the greatest production in U. muhlenbergii are spread out over most of the year. When water is supplied after periods of dryness, the two species absorb it at different times and from different sources. This difference supports current ideas about niche segregation in plants but represents the first time it has been demonstrated for lichens.