Lichen competition: two-dimensional warfare in slow motion

Lichens, a symbiotic association between a filamentous fungus and an alga, are often dominant in stressful environments such as the surfaces of rock and tree bark. Under these conditions, lichens experience extremes of temperature, moisture supply, and low availability of nutrients. As a consequence, lichens sequester a high proportion of their carbon production for stress resistance rather than for growth. Hence, as a group lichens are particularly slow growing organisms with many species growing at less than 2mm per year and some at less than 0.5mm per year. Whether or not competition occurs between lichen thalli in these communities is controversial. This article discusses the evidence that competition occurs between lichens on rock and tree bark and assesses whether competitive effects are likely to be important in structuring these communities.