Lichens are the dual organisms of the biological world. Each lichen consists of a fungal body that houses algal or cyanobacterial cells, with each partner contributing something the other cannot produce: the fungus offers shelter and mineral uptake, while the photosynthetic partner supplies sugars. Biologists once regarded this arrangement as a straightforward symbiosis; recent work has revealed a more complicated picture in which multiple fungi and multiple photosynthetic partners can live together in the same thallus, and in which the partnership's stability depends on the particular environment in which the organism grows.

A lichen colonising a bare rock is not engaged in a rapid process. The growth rate of a single colony, once established, is often less than a millimetre a year for crustose species that lie flat against the surface, and only a few millimetres a year for the foliose and fruticose forms with a more three-dimensional structure. These slow rates make lichens useful as natural chronometers: the diameter of an undisturbed colony can, under the right conditions, indicate how long the surface has been exposed. The geologist Beatrice Thornton has used this method to date the retreat of small glaciers in the Alps, reading the oldest lichen colonies on a moraine as a lower bound for the time since the ice left.

The dating method depends on knowing a local growth curve, which must be calibrated against surfaces of known age - gravestones, dated buildings, documented rockfalls. In regions where such calibration is possible, lichenometry, as the technique is called, can be accurate to within a few decades for surfaces up to several centuries old. Beyond about five hundred years, the growth of individual colonies is no longer linear, and the method becomes unreliable. Thornton's Alpine work has nevertheless allowed her to date movements of small ice lobes that were too minor to be recorded in the historical archive.

Pollution shortens lichen lifespans dramatically. Sulphur dioxide, the characteristic pollutant of coal-burning cities, damages the chlorophyll of the algal partner and, in strong concentrations, kills the colony outright. The effect is severe enough that the distribution of lichen species has been used since the 1970s as a map of urban air quality. A biologist walking outward from a city centre can observe a characteristic zonation: no lichens in the most polluted centre, crust species tolerant of acidic conditions in a middle zone, and the richest, most pollution-sensitive species only in the outermost suburbs. Following the decline of coal burning in most European cities, the lichen communities have been advancing back toward the city centres, providing an unusually direct measure of air-quality improvement.

The ecological functions of lichens in natural settings are easy to overlook because lichens rarely dominate a landscape's appearance, but they are considerable. In boreal forests, reindeer and caribou depend on several species of Cladonia as a major winter food, and the structure of these ground-level carpets determines whether the animals can sustain themselves through the cold months. On cliff faces and bare tundra, nitrogen-fixing cyanobacteria housed inside lichens can be among the principal sources of nitrogen for the ecosystem, supporting the eventual establishment of vascular plants. Lichens also produce an extraordinary range of secondary metabolites, some of which have been developed into medicines, dyes, or perfume ingredients.

The sensitivity of lichens to atmospheric chemistry has made them valuable for long-term environmental monitoring. Because lichens absorb most of their nutrients directly from the air, trace elements present in the atmosphere accumulate in their tissues in proportions that broadly reflect ambient levels. Analysis of lichen samples from archive collections has been used to reconstruct historical trends in lead, cadmium, and several other pollutants. More recently, analysis of radioactive isotopes in lichens of Lapland allowed researchers to measure the regional fallout from the Chernobyl accident and to track its slow decline over the subsequent decades.

Lichens are also notably resistant to environmental extremes that defeat most other organisms. Some species survive prolonged desiccation, rehydrating and resuming photosynthesis within minutes of becoming wet. Others tolerate vacuum and ultraviolet radiation that would kill nearly every other eukaryote; in a well-known experiment, specimens of Rhizocarpon geographicum spent eighteen months attached to the outside of the International Space Station and were returned in viable condition. Such resilience has encouraged speculation, though not conclusive evidence, about the possibility of lichen-like organisms surviving on Mars.

The pressures facing lichen communities are nevertheless significant. Atmospheric nitrogen deposition, the by-product of intensive agriculture and motor vehicle emissions, fertilises the growth of a small number of nitrogen-loving species at the expense of the many species adapted to nutrient-poor conditions. Climate change is shifting the ranges of some species and, in Arctic regions, threatening the Cladonia carpets on which reindeer depend. Thornton has observed that the very characteristics that make lichens useful indicators - their dependence on atmospheric chemistry, their slow growth, their narrow habitat requirements - also make them vulnerable to the larger shifts now under way. The century ahead, she has warned, is likely to be uncomfortable for organisms that have survived for hundreds of millions of years precisely because they do not change quickly.