Academic Reading — Test 145

Reading passage

Lichens are among the most curious organisms found in the woodlands of Britain. Although they are often mistaken for simple plants or mosses, a lichen is in fact a partnership between two very different forms of life: a fungus and an alga, or sometimes a cyanobacterium. The fungus provides a protective structure and absorbs moisture, while the photosynthetic partner produces sugars using sunlight. This close relationship allows lichens to colonise surfaces that few other organisms can tolerate, including bare rock, old walls and the bark of trees. Because they have no roots and no waxy outer skin, lichens take in water, minerals and gases directly from the surrounding air. It is precisely this feature that makes them so valuable to scientists studying pollution. Unlike most plants, lichens cannot filter what they absorb. Whatever is dissolved in rainwater or carried on the wind passes straight into their tissues. As a result, they accumulate substances from the atmosphere over long periods, and many species are extremely sensitive to chemical change. Sulphur dioxide, a gas released mainly by the burning of coal and oil, has historically been the most damaging pollutant for British lichens. During the industrial expansion of the nineteenth and twentieth centuries, large areas surrounding factories and cities became almost entirely free of lichens. Naturalists began to notice these bare zones, sometimes called "lichen deserts", and realised that the absence of certain species could reveal a great deal about local air quality. This observation laid the foundation for the use of lichens as bioindicators. A bioindicator is a living organism whose presence, absence or condition reflects the state of the environment. Lichens are particularly well suited to this role for several reasons. They are widespread, long-lived and present throughout the year, so they can be studied in any season. They also respond to pollution in predictable ways, with some species disappearing at low concentrations of sulphur dioxide while others survive in heavily contaminated air. By recording which species grow on tree trunks at a given site, ecologists can estimate the level of pollution without the need for expensive measuring equipment. This makes lichen surveys an affordable method that can be carried out by trained volunteers as well as professional researchers, and large community programmes have been organised across the country to gather such data. Not all lichens react in the same manner, and their varied tolerance is the key to their usefulness. Broadly, lichens are grouped by their growth form. Crustose lichens form a flat crust pressed tightly against the surface and tend to be the most resistant to pollution. Foliose lichens have leaf-like lobes that lift slightly away from the bark, while fruticose lichens are shrubby or hair-like and hang or branch outwards. The bushy fruticose types, which expose the greatest surface area to the air, are generally the least tolerant and vanish first when pollution rises. Consequently, a woodland rich in hanging, beard-like lichens is usually a sign of clean air, whereas a site dominated only by tough grey crusts often indicates a more polluted setting. The story of British lichens over recent decades has been one of cautious recovery. Following clean air legislation introduced after the middle of the twentieth century, the burning of coal in homes and industry declined sharply, and concentrations of sulphur dioxide fell across much of the country. Many sensitive species that had retreated to the cleaner west and to remote uplands began to spread back towards areas they had once abandoned. However, the picture is not entirely positive. A new chemical threat has emerged in the form of nitrogen compounds, released largely by vehicle exhausts and by agriculture. These compounds act as fertilisers, encouraging a small number of nitrogen-loving species to flourish while suppressing many others. Scientists now warn that nitrogen pollution may be reshaping lichen communities just as sulphur dioxide once did, even though the overall appearance of woodlands seems healthy. For all these reasons, lichens continue to serve as a quiet but reliable witness to the condition of the air. They require no electricity, no laboratory and no constant supervision, yet they record changes that might otherwise go unnoticed for years. A patient observer who learns to identify even a handful of species can read a great deal from the bark of a single oak. As concern about air quality grows, these unassuming organisms remain one of the simplest and most elegant tools available for monitoring the invisible health of the environment, and their long-term study offers an inexpensive complement to modern instruments.