Lichens: Nature's Air Detectives

A partnership disguised as a plant

Look closely at an old wall, a tree trunk or a weathered rock and you will probably spot crusty grey-green patches, tiny orange splashes, or little grey 'shrubs' barely a centimetre tall. Most people walk straight past them, assuming they are some kind of moss or stain. In fact they are lichens — one of the strangest and most useful life forms on Earth. A lichen is not a single organism at all, but two life forms living together as one.

Two lives, one body

A lichen is a symbiosis — a close partnership between two very different organisms:

• A fungus, which forms the tough outer body, anchors the lichen and soaks up water and minerals.
• An alga (or sometimes cyanobacteria), which is green and carries out photosynthesis, making sugary food from sunlight.

Each partner gives the other something it cannot make alone. The alga supplies food; the fungus supplies shelter, moisture and protection from drying out. Together they can survive in places where neither could live by itself. It is one of nature's most successful teams — so successful that lichens cover an estimated 6% of Earth's land surface.

To understand the photosynthesis half of the deal, see Photosynthesis Explained, and for the fungal half, Fungi and Mushrooms.

Survival champions

Lichens are astonishingly tough. Because the partnership is so self-sufficient, lichens grow where almost nothing else can:

• Bare rock on mountaintops, where they are often the first life to arrive.
• Tree bark, walls, gravestones and roofs.
• Scorching deserts and the freezing Arctic tundra, where reindeer depend on "reindeer lichen" for winter food.

Lichens can even shut down completely when dry, going dormant, then spring back to life when moisture returns. Scientists have shown that some lichens can survive exposure to the vacuum and radiation of outer space. But there is a trade-off for all this toughness: lichens grow incredibly slowly, often less than a millimetre a year. Some large patches in the Arctic are thought to be thousands of years old.

Pioneers that build soil

Lichens are pioneer species — among the first life to colonise bare ground after events like volcanic eruptions or retreating glaciers. As they grow on rock, they release weak acids that slowly break the surface down, and their dead remains add organic matter. Over centuries this helps create the first thin layer of soil, paving the way for mosses, then small plants, then larger ones. In this way lichens quietly kick-start whole ecosystems. You can connect this to Soil and Why It Matters.

Nature's air detectives

Here is the property that makes lichens famous: they are superb bioindicators of air pollution. Unlike plants, lichens have no roots and no protective waxy skin. They absorb water, minerals — and any pollutants — directly from the air across their whole surface. They cannot flush these substances out.

This makes them extremely sensitive to dirty air, especially to sulfur dioxide (released when fossil fuels burn) and to excess nitrogen compounds. In heavily polluted cities, delicate lichens vanish, leaving only the very toughest species or bare bark — historically these have been called "lichen deserts." As you move into cleaner countryside, more and more lichen species appear, including bushy, branching kinds that pollution wipes out first.

By recording which lichens grow where, scientists can map air quality across a whole region without expensive equipment — and because lichens live for years, they reveal pollution levels over time, not just for a single day. When laws cut sulfur emissions in many countries, sensitive lichens gradually returned, providing living proof that the air was getting cleaner.

Reading the clues

Lichens come in three main growth forms, and the mix tells a story:

Growth form	Looks like	Pollution tolerance
Crustose	Flat crust stuck to the surface	Most tolerant — survives dirtier air
Foliose	Leafy, lobed, slightly lifted	Medium sensitivity
Fruticose	Bushy, branching, hair-like	Most sensitive — needs clean air

Lots of bushy, fruticose lichens usually means clean air. Only flat crusts surviving can mean the air is more polluted.

Safe activity: a lichen air survey

This is a safe outdoor investigation — you only look, you never eat or rub the lichens, and you wash your hands afterward.

1. Choose two sites to compare — for example a busy roadside and a quiet park, or trees near a road versus trees deeper in a wood.
2. Pick similar trees at each site (the same species and similar age if you can) to keep things fair.
3. Look at the bark and count growth forms. At each tree, record how many crustose, foliose and fruticose lichens you can see. Take photos instead of picking anything.
4. Compare your sites. Does the cleaner-looking site have more bushy, fruticose lichens? Does the busy roadside have fewer species?
5. Draw a conclusion. What does the lichen mix suggest about the air at each place?

To make your survey trustworthy, keep things fair — compare similar trees and count the same way at each site. Learn how in What Is a Fair Test? Variables Explained.

Why this matters

Lichens prove that some of the most important organisms are the ones we overlook. They build soil, feed Arctic wildlife, and act as a free, living monitoring network for the air we all breathe. Next time you pass a crusty patch on a wall, remember: you are looking at a tiny partnership that may be older than your grandparents — and quietly reporting on the quality of your air.