Crustose lichen Both attractive and intriguing, these very sensitive plants act like passive sponges and, so far as air pollution goes, their effect is proven.  They’re also used as ecological indicators for such projects as monitoring the changes in forest after logging and being sensitive to humidity and light levels, are the first to give warning of malaise.

Crustose lichen growing on dolerite, showing fruiting bodies

Lichens are a combination of fungus and either algae or cyanobacteria, resembling Jurassic crusts on trees, stones and walls of ancient buildings, as well as not so ancient surfaces such as glass, bitumen, rubber, concrete, roofing tiles and paint. So how do they get their food growing on these amazing places? Basically it’s nutrients dissolved in rain water.
They are found everywhere, exploiting whatever niche is available.  Some even occur below the high-water mark on seashores, or semi - permanently submerged in rivers and lakes They’ve been found attached to the feet of seabirds or on bits of driftwood that float around the world and there are spores in the air flying around. Even so, some of the enormous distances between sightings of particular species is unbelievable e.g. south east Tasmania and the Rift Valley of East Africa.
Fruiting cups

       Fruiting bodies of lichen growing on rotten wood

Pink spores
In Europe in particular, lichens are disappearing due to air pollution but in Tasmania there are still vast numbers. Here, the most well known and attractive crustose lichens are seen on dolerite rocks, forming platelike mats which appear to die off during drought or heat, yet miraculously come to life after the first rainfall and in this way survive for many years. 
Tip for gardeners: paint a little milk on to the surface of new landscaping rocks and the lichens will soon develop.

PART ONE                   PART TWO