Why is lichen a good determinant of deglaciation? Becasue it is the first organism to inhabit organically barren sediments and rock outcrops. So, by measuring the size of a lichen patch - scientists can determine the timing of glacial retreat, temperature increase, moisture content, and the amount of time it takes for organisms to reinhabit the opening niches. Lichen draws other organisms that eat it. They leave droppings, fertilizing the organically poor soils and making it possible to grow other vegetation - drawing more and a greater variety of animals - etc., etc., etc... I believe, if I recall my biology correctly, that lichens are deposited as spores that are easily carried by the air and wind currents over very long distances.
Just thought I'd put a posting up explaining something that came up in today's class.
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