Discovering a liking for lichens …

What is happening in the picture below? Some kind of northern English re-enactment of Jerusalem’s Wailing Wall maybe? Why else might anyone need to peer at a building from such a close distance? The answer? Lichens. I spent Saturday at the Field Studies Council’s Malham Tarn Field Centre in the Yorkshire Dales on their introductory course on lichen identification run by Allan Pentecost. Tarn House, the main building of the field centre, not only gives us shelter from the Yorkshire drizzle, it’s walls, covered with many different types of lichen, are an integral part of the course.


Lichen hunting on the wall of Tarn House, Malham Tarn Field Centre, April 2014.

I’ve known Allan Pentecost for thirty years now. It was at Malham, in fact, that I first met him, spending a day walking through the limestone scenery whilst being introduced to the tufa-forming algae of the area that were his primary interest. However, his interests could never be confined by the narrow specialisation that is de rigour for successful academic scientists these days. He brings his natural historian’s eye to freshwater algae more generally and, as I learned today, is also extremely knowledgable about lichens. Next month, he and I will be teaching our own course on algal identification at the Freshwater Biological Association but, today, I am here as a student.


Allan Pentecost teaching about lichens at Malham Tarn Field Centre, April 2014.

Lichens have fascinated me for a long time, though I have never got beyond recognising a few of the more obvious species (see “Upper Teesdale in March“). However, the term “species” is, itself, problematic when used for lichens as they are, in fact, composed of two separate species, a fungus and an alga, living in a mutually beneficial (“symbiotic”) union. That’s what piques my interest, as someone whose primary interest is the algae. We find lichens in many habitats where algae cannot grow, yet we also saw, at Malham, algae and lichens growing side by side. What do algae get from the partnership that they cannot get when living alone?

The lichens on the walls of Tarn House were not the most spectacular that we saw but my eye was drawn, in particular, to the bright orange patches of Xanthoria parietina, which formed crusts on the wall. This one, in particular, is associated with locations where there are birds, as their droppings provide a steady supply of fertiliser for the lichen. Having been gently schooled by Allan in the importance of identifying the type of vegetative propagules as a first step to naming lichens, X. parietina confounded us by not having any such structures. This creates a particular problem as the sexual reproductive stages transfer only the fungal component of the lichen. The algal cells, it was discovered, travel by a different route: via the faeces of tiny little mites which feed on the lichen. This is no happy marriage of alga and fungus; rather, it seems, it is a ménage à trois.

More about Malham’s lichens in the next post.


Xanthoria parietina on the wall of Tarn House, Malham Tarn Field Centre, April 2014. Left hand image scale bar: one centimetre; right-hand image: close up showing the cup-like reproductive bodies, apothecia, each about a millimetre across.


Meier, F.A., Scherrer, S., Honegger, R. (2002). Faecal pellets of lichenivorous mites contain viable cells of the lichen-forming ascomycete Xanthoria parietina and its green algal photobiont, Trebouxia arbicola. Biological Journal of the Linnean Society 76: 259–268.

4 thoughts on “Discovering a liking for lichens …

  1. As you probably know, Xanthoria is common on rocky shores in the upper littoral. I suppose this is a nutrient-poor habitat. I wouldn’t have thought seabirds were significant for its nutrition but who knows. Perhaps it’s not really the same species.

    • You had me searching through my field notebook to check that I really had written down Allan’s comments about the preferences of X. parietina correctly. I also found some experimental verification in
      Armstrong, R.A. (1984) The influence of bird droppings and uric acid on the growth of five species of saxicolous lichens. Environmental and Experimental Botany 24: 95-99. The various comments on its ecology that I have read suggest that it has a very broad niche.

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