January 2018

Stag's Horn Lichen

Since the middle of November, the weather has been decidedly unfriendly, mostly cold, often dull and dark, and frequently wet.  Whilst many of us have been stuck in traffic jams on icy roads, nature has been getting on with it, or not depending on the species.  What I mean is that many plants and animals have been putting into place their survival strategy – lying dormant or hibernating or maybe just changing their diet to whatever is palatable and available.  One group of organisms that seems largely unaffected by the seasons are the lichens.  Unnoticed by many people, they are all around us growing on our houses, our pavements, and on our trees and shrubs.  There are about 1800 different species growing in the British Isles and identification is mostly for specialists, but they are interesting organisms with an amazing structure that can be revealed with only a simple hand lens.

Lichens are symbiotic, which means they are formed of more than one type of organism.  Usually they are composed of a fungus and an alga (singular of algae, in case your Latin is a little rusty), or a fungus and a cyanobacterium, or sometimes fungus, alga, and cyanobacterium.  Like the mitochondria, the symbiotic bacteria in our own human cells, the algae and cyanobacteria provide energy for the fungus, and in the case of the cyanobacteria, they also provide nitrogen that the fungus cannot otherwise fix. 

Crustose Lichens growing on Cherry tree bark.
The cups and bumps relate to spore production

There are four types of lichen that are based on the form they take.  There are crustose lichens; these grow on a surface, a rock say, and are completely attached to that surface – nothing sticks out from the surface.  You can find these on paving stones or on tree bark or a good place to find them is on gravestones.  Squamulose lichens are very similar but have a more scale-like structure. 

Crustose lichen growing on concrete

They can be very small and hard to distinguish from crustose lichens.  Foliose lichens are, as their name suggests, leaf-like and the ‘leaves’ are only attached to the surface at their base.  Fruticose lichens are more bush-like and can look more like hair or coral – you will find lots of fruticose lichens on the shingle at Dungeness or Rye Harbour, but you are just as likely to see them growing on your apple tree, though they may well be different species.

One very useful service that lichens perform is as indicators of air quality.  They thrive on clean moist air, but can be wiped out by polluted air.  By counting the occurrence of certain species and following their growth and decline, experts can see what pollutants are in the air, even if there is only sporadic exposure, as well as how serious the levels of pollution are.

An example of Foliose lichen growing on Willow

Lichens reproduce sexually or by shedding part of their structure (vegetatively).  Sexual reproduction involves spores, which are fungal and so don’t contain both halves of the symbiosis.  This means that the spore has to find a suitable species of algae to successfully reproduce – clearly a hit and miss process.  Vegetative reproduction is a more certain strategy because all parties to the symbiosis are present in the shed part of the lichen which then just has to land on an appropriate surface.  Many lichens use both strategies.

There’s no doubt that lichens are a strange and wonderful part of nature and it’s well worth looking around next time the weather allows to see how many different ones you can find.  But make sure that they are not mosses or liverworts because they are strangely different, or rather differently strange.

A Fruticose lichen growing on Willow