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

December 2017

As I write this, the weather has taken a relatively sharp turn as we moved from autumn to winter.   The temperature has dived from a relatively mild 15°C to a decidedly chilly 5-6°C.  This has the effect of driving many of us indoors, venturing out only when fully wrapped.  One insect that also likes to be indoors is the common green lacewing.  The lacewing overwinters as an adult (as opposed to hibernating as an egg or caterpillar) and if it can, it will find a way inside your house where it is warm and cosy and safe from predators.  Once inside your house, a remarkable change takes place – it goes from bright green to a delicate pink colour.  Just why this should happen is unclear – perhaps it is better camouflaged that way despite your choice of décor – but in the spring it will turn back to its natural green colour.  In the wild, lacewings hibernate naturally under leaf litter or in other rough and untidy places you may have in your garden, so perhaps pink is less conspicuous in such places.

Common green lacewing - Chrysoperia carnea

When your lacewing leaves your house in spring it will find a mate and very quickly lay a batch of green eggs.  The eggs also change colour to grey before the emergence, in a few days, of the lacewing larvae.  These are one of nature’s most voracious predators and will eat the larvae and adults of many different species of insect.  Aphids are one of their favourite prey insects and they will chomp their way through a greenfly infestation in very short order.  When I say chomp, that is not quite accurate as they have no biting mouthparts.  Instead, they adopt a similar technique to spiders by injecting their prey with digestive enzymes and sucking out the resultant ‘soup’.  Their appetites make them popular with farmers and gardeners who can buy lacewing eggs as a non-chemical pest control.  (You can even buy them in batches of 1000 from Amazon!)  Once they have cleared your rosebush of greenfly, however, their predatory instincts reduce them to cannibalism until they have eaten enough to spin a cocoon from which the adult insect will emerge.  The entire life cycle takes only about 4 weeks, so that there will be several generations produced before the colder weather drives them in search of a warm winter home.

Common green lacewing (Chrysperia carnea) found in our moth trap

If you want to encourage lacewings to your garden – and with an appetite for aphids like theirs why wouldn’t you? – then you can make a ‘hotel’ for them.  The ‘hotel’ will be similar to the solitary bee hotels that all the garden centres and supermarkets sell nowadays full of short lengths of bamboo for the bees to nest in.  To make one, simply cut the top off a 2 litre plastic bottle (if nothing else it will stop the bottle from ending up in the ocean!), roll up a length of corrugated cardboard and put it inside the bottle so that the ends are protected from rain.  Then hang your hotel in the fork of a tree or under the eaves of your shed, pointing downwards so that water can drain out.  This may attract ladybirds as well as lacewings, a double blow to next year’s aphids.

Here are a few other species of lacewing proving that they are not all green or all that common.

A well marked species found on comfrey. Chrysopa perla

A brown lacewing or Alder Fly, possibly Sialis lutaria

Another species found in the moth trap, possibly Sympherobius elegans