February 2018

I like writing this column because it forces me to check my facts before I write it and that means that I find stuff out.  We had some unusual visitors to our bird feeders recently – redpolls.  This was very unusual simply because we have never seen them on there before.  Redpolls are a small finch that breeds in this country, and we also get an influx from Scandinavia in winter.  When I looked up redpolls, it turns out that they could have been one of two species – the common redpoll, or the lesser redpoll.  Apparently these have only recently been split into two species, more recently than 2001 when my bird guide was published.  My bird guide did however, mention two sub-species depending on where in Scandinavia they bred, that correspond to the new species division.  So did we see Lesser Redpoll or Common Redpoll?  I haven’t a clue because the differences are small (one is slightly smaller and paler than the other) and I wasn’t expecting there to be two possible species when I saw them.  But it was a delight to see them and as long as they know the difference that’s all that matters.

Common or Lesser Redpoll - Carduelis flammea or cabaret

In last month’s column I briefly mentioned mosses and liverworts.  Apart from the fact that I could probably point out mosses or liverworts if I saw them, that was about all I knew, so I looked them up and it turns out that they are a very interesting group of plants.  As a group they are known as bryophytes and the group includes another set of plants that I had never heard of – hornworts.  Apparently hornworts are quite scarce in the UK which is probably why I haven’t come across them before (and if I had I probably would have thought them to be liverworts).  One thing about bryophytes is that they are not vascular plants.  Vascular plants (the plants we are more familiar with) have flowers, branches, and even trunks that contain sap which flows through the plant in a similar way to blood in veins – hence the term vascular.  Vascular plants grow, flower, set seed, which then germinate to form new plants, but bryophytes complicate matters with an extra stage. 

Bryophytes produce spores; those spores, usually dispersed by the wind, land somewhere favourable and grow into a new plant, but the difference between spores and seeds is that the DNA in seeds has the chromosomes from both parent plants, but bryophyte spores only contain the DNA from one parent.  (The technical term is haploid.)  When the plant that grew from the spore grows into a moss or liverwort, that stage cannot then produce spores.  Instead each plant is either female which produces eggs, or male which produces sperm.  The sperms are then usually dispersed by water (which is why bryophytes usually grow in very damp places) or by insects that carry sperm from one plant to another.  This egg and sperm stage is called the gametophyte stage, but you probably don’t need to know that.  The fertilized eggs are then dispersed from the gametophyte, again usually by water and having both the male and female DNA, they can then grow into the spore producing generation (called sporophytes, should you be interested).

From studies of their mitochondrial DNA it has been suggested that liverworts may well have been the first land plants to evolve from green algae more than 400 million years ago.  There are thought to be more than 1,000 species of bryophytes found in the UK and more than 20,000 worldwide. Though they are often overlooked, mosses and liverworts are an important part of the ecology and support a wide range of insects and other arthropods, not to mention the many birds that feed on those smaller creatures.  One day I hope to find a creature that makes its home only in moss – the tardigrade bear – that is almost microscopic and not actually a bear, but no less interesting for all that.

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