My most recent visit to Ennerdale and the River Ehen almost did not happen: unexpected overnight snowfall led to my wheels spinning on the Whinlatter Pass before I retraced my steps to Braithwaite and followed roads at lower altitudes around the outskirts of the fells. Fieldwork in the morning took place amidst intermittent snow showers but, by the afternoon, it was dry if not quite as balmy as the visit I described in “Croasdale Beck in February”. “Unseasonable”, I was reminded, is a two-edged term.
There was little incentive to linger with my arm in the agonisingly cold water, so this post is about some algae growing on dry land that caught my eye. Amidst the gravel in a farmyard in Ennerdale Bridge I saw some dark brown leathery growths that I recognised straight away as the Cyanobacterium Nostoc commune (see “Nosing around for blue-green algae …”). It looks rather nondescript, even slightly unsavoury, with the naked eye but, under the microscope, the rosary-like structure of the filaments suspended within a jelly-like matrix is revealed. The slightly larger cells with thicker walls and lighter contents are the heterocysts, responsible for fixing nitrogen from the atmosphere (fulfilling the same function as the nodules on the roots of legumes).
A patch of Nostoc communein a farmyard in Ennerdale Bridge in April 2019. The picture frame covers about 30 centimetres.
Nostoc communefrom Ennerdale Bridge under the microscope. Scale bar: 10 micrometres (= 100thof a millimetre).
This type of coarse, well-drained gravel is a good habitat for Nostoc and, once you know what you are looking for, it is a common sight on gravel driveways, usually to the annoyance of the owners. If there is only a small amount, the best way to control it is simply to pick up the colonies and toss them onto the compost heap. However, once it is established, this can be a big undertaking and many people are quite happy to tolerate some of this brown gunk on their driveways. On the other hand, it can sometimes get out of hand and the consequences of not doing anything are well illustrated by the photograph below. The Nostoc colonies have spread but these, in turn, have created a habitat into which first mosses and later grasses can establish.
This small farmyard on the edge of the Lake District contains, in short, the first stage of an ecological succession. We could think of a gravel driveway as a mini-desert, as the copious Cumbrian rainfall will not be retained in the surface layers, making it hard for plants to survive. However, if a tough organism such as Nostoc is able to establish itself, then this, in turn, will trap water and make the driveway more amenable to slightly more fussy organisms such as mosses. As the moss and Nostoc grow together so, eventually, grasses are able to establish too. Were there to be no interruption to this process then, eventually, decades later, we might even see trees growing on this driveway.
It is hard to imagine, but just about every type of terrestrial habitat started out, aeons ago, as a bare rock surface. Various forms of physical weathering start the process of breaking this up allowing, over time, organisms such as Nostocto get a foothold and convert the virgin surface into a mature ecosystem (you can read about another example in “How to make an ecosystem”). It may take centuries for this to happen in the natural world, so it is particularly fortuitous to see this human-assisted succession so well developed. At some stage, I suspect, the owner will decide that enough is enough, and rake the gravel. Meanwhile, however, we have a rare opportunity to reflect on the role that primitive micro-organisms play in shaping even the grandest of our natural habitats.
A lawn of Nostoc, moss and grass growing on a gravel driveway in Ennerdale Bridge, April 2019.
Miles, J. & Walton, D.W.H. (1993). Primary Succession on Land. Special Publication of the British Ecological Society 12, Blackwell Scientific Publications, Oxford.