One of the first things you discover in conservation is its hierarchy of interests. People love nature, but they love some of nature more than others, or at least give it more attention.

Top of the Pops are the birds, and it is not hard to see why. They are active (mostly) in daylight, many of them are reasonably easy to spot, they can be colourful, they can be dramatic, and they can sound fantastic. From the dawn chorus to starling murmurations over Brighton Pier, they provide some of the most impressive and accessible experiences nature has to offer in the UK.

For some of those same reasons, but not all, butterflies and flowering plants (except grasses) are pretty popular too. Mammals (with some exceptions), reptiles, fish and amphibians can be a bit harder to find, but they also attract a lot of justified curiosity.

With the fungi, moths, bees and dragonflies, we are into the realms of the more niche interest, but many generalist nature-lovers can name a few at least. Beyond these groups, there are the occasional well-known species (glow worms and stag beetles, for example) but we are deep in the territory of small communities of specialist enthusiasts.

In the UK, there are literally thousands of species of beetle, bug and fly, spider, slug and wasp to name but a few broad groups. Numbers of these could be seen casually on any walk in a local wood or park, but hardly any of us could name even a few. The same is true of the lichens. How many people do you know who could name 10 lichens commonly found on trees in Surrey? Most likely, I would guess, the answer is no-one.

Of course, there are reasons why lichens do not set everyone’s pulse racing. It is true, I will admit, that some look rather more like a patch of ageing paint than a living thing (see picture 1), although the fact that they are nonetheless alive is, I think, remarkable enough. But others offer more for the eye: there is beauty to be found in lichens, if you care to look closely.

In the dismal grey of an English winter, the light apple-green of Flavoparmelia caperata stands out against dark, rain-soaked wood (picture 2) as does the rich yellow of Xanthoria parietina (picture 3).

Both of these are very common in England these days, for reasons I come to below. Then there are “shrubby” lichens (picture 4) which can clothe whole branches in silvery grey: quite a sight, even from a distance.

“Yeah, but”, I hear you cry, “why should I reallycare about lichens?” (Incidentally, this “why should I care” question is one that no nature-lover should ever ask, and you should be ashamed of yourself, but courtesy demands that I have a go anyway.)

An axiom of ecology is the interconnectedness of all things: lichens are a source of forage and shelter for invertebrates which are themselves a source of forage for larger creatures. To take an example, at Newdigate in June, a Brussels Lace moth (below) was recorded by the Surrey Moth Group. A real rarity for Surrey (it is commoner in the south-west), its larvae are lichen-feeders and it is by no means the only moth, let alone the only invertebrate that is.

The diversity and abundance of invertebrates in woodland has been found to be positively related to lichen abundance: more lichens, more invertebrates and that is likely to be good for species higher up the food chain as well. Last autumn, I saw Goldcrests at Newdigate foraging for insects among the lichens on the trees and that is one of many bird species that also make use of lichens for nesting material. So, it seems to me, if you care about birds, you should care about lichens too.

There is, though, one other reason to take an interest in lichens, which is their role as environmental indicators. Many lichens have filaments, called rhizines, securing them to a surface, such as the bark of a tree, but these are not roots. A lichen takes no nutrients from the surface it grows upon, only from the air around it. For this reason, lichens can be highly susceptible to airborne pollutants, but some have greater susceptibility than others.

More than fifty years ago, a framework was drawn-up which related the presence or absence of different lichen species or groups to the levels of sulphur dioxide in the air. Sulphur dioxide pollution has since reduced dramatically, but the same approach has now been used for another airborne pollutant which remains very much a current problem: “fixed” nitrogen.   

As anyone doing GCSE Chemistry will tell you, the earth’s atmosphere is mostly elemental nitrogen gas (N₂), but (as the GCSE Biologists will tell you) plants have not evolved the capacity to assimilate nitrogen gas. All organisms need nitrogen to grow, but plants obtain it through nitrogen compounds (such as ammonia, NH₃), which are far rarer than gaseous nitrogen*.

Many natural habitats are nutrient-poor, including for accessible nitrogen, and the plants which occupy them are adapted to such conditions. Pollution from airborne nitrogen compounds is a threat to the extent that it may upset a natural balance giving a few species an advantage at the expense of the others. The potential result is a loss of species diversity.

This year, I did a little survey of lichens on oak trees in the grass-scrubland area at Newdigate above the lakes. Details of the framework I used are available at the Air Pollution Information Service website, which has a lot of other interesting information besides.

The framework tests for “dry” nitrogen pollution specifically: these are gaseous nitrogen compounds – mainly oxides produced in fossil fuel combustion (“NOx”) and ammonia gas from agriculture. Both forms can also be deposited “wet”, meaning they have first dissolved in rainwater, but that form of pollution is not measured by this lichen test.

My results are shown in picture 6. Nitrogen-tolerant lichens (such as Xanthoria parietina, picture 3) were more frequently present on the oaks than nitrogen-sensitive lichens, indicating a degree of nitrogen-pollution. I surveyed trunks and branches of only 5 trees (a small dataset), and cannot entirely rule-out the possibility that I have simply fluffed the lichen ID, so this is a bit of fun rather than proper science, but perhaps that result is not a great surprise.

In a recent article in British Wildlife, ecologists working at Butcherlands in Sussex noted that bramble may be particularly promoted by airborne nitrogen pollution: Newdigate also has a lot of bramble. We are not short of nettles in some areas too, and they are another nitrophile.

None of this is a reflection on the Reserve itself. Instead, it emphasises that no small area maintained for wildlife can isolate itself from the wider regional environment it exists within. The south-east of England is the zone of greatest dry nitrogen pollution from NOx in the UK and this is perhaps a cause of Newdigate’s situation. Solving that problem cannot be done at the level of an individual reserve: it is a matter of regional and national policy.

Still, I don’t want to paint an unduly negative picture. We’ve been doing a fair bit of recording at Newdigate this year and, despite the threats from airborne pollution, there is some impressive species diversity. I hope to write about the heroes of the Surrey Moth Group and some of what they have found next time.

Stephen Woodcock
Voluntary Warden, Newdigate Brickworks Nature Reserve

*Some plants, known as nitrogen “fixers”, have symbiotic relationships with bacteria, which are able to assimilate gaseous nitrogen and pass the ammonia to the plant. The same is true of some lichens.