Unseen biodiversity of Glen Affric, part 1

This year I’ve been making regular trips out to Glen Affric, usually once a week, to photograph the Caledonian Forest and its associated biological diversity. Although I’ve been visiting the glen since 1979, and have been going there regularly ever since, especially after Trees for Life began practical work there in 1989, I’ve stepped up the frequency of my visits this year. I’ve done this as I wanted to develop a better feel for the changes that occur throughout the seasons and to get to know in more depth at least some of the cycles of Nature that occur in the glen.

Although I’ve been posting blogs about some of these trips during the year, they only cover a few of the journeys I make. A lot of what I see and photograph during them doesn’t get featured, so to address this I’m writing this blog as a sort of ‘best of Glen Affric biodiversity’ highlights from the past year. I’m aiming to publicise some of the species, and interactions between species, that are not immediately obvious and therefore may be missed by most visitors to the glen.

I’ll do this in a mostly chronological sequence, starting from the beginning of the year. In fact there’s not a lot to see then in terms of species diversity, as many bird species have migrated south for the winter, insects have gone into their pupal phase, annual plants have died back and most fungi have finished fruiting for the year.  At this time my attention often turns to lichens and bryophytes (mosses and liverworts) as they persist throughout the year, and in fact are often more conspicuous then, because there are no leaves on the trees and understorey plants to obscure the view of them.

Winter snowfalls cover up many of the plants on the forest floor, but they do provide an alternative benefit, by providing the canvas on which birds and mammals leave visible signs of their presence. Thus when I was out in the Cougie area of the Glen Affric National Nature Reserve in March with my former Trees for Life colleague, Dan Puplett, who is very knowledgeable about animal tracks and signs, he pointed out the tracks of various mammals, including pine marten (Martes martes), fox (Vulpes vulpes) and badger (Meles meles).

 

 

 

It’s rare to see these animals in the flesh, as they are mostly nocturnal and very good at avoiding people, but it was satisfying to know they’d been there not long before us on this snowy day in the middle of March.

Apart from the animal tracks there wasn’t much to see as there were several inches of snow covering everything and it was a grey and very dull day. However, as we were walking around I was astonished at a certain point to see an insect sitting on the snow. Taking a closer look I recognised it as a stonefly and it was subsequently identified by the retired professional entomologist Colin Plant as being a female of a species called Leuctra geniculata. Stoneflies are a group of insects that spend the larval stages of their lives as nymphs in freshwater. When a nymph is ready to pupate it climbs out of the water and attaches itself to a rock or a tree trunk, from which the adult emerges after pupation. This female must have emerged a few days earlier, sensing it was spring when the weather was warmer, only to get caught out by the subsequent heavy fall of snow.

Not long after that snowy day in March I made another trip to Chile, to continue the photography and research work for the book I’m working on about the Araucaria (monkey puzzle tree) forest there. After that, I was in Ireland to speak at a conference about restoring native forests there, so it was early May before I had a chance to return to Glen Affric. By then the new leaves had emerged on the deciduous trees, and when I was looking at a young downy birch (Betula pubescens) beside Loch Beinn a’ Mheadhoin, I spotted a stonefly, which turned out to be another female of the same species (Leuctra geniculata). It was almost as though the forest had provided me with a direct link between my two visits to the glen, bridging the gap when I’d not been there …

Some of what I photographed that day, and from my next visit a week later, featured in the blog, ‘Return of the leaves, revisited‘ that I posted in July. However, there’s never enough space in my blogs to include everything I encounter, and one subject that got omitted then was a wolf spider (Pardosa trailli). Wolf spiders are unique for the way the females carry their eggs, which are in a sac that is attached to the spinnerets at the end of the abdomen. Once the spiderlings hatch out, they climb up on to their mother’s abdomen, where she carries them until they are large enough to take care of themselves – usually a period of several weeks.

May is the month when new life fully emerges in the Caledonian Forest,  and I made several trips to Affric during it. One of the places I visited then at regular intervals is a rock pool beside the Affric River at the Dog Falls car park. Over the years I’ve repeatedly seen tadpoles of the common frog (Rana temporaria) in the pool, and there were a large number of them there again this year. Although the pool is only about 10 metres from the car park and four metres from the trail that goes to Dog Falls, I suspect that very few of the visitors to the glen actually notice the tadpoles there.

Another favourite site that I made repeated visits to over the course of the spring months was a dog rose bush (Rosa canina) growing near the road in the glen. The bush usually has aphids feeding on it, and they are one of the groups of invertebrates that I have a particular interest in. By the second half of May the aphids were there again in good numbers, and thanks to the expertise of Bob Dransfield I was able to get them identified as being a species called Macrosiphum euphorbiae.

This is known commonly as the potato aphid, because it can infest potato plants in large numbers. Like many aphids, it has a complex lifecycle, which includes alternating between hosts, with rose bushes being the primary host and other species such as potato plants being the secondary host. It can also reproduce by parthenogenesis (ie without having to mate with a male first) and is viviparous, meaning that a female gives birth to live young, instead of laying eggs as most insects do. This enables aphid populations to build up very quickly, and when I returned to the dog rose in late May, I was able to photograph the birth process as it happened.

That same day I made a very interesting discovery when I spotted a strange-looking insect on the road in the glen, near Dog Falls. Its body was striped in yellow and black, like a hoverfly or a wasp, but it clearly wasn’t either of those, and looked more like a cranefly from the way it was holding its wings. When I sent it to Peter Chandler, the specialist who helps me with the identification of Diptera, or two-winged flies, he replied stating it was a cranefly called Ctenophora flaveolata. This is an uncommon species that breeds in the dead wood of primarily beech trees, and there are no previous records for it north of the Lake District in the UK. It’s a mystery therefore how it came to be in Glen Affric, especially as there are no beech trees there in which it could have bred, and this led to the publication of a note about the discovery in the Dipterist’s Digest, which Peter edits. Because I found it on the road, it’s possible that it was transported inadvertently by a car that had come up from England, but there’s no way of knowing if that’s the case.

By this time of year, large numbers of insects are on the wing, and there were plenty about in Glen Affric in late May, including a range of micro-moths. One species that I saw various individuals of was the yellow-barred gold micro-moth (Micropterix aureatella). This is a fairly widespread species in the UK, but is more concentrated in the north and west, and has beautiful metallic colours on its tiny body. I saw quite a lot of them in Affric over the next couple of weeks, but I suspect most visitors to the glen didn’t notice them.

On the same day I also came across a birch tree with a group of very bright, almost fluorescent, pink galls on some of its leaves.  I hadn’t seen these before, but there were quite a number of leaves on the tree that were galled like this, and I soon spotted more on other nearby trees as well. The galls looked almost surreal with their vivid intense colour, so I thought they’d be easy to identify. At home that evening a quick Internet search revealed that they are induced by a mite (Acalitus longisetosus).

I’ve had a long-standing interest in plant galls, and have photographed many different ones in Glen Affric during the past 20 years. I was therefore rather surprised to discover these galls, as I’d never come across them before, and I wondered how I could have missed them, when they are so conspicuous on the leaves. I suspect it may be a case of the population of the mites that induce the galls going through cycles of fluctuating abundance over the years, like many invertebrates do, and that 2018 was a good year for them. I haven’t been able to find any information that would confirm or refute this idea, but I’ll certainly be keeping my eye out for these galls in future years, to see if there is any pattern or cycle to their appearance on birch trees.

On another birch tree just a few feet away, some movement on the trunk caught my attention as I was walking past. Stopping to have a closer look, I saw that it was a spider with an unusually-shaped abdomen. It seemed quite distinctive to me, and when I contact my friend the arachnologist Edward Milner, he said it was a species called Cyclosa conica, which has no common name but is unmistakeable because of its unique body shape.

I watched the spider for a few minutes as it moved around on the tree trunk, and at a certain point it seemed to disappear completely. I’d taken my eyes off it for a few moments to adjust the exposure settings on my camera, and when I looked again I couldn’t see it. The spider was still there though, but was sitting on a patch of heather rags lichen (Hypogymnia physodes) on the birch trunk, where it was superbly camouflaged. The pattern and coloration of its abdomen perfectly matched that of the lichen, making it virtually invisible. If you haven’t spotted it yet in this image, it’s on the lower right part of the lichen.

Although spiders are predators, they are also prey for other organisms, so being camouflaged against their background helps to reduce the risk of being caught and eaten. The risk of predation that spiders face was reinforced a few minutes later, when I came across two spiders on the unfurling frond of some bracken (Pteridium aquilinum). One of these, a wandering crab spider (Philodromus aureolus), had killed the other one (Metellina mengei), and was presumably about to consume it.

On my next visit to the glen, on 5th June, I returned to the rock pool beside the Affric River at the Dog Falls car park, to check on the tadpoles there. Last year I had been doing the same thing, but I was surprised one day to find that all the tadpoles had vanished, before they had metamorphosed into frogs. I don’t know what happened, but I suspect there may be have been a flood that flushed them all out of the pool. This year that wasn’t a problem,  and when I arrived at the pool on 5th June I observed that the tadpoles had become froglets, and were in the process of dispersing from the pool.

The froglet stage is an intermediate phase in the metamorphosis from tadpole to frog, in which the front and back legs have fully formed and the froglets are able to come out of the water, but they still have vestigial tails that have not yet been fully re-absorbed into their bodies.

 

 

 

 

 

 

 

When I left the rock pool area where the froglets were, I noticed a discoloured area on one of the leaves of a blaeberry plant (Vaccinium myrtillus) that was growing beside the footpath. I almost walked right past it, but experience has taught me to pay attention to even the smallest indication of anything that appears to be different in Nature. I was glad I did this time in particular, because when I looked closely at the leaf I recognised the discolouration as being caused by a plant gall. I’m familiar with the galls induced by the blaeberry redleaf fungus (Exobasidium myrtilli) on blaeberries, but this looked different so my curiosity was piqued.

I wasn’t sure what it was so I sent some photos to Margaret Redfern, who helps me with gall queries. She in turn forwarded them on to Brian Spooner, an expert on fungi at the Botanic Gardens in Kew in London, and he asked to see the specimen. When I sent that to him, he identified it as a species related to the blaeberry redleaf fungus which is called Exobasidium arescens. There are apparently only a handful of records for this species in Scotland, clustered in the Cairngorms National Park, and this is almost certainly the first instance of it being found west of the Great Glen. Because it’s a significant discovery, Brian and I have been asked to write a brief article about it for Cecidology, the journal about galls in the UK, and that will be published next spring.

A week later, in the middle of June, I came across a good example of parasitism in the glen, when I noticed a harvestman on a leaf, with small red shapes atttached to its legs. Harvestmen are predators that are in the same Class of invertebrates as spiders, mites and ticks, and they are parasitised by the red larvae of mites, which feed on their body fluids. In June I saw a lot of harvestmen with mites on them, and this individual had 5 on its legs, some of them larger than others. The larvae appear to do no lasting damage to their hosts, and drop off when they’ve finished feeding, to pupate and become eight-legged adults.

Thanks to the expertise of Joanna Makol of Wroclaw University of Environmental and Life Sciences in Poland, I was able to get the mite larvae identified as being the species Leptus phalangii, while Paul  Richards, a specialist in harvestmen, identified this one as being Platybunus triangularis. In the following weeks I found quite a few specimens of the most common harvestman in the forest (Mitopus morio) with mite larvae attached to them,  and those turned out to be the same species, Leptus phalangii. While I’ve seen these on harvestmen in previous years, there seemed to be more of them than usual this year. Perhaps the weather conditions this spring were particularly conducive to an expansion of their population?

By the middle of June, spring-flowering plants such as wood sorrel (Oxalis acetosella) and dog violets (Viola riviniana) have already finished blossoming for the year. However, other species are beginning to bloom then, ensuring that there is a continuous supply of nectar throughout the growing season for the insects that act as plant pollinators. The aphids that I had seen on the dog rose bush in May had disappeared, no doubt having moved on to their secondary host plant species, but the roses themselves were in blossom now and attracting insects, such as this fly (Phaonia angelicae). Flowering species like this provide nectar for visiting insects, which they use as a rich food source, and in turn the insects transfer pollen from one plant another, thereby enabling cross-fertilisation and subsequent seed production to take place.

While bees and hoverflies are perhaps the insects that most people associate with pollination, there are many others that also fulfil this function. A wide variety of beetles act as pollinators, as do various members of the order Diptera, or two-winged flies. A few days later, on 21st June (the summer solstice), not far from the dog rose bush I found some examples of serrated wintergreen (Orthilia secunda) on the forest floor, and there were both beetles and a two-winged fly visiting one of the plants.

It was only a few metres further from there to where I had found some aphids (Amphorophora gei) on the flowers of water avens (Geum rivale) a few days previously. I wrote a blog about this discovery during the summer, as I suspect it was the first record for this species of aphid in Glen Affric. Now, as I looked around, I found a much larger patch of water avens on the other side of the road, very close to the steep drop into the Affric River gorge, and there were more of the aphids on those plants as well.

My day in Glen Affric on the solstice was a memorable one, because I kept finding more and more species of interest, even though I stayed in a relatively small area, not far from Dog Falls itself. On the unfurling frond of some bracken I noticed an unusually-shaped insect that I immediately recognised, although I hadn’t seen it in real life before. It was a snakefly (Atlantoraphidia maculicollis), and its common name is derived from its elongated prothorax, which gives its body a somewhat snake-like appearance. There are 4 species of snakefly in the UK,  and although I’d seen photos of them in guidebooks and on the Internet before, this was the first time I’d actually encountered one in person.

I spent a little while at Dog Falls itself and at one point some movement on the leaning trunk of a young birch tree there caught my eye. Taking a closer look, I discovered there was a moth resting on the trunk, perfectly camouflaged against the bark. Although it was only a few feet from where I was standing, its presence was completely invisible to me until it moved slightly. Thanks to the expertise of Roy Leverton I was able to get it confirmed as being the mottled beauty moth (Alcis repandata). I wondered how many other moths I had missed that day, because they hadn’t moved at all and had therefore not drawn my attention?

Also right at Dog Falls, there’s a young Scots pine sapling (Pinus sylvestris) about 2 metres in height, growing amongst some rocks. When I’d had a close look at it on 12th June, I’d found there were sawfly larvae (Neodiprion sertifer) feeding on some of its needles. This is a species that feeds gregariously and can sometimes reach pest proportions in commercial monoculture plantations of pines. By contrast, in natural pine forests like those in Glen Affric, its population is in balance with the other components of the much more diverse ecosystem, so although some trees become infested there is no serious damage done overall.

I spent some more time watching these larvae on the solstice, and I noticed something then I hadn’t been aware of before. I observed that the larvae were only eating the needles from last year’s growth, and were leaving the new needles of the current year (which hadn’t reached their full size yet) alone and untouched. This struck me as being slightly unexpected, as the new growth of plants, being tender and full of sap, is often the prime area targeted by herbivorous insects. When I contacted Andrew Halstead, a specialist in sawflies, about this he replied that it is normal behaviour for this species to target the older needles. He suggested that the new growth possibly has chemical compounds in it that make the needles unpalatable, thereby providing the pine with protection for its vulnerable young shoots.

Close examination of the pine twigs revealed how thorough the larvae are in eating the needles. Where they had been feeding, only tiny brown stubs at the base of each former needle remained – the rest of the needles, the entire green section of them, had been consumed. Over time a dense concentration of the larvae like this can strip entire branches of their needles. and it is this behaviour that causes problems for commercial forestry.

Over the next few weeks, throughout July and into early August, I checked on these larvae during every trip I made to Glen Affric, observing their growth in size and their progression as they moved to different branches of the young pine. Eventually, by the middle of August, the larvae were all gone – presumably they had pupated and the adult sawflies had flown away. The pine itself still had a lot of branches that hadn’t been affected by the larvae, and even those had been infested still had the current year’s new needles on them, so I was confident the tree would recover and keep growing in future years.

Not all of the larvae would have survived to pupate into adults though. Like the herbivorous larvae or caterpillars of butterflies and moths, they are unable to fly or escape by moving quickly away, and therefore are vulnerable to predation, especially by birds. Whereas the larvae of some sawflies (and those of many moths and butterflies) rely on camouflage to reduce the risk of predation, these pine sawfly larvae use a different technique to deter potential predators. When they feel threatened, they twitch their bodies suddenly and rapidly, with their front ends rearing up in a ‘threat’ pose that is intended to scare away their enemies.

The remarkable thing is that the larvae all engage in this behaviour in synchrony with each other. They all twitch at the same time, and this mass movement gives the impression of a much larger organism, thereby presumably greatly increasing its effectiveness in scaring away potential predators. The larvae of a number of different sawfly species exhibit this same behaviour of coordinated twitching, and the result is a Medusa-like effect, as the simultaneous writhing of the larval bodies resembles the fabled serpentine hair of the mythical woman in ancient Greek legends.

Still photographs cannot do justice to this behaviour, as the camera’s flash freezes the motion in a single position. However video captures it perfectly, so here’s a brief compilation of some footage of the larvae twitching in response to my arm moving near them (off camera!), to simulate the shadowing effect of the approach of a predator:

https://www.youtube.com/watch?v=6F1jvPy4RdU;vq=hd720&rel=0&showinfo=0

By including some photos of these larvae from July and August, I’ve got a bit ahead of my narrative describing the biological diversity I encountered in Affric during the course of the year. I’ll continue the correct chronological sequence in part 2 of this blog, but to finish now here’s another photo from my day there on the solstice, of a rather colourful moth – the brimstone (Opisthograptis luteolata).