Thursday, 31 May 2007

The invisible tree

Writing in 1996, the author Richard Mabey referred to the native black poplar as the invisible tree. Here was a tree of immense presence and with a strong cultural history, that had ‘passed so thoroughly out of common knowledge’. How could we have let this happen?

The black poplar (Populus nigra subsp. betulifolia) is a tree of deep alluvial soils, largely restricted to lowland river floodplains across southern Britain, northern France and western Germany. As our landscape has changed over recent centuries, so the drainage of land has seen the loss of black poplars from large parts of their former range. The most recent estimates suggest that there may be only 7,000 individuals left in Britain, most of which are males (black poplars are dioecious, the male and female catkins appearing on separate trees). The loss of these majestic trees had largely gone unnoticed until, in 1973, Edgar Milne Redhead, recently retired as deputy curator of the herbarium at Kew, took an interest in the species. Milne Redhead’s work revealed the extent of the loss and demonstrated that the remaining individuals had almost certainly originated as cuttings from a population of just 600 trees. Most of the trees were isolated and there was little hope that they would be able to secure natural pollination and seed set without intervention. There was also the threat of hybridisation with the various Italian black poplars favoured by local councils for tree planting schemes. The almost complete absence of seedlings within Britain is something that has been noted by many authors.

Here in Norfolk, the most recent survey revealed just 54 male trees, together with an impressive female tree on the village green at Old Buckenham. A map included in the most recent county flora shows that most of these trees are in the central southern part of the county, while just over the border into Suffolk there are many hundreds of trees. However, to really appreciate this tree at the landscape scale you have to visit the Vale of Aylesbury, where the presence of these trees can really be felt. The short trunk section, with its rough bark and bosses, almost always leans at an angle, supporting above it a rich green and luxuriant canopy.

The conservation of the black poplar will require a careful balancing act, securing the future of the species by establishing more individuals, while at the same time ensuring that these individuals are planted correctly within a cultural landscape. Many of the cultural traditions associated with the tree stem from its scarcity and position within the landscape (as boundary markers or centrepieces for village festivities) and so newly established trees must fit into this pattern.

Wednesday, 30 May 2007

Look before they leap

Rake over a handful of woodland leaf litter and you will almost certainly see any number of tiny creatures. Some will remain where you find them, some will move slowly away, seeking out shelter from the daylight to which they have been exposed, and others will spring away from you like miniature fleas. You probably know the last of these animals – the ones that leap – as ‘springtails’. They are familiar because they are found in many different habitats and are just as easily encountered when you tidy your flowerbed or turn over a patch of seaweed on the tide line. What you may not appreciate is just how many of them there are and just how successful they have been.

It has often been said that springtails are the oldest known group of insects, having existed on the Earth for at least 400 million years. Although the limited fossil evidence does indeed demonstrate this ancient heritage, springtails are only distantly related to true insects and are considered ‘primitive’ in comparison with their more diverse cousins. However, complexity does not always bring success and it is a testament to the functionality of springtail design that their basic body plan has changed relatively little over the last 40 million years. While evolutionary pressures are strong, a design that works can remain little changed over very long time periods. Charles Darwin paid little attention to the springtails, describing them as ‘wingless, dull-coloured minute insects’ but his close friend Sir John Lubbock was fascinated by them, writing, in 1893, a famous monograph on them. Lubbock described some 130 species (6,500 are recognised today), with many beautifully illustrated by A T Hollick, a deaf and dumb artist of precocious talent.

Lubbock gave these creatures the group name of Collembola, derived from the Greek words ‘colle’ (glue) and ‘embolon’ (piston) because of an adhesive organ on their undersides, used to attach them to slippery substrates. The name ‘springtail’ comes from a second appendage, the ‘furcula’, which enables the springtail to leap away from danger. Under normal conditions the furcula is held in place against the underside of the body by a catch. This catch is released if the springtail senses danger and the furcula slams into the substrate, launching the springtail into the air. Some species are able to reposition the furcula in flight so can jump again as soon as they land; others need a period of rest between jumps before they can fire again. With up to 40,000 springtails per square metre in good habitats you would think that we would take more interest in them. However, their tiny size seems to be a barrier to our interest, which seems a real shame.

Tuesday, 29 May 2007

The dragons stir and search for damsels

It’s early morning and there is just enough warmth in the sun’s rays to rouse slumbering dragons. Perched with wings outstretched and hanging motionless on the reeds and sedges are four-spotted chasers and hairy dragonflies. These are perhaps the first of our dragonflies to appear on the wing, to be followed over the coming weeks and months by other species. Also on the wing, floating just above the vegetation like decorated matchsticks, are the first of the damselflies – in this instance azure and blue-tailed damselflies. Once known as the ‘Devil’s Darning Needles’ these tiny creatures will have recently emerged, shedding the skins of their aquatic existence.  It is these creatures that the hairy dragonflies, in particular, will soon be hunting.

Early morning is the best time to really appreciate the beauty of our dragonflies. Perched, not yet at operating temperature, it is possible to make a close examination of them, taking in the jewelled beauty of their apparel, the delicate venation of their wings and their large globular compound eyes. These are an important feature, for all dragonflies are fierce predators – taking anything from small flies to butterflies. Good vision, manoeuvrability and a burst of pace are essential for such active hunters.

Male four-spotted chasers are highly territorial, choosing a suitably prominent perch from which they can launch sorties towards any other dragonfly that wanders into their airspace. If this turns out to be a female four-spotted chaser then a very brief mating will follow, in flight, after which the female will break away to initiate egg-laying. This is one of the easiest species to recognise, with the combination of its dark tapering body and dark wing spots (four on each of the two pairs of wings) there is little else with which it could be confused. Once airborne the four-spotted chaser, as its name may suggest, is a fast flier, busy and energetic as it moves about around favoured still water habitats.

While the four-spotted chasers are mostly still perched on vegetation, the hairy dragonflies are on the wing, actively hunting for prey and patrolling low through the reeds and sedges. They seem to knock into plants; are they trying to flush prey? Related to the other hawkers (a group of dragonflies which includes the migrant hawker, Norfolk hawker and the impressive emperor dragonfly), they are typically ‘hawker-shaped’, fairly stout but smaller and more compact than some of the other species. At this time of the year they are the only hawker on the wing, the males resplendent with an apple green thorax and an enamel blue and black abdomen. Why not check them out for yourself by visiting one of our wetland reserves.

Monday, 28 May 2007

Bugles herald breeding cranes

The RSPB reserve at Lakenheath is turning out to be something of a triumph. Just 11 years ago much of the reserve was carrot fields, bordered by the River Little Ouse and a series of washes, where passage waders and dancing terns could be seen. The reedbeds that now cover much of the reserve show what can be done with careful planning and control of water levels. It was hoped that the reedbeds would attract breeding bitterns but it is another bird that has stolen the limelight by choosing Lakenheath as its home. This is the common crane.

Cranes are truly majestic, with a seven foot wingspan and elegant courtship display, these imperial beauties look oddly out of place when seen feeding on our farmland. Although there is little, if any, evidence to prove that cranes bred in Norfolk before  1981, it is likely that they did. Of the 300 or so place names in Britain associated with the crane only two are in Norfolk but there are many others concentrated in the Fenland of Lincolnshire and Cambridgeshire. This area would have been used by cranes from more northerly breeding populations, passing through south in autumn or wintering here. It may also have supported breeding and records do show that crane featured prominently on the menu at important banquets up until about 1600. After this time the numbers fell dramatically and from then right through until the 1970s, cranes were very rarely encountered in England. Then, on 15th September 1979, two cranes appeared at Hickling. These overwintered and then moved up to the North Norfolk coast the following spring; attempting their spring migration, but clearly deterred by the size of the North Sea crossing, the birds returned to Broadland. Over the following years other birds joined them and then finally, in 1982, they bred successfully. Since then, numbers have increased very slowly with occasional nesting attempts producing young.

This all makes the arrival of breeding birds at Lakenheath that much more unexpected. Earlier in the year there were seven individuals present, arranged in what appeared to be three pairs with a lone young male attempting to muscle in on already established bonds. He has since disappeared, possibly along with one of the other pairs, but two pairs of these wonderful birds appear to be in residence on the reserve, typically keeping a low profile but proclaiming their presence through their loud bugle calls at dawn and dusk. With more birds now using a westerly migration route that brings them into East Anglia from Fennoscandia, it is likely that the number of cranes will continue to grow and we will increasingly hear their echoing calls across our reedbeds.

Saturday, 5 May 2007

Larks rise

Norfolk has a number of nature reserves that are, quite rightly, famous for particular species of bird. The Norfolk Wildlife Trust reserve at Weeting Heath is one of these, hosting several pairs of nesting stone curlews each year. These rather unusual waders are not easy to spot, so the hides at Weeting (together with the helpful wardens and volunteers) provide an ideal opportunity to see these rare birds. Another species that can be seen on the reserve at the moment is the woodlark, and a few weeks ago I was fortunate enough to be able to watch a pair of woodlarks, nesting just in front of the east hide. The nest itself was hidden from view, a depression within a patch of longer vegetation, but I was able to watch what I assumed to be the male bird arrive and softly call his mate off the nest. The male bore a unique sequence of tiny colour rings fitted to each leg, enabling his identity to be established and helping local researchers understand the pattern of movements undertaken by these enigmatic little birds. I duly noted the sequence in my notebook and reported it to the British Trust for Ornithology at Thetford, who operate the national ringing scheme on behalf of the British and Irish governments. Just last week I received information back, informing me that this particular individual had been ringed as a chick on 28th April 2004 at a nest site some 3km north of Weeting. Interestingly, in addition to my record from the 8th April, another birdwatcher had seen the bird on the 3rd, also at Weeting. Colour-ringing is a very useful technique since it allows observers to identify individual birds in the field without having to recapture them.

This particular pair of woodlarks would probably go on to rear another two or three broods this summer and they are part of a healthy and rapidly expanding population. The population first began to expand in the late 1980s, facilitated by an expansion in the amount of restored heathland and young plantation woodland. Thetford Forest has been the focus for much of this expansion and the number of breeding territories within the forest had increased from 48 in 1980 to 446 territories by 1997 (when the last national survey was carried out). There has also been a tremendous increase in the number of woodlarks nesting on the surrounding heathland, thanks primarily to the introduction, in 1988, of the Breckland Environmentally Sensitive Area (ESA) Scheme. It now seems likely that Breckland, together with the Suffolk Sandlings, supports just under half of the national population. Mind you, there is still some way to go before the species re-occupies all of its former range.

Friday, 4 May 2007

Wasp regulates butterfly population

One of the joys delivered by the arrival of spring is the opportunity to sit out in the garden and watch wildlife closer to home. One of the creatures to make an early appearance in the garden this year has been the holly blue butterfly. Over recent days several individuals have been seen together flitting around the holly tree and over the old boundary wall, covered in a mass of ancient ivy. This small butterfly is the only blue on the wing at this time of the year and it is also the one most likely to be encountered in gardens. Pastel blue in colour, it can be separated from the common blue (which may visit more rural gardens later in the year) by the lack of any orange markings on the underside of its wings. The holly blue also differs in its behaviour; while other blues fly low over the ground, the holly blue flies high around bushes and shrubs.

In spring, the butterfly lays its eggs on holly, the caterpillars timing their emergence to coincide with the availability of tender new buds. Later in the year a second generation (and in some years also a third) will be reared on ivy. The number of generations each year is determined by temperature; long, warm summers will allow a third brood in the south of the country but only a second brood in the north of England, where a single brood is more typical.

The larvae, feeding on flower buds, berries and terminal leaves can be found with a little bit of effort, as can signs of the damage caused by their feeding behaviour. At this stage, they are vulnerable to a parasitic wasp called Listrodomus nycthemerus. This wasp almost exclusively attacks the holly blue and, in some years, can inflict substantial damage on the population. The female wasp deposits an egg into the holly blue caterpillar, without killing it. The egg remains within the body, unhatched, until the butterfly enters its pupal stage. At this point, the wasp egg hatches and the emerging larva proceeds to eat the entire contents of the pupal case and, by doing so, kills the butterfly, before completing its own life cycle. The occurrence of such parasitization is thought to be behind the strong cyclical fluctuations seen in holly blue populations. As the butterfly becomes more abundant, so wasp numbers build up to the extent that they eventually cause the butterfly to decline in abundance; a wasp decline follows and the cycle starts over again. These fluctuations, and the long-term increase in this species, are monitored by Butterfly Conservation. For more information on their work, contact the Norfolk Branch through Hilary Beynon 01493-740499.

Thursday, 3 May 2007

A leisurely lunch

There is one particular stretch of the local river that is often frequented by grass snakes. No doubt, this reflects the availability of suitable prey, plentiful cover and the nesting opportunities provided by decaying waterside vegetation. On occasion these snakes may be seen, or more likely heard, as they slip away into the water following the approaching footsteps of a passer-by. However, thanks to the eagle-eyed attentions of one of my work colleagues, one particular snake was spotted the other day, unable to slip away because it was in the process of swallowing a sizeable toad.

Grass snakes are not sit and wait predators but are active hunters of toads, frogs and newts. As such, they associate with ponds and rivers and will readily take to the water, swimming with lateral undulations of their body and sometimes diving underwater (where they can remain for up to 30 minutes). This particular individual was a large snake and, since it is known that large grass snakes prefer to take larger prey, it was not really a surprise to see it tackle such a large toad. The toad in question was very much alive and had inflated its body by gulping in air. This would make it more difficult to swallow but was very unlikely to alter the outcome of the contest. Grass snakes have sharp recurved teeth and this would make it impossible for the toad to extricate itself from the deadly grip.

Grass snakes do not really have any special adaptations useful for overcoming their prey, so the victim is typically swallowed alive and head first. With larger items the snake opens its jaws as wide as it can and then works them sideways along the prey. Very slowly, the victim is consumed. Unfortunately for the toad, death does not come quickly, and it may survive in the gut of the snake for some time, only slowly succumbing to suffocation or the action of the digestive juices. There are records of amphibians, swallowed in this way, surviving; grass snakes will often regurgitate their most recent meal if captured or threatened and there are cases of regurgitated frogs hopping away, seemingly unharmed.

The size of this snake suggested that it was a female (females are larger than the males) and, this being the case, she is likely to consume another four toads over the course of the summer. Despite being smaller, male grass snakes will average eight toads a year. This is because the female grass snake does not feed for some six weeks during pregnancy and the egg-laying period. In this particular case, the female may have been struggling with her leisurely lunch but it would keep her going for some time.

Wednesday, 2 May 2007

Black Terns grace Livermere lake

As I noted last month, Livermere Lake sits within a very open landscape, just over the county border towards Bury St. Edmunds. It has a reputation for attracting unusual waterbirds, including many passage migrants, and a visit in late April or early May can be particularly rewarding. Just the other day I was fortunate enough to see both my first little ringed plover of the year and my first black terns. The terns certainly stole the show. Playfully pitching about in their characteristic, almost erratic, flight, they frequently rose and then dipped down towards the water’s surface. These small, compact birds, short-winged and tailed, are marsh terns – distinctly different in appearance from the larger, more familiar, Sterna terns (like common, arctic and sandwich), which are more commonly seen.

The black tern can best be described as a fairly common passage migrant, seen across eastern counties during spring and, to a lesser extent, autumn. While the last of the spring passage birds may be seen in Norfolk as late as mid-May, the first autumn birds can appear just a few weeks later at the beginning of July. The spring birds will have travelled up the Atlantic coast of Africa and Europe from coastal wintering grounds in Senegal, the Gambia and Namibia, while others will have turned right through the Straits of Gibraltar to enter the Mediterranean and then head overland across continental Europe. All of the birds will have been heading for breeding areas within Eastern Europe and the former Soviet Union. Historically, however, many would have headed to Eastern England to breed in the Fens and the Broads, a pattern that no longer occurs. Writing in 1668, Sir Thomas Browne noted how the black tern was ‘common about Broad waters and plashes not farre from the sea”. One particular area of alder carr near Upton was home to many hundreds of pairs in the 1600s. Unfortunately, this particular colony disappeared soon after the introduction of the 1799 Upton Drainage Act. Drainage of the favoured wetland areas no doubt played a major part in the demise of this bird but the more direct actions of Man were also contributory. Egg-collecting was rife and the last Nineteenth Century nesting attempt (Sutton Broad, 1858) ended when the eggs were stolen and the adult birds shot. Just over 100 years later, four pairs nested at Welney but because of falling water levels none of the young survived. The last successful nest was also at Welney (in 1975), a poignant reminder of a breeding species effectively lost from our much-changed countryside. There are occasional records of over-summering birds and, with the establishment of new inland reserves, it is just possible that we will see further nesting attempts in the future.

Tuesday, 1 May 2007

Moth follows hedgerow decline

Despite setting off under the sun and spring warmth of inland Norfolk, I arrived at Holme NWT reserve to find a light fog drifting in off the sea and a chill in the air. This served to deaden the sounds of waders feeding just beyond the dunes and lowered my expectations of being able to photograph newly arrived migrants in good light. My spirits lifted somewhat with sight of my first wheatear of the year, a male posturing in typical head-up pose. However, it was not the spectacle of this bird, or of the ring ouzels that mingled with a late-departing fieldfare, that made my visit memorable. Instead, it was the discovery of a larval web of an increasingly uncommon moth, the small eggar, on a small hawthorn bush just a few dozen metres from the visitor centre.

While the adults of this species are on the wing between February and March, they are rarely seen and only appear at light traps occasionally. Quite thickset, these squat little creatures belong to a group of moths known as the Lasiocampidae, which includes more familiar species like fox moth, drinker and the lackey.

The eggs of the small eggar are laid in clusters on blackthorn or hawthorn bushes (occasionally elm, grey willow and spindle, deposited by the female and covered with fine hair-scales dropped from the tip of her abdomen. From these eggs, tiny greyish-black, red-haired, caterpillars emerge. Almost immediately, the caterpillars spin a larval web made of fine silk, which appears to offer some protection from the unwelcome attentions of predators – though possibly not parasites; the species is known to suffer high levels of parasitization. As they grow, the caterpillars alternate between bouts of daytime sunbathing within the web and nocturnal excursions in search of food. Each web will support many dozens of caterpillars all sheltering within its confines through until just before pupation. Interestingly, some individuals entering pupation may remain within the resulting chrysalis for two or even three winters, rather than simply emerging the following year. This could be a mechanism by which the impacts of parasites on subsequent generations are reduced.

The webs themselves would once have been a familiar sight over much of southern Britain. However, the widespread removal of traditional hedgerows has resulted in a very large decline in the small eggar population, to the extent that conservationists are now concerned for its future. Indeed, many local Biodiversity Action Plans include the species in their list of objectives, Even in those counties, like Norfolk, where the species still occurs, this moth can best be described as decidedly uncommon. Finding a larval web, then, is something memorable.