Early June at Newhaven's Castle Hill LNR: continental visitors arrive

Week 10 of the 2016 transect-walking season

 

Butterfly numbers were again lower than expected with only seven species seen during the walk. The weather during the walk was warm and sunny with only light winds, but this was the first day in a while that the weather was not cool, cloudy and windy. Despite the tendency to think that numbers are poor, upon comparison with most other years numbers are about average - and one species, the Green Hairstreak (Callophrys rubi) has already had its best year to date. Eighteen butterfly species have been recorded so far during 2016.

Diamond-back Moth (Plutella xylostella)

Two overseas species spiced up proceedings: the tiny Diamond-back Moth (Plutella xylostella) and the much larger - and much more noticeable - Painted Lady (Vanessa cardui). The latter was seen flying in off the sea in a northerly direction and these are possibly the vanguard of a greater immigration which may occur over the coming days. These are a strong, unmistakeable insect and there is always a hope that the event of 2008 will be repeated, when many, many millions of these butterflies piggybacked across three generations from the African Atlas Mountains into the UK and as far north even as the Arctic Circle, before their offspring made the return journey back south at the end of their season. 

The Diamond-back Moth is in the midst of a similar immigration event, which hit Newhaven on 30th May, but apparently from the east rather than from southerly climes. I have recorded more than ten thousand of these moths in Newhaven during the past week and I saw an additional 400 - well, 398 - during this walk. Many of these were nestled amongst hoary cress and kale along the cliff bottom and their caterpillars will feed on these and other plants of the cabbage family during the summer. There were without doubt many thousands more than the 400 that I counted, but transect-walking rules are not to be broken, so I counted only what flew up before me as I stepped along the route! I have written a separate report about the Diamond-back Moths which provides more information about this tiny species and the current immigration event.

Large Skipper (Ochlodes faunus)

Whilst these and other exotic species add colour to the nature reserve, we are inevitably more interested in the native species which breed each year at Castle Hill. Numbers fluctuate annually, in response to many variable and confounding factors, and not least the weather. These species have evolved to take the knocks and then make hay when conditions are more favourable. This year is proving to be more or less average when compared with some previous years. It was pleasing to see the emergence of the Large Skippers (Ochlodes faunus), albeit in only small numbers, but these feisty little butterflies, which lay their eggs on Cocksfoot grass, should increase in number during the next few weeks. Common Blue (Polyommatus icarus) numbers are increasing by the week and the first brood should peak in the next two or three weeks, as should the day-flying moths the Mother Shipton (Euclidia mi) and Burnet Companion (Euclidia glyphica), which can be found amongst herbaceous areas along the tops of the cliffs.

Earlier in the year I made a chance observation of a Raven raiding the Barrow Head cliff face hollows which are used as nest holes by a small group of Jackdaws and feral pigeons. The Jacks were perturbed and apparently helpless on that occasion as the Raven leered menacingly into each hole, but there was nothing to steal away at the time (egg-laying time was still in the future) and it flew off empty-handed. I witnessed a repeat raid today, probably by the same Raven. The response of the Jacks could not have been more different: whereas they previously appeared to yield helplessly to their preponderant corvine cousin, today they were a tight fighting force. Arranged in formation they swooped in turn, five Jacks deep, to mob their attacker, they swarmed around, jostled, grappled, set upon, overran and comprehensively saw it off. Within the space of thirty seconds the egg thief was utterly routed. It was spectacular stuff! The Jacks had a most valuable resource to lose this time in their chicks, which must now be very close to fledging. I think this is an example of the concept in the study of behavioural ecology known as the incumbency advantage.

Krebs & Davies (1993) eloquently explained at length how an incumbent (or owner of a resource) has an advantage over a challenger because the incumbent knows or understands the resource at stake, whereas the rival does not possess such detailed knowledge. I am not a behavioural ecologist, but I believe the battle over the Jackdaw chicks (the resource) was an example of this. The investment by the several adults and the bonds with the chicks resulted in a much greater challenge to the Raven than when their nest holes were empty. The hypothesis of the incumbent advantage is that the owner will fight more strongly than the less certain challenger and almost always win. Also, they have more to lose than the predator has to gain. The stakes vary and there are plenty of examples in nature when the predator wins the prey, but I was stunned and mightily impressed by the ferocity of the parent birds on this occasion.

The reserve is easing itself into the summer season now. Grasses are shooting skywards and flowering, along with Thrift and the Common Blue's food-plant Bird's-foot Trefoil on and above the cliff faces. There are the usual concerns about humans' conflicting interests and multiple uses of the nature reserve, but I am not going to bore the reader with a personal diatribe until I have taken the time to convey my feelings - my disgust even - in a more positive light.

The full list for Week 10 (24 species)

Butterflies (7 species) Large Skipper (Ochlodes sylvanus)  3
Small White (Pieris rapae)  1
Green-veined White (Pieris napi)  1
Speckled Wood (Pararge aegeria)  2
Small Heath (Coenonympha pamphilus)  1
Painted Lady (Vanessa cardui)  9
Common Blue (Polyommatus icarus) 12
Moths (17 species) Diamond-back Moth (Plutella xylostella)  398
Common Tubic (Alabonia geoffrella)  1
Light Brown Apple Moth (Epiphyas postvittana) 1
Common Marble (Celypha lacunana)  3
Plum Tortrix (Hedya pruniana)  5
Knapweed Bell (Epiblema cirsiana)  1
Regal Piercer (Pammene regiana)  1 – first site record
Grey Gorse Piericer (Cydia ulicetana)  15
Lead-coloured Drill (Dichrorampha plumbana)  1
Garden Grass-veneer (Chrysoteuchia culmella)  1
Meadow Grey (Scoparia pyralella)  1
Little Grey (Dipleurina lacustrata)  2
Lackey (Malacosoma neustria)  1
Yellow Belle (Semiaspilates ochrearia)  1
Angle Shades (Phlogophora meticulosa)  1
Mother Shipton (Callistege mi)  1
Burnet Companion (Euclidia glyphica) 4
Other highlights Grove or Brown-lipped Snail (Cepaea nemoralis)  1 – not previously found at CHLNR
22-spot Ladybird (Psyllobora vigintiduopunctata)  1
Dock Bug (Coreus marginatus)  7
Green Shield-bug (Palomena prasina)  1

Common Tubic (Alabonia geoffrella)

Garden Grass-veneer (Chrysoteuchia culmella)

Cutting brambles at Barrow Head

Restoring a mixed grassland habitat for the Common Blue butterfly (Polyommatus icarus) on the slumps at Castle Hill LNR in Newhaven

In February, the sun-splashed, salt-stunted grassy slumps beneath the cliffs at Barrow Head are largely absent of human activity. The occupation by hordes of barbecue-brandishing sun worshippers and their competing beat boxes are still five and a half months away; for now only the occasional dog walker ventures off the ribbon of vegetative shingle onto the slumps. The rabbits and voles need concern themselves only with the fox and the kestrel amongst their daily anxieties.

The Barrow Head rabbits are a depleted race; once numerous enough, perhaps, for the headland name to have been re-Christened in their name as their burrows became the noticeable feature following the geological demise of the barrowland. The rabbits have melted away themselves over the decades and their nibbled turf has become neglected so that the few remaining burrowers cannot compete with the advance of successional vegetation: the slithering bramble and brittle-boned elder which have slowly and ominously emerged the victors in a turf war across the slumps. The salt-laden Channel winds do their best to slow the advance, but these are winds which lack the fury of an Atlantic or North Sea gale. Channel winds are soft in a way which the Welsh writer Edward Thomas understood in his 1909 book The South Country: 

...the South is tender and will harbour anyone; her quiet people resent intrusion quietly, so that many do not notice the resentment. These are the "home" counties. A man can hide away in them. The people are not hospitable, but the land is.

Common Blue (Polyommatus icarus)

The slumps are a habitat in flow, particularly so in February. The chalk cliffs of Barrow Head are topped with layers of change: London and Woolwich clays, sands, raised beach, lignite and a rendzina crowned with short-cropped fescue and thrift. These flow, perhaps in emulation of the Ouse river a stone's throw to the east, from the clifftop downwards in a glacial dance onto and into the slumps, plump with the weight of the winter's rain. Castle Hill's barrows, after which the small headland is named, have long since melted away into the slumps below, noticed or not by the eyes of man. But the flow is not always glacial in character, a smooth progression of melting clay or fossilised oyster bed dancing across deep time; it is seasonal, sudden and catastrophic and some February inspections reveal the gashes and scars of a sudden night time slip or collapse of sandstone.

Wild thyme and trefoil

The sandy, ligneous soil forming the slumps is well drained and offers a suitable habitat for Sedum: biting and white stone-crops which flower sulphur and white in the summer. This two-tone tableau carpets the exposed soils, filling the gashes and scars of winters past. Amongst these one can find burrowing invertebrates such as solitary bees and wasps, beetles and hunting spiders. Their exposed habitat relies on change. If the flow from above was to cease, the barren ribbon of soils would engage in a succession of the grasses and sedges which can be found a little lower down the slumps and, as their roots take hold of the soils, rhizomes of bramble would sneak in and begin to suffocate the lower plants and their denizens. The burrowing insects would be replaced by cone-heads and Roesel's bush-crickets, which already chirrup and drone further down the hill, but the loss of the marginal habitat and its tenants would be very difficult or impossible to replace once it is gone and the character of the place would become more uniform, less diverse.

It is a mistake to look upon the Barrow Head slump and see only a wasteland of slipped ground, useful only as a site for disposable barbecues, a resting place of windblown litter or to utilise as an outdoor latrine. There are contours of life here: narrow bands of neighbouring communities which exploit ribbon-like bio-halines. These specialist niche-dwellers of the slumps collectively amount to the richest biological community anywhere within the Castle Hill Local Nature Reserve. The resident stonechat pair seem to know this. These ribbons require a well-considered eye and hand to ensure they are performing well. The suffocating tangle of bramble must be held in check and this is the reason why our work-party is visiting today.

Thrift

We arrive with our tools, down them and stand in a cluster to ponder the landscape. The slumps' grassland areas have been choked by encroaching bramble during recent years and it came to our notice the previous year when numbers of butterflies experienced a significant decline. The grassy undulations, now smoothed by crowns of bramble, were in previous years host to a colony of common blue butterflies, which laid their eggs on bird's-foot trefoil. Mixed clovers fed the Mother Shipton and Burnet Companion day-flying moths; countless noctuid moth species nibbled away at grass roots and, as adults, they shared the shelter of the grass tussocks with foreign visitors such as Silver Y and Rush Veneer. The loss of grass and flowers means also the loss of the invertebrates which rely upon them. The birds which are fed by their lower trophic levels shelter in the bramble perhaps less certain of their next meal.

Our group of friends have pondered how to tackle the thicket. We debate the need for sheep - their grazing power would do the trick! To fence the sheep in would be a simple and satisfying task, but fencing out humans and their dogs is considered by all to be an impossible goal. Everything growing before us has endured despite the behaviour and activity of many people and their pets and any livestock, however lookered, would be placed in a precarious situation, however optimistic or diligent we may be.

Sedum, sulphur and white growing above the Barrow Head slump

We agree to leave in place a protective skirt of bramble a few metres thick along the cliff bottom and to remove anything below it. There are some considerations and negotiations to be made: what will happen to the cherished colonies of Roesel's bush-cricket and long-winged cone-head, which we recorded around the thicket the previous summer? We agree that the next generation of these species will emerge from their protective grasses as nymphs and, if desired, shall migrate slightly uphill to the remnant bramble. Where shall we have a fire? Must we scorch and lose another patch of grassland? After much debate and concern about the practicalities of carrying a prickly cumbersome mass of bramble down to a remote fire site and the prospect of sacrificing a remaining patch of grass to a fire, we agree to cut a patch of bramble and build the fire on the exposed ground. How should we actually cut the bramble? It appears to be more involved than lopping and sawing through the scrub of gorse, blackthorn, hawthorn and sycamore on the northern slope of the reserve. We decide to learn through doing and get to work.

We agree to leave in place a protective skirt of bramble

Tired arms

Burning the tangled, rolled fleeces is a pleasurable affair

It quickly becomes apparent that cutting bramble is hard work. One has to adopt a crouching posture, so as to work at the level of the lower stems, but also to ensure a reasonable working distance from the thorns. Legs and arms are not as protected as hands and, soon after cutting commences, as body temperature rises and layers are cast off, they are protected less still. A few explorative snips with a lopper persuade me that progress can only be made with a less discriminatory set of shears: several cable-thick stems may be cut at once instead of a single stem at a time. I swap tools and progress through the stems. We learn that the best method for removal is to imagine we are shearing a sheep and begin working with a partner, cutting and rolling, cutting and rolling, one shearing through the spinous fleece, bristling with thorns, the other rolling the cut fleece over, pulling it back to enable the next cut.

The hard work takes its toll and my forearms tire too quickly. The slashes of my shears slow, soften and cease. The lactic acid burn would penetrate so deeply into seldom-used muscles that I would not enjoy full use of my aching arms for six full days. When I stop and sit by a cleared patch I notice for the first time just how much new habitat we have created. If the potential for blue butterflies could be measured in satisfaction, we had enjoyed a very satisfying morning of hard work. The resident stonechats, perched almost within an aching arm's reach behind me, clack in appreciation.

Burning the tangled, rolled fleeces is a pleasurable affair. The flames take hold of the brambles with enthusiasm. I stand a distance from the heat and imagine that the potential for harm held within each thorn can be measured as a unit of energy. As the flames sweep through the newly added bundles, the crackles and pulses of flame mark the demise of one thorn after another. No longer shall they catch hold of my boot or pierce my skin; no longer shall their stems and leaves shade the trefoils and their butterflies from the sun.

...we had enjoyed a very satisfying morning of hard work.

Barrow Head and restored slump

Cleared bramble, exposed land

Cleared bramble and fire site