26 July 2011

Xysticus & Agroeca spiders

The growing number of insects in the newly cleared area around the former Austford Farm is providing plenty of food for spiders and other insectivorous creatures.

Xysticus cristatus (below) is one of the crab spiders that conceals itself in flower heads waiting for an insect to alight before grabbing it.

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It is rather like going to the pub and having a giant cannibal spring up from behind the bar and start sucking your blood just as you settle down for your pint.

The unprepossessing blob of mud on a rush stem below is a completed egg case of one of the fairy-lamp-spiders, Agroeca proxima or A. brunnea.20110710 BHW Agroeca brunnea egg  case 080

I wrote about this in the Easter eggs entry for 12 April 2009 and posted a photo that shows the spider's egg case before she covers it with mud.  This outer coating is, of course, a good camouflage, or rather deception, as it looks just like a blob of mud thrown up by a passing cart or galloping animal.

The casing is resistant to rain as the mud is strengthened by the spider's silk like fibre reinforced concrete or the addition of straw to mud bricks in medieval times.

Naturalist Edward Connold wrote in his Gleanings from the Fields of Nature (1909) that he first noticed these mud-plastered cocoons in 1893 when he found them in large numbers "in an open wood near Hastings."  I wonder if this was Brede High Wood.

He also wondered about the considerable effort it must take the spider to first weave the egg cocoon and then transport wet mud up a rush or grass stalk.  Other spiders do not go to such great lengths to protect their egg cocoons, yet Agroeca's efforts do not seem to result in a significantly larger number of members of the two species.  Maybe there was a situation in the dim and distant past when a mud-covered egg case did give it some slight advantage over their competitors and the ability to create these in their present form was naturally selected.

24 July 2011

A note on dodder, Cuscuta epithymum

Common dodder, Cuscuta epithymum, is a low-growing parasitic plant with red, thread-like stems and pink flowers in summer. It was first recorded in Brede High Woods in 1994 in a small open area (TQ790202) that is part of what was known in pre-reservoir days, i.e. before 1930, as The Hothes (now part of Compartment 4b), indicating a rough grazing area of gorse and heath. It is currently normally referred to as ‘Sedlescombe Heath’. Dodder was parasitic on heather, Calluna vulgaris, here, but will also use a wide range of other plants as host, such as wood sage, bracken, bramble and various grasses.  It has even been found on lousewort, Pedicularis sylvestris, which is itself partially parasitic.

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It was described as common in Sussex by Arnold in 1887, but had become relatively scarce by the time Hall’s Sussex Plant Atlas was published in 1980. Currently its main East Sussex locations apart from Brede High Woods are Ashdown Forest, Chailey Common and Hastings Country Park and it was also widespread on the Downs in the past, but it continues to be a generally declining plant. The Online Atlas of the British and Irish Flora says “The loss of lowland heath, ploughing of chalk downlands, and an increase in scrub have caused a decline in this species since 1930.”  Whilst it is still locally abundant in southern England, distribution maps suggest a continuing substantial decline.

In the early years of the 21st century it disappeared from its only known location in Brede High Woods, but another patch was discovered in 2009 some 200 metres north of the original site. Following clearance of conifers and broadleaves from formerly open areas in late 2009 it appeared in some abundance in 2011, particularly in the southern part of 4b that had been an oak plantation and it was also found in Compartment 5a where there had been dense conifer cover for many years.

It is well known that seed can retain viability under unfavourable conditions for many years (Meulebrouck, 2009) and it will often, though not always, colonise fire sites in the heathlands where it grows (Rich et al., 1996).

Dodder only flourishes in the early successional stages of heathland and other habitats and management is important in ensuring its long-term survival. Meulebrouck (2009) recommends for Belgian heathlands   “a combination of cyclical management by mowing, burning and shallow turf cutting” with seven- or ten-year management cycles on patches containing dodder, a technique that can be successful even at small scales.

Meulebrouck further points out that livestock grazing is an important management measure for lowland heaths. The positive effect of extensive grazing on both the presence of dodder populations and the long-term metapopulation viability, indicates that grazing is a beneficial and valuable conservation tool for dry heathlands. A mosaic of pioneer phase patches of heathland regeneration are not only important for dodder, but for other plants and their associated faunas that flourish in bare, or thinly vegetated, open habitats.

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Arnold, F. H. (1887) Sussex Flora. Hamilton, Adams & Co, London

BSBI et al. (2011) Online Atlas of the British and Irish Flora. http://www.brc.ac.uk/plantatlas/index.php?q=plant/cuscuta-epithymum

Meulebrouck, Klaar (2009) Distribution, demography and metapopulation dynamics of Cuscuta epithymum in managed heathland. Doctoral thesis for the Catholic University of Leuven, Belgium. http://dfwm.ugent.be/lavobo/docs/Doctoraten/Klaar_Meulebrouck_doctoraat.pdf

Rich, T., Donovan, P., Harmes, P., Knapp, A., Marrable, C., McFarlane, M., Muggeridge, M., Nicholson, R., Reader, M., Reader, P., Rich, E. & White, P. (1996) Flora of Ashdown Forest. Sussex Botanical Recording Society.

21 July 2011

Cricket and leaf mine

I was photographing the mine of the agromyzid fly Agromyza flaviceps in a leaf of hop by the footpath east of the old Austford Farm when a speckled bush-cricket (Leptophyes punctatissima) must have made a runner beneath the lens.

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I have to confess I did not even notice it until I got home and downloaded the picture.

20 July 2011

Summer butterflies 2011

If the sun shines, this is one of the best times of the year for butterflies.  The browns are among the commonest with small heath, gatekeeper and meadow brown all on the wing.  It also seems to be a good year for ringlet (Aphantopus hyperantus), often a difficult species to photograph because they never seem to stop fluttering about among the brambles and long grass.  Unless, of course, they are preoccupied:

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The plantation clearances of 2009 have given long stretches of edge habitat where woodland meets more open ground where nectar-bearing flowers can flourish.  This year the silver-washed fritillary seems to have spread westwards and it is good to see this dramatic butterfly still doing well.

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Sadly, though perhaps it is important for the butterfly, this fritillary tends to move suddenly  to fresh fields and pastures new perhaps, like many other invertebrates, to evade the predators, parasites and pathogens that always have an eye to the main chance themselves.

16 July 2011

Three heathers

We have discovered a quite extensive colony of cr0ss-leaved heath (Erica tetralix) towards the north of Compartment 5a (Map ref. TQ792204).

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This area was cleared of conifers less than 18 months ago and these plants must have survived in the gloom for many years before the light returned.  Cross-leaved heath tends to prefer wetter spots than the other two Sussex heathers and, although the habitat here seems pretty dry, the plants grow in a fairly narrow west to east band where there may be a spring line or damper conditions.

The first of the heathers to flower is bell heather (Erica cinerea) with bright purple flowers and a preference for dry banks.  It is not all that common in Brede High Woods but, hopefully, it is increasing.

20090810 BHW Erica cinerea

The latest heather is ling (Calluna vulgaris) which is a rather pale mauve and has only just started.  This is the commonest of the three species in the woods and is doing well in many places where there is acid soil.

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9 July 2011

Grassland wildlife

Yesterday I talked about some of the new grasslands in Brede High Woods that have developed since conifer plantations were cleared in 2009.

As well as grasses and other plants, they are also attracting a wide range of invertebrates.  One of the most dramatic is Roesel's bush cricket (Metrioptera roeselii) which sings like a Savi's warbler (if you know what that sounds like) and takes to the wing very readily.

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Mainly recorded from Rye Harbour in our area, this species now seems to be spreading, maybe in response to climate change.  I saw several in Compartment 4c which was a dense Christmas tree plantation less than two years ago and it is encouraging that creatures such as this can colonise new areas so quickly.

Grassland butterflies like meadow browns, small heaths and the summer skippers have all increased in numbers since last year, and I found both small and Essex skippers (Thymelicus sylvestris & T. lineola)  to be present in some numbers.

Essex skippers have black patches on the underside of the tips of the antennae:

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Whereas small skippers are dusky orange at the tips of the antennae:

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Small skippers tend to prefer Yorkshire fog grass in their early stages, and there is plenty of that in the 'new' fields, but Essex skippers usually go for cock's foot which is not nearly so common in the area.  Perhaps here the larvae are feeding on another species of grass.

8 July 2011

Seas of grass

The compartments from which the conifers were cleared in late 2009 have produced a wealth of fresh, new vegetation and many areas are now waving seas of grass.

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Generally they consist of two dominant species, Yorkshire fog (Holcus lanatus) and common bent (Agrostis capillaris).  The Holcus is pale fawn, almost white and the Agrostis a shimmering brown.  Both tend to occur in wide patches several metres across giving a variegated appearance to the sward that constantly changes as it is combed by the wind.

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I often wonder how grasslands like this can appear so quickly after woodland clearance, but the seeds of the Holcus, and I suppose of the Agrostis too, are known to be able to remain viable for many years producing "a large, persistent seed bank" (Cope & Gray, 2009, Grasses of the British Isles).  So, these billowing fields have arisen as children of the pre-conifer grasslands of the early or middle part of the last century.

As well as being a joy in their own right, the grasslands are attracting other species and there were many butterflies such as skippers, small heaths and meadow browns, as well as a chorus of grasshoppers and happy bumble bees.  Nectar for the flying adult insects is largely supplied by brambles at the woodland edges and biodiversity should increase dramatically as more flowers appear.

This in turn will benefit small animals and seed-eating birds with further benefits up the food chain.

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I like the flying insect top right in this picture of Yorkshire fog at anthesis.

7 July 2011

The blue and the scarlet

I was very pleased to have refound the blue pimpernel by the track to the south of the old wood yard not far from the site where I first saw it maybe 20 years ago.

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There are two 'blue pimpernels' in the British Isles, but they are very difficult to tell apart.  Our common scarlet pimpernel (Anagallis arvensis subsp. arvensis) with, surprise, surprise, scarlet flowers has a blue form (wait for it) Anagallis arvensis subsp. arvensis forma azurea.  The other one is Anagallis arvensis ssp. foemina.  The defining difference is found in the small hairs along the outer edges of the petals.  In A. a. a. azurea these hairs have three cells and the end one is globular.  In A. a. foemina they have four cells and the end one is oval.

pimpernel petals

The cells do not, of course, have numbers engraved on them.

I looked, under a high power microscope, at a flower from Brede High Woods and another blue one from a colony that has been flourishing in our garden for ages and the one from the woods is the true blue pimpernel (A. a. subsp. foemina).

All this made me wonder what these microscopic hairs on the petals are for.  The end cell in both species is reddish and glandular so I assume it contains some special chemical.  What for?  Perhaps if the plant is lightly crushed underfoot, the chemical is released and attracts potential pollinators, but this seems a rather complicated way to evolve to achieve an end much more easily achieved by other plants without such devices. 

An ancient introduction, A. a. foemina, is now quite rare and  according to the New Atlas of British and Irish Flora may be declining, probably due to more intensive weed control in arable fields, though some records may be from bird-seed.