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This is where I post various musings about wildlife and ecology, observations of interesting species (often invertebrates)
and bits of research that grab my attention. As well as blogging, I undertake professional ecological & wildlife surveys
covering invertebrates, plants, birds, reptiles, amphibians and some mammals, plus habitat assessment and management
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Tuesday, 26 July 2011

Diversity: it's the talk of the chalk

The south-east of England is known for a number of things - too many rich bankers in over-priced houses, an excessive number of golf clubs (there's a correlation here I think!) and easy access to France by tunnel. However, from an ecological point of view, it is also well known for its calcareous grasslands growing on the cliffs and Downs pushed up when the ripples of the 'Alpine orogeny' (formation of the Alps) buckled what is now the south coast of England.

These grasslands are often exposed to the weather, hot when sunny, slightly alkaline (pH around 7.5-8), windswept, free-draining (so, often dry), low in nutrients due to leaching, and often closely grazed (by rabbits and sometimes sheep). Hence, though conditions are too challenging for more ruderal plant species, millennia of such conditions mean that there is a diverse array of plant and animal species adapted to grasslands of this type. Now, I don't intend to attempt a summary of the ecology, natural history and conservation management of southern England's chalk grasslands - these are covered elsewhere (e.g. Rodwell 1992, Crofts & Jefferson 1994). Instead I simply wish to provide a snapshot of what is visible during a brief visit - in this case during an Open University field trip in July 2011.

The site was the top of the cliffs just east of Birling Gap in East Sussex (near the more famous headland of Beachy Head). The cliffs are chalk with flint beds and have their own interesting flora and fauna such as the splendid lygaeid bug Henestaris laticeps - a specialist of cliff-faces where Buck's-horn Plantain (Plantago coronopus) grows, and easily identified by its stalked eyes.

H. laticeps (5-6mm long) showing stalked eyes. The similar H. halophilus is found in saltmarshes and has shorter eye-stalks. The function of the stalks remains unknown though noted by Southwood & Leston (1959).

However,cliff specialists might one day form a post (or series) of their own, so to keep the focus squarely on the cliff-top grasslands, let's take a walk uphill and see what can be found...

As this is a grassland, it seems sensible to start with the plants. Chalk grassland is noted for its high diversity - often a mosiac of mat-forming and rosette species with a scattering of upright species. These low-growing species are well placed to spread their leaves to maximise photosynthesis as well as spreading their roots to capture water as it swiftly drains through the soil. Those with rosettes are low-growing only until it is time to flower, at which point a spike grows quickly, flowers and sets seed, hopefully avoiding grazing or other potential hazards. Rosettes of some species such as P. coronopus may have downy hairs in the centre which trap moisture (including dew) and may also disrupt airflow sufficiently to reduce water loss through evapotranspiration.

Squinancywort (Asperula cynanchica), a prostrate mat-forming plant with small pinkish-white flowers and tiny linear leaves.
In the face of limited water and nutrients, other species manage the energy balance a different way, growing a larger stem and becoming upright. Some of these, such as Viper's Bugloss (Echium vulgare) shown below protect their investment in biomass (which may be offset against less rapid reproduction and/or reduced energy reserves) by developing bristles or spines to deter herbivores.

A clump of Wild Mignonette (Reseda lutea)
The small yellowish flowers of R. lutea.
The blue and pinkish flowers of E. vulgare (bristly hairs can be seen).
In the absence of abundant nutrients, ruderals (which are stronger competitors than chalk grassland plants when conditions are favourable) are absent, or heavily disadvantaged, and so those species which compete poorly but can tolerate chalk grassland conditions are able to form the high-diversity mosaic mentioned, in some cases taking advantage of opportunities to colonise bare patches of soil (a thin rendzina) produced by erosion and other processes. In this case, for those interested in the NVC, or National Vegetation Classification (Rodwell 1992), although I have not formally surveyed/analysed the site, it appears to be close to a CG2 Festuca ovina-Avenula pratensis grassland; with abundant Carex flacca, Lotus corniculatus and Thymus polytrichus (among others) it may be CG2a, the Cirsium acaule-Asperula cynanchica sub-community. This is an informal assessment, so apologies to the site managers if it's wrong!

Leaving the NVC aside, such a high diversity of plant species naturally suggests a high diversity of invertebrates. I'm not aware of a species list for the site (I'd love to survey it!), but even a single visit on a moderately warm, fairly cloudy day with a little rain provided numerous interesting sightings.

Yes, I know, more beetles in copula... this time it's the Bloody-nosed Beetle Timarcha tenebricosa on the gorse fringing the grassland along its northern edge, parallel to the southern cliff edge. These are large (up to 18mm) leaf beetles which get their common name from their ability to ooze red liquid when disturbed. This pair was too busy to bother (as were several others).
A shell of the 'Garden Snail' Cornu aspersum (formerly Helix aspersa) which has been used as a nest by one of the Osmia bee species that make shell-nests.
These Osmia shell-nesters use the internal coils as they might any other handy crevice and there are three species relevant here; O. aurulenta, O. bicolor and O. spinulosa (also known as Hoplitis spinulosa). Although somewhat anecdotal, Andrewes (1969) notes that O. aurulenta closes its shell-nest with a plug of felt-like plant material as seen here (it is not dung, although it does appear so) while O. bicolor uses a fine 'rubble'  of shell and stone fragments and covers the shell with a mound of plant material (Edwards 1998) which was not seen in this case, though a shell hidden under the edge of gorse had been chosen, assuming no movement of it by humans. Edwards (1998) also lists H. aspersa as a shell used by O. aurulenta, but not O. bicolor; similarly, Edwards & Roy (2009) note that H. spinulosa uses smaller (i.e. medium-sized) shells such as Cepaea nemoralis and is not suited to the hard-grazed grassland seen at Birling Gap. So, my tentative suggestion is that this is a nest of O. aurulenta (a scarce/Notable species and probably the least common of the three mentioned here) - however, observations of adults would be needed to make a certain identification to species. Maybe on my next visit!


References

Andrewes, C. (1969). The Lives of Wasps and Bees. Chatto & Windus, London.
Baldock, D.W. (2008). Bees of Surrey. Surrey Wildlife Trust, Woking.
Crofts, A. & Jefferson, R.G. (eds.) (1994). The Lowland Grassland Management Handbook. English Nature, Peterborough/Wildlife Trusts, Lincoln.
Edwards, R. (ed.) (1998). Provisional Atlas of the Aculeate Hymenoptera of Britain and Ireland. Part 2. BWARS/BRC, Huntingdon.
Edwards, R. & Roy, H. (eds.) (2009). Provisional Atlas of the Aculeate Hymenoptera of Britain and Ireland. Part 2. BRC, Wallingford.
Rodwell, J.S. (ed.) (1992). British Plant Communities 3: Grasslands and Montane Communities. CUP, Cambridge.
Southwood, T.R.E. & Leston, D. (1959). Land and Water Bugs of the British Isles. Warne, London.

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