From Thorn to Orchid

Last Summer I got to revisit an old haunt in South London where I used to volunteer with the London Wildlife Trust. I was very excited about returning to Hutchinson’s Bank Nature Reserve (it is in the suburb of New Addington and easily reached by tram from Croydon). I left the restoration project when I moved ‘North of the River’ some years ago and had not seen the final transformation from scrubland back to chalk grassland. I was not disappointed – this site is a bit of a treat even when the weather isn’t at its finest.

The reserve was taken on by LWT (on behalf of Croydon council) because of the potential to enhance the scrubbed over chalk grassland through habitat restoration & management work and by building on the planting and maintenance already undertaken by a group of dedicated locals who had successfully introduced small patches of Kidney vetch (Anthyllis vulneraria) and Greater yellow rattle (Rhinathus angustifolius). Because of these locals who were actively involved there was also already a very impressive list of butterfly and orchid records associated with the site.

Lowland calcareous grasslands form over shallow limestone-rich or chalky soils which have a typically high pH, low nutrient levels and tend to be free draining. Because they favour these particular conditions, chalk grassland plant species are called calcicoles (lime-loving plants). Much of Hutchinson’s Bank Nature Reserve is, as the name implies, on the slope of an embankment which aids with the drainage of rainfall, and the fact that the slope is south-facing ensures fairly warm conditions throughout the Summer months.

hutchinsons-bank-montage

Clockwise from top left: Tibellus oblongus; mating Robberflies, Machimus atricapillus with prey; Longhorn beetle, Rutpela maculata; Marbled white, Melanargia galathea on scabious; Small Blue, Cupido minimus on Anthyllis vulneraria; Pyramidal orchid, Anacamptis pyramidalis; Volucella pellucens; Philodromus sp.; Chrysotoxum bicinctum; Oncocera semirubella; Roesel’s Bush Cricket, Metrioptera roeselii.

It is estimated that there is between 25,000 ha and 32,000 ha of chalk grassland in the UK1 where it is considered a nationally rare habitat. Calcareous grasslands have been described as being equivalent to coral reefs in terms of their species richness, and though this can be seen in small areas, the comparison doesn’t really hold once you increase the scale of the compared areas. As you increase the study area on a coral reef, you will continue to find new species at a higher rate than in chalk grasslands where you will fairly quickly find all the resident species, relatively speaking.

This notwithstanding, calcareous grasslands are highly species rich with a single square metre supporting between 50 and 60 species of vascular plant (including 37 Red Data Book species). As a result of this habitat heterogeneity, we find variation in vegetation structure and large numbers of different food plants which cater for one of the most diverse insect communities in Britain.2

What makes these habitats especially rare is the fact that they are remnants of Mesolithic  agriculture; established about 9,500 to 5,000 years ago when forest cover was cleared for growing crops and rearing domestic animals which continued well into the Neolithic era. The highly porous soils meant that nutrients leached away and that these largely-unfertilized fields eventually lost productivity and were abandoned for new sites. But while they were productive, they were kept clear of encroachment by scrub and the succession to closed-canopy forest was inhibited.2, 3, 4 These cleared areas would then support grass swards and herbs associated with both steppe and Meditteranean vegetation types whose seeds had previously lain dormant in the soil seed bank. This anthropogenic land management system involves quite a specific regimen, and though supported by some historical pollen records and fossilised beetle fauna, it remains unresolved.4, 5 

In 2000, Frans Vera proposed a new hypothesis to explain open patches of land (much like savannahs) based on the same evidence but concluded that these areas were maintained by large herbivores such as auroch, wild horses and deer. The Vera Hypothesis, as it has come to be known, remains controversial and has become the basis for a large-scale rewilding experiment at Oostvaardersplassen in the Netherlands. It is likely, in my view, that a mosaic of open areas was first created for agricultural use and then maintained by browsing and grazing of ungulates.

With this in mind, it is therefore interesting to view a map of Hutchinson’s Bank Nature Reserve from 2012 which shows the management plan for different areas including removing topsoil (the most recent land use was modern agriculture, rotational grazing and cutting back scrub. These accepted chalk grassland management practicesare very similar to those used by Mesolithic farmers ~9,000 years ago.

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London Wildlife Trust map and management plan for Hutchinson’s Bank Nature Reserve from 2012.

The largest threat to chalk grassland ecosystems is therefore a lack of correct management which leads to encroachment of scrub and eventually reforestation. Add to this past (and perhaps recurring) socio-economic pressures to develop high-yield crops and provision of housing, and the threat becomes compounded. With only 29% of lowland calcareous grasslands assessed as SSSI being described as favourable by the Joint Nature Conservation Committee, there is real cause for concern. However, an additional 40% of sites are described as “unfavourable recovering”, but without any indication of what that means for each site in terms of actual improvement over time I am unsure of how much solace one can draw from that number.

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Female larva of the nationally scarce Drilus flavescens that I found at Saltbox Hill SSSI in 2011.

It was on one of my volunteering days in August 2011 that we went to another chalk grassland managed by LWT nearby. We were here to survey the vegetation, plot the exact perimeter and identify areas for habitat management.

Saltbox Hill SSSI is located near Biggin Hill airport and is on a very steep hillside with ancient woodland on the ridge of the hill. With an impressive species list and located near the home of Charles Darwin, this area undoubtedly has natural history kudos, and it was here that I found one of the strangest looking insects that had me puzzled for quite some time.

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NBN atlas map showing the distribution of Drilus flavescens in the UK.

 

Some small square sheets of corrugated iron had been set out to act as refugia for the resident slow worms and snakes. Sitting on the edge of one of these sheets was a segmented, rather hairy, caterpillar-like insect. I was completely stumped. I just about managed to get a photo with my phone and as soon as I got home I turned to the internet for help. iSpot is a very useful resource for these baffling discoveries – experts and amateurs alike will help with an ID of any species from a photo and some habitat information. Within a matter of hours I had an ID of Drilus flavescens. Turns out my insect was the female larva of a highly sexually dimorphic beetle found in chalk grasslands. It has a very limited range and is classified as scarce in the UK. Fascinatingly, the males look more like traditional beetles as adults, while the females remain looking much like their larval form. You can find more information at Mark Telfer’s excellent website here.

A visit to a chalk grassland in Summer is a complete sensory immersion. I implore you to go and walk through the grasses skirting the ant mounds; smell the heady herby scents of wild thyme and oregano as you brush past; be surrounded by the buzzing of bees and flies and the soft susuration of grasshoppers; and be dazzled by the sight of brightly-coloured flowers and dancing butterflies. These are spaces that celebrate the wonder of life. I am heartily looking forward to another visit this year.

 

References:

  1. Price, E.A.C. (2003) Lowland Grassland and Heathland Habitats (Habitat Guides Series), Routledge, London and New York.
  2. Mortimer, S.R., Hollier, J.A. and Brown, V.K. (1998) Interactions between plant and insect diversity in the restoration of lowland calcareous grasslands in southern Britain. Applied Vegetation Science 1: 101-114.
  3. Willems, J.H. (1983) Species composition and above ground phytomass in chalk grassland with different managementVegetatio, 52, 171-180.
  4. Robinson, M. (2014) The ecodynamics of clearance in the British Neolithic. Environmental Archaeology. 19 (3), 291-297
  5. Bush, M.B. and Flenley, J.R. (1987) The age of the British chalk grassland. Nature329 (1), 434-436.
  6. Butaye, J., Adriaens, D., and Honnay, O. (2005) Conservation and restoration of calcareous grasslands: a concise review of the effects of fragmentation and management on plant species. Biotechnologie, Agronomie, Société et Environment. 9 (2).
  7. Crawshay, L. (1903). On the life history of Drilus flavescens, Rossi. Transactions of the Entomological Society of London, 51, 39 – 51.

 

 

Spec_accum

An example of a species accumulation curve for 27 taxa found across 9 sites. The green area on either side of the line indicates the confidence interval. Notice how the line increases and then starts to level off (reaching an asymptote) – this indicates that by site 8 all of the taxa have been recorded. It starts with a sharp rise as we discover new and common species, and then the rate slows because we are now only adding rarer species. If this figure represented species accumulation in a chalk grassland, a coral reef by direct comparison would show a more continuous rise and which would require far more sampling before plateauing as in this figure.

 

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