An Ecology Fieldtrip

David Beeson, September 2022

For those of us living in northwest Hampshire we are used to a chalky landscape cut through by clear rivers and streams. The river valleys are lush, alkaline and covered in the remains of C17, C18 and C19 water meadows (see article). So, my ecology group opted to visit South Dorset with its areas of dry sandy-gravelly soils yielding heathlands with distinctly different ecology.

The geology of the area means that the top layers of ‘soil’ are seldom soil at all. The water drains so easily that the plants live mostly in ultra-dry conditions and are xerophytes – plants with small, tough leaves and growing mostly to just centimetres in height. With their accumulation of energy, and so organic matter, so limited their substrate contains precious little humus and nutrient recycling is low. The open heaths are dominated by ling (Calluna vulgaris) with the driest places showing bell heather (Erica cinerea). The Dorset heath also occurs here (Erica cilliaris).

The odd thing is that rainfall, over the centuries, has washed many nutrients out of the top sandy layers and the iron is deposited as a water-impermeable iron pan … resulting in the lower areas being very wet indeed. Here, cross-leaved heath (Erica tetralix) dominates, and the environment is acidic.

Even if organic matter accumulates on the open heath it forms a thin layer and no subsoil develops.

With dry soils and few nutrients growth rates are minimal and food chains short and unusual. When I live-trapped for small mammals in these areas I found none. The food chain is mainly ling > small insects > lizards > smooth snake. Even adders struggle here and prefer to dwell around the local RSPB farm. Predatory birds have little to pluck from the ground and stick to the wooded zones and coastal marshes.

Gorse (Ulex europaeus) is a spiny plant that can dominate if grazing or burning does not occur. Growing to a couple of metres in height and with yellow blooms that, to me, have the aroma of coconut it provides above-ground nesting sites. At ground level, a more amenable plant is the dwarf gorse (Ulex minor) found on thinner soils and in more open locations.

The star of the area, and a challenge to locate, is dodder (Cuscuta epithymum) – a parasitic climber without any hint of chlorophyll. It has thread-like red stems, no roots at maturity, and leaves reduced to useless scales. For nutrition it latches on to ling, extracting both water and organic materials. In September 2022, its white-pink flowers had gone, and its stems had been largely desiccated by the hot summer.

Adjacent to Coombe Heath is a small wetland whose pH surprised me in being near neutral and not around pH 4 as I had predicted. As it was not acidic it had many water invertebrates, including carnivorous water boatmen. The special organism was, however, raft spiders that patrol the water’s edges and consume small insects. Regrettably, they had seen us coming and had hidden.

However, the Arne RSPB nature reserve where we were based has one area to the east, that has sufficient soil quality for fields and woodland to have been developed. Here the ecology is quite different with oak, birch, sweet chestnut and Scots pine. It is probably an area long ago reclaimed from Poole Harbour and it has river deposits on its surface.

With better soils allowing a range of deciduous trees and shrubs to develop, and areas of modest farmland, the animal life is more diverse. We spotted small herds of (non-native) sika deer, grey squirrels and plenty of signs of rabbits.

Sika deer are Asian, yet thrive here, so need human control and their population is (sensibly) well down on previous years.

With the deer’s rut fast approaching, at least one male was in close consort with one herd of females, another building up its strength by feeding on the salt marsh.

Surrounding the area to the north, south and east are rivers and Poole Harbour. It is tidal and the exposed muddy fringes are rich in both algae (hence organic food) and invertebrates and they draw in predators. Waders, ducks and gulls are common. Egrets stalk the narrow inlets, terns surface dive for fish and there is an osprey nest – although its single chick was caught and killed by a goshawk.

In the distance, the view incorporates the chalk hillsides near Corfe and Corfe Castle itself.

Arne RSPB is part of The Purbeck Heaths National Nature Reserve that stretches from Wareham in the northwest down to Studland on the southeast coast. It comprises one of England’s largest wildlife-protected zone. Most of the fences have been removed to allow free movement of larger organisms and some semi-domesticated pigs have been released to break up some soils and to encourage annual plants to again flourish.

The name ‘heath’ is given to lowland areas dominated by heathers, more upland and wetter places are called moors. Heaths are an internationally very rare habitat and their wildlife is often endangered.

The place is not without serious challenges. My ecology group’s second day had to be scrapped as human activity (a BBQ) had burned out our sand dune study site and human pressure is strong, especially along the coast. Some newly acquired land had previously carried exotic conifers. While the large trees had been removed, a strong seed base remains and thousands of seedlings will need removal – a daunting task for volunteers.