The sand dunes lie at right angles to the prevailing wind and form a series of ridges, running roughly north-south, with lower lying areas between their crests. The long, linear depressions formed when a length of beach has been enclosed by newly formed dunes are known as "swales", whilst the smaller hollows are called "slacks".
The slacks and swales are sheltered from the wind and salt spray. They are often quite damp and may support temporary or even permanent ponds, because at certain times of the year the water table is not far below the surface. Also, plant and animal remains tend to accumulate in the slacks, which aids water retention. This means that species which cannot tolerate the harsher conditions on the crests of the dunes can become established.
Further inland, the slacks contain species that are common to damp pasture land, including many grasses, vetches, Hairy Hawkbit and Stinking Iris. The pale fleshy flowers of the parasitic Broomrape may be seen in the summer. Many of the plants found here provide food for the larvae of moths. Burnet Moths may be seen on Bird's-foot trefoil and their caterpillars construct their cocoons high up on grass stems where it is difficult for predators to reach them. The orange and black ringed caterpillar of the Cinnabar Moth feed on ragwort.
The insect and plant life of the slacks attracts many different birds, such as Skylarks, Yellowhammers and Magpies. Larks nest amongst the grass but are more likely to heard than seen. Many birds such as Linnets, Bramblings and Chaffinches appear in the area during the autumn and winter but nest further north during the summer. The Short-Eared owl preys on the many small mammals found in the dunes.
The fixed dunes were once mobile dunes, like those nearer the sea. Over time, new dunes have formed to seaward and interrupted the supply of fresh sand. The once dominant Marram grass and Lyme grass have been replaced by new plant communities and thin, impoverished soils have developed on what was raw sand.
The fixed dune grassland is dominated by Red Fescue Grass and the spikes of small, yellow flowers of Lady's Bedstraw. A wide range of flowering plants including Restharrow, Bird's foot Trefoil and various types of vetches and clovers provide food for may different moths and butterflies. Particularly apparent during the summer months are Common Blue and Meadow Brown butterflies together with Cinnabar and Six-spot Burnet moths. Many animals can be found here, including invertebrates like the Banded Snail.
The fixed dune grassland is rich in low-growing herbaceous plants and grassed because the low levels of nutrients in the soil prevent may other more aggressive species from growing. The grassland was, in the past, maintained by the grazing of cattle and sheep. This system of management lapsed early in this century and since then no agricultural activity has taken place on the dunes. The absence of grazing has encouraged the development of scrub, except where rabbit grazing is occurring. Rabbits are now the only large grazing animals on the reserve and are vital to the survival of the grassland areas.
One of the major aims of the management of the Reserve is to preserve the species diversity within the dune grassland, which has been threatened by the increasing amount of nutrients (especially nitrates) in the soil. the nitrates come from plants like vetches, clovers and Sea Buckthorn which are able to convert atmospheric nitrogen into the soil and taller, more aggressive plants begin to grow. This process is being reversed by regular cutting back, clearing and treating the invasive vegetation. Also, the existing grassland is mowed after the plants have flowered and set seed.
The scrub which surrounds the grassland is mainly composed of Sea Buckthorn, with patches of Sallow, Elder and Hawthorn. These bushes offer cover for a variety of birds, including the Blackcap. Cuckoos may be seen in the summer. The female Cuckoo lays her eggs in the nests of many different species, including warblers and pipits.
During the last hundred years or so, a number of alien species have been introduced to the area. The most obvious of these are the Evening Primroses, whose large yellow flowers open at night, providing nectar for visiting moths.
The foredunes are the first dunes to form in the dune system and the develop above the line of wave-deposited debris (strandline) where only the strongest waves during very high tides will reach.
The strandline may slow the wind-blown sand sufficiently so that fast-growing, pioneering plants can start to grow. At Berrow, Lyme grass is among the first plants to become established. This tall grass is able to keep pace with the sand that accumulates around it by rapidly growing upward and outward, slowing the sand down and forming a narrow line of low dunes. The sand accumulates even more quickly when Marram grass arrives. Like Lyme grass it forms tussocks and grows rapidly in response to being swamped by sand.
At this stage, the dunes are know as "yellow" or "white" dunes, because of the areas of bare, uncolonised sand. There is little water or nutrient in the dunes and they are unstable, so that only a few other plants can survive, such as Sand Sedge, Sand Couch and Sea Spurge. However, the dunes slowly become enriched by decaying plant material and the droppings from the rabbits which feed here. Mosses and lichens cover the bare ground between the plants; the dunes are sometimes called "grey" dunes at this stage.
Marram and Lyme grass thrive when fresh sand is being added to the dune, but they die out if the supply ceases. This can occur when a new line of dunes forms upwind. The loss of these grasses, or excessive trampling, can leave the dune open to wind erosion, which may be so great that a gap (blowout) is punched through the dune. The sand blown out of the foredunes will accumulate downwind and as long as it is fresh it will be recolonised by Marram grass and the dunes will once again become stable. Conservation work is being carried out to stabilise the parts of the foredunes which are most prone to windblow.
Dune stabilisation can be achieved in a number of ways, but the most cost-effective method is that of sand fencing. The fences are built across the blow out, at right angles to the prevailing wind. They may be constructed from wooden posts and wire netting or, as will be the case at Berrow, from cut branches of Sea Buckthorn, woven together. Wind speed is reduced by the fence, allowing the sand grains to drop out and build a replacement dune.
Inland, the dunes rise to a height of about 15 metres and the crests are covered in thickets of Sea Buckthorn, Blackthorn and Hawthorn. In summer, the tall, yellow flowers of the Evening Primrose can be seen all over the foredunes.
About 15 years ago, a number of ponds could be found in the dune slack areas. These areas remained wet throughout most of the year, supporting wetland plants such as Marsh Pennywort, the less common Narrow-Leaved Reedmace, Meadowsweet, Wild Mint and three or four species of orchid, including the Heath-Spotted Orchid and the Marsh Helleborine.
In recent years, due to a lowering of the water table in the area, these ponds have dried out completely or become dry during the summer months. Investigations have shown that the water table exists at a depth of about half a metre below the surface and a number of ponds have recently been dredged. The ponds are now beginning to re-establish with plants such as the Common Reedmace and Gypsywort.
A rich invertebrate fauna is associated with the wetland habitats with fourteen species of dragonfly having been recorded in the area, including the Hairy Dragonfly and the Ruddy Darter. Beetles are also well represented, with many notable species, such as the nationally rare Greater Silver Diving Beetle.
The Sallow bushes growing nearby provide protection and food for many insects, including the larvae of the Buff-Tip Moth and the Lackey Moth, whose caterpillars leave a tent of silky threads behind, when they hatch. Many smaller insects, such as Greenfly, find Sallow a suitable food plant and consequently a variety of insect-eating birds can be seen feeding around these bushes. Two nationally rare species of Soldier Fly are also present.