The Ecology of Limestone Grassland on Portland
Introduction
Portland from the north
The island of Portland is a wedge-shaped mass of limestone projecting out into the English Channel. The island’s maritime location excludes much of the wildlife to be found in more sheltered parts of the county. Many species of trees that are common elsewhere in Dorset, such as Oak, Field Maple and Hazel are not found on the island and there are few wild mammal species present. The alkaline nature of the island’s limestone soil excludes any of Dorset’s famous heathland flora from growing, with associated birds such as Dartford Warbler and Nightjar only occurring on migration. However, this same thin, alkaline soil is also the source of Portland’s limestone grassland habitat which today supports such a huge diversity of flowers and insects.
Strip field with herbivore
History
In prehistoric times Portland would presumably have had far more tree cover than today, although probably not the continuous Oak-dominated woodland that would have covered the rest of Dorset. Whatever tree cover there was would have been removed by the earliest inhabitants, leaving the landscape open and suitable for cultivation. By the time of the Norman Conquest sheep farming had become the most important form of agriculture, as recorded in the Domesday Book. The continuous grazing of herbivores since then has given rise to the short, herb-rich turf that still dominates much of the island today, although the herbivores are now smaller and have longer ears!
The importance of low fertility
Ivy Broomrape and Horseshoe Vetch
Sheep-grazing on Portland and other areas of limestone grassland, such as the South Downs, resulted in a lowering of the fertility of the underlying soil. This was due to the constant removal of nitrogen as the sheep ate the grasses and the proteins contained in them. The traditional practice of penning the sheep in at night would have increased this effect by reducing the amount of nitrogen returned to the soil in the sheep’s excrement. The result was a soil high in minerals (from the limestone below) but low in the nitrates necessary for rapid plant growth. A wide variety of small, low-growing plant species were able to grow in these conditions, providing the sheep with sufficient nutrients as long as the sheep were kept at a relatively low density. In these conditions low-growing plants were able to flower and set seed, the most successful doing so in a short space of time thus reducing the risk of being eaten by a sheep. This allowed a large number of species to grow together and gave rise to the high level of plant diversity that is found today in the grassland around Portland’s cliffs and quarries.
Silver-studded Blue
The insects found on these plants are those already dependent on the particular conditions produced by such short turf. Butterflies such as the Silver-studded Blue benefit from being so close to the warmth of the soil and are able to complete their life cycle faster than if forced to live in higher-growing plants. Ants are also abundant in this warm, thin soil, and supplement their diet with sugary secretions produced by the butterfly larvae. In return the ants protect the caterpillars and carry the pupae into their nests to spend the winter away from predators and frosts.
Skylark in farmland
If this soil is ploughed and fertilised, as has happened over much of the Dorset Downs, the whole system is destroyed and will not return to its former state until the nitrogen level is low again, which may take a great many years. Fertilised soils encourage the growth of fast-growing species such as coarse grasses, thistles, docks and nettles but hold a far lower number of species than before. The height of the vegetation results in shaded, cooler soils unsuitable for blue butterflies and the ants they depend on. On the other hand some species prefer these conditions; for example Red Admiral butterflies feeding on nettles and Skylarks nesting in the long grass.
Before and after a natural restoration
Restoring land after quarrying
The bare rock and rubble left when a limestone quarry ceases operation is naturally low in nitrogen. The lack of any soil cover inhibits plant growth at first but the action of the weather and pioneer species such as Teasel and Viper’s Bugloss soon starts to produce a thin, nitrogen-poor soil. Within a few years many of the species previously found here will return as seeds are blown in from the surrounding areas. This process can be seen in process in many parts of the island, with butterflies generally returning to feed on the plants after 10 - 15 years.
The desire to speed up this process has led to the use of seed mixtures obtained from seed suppliers. Whilst such mixes may contain the same species that grow on Portland, the seed is often obtained from faster-growing, more vigorous plants. The result is a landscape that becomes quickly green, but does not support the butterflies that need the warmth provided by plants growing close against the soil. Eventually these introduced forms are likely to be replaced by the better-adapted local strains, but the natural colonisation process will have only been slowed down by introduction of non-native seed.
Black Mustard
Any topsoil kept on site from when the quarry was opened is likely to be high in nitrogen, particularly if the land was previously farmed. This soil should never be used in areas intended to revert to limestone grassland, but should be kept for areas requiring high fertility, such as playing fields, gardens and for tree planting. Agricultural soils on Portland also contain a high level of Black Mustard seed, a plant used in crop rotation as a “green manure”. This vigorous plant forms an uninterrupted stand a metre high, preventing any other plants from growing.
Bee Orchid
A more environmentally friendly restoration method would be to use seed collected locally and broadcast directly onto the limestone rubble, without any introduction of soil. Sections of turf could also be imported from nearby areas, providing species-rich islands from which plants could spread out. As it would not be desirable to harm previously undisturbed areas this turf should be obtained from land destined for development such as new quarries or road schemes. A third option would be to collect local seed in advance and produce plants for planting out at a later date. It is intended that all three methods are to be tried in the restoration of Independent Quarry.
Bob Ford, April 2003