The Ecology of Portsdown Hill SSSI
Today Portsdown Hill SSSI has a mosaic of vegetation types. The vegetation ranges from fine species-rich chalk grassland (about 5%) to woodland (about 10%), with scrub and coarse grassland making up the rest, (see fig. 5). The chalk downland species tend to be associated with nutrient-poor areas where vigorous plant growth is limited, i.e. with thin topsoil on banks or old trackways. With little competition between plants on these stressful sites many plants that are adapted to these conditions can grow together. Where topsoil is thicker the more competitive species take over. This tends to lead to the dominance of only a few vigorous species.
Although the short grassland contains more species than the other habitats, tall grass and scrub add to the site's overall diversity. Each habitat supports a range of associated species and many animals exploit different habitats at different parts of their life cycle. For instance the larvae of many beetles live in wood. Whilst the adults forage for nectar and pollen in flower-rich grassland. Even specialist downland insects benefit from the shelter provided by taller vegetation.
Today, where the topsoil is thin the vegetation is short and diverse and where the soil is deeper the vegetation is taller and rather species poor, however it has not always been like this. Fifty years ago and for centuries beforehand short species-rich grassland dominated the whole area despite the fact that the same variations in soil depth occurred. This was due extensive grazing. Ongoing removal of leaves by livestock is another sort of stress that the specialist downland plants were able to cope with but nutrient demanding plant species were not. Therefore, the proportion of each type of vegetation type changes with management as well as soil depth.
Grazing livestock also remove nutrients as they graze, thus favouring the downland species. Although they return some nutrients back to the ground in the form of dung, they incorporate most into their bodies. When they leave they take the nutrients with them. The dung can be seen as a form of modified compressed vegetation from which many nutrients have been extracted. The dung itself is an essential habitat to many uncommon insects, e.g. the Hornet Robberfly.
Myxomatosis This disease is a rabbit-specific influenza from Brazil. It was introduced to Britain in the 1950's in an attempt to control the massive rabbit population. At the time there were 100 million rabbits in Britain -myxomatosis killed over 99% of them.
When cattle grazing finished in the 1950s it coincided with the loss of the rabbit population due to the disease, myxomatosis. This meant that the grazing pressure was suddenly lifted. Since then the fine downland turf has been gradually invaded by tall grass and bushes, except where the soil is still too poor for the nutrient demanding vegetation to grow. At the current rate of expansion it will take only 20 or so years without management before these areas are covered in bramble and clematis and the entire hill is covered in scrub.
Chalk downland soil - a difficult place for plants to grow
Chalk downland soil is strongly influenced by (surprise!) chalk - a soft white limestone rock. Chalk contains few nutrients and rainwater runs through it easily. This means plants find it difficult to grow vigorously on chalk because they are limited by a shortage of water and nutrients.
An additional stress that downland plants face is grazing. Livestock eat leaves and in so doing not only suppress the vegetation but also remove nutrients.
What this means is, the growing conditions found on traditionally managed downland prevents vigorous plants from dominating. Many small slow growing plants can co-exist in these conditions and because they are not competing with each other they can grow close together. The result a rich, diverse habitat.
In the absence of grazing organic material builds up from the accumulated leaf litter. This retains water and nutrients and thus promotes those plants that can take advantage of the increased fertility. Grazing intolerant species can also move in.
In the last decade a programme of scrub clearance and grazing has reversed expansion and downland loss. In several parts of the hill winter grazing with cattle has suppressed the rank grass and allowed downland species to increase.