The wildlife value of hedges

Forests once covered most of Britain, and because of this, the country’s flora and fauna is mainly of woodland origin, and has had to adapt as best it can to the disruptive effects of man’s clearances.

Hedgerows provide a partial substitute for lost woodlands. They are essentially woodland edges without the woods, forming an ‘ecotone’ or border area between two other ecosystems. At their best, hedgerows can share the creatures of both other systems and be doubly rich in wildlife. On many farms they form the major non-crop habitat. However, many hedgerows today have become gappy and bare at the base due to poor management, with low floristic diversity and limited wildlife value.

Hedge shrubs

The most widespread hedge shrubs, hawthorn and blackthorn, are essentially scrub plants adapted to the colonisation of open lands. Hawthorn has been widely planted for hedgerow creation. The presence of woodland species may indicate the hedge is of woodland origin. In ancient hedgerows, the shrubs and trees themselves may represent important genetic reservoirs, linking back to the ancient woodlands from which they came.

Hedges in arable land tend to have a low number of woody species. This may be due to over-frequent trimming and use of herbicides, or due to lack of nearby seed supply from woodlands and other hedgerows. Because of this tendency, arable hedges with few woody species may be older than pasture hedges with more woody species.

Hedgerow shrub species are described in detail here.

Herbaceous flora of hedgerows

The herbaceous flora growing under and to the side of hedgerows have certain characteristics not commonly found elsewhere on agricultural land. This strip of non- woody plants is often on a bank, giving better drainage than the surrounding land. South facing banks have higher temperatures than the adjacent field. The hedge provides shelter and varying degrees of shade. Herbaceous flora in arable fields is not subject to cutting or grazing during the main growing season, although it may be treated with herbicide early in the year.

Hedgerows have a characteristic community of herbaceous hedgerow plants which, although containing few rarities, is distinctive in the combination of species, the structure of the vegetation and the prevailing microclimate. The species composition of hedgerow ground flora is mostly related to adjacent land use, rather than to the shrubby species in the hedge, or to the hedge management regime. In one survey (Cummins and French, in Watt and Buckley, 1994) only 15% of the hedgerow flora was associated with woodlands, the remainder being species typical of grassland or abandoned ground. Only mixed hazel hedges were associated with the woodland type of ground flora. The most species-rich hedge-bottoms are those next to grasslands, the poorest being those adjacent to arable crops. A trend towards reduced diversity of herbaceous species occurred between 1978-90, reflecting a further move towards impoverished ‘arable’ type hedges (Boatman et al, in Watt and Buckley, 1994).

Hedgerows next to arable fields typically have a ground flora which is impoverished due to the impact of agricultural operations in the adjacent field. It is usually dominated by a few species such as nettle (Urtica dioica), oat-grass (Arrhenatherum elatius) and hogweed (Heracleum sphondylium). Also common are the perennials rough- stalked meadow-grass (Poa trivialis), creeping thistle (Cirsium arvense), lyme-grass (Elymus arenarius) and bindweed (Convolvulus arvensis), and the annuals (Bromus sterilis) and cleavers (Galium aparine). The farmer therefore views the hedgerow as a source of weeds, though in using herbicide along the hedge bottom, and by addition of nutrients from mis-applied fertiliser, the likely effect is to further encourage the growth of aggressive, weedy species. Some arable farmers adopt the practice of keeping wider field margins which are regularly mown, so preventing the germination and growth of unwanted species.

The hedgerow flora and field margins of tussocky grasses have an important role in providing overwintering habitat for beneficial insects. Herbaceous hedgerow flora are also important habitats for butterflies, bumble bees and hover- flies. Cover at ground level is essential for many small mammals and reptiles. Butterflies require particular food and egg-laying plants, but for most other organisms, a general habitat giving food, shelter and cover is sufficient, and the species composition is not important.

Birds

The value of a hedgerow to birds depends entirely upon the management of the hedgerow and adjacent crops, and the position of the hedgerow in relation to other habitats in the area. The farmland bird population is very much in the hands of the farmer. Birds are relatively mobile animals, and their chosen home ranges are influenced by the availability of shelter and nest sites, and the relative abundance of organisms lower in the food chain, on which they depend.

The majority of birds in farmland rely on trees and tall hedgerows, and fewer birds are found in intensively managed hedges. Studies have shown that on average there will be twice as many birds in a hedge 13’ (4m) high, than in a hedge less than 6’ (2m) high. The abundance of individual bird species and species numbers are directly related to the amount and type of management the hedgerow receives. Hedges become of much less value to birds and other wildlife where the following occur:

• annual hedge trimming.
• flailing of verges, ditches and banks beyond what is needed to prevent scrub encroachment or ensure efficient drainage.
• the use of herbicides on verges and hedge bottoms.

Birds prefer overgrown, ‘neglected’ hedges, or hedges in the last few years before laying. Newly laid and regularly trimmed hedges, as required by the farmer for ease of management, being proof against stock, and to lessen competition with crops, are of lesser value to birds. The corridor effect is not relevant for birds, as they are mobile. Studies have shown that a heavily trimmed hedge devoid of berries frequently supports fewer birds than a bare grassy verge or ditch with no hedge. There are of course some species such as the skylark, corn bunting, meadow pipit and game birds which require open land, and whose presence does not depend on hedgerows.

In practice, hedgerows are the only remaining semi-natural habitat on many farms, and as such are important for bird populations. With less frequent trimming, management costs can be reduced while increasing their value for birds and other wildlife. The relative importance of hedges for birds increases on farms where there are no other woodlands.

Managing hedges for birds

Pollard, Hooper and Moore (1974) distinguish by management seven types of hedge. Bird species and numbers tend to increase from the first to the last. The types are diagrammed below.

Mixed hedges with plenty of hawthorn are the most favoured by birds. Thorn provides good cover, comes into leaf early in the year, supports an abundant insect life and provides berries.

Game birds

The pheasant (Phasianus colchicus), partridge (Perdix perdix) and red-legged partridge (Alectoris rufa) are important game birds closely associated with hedgerows and hedged fields. Loss of hedgerows has reduced populations of all three, though pheasant populations are maintained by hand-rearing.

Only the partridge is native to Britain, where it depends almost entirely on agricultural land. Partridges usually nest in the hedgerow verges rather than in the hedge bottoms. The width of the verges is more important than the width of the hedge, but the hedge provides necessary cover. Nesting partridges dislike seeing neighbouring pairs, so the more their sight lines are blocked by hedge shrubs, the more nest territories can occur in a given area. The verges provide a vantage point with low cover and a dry, sheltered bank to raise chicks. From the keeper ’s viewpoint, nests concentrated along hedgerows are easy to keep track of and ground predators can be more easily controlled.

Pollard, Hooper and Moore (1974) conclude that while hedges are essential for the survival of many species of birds on individual farms and in large unwooded areas of the countryside, they are not needed for the survival of species in the country as a whole. However, as total loss of woods and copses increases, hedgerows become more important for bird life.

Small mammals

Hedgerows are important habitats for small mammals on farmland. Their value increases on arable land subject to frequent cultivation, which disturbs the field as a habitat.

Shrews and voles use hedgerows as permanent habitats, although they feed in adjacent cereal fields when the crop is tall enough to give cover. Hedgerows also act as important dispersal corridors, especially for the water shrew. Harvest mice use hedgerows as winter refuges.

Wood mice are least affected by the presence of hedgerows, as they are very opportunistic species that adapt well to surviving in open grassland or arable fields without hedges or woodland.

The abundance of small mammals along hedgerows attracts predators, especially weasels. Foxes and birds of prey also exploit this ‘hedgerow harvest’.

Amphibia and reptiles

Hedges, and their associated banks and ditches are important habitats for amphibia and reptiles. Amphibia find damp places in the ditches, hibernation burrows in the banks and warming-up places in the shelter of hedges. Banks may be more important than hedges for reptiles. Studies of vipers in Purbeck, Dorset, showed that they used the banks for hibernation, spring feeding and autumn nesting. Banks were also important migration routes between the dry winter habitat and the wet summer feeding areas. Hedgerow removal would seriously damage the life pattern of these snakes, and possibly other reptiles as well.

Invertebrates

As with birds, most British invertebrates are of woodland origin, and therefore find hedges a relatively good substitute for woods. Hedge shrubs come into leaf early in the year, well before forest trees such as oak, elm and beech, and this early growth combined with early flowering provides a bonanza for spring insects. The number of invertebrate species which feed on a particular shrub or tree species has been shown to relate to the length of time the tree has been part of the native flora, and its relative post-glacial abundance.

Hedgerow shrubs are obviously important for insects, even given the species overlap between different plants. Over a hundred types of moth feed on hawthorn alone, and nearly as many utilise blackthorn. Invertebrates which depend on hedgerow buds and flowers often suffer from late winter or spring trimming. The best hedges for invertebrates are those which are overgrown.