The UK Habitat Action Plan for species-rich hedgerows defines them as hedges "which contain five or more native woody species in a 30 metre length". Hedges that contain less than 5 woody species (trees and shrubs) in each 30m, but have a rich variety of herbaceous plants at their base, are also included. Hedges which consist mainly of privet, yew or non-native trees (including beech) are excluded.
Hedges are important for many landscape, cultural, agricultural and archaeological reasons. They perform a variety of farming functions such as protecting and enclosing animals, and act as windbreaks that help control soil erosion.
From a biodiversity point of view, hedges are vital habitats and sources of food and shelter for wildlife, as well as providing corridors to help wildlife move around.
There are regional variations in the ecological value of hedges. Over half of the hedgerows in Northern Ireland are species-poor. They are dominated by hawthorn and common in lowland areas. Hedges dominated by gorse are common in the Mourne and Sperrin mountains. Species-rich hedges in County Fermanagh are dominated by hazel, willows and blackthorn. Fermanagh hedges were often associated with a ditch which helps to increases the overall species diversity within and around the hedge.
The need to help wildlife contend with climate change means that there is an increasing focus on improving existing habitats by increasing their size and protecting them from damaging activities on adjacent land. Butterfly diversity tends to be greatest along hedgerows that are high and wide, as well as being rich in plant species. The large number of animals and plants found in hedgerows reflects the complexity of the habitat. Hedgerows include elements of woodland especially woodland edge and scrub, and also of grassland.
Hedgerows are a very old way of identifying boundaries. A basic pattern of hedges and fields was established in Britain in about 1000 years ago and in some areas has changed very little.
Although Northern Ireland has the highest density of field boundaries in the UK, hedges in Ireland are generally much younger than in Britain. The majority being planted between 1750 and 1850, and often with mixed species. Townland hedges are considered the oldest, most ancient, hedge types in Ireland. They generally have a greater variety of tree and shrub species and are associated more with woodland herbs.
Hedgerows are rich habitats for all kinds of wildlife including plants, birds, mammals and insects. Over 170 species of trees, shrubs and flowers have been found in hedges here in Northern Ireland.
Hedgerows provide valuable nesting habitat and song posts for breeding birds. In Northern Ireland 36 bird species regularly rely on hedgerows for breeding, shelter and feeding and more bird species occasionally use hedges amongst other habitats for various purposes. Wood mice (also known as field mice in Northern Ireland) use hedges for cover and move long distances along them.
Other mammals found in or around hedges include foxes, badgers, rabbits, stoats, hedgehog, Irish hare and bats. Hedgerows act as corridors for many species, including reptiles (the viviparous lizard is Northern Ireland’s only land-based reptile) and amphibians, allowing dispersal and movement between other habitats.
Hedgerows are the primary habitat for at least 47 species which are causing conservation concern in the UK many of which are included within the UK Biodiversity Action Plan as well as being specifically protected in UK and EU law.
Although hedgerows in Northern Ireland have not been removed on the same scale as those in Britain, there are a number of reasons why hedgerows are under threat. Lack of time and labour, as well as an increase in costs are the primary reasons for farmers not implementing good management of hedgerows.
Increased stocking rates, particularly of sheep, can lead to hedgerow damage and the need to fence fields. Neglect (for example, not cutting a hedge) can lead to hedgerows changing into lines of trees and the development of gaps. Using incorrect machinery, as well as cutting too often and at the wrong times, can also lead to poor hedgerow structure.
The use of chemicals (fertilisers, herbicides and pesticides) right up to the bases of hedgerows can lead to nutrient enrichment and a decline in species diversity. Hedges have been removed for many reasons, including for agricultural and development purposes, for power lines and road re-alignment and the construction of sight lines.
Lack of traditional hedgerow management such as coppicing or laying has led to hedges growing tall or becoming gappy, though this trend is now being reversed through new incentives for positive management.