Action: Plant new hedges
- Two studies from France and the UK compared newly planted hedges with control areas. Both (including one replicated trial) found newly planted hedges had higher abundance, species richness or diversity of beetles or spiders than crop fields or field margins. The replicated study also found vascular plant species diversity and grass species richness were higher in newly planted hedges than recently established grass field margins. A review found newly established hedges supported more ground beetles than older hedges.
- A small-scale study from the UK found that local hawthorn plants exhibited better growth and were more stock proof than those of eight other provenances. A literature review found lower pest outbreaks in areas with new hedges. A replicated study in the UK found that the diversity of arthropods supported by newly planted hedges varied between seven different plant species
- An unreplicated site comparison study in Germany found that two out of 85 ground beetle species used newly planted hedges as stepping stones for dispersal. Results from the same study found that invertebrates that moved passively (attached to mammals and birds), such as snails, benefited most from the hedge-islands compared to actively moving ground beetles and harvestmen.
Agricultural intensification, which has included increasing field size and pesticide use, has resulted in a loss of field margin habitats, such as hedgerows. These features can provide a relatively undisturbed habitat for wildlife in intensively managed agricultural landscapes. Hedge planting has therefore been investigated to determine whether it can enhance biodiversity; evidence to date focuses mainly on invertebrates.
Supporting evidence from individual studies
An unreplicated site comparison study from 1982 to 1991 in western Germany (Gruttke 1994) (same study as Gruttke & Willecke 2000) found that only two ground beetle (Carabidae) species (out of 85 sampled) used a sequence of young hedge plantations as stepping stones for their dispersal. Two forest or forest-edge ground beetle species, present in nearby semi-natural habitat, gradually appeared along a meadow and hedge strip over the nine years following hedge planting (1982 to 1990). Twenty-five ground beetle species from the semi-natural habitat showed no tendency to use the hedge plantations as stepping stones. In 1982, nine small hedge islands (each 400 m2) were planted at intervals along a 10 m-wide meadow strip, attached at one end to mixed wooded and open semi-natural habitats (woods, hedge fragments, ponds surrounded by small reeds, wet and dry meadows), and extending 1.6 km into arable fields. Ground beetles were sampled using six pitfall traps/section in hedge islands and meadow strips from 1982 to 1990. Semi-natural habitats and adjacent arable fields were sampled from 1990 to 1991.
A small-scale study in 1996 in France (Fournier & Loreau 1999) found that ground beetle (Carabidae) diversity declined with distance from a newly planted hedge in intensive arable farmland. Rare ground beetle species decreased and the most abundant species Pterostichus melanarius became more dominant with distance from the hedge. The hedge was planted in 1995 and comprised two 200 m sections of shrubs divided by a 100 m section of mixed fodder crop (oats and cabbages). It was separated from the adjacent barley crop by a 9 m-wide zone planted with oats and sorghum. Ground beetles were sampled using pitfall traps in the hedge (15 traps) and at 10-110 m from the centre of the hedge (four traps at each of five distances). Traps were emptied every 2-4 weeks (April to mid-October 1996). Fenced pitfall traps (12 in the hedge and three 110 m into the crop) were used to estimate absolute densities and were emptied every day for eight days in June 1996.
A 1999 review of literature (Kromp 1999) found two unpublished studies showing that newly planted hedges supported field species of ground beetle (Carabidae). In one study, the youngest hedge, three years old, had more ground beetles than 5-, 9- or 40-year old hedges. Another study in Germany showed that newly planted hedges linking patches of semi-natural habitat were not used as corridors by forest or openland ground beetle species (Gruttke 1994).
The same unreplicated site comparison study as (Gruttke 1994), between 1982 and 1998 (Gruttke & Willecke 2000) found marked differences in the effectiveness of the hedge-island and meadow habitat strip as a dispersal corridor for four invertebrate taxa: ground beetles (Carabidae), harvestmen (Opiliones), spiders (Araneae) and snails (Gastropoda). Nine years after planting, the hedge-island and meadow strip did not (or not yet) function well as a dispersal corridor for ground beetles or harvestmen. Snails were the best colonizers, with the highest proportion of species migrating to the strip, including target woodland species. The authors suggest that passive travel by small snails on mammals or birds may have contributed to this. Spiders also had a high proportion of immigrating species, but many of them were not present in the source habitat and may have passively ‘ballooned’ in from the surrounding area, rather than using the strip as a dispersal corridor. The authors conclude that while the hedge islands appear to be working as stepping stones for species able to travel passively, this is not true for actively moving invertebrates, such as ground beetles or harvestmen, perhaps because of the age, size or connectedness of hedge islands at the time of study. In addition to the sampling regime described in (Gruttke 1994), invertebrates were sampled from the surrounding area in 1992-1994 and 1997-1998. Spiders, harvestmen and ground beetles were sampled using pitfall traps and snails were sampled by flotation (in 1984, 1987 and 1990).
A 2000 literature review (Holland & Luff 2000) looked at which agricultural practices can be altered to benefit ground beetles (Carabidae). It included one study (Fournier & Loreau 1999), which is outlined above, that found a greater diversity of ground beetles near newly planted hedges. Another study (El Titi 1991), of whole farming systems, found lower pest outbreaks in areas with new hedges on farms managed under integrated farming.
El Titi A. (1991) The Lautenbach project 1978-89: integrated wheat production on a commercial arable farm, south-west Germany. Pages 209-231 in: L.G. Firbank, N. Carter, J.F. Darbyshire and G.R. Potts (eds.) The Ecology of Temperate Cereal Fields, Blackwell, Oxford.
A replicated study in 1998 and 1999 in mid-Wales (Hayes et al. 2001) found that seven species planted in two hedgerows in semi-upland farmland supported significantly different numbers of arthropods: common gorse Ulex europaeus (1007 arthropods), sessile oak Quercus petraea (436), blackthorn Prunus spinosa (381), hawthorn Crataegus monogyna (258), silver birch Betula pendula (180), rowan Sorbus aucuparia (110) and ling heather Calluna vulgaris (53). Sessile oak was the most diverse in terms of arthropod orders, with 13 out of 15 orders recorded, two of which were not found on any other host. Hawthorn and common gorse were the next most diverse, each with one unique arthropod order. Common gorse, sessile oak, blackthorn and rowan between them had representatives of all 27 families of beetles (Coleoptera), true bugs (Hemiptera) and moths and butterflies (Lepidoptera) recorded in the study. All planted species had a similar or better growth rate than the commonly planted hawthorn, apart from sessile oak and ling heather. Planting was undertaken in 1996 within the fenced (2 m-wide) margins of two fields. Margins were divided into eight 6 m plots, which were planted with a double row of 30-40 plants of each species, replicated across three blocks. Invertebrates were sampled by tree beating at five points/plot in June, August and September (1998-1999).
A randomized, replicated small-scale study from 1995 to 1997 in mid-Wales (Jones et al. 2001) found that hawthorn Crataegus monogyna plants, propagated from seeds sourced from a local hawthorn population (local provenance), exhibited better growth and had a more stock proof growth form than those of eight other provenances. Local plants had the latest bud-burst, least severe mildew symptoms and more thorns compared to those of other provenances (four British, four continental European). Hawthorn of local provenance grew tallest at the upland site, but was relatively slow-growing at the lowland site. In terms of establishment, fenced plots had lower hawthorn mortality than unfenced, sheep-grazed plots (4% vs 100% mortality respectively) at the upland site. Mortality was low at the lowland site (fenced: 1%, unfenced: 3%). Fenced plots with mulching had approximately 320% greater growth than unmulched sections. One experimental hedge was established at one upland and one lowland site, both sites were grazed by sheep. The two hedges had three replicate blocks of four 10 m strips that were either fenced, mulched or fenced and mulched. Within each strip, nine plants of each provenance were planted in a random order in 1995. Each plant was measured (February 1995-1997), scored for powdery mildew Podosphaera clandestina (July and August) and the date of bud burst was recorded (1995-1996).
A replicated study in winter 2002 in Oxfordshire, UK (Pywell et al. 2005) found that the total abundance, species richness and diversity of beetles (Coleoptera) and spiders (Araneae), as well as abundance and species richness of rove beetles (Staphylinidae) was higher in hedge bases than in field margins, but there was no difference between recently planted (2-5 years old) and mature hedgerows (40-60 years old). Grass cover was lower, but the number of grass species higher, in the bases of recently established hedgerows compared with recently sown grass margins (3-4 years old). The diversity of vascular plant species was greater in recently established and mature hedgerows, as well as mature field margins (ca. 50 years old) compared with recently sown grass margins. The bases of recently planted hedgerows had fewer vascular plant species and lower cover of tall perennial wildflowers and mosses compared with mature field margins. Five geographically separate replicates of each of the four habitats were sampled for beetles and spiders in February 2002 by taking 12 soil core samples in a 70 m-long sampling section. Percentage cover of vascular plant species, moss and bare ground was estimated, and biomass (dry matter) and organic carbon content were measured.
- Gruttke H. (1994) Dispersal of carabid species along a linear sequence of young hedge plantations. Pages 299-303 in: K. Desender (ed.) Carabid beetles: ecology and evolution. Kluwer Academic Publishers, The Netherlands.
- Fournier E. & Loreau M. (1999) Effects of newly planted hedges on ground-beetle diversity (Coleoptera, Carabidae) in an agricultural landscape. Ecography, 22, 87-97
- Kromp B. (1999) Carabid beetles in sustainable agriculture: a review on pest control efficacy, cultivation impacts and enhancement. Agriculture, Ecosystems & Environment, 74, 187-228
- Gruttke H. & Willecke S. (2000) Effectiveness of a newly created habitat strip as dispersal corridor for invertebrates in an agricultural landscape. Environmental Encounters Series: Workshop on ecological corridors for invertebrates: strategies of dispersal and recolonisation in today's agricultural and forestry landscapes, Strasbourg, 67-80.
- Holland J.M. & Luff M.L. (2000) The effects of agricultural practices on Carabidae in temperate agroecosystems. Integrated Pest Management Reviews, 5, 109-129
- Hayes M.J., Jones A.T., Sackville Hamilton N.R., Wildig J. & Buse A. (2001) Hedgerows of the World: Their Ecological Functions in Different Landscapes. 10th Annual Conference of the International Association for Landscape Ecology, Birmingham, UK, 339.
- Jones A.T., Hayes M.J. & Hamilton N.R.S. (2001) The effect of provenance on the performance of Crataegus monogyna in hedges. Journal of Applied Ecology, 38, 952-962
- Pywell R.F., James K.L., Herbert I., Meek W.R., Carvell C., Bell D. & Sparks T.H. (2005) Determinants of overwintering habitat quality for beetles and spiders on arable farmland. Biological Conservation, 123, 79-90