Action: Connect areas of natural or semi-natural habitat
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- All four studies (including one site comparison and two replicated trials) from the Czech Republic, Germany and the Netherlands investigating the effects of habitat corridors or restoring areas of natural or semi-natural habitat between existing patches found some degree of colonization of these areas by invertebrates or mammals. However for invertebrates one unreplicated site comparison reported that the colonization process was slow (Gruttke 1994), and three studies found that the extent of colonization varied between invertebrate taxa.
- One small, replicated study from the Czech Republic investigated colonization of two bio-corridors by small mammal species. It found more small mammal species in the bio-corridors than in an adjacent forest or arable fields.
- All three studies from Germany and the Netherlands looking at the effects on invertebrates found mixed results. One replicated study found more species of some wasps (cavity-nesting wasps and caterpillar-hunting wasps) in grass strips connected to forest edges than in isolated strips. An unreplicated study found that the abundance of three ground beetle species substantially increased in an arable field undergoing restoration to heathland but that typical heathland species failed to colonize over the 12 year period. One study found that two out of 85 ground beetle species used a meadow and hedge-island strip extending from semi-natural habitats into arable farmland. In the same study the habitat strip did not function well for ground beetles and harvestmen but was colonized by snails and spiders.
This intervention involves the creation of habitat corridors between currently isolated natural/semi-natural habitats or the restoration of natural/semi-natural habitats between existing patches.
Habitat fragmentation, as well as destruction, may be an important driver of population declines. Small areas hold fewer species than large ones and if individuals are unable to cross areas of converted habitat then populations in separate habitat patches will become isolated. This potentially makes them more vulnerable to extinction, from natural variations in birth and death rates or sex ratios, from inbreeding depression and from outside pressures; both natural (such as storms or wildfires) and man-made (such as hunting or continued habitat loss). However the precise effects of habitat fragmentation, as opposed to loss, are debated (e.g. Fahrig 1997).
Theoretically, the number of species surviving in a habitat fragment is determined by its size and its effective distance to other habitat patches (MacArthur & Wilson 1967). Connecting remaining areas of natural or semi-natural habitat is therefore often seen as a way to increase the viability of populations, but there is considerable debate as to the effectiveness of such ‘wildlife corridors’ (e.g. Beier & Noss 1998).
MacArthur R.H. (1967) The Theory of Island Biogeography. Princeton University Press, Princeton, New Jersey.
Fahrig L. (1997) Relative effects of habitat loss and fragmentation on population extinction. The Journal of Wildlife Management, 61, 603–610.
Beier P. & Noss R.F. (1998) Do habitat corridors provide connectivity? Conservation Biology, 12, 1241–1252.
Supporting evidence from individual studies
An unreplicated habitat restoration study from 1973 to 1984 on a heathland reserve in the Netherlands (van Dijk 1986) found a substantial loss of ground beetle (Carabidae) species in an ex-arable field undergoing restoration to heathland over the 12-year period. Many of the ground beetle species that disappeared or decreased were able to disperse and capable of flight. The adjacent heathland and a young coppiced oak forest did not lose any species characteristic of their respective habitats over the same period. The numbers of several ground beetle species (Amara communis, Pterostichus versicolor, A. lunicollis) increased substantially in the field over the 12 year period, and the authors attribute this increase to the restoration process, which involved management to promote nutrient impoverishment of the soil. A small group of species that favour dense heather (Calluna spp., Erica spp.) vegetation and that were found in the adjacent heathland had not colonized the restoration field by the end of the study. Cultivation of the ca. 5 ha field ceased in 1972, prior to which it had mainly been used for growing wheat. The vegetation was thereafter mown annually and the cuttings removed in order to impoverish the soil. Sets of three pitfall traps (25 x 25 cm, 10 m apart) were established in the restoration field, the heath, the forest and a 3-4 m wide sand bank running between the field and the forest. Ground beetles were sampled weekly throughout the year for 12 years.
An unreplicated site comparison study from 1982 to 1991 in western Germany (Gruttke 1994) (same study as (Gruttke & Willecke 2000)) found that out of 85 ground beetle (Carabidae) species sampled, only two used a young habitat strip as a dispersal corridor. The two ground beetle species (Carabus nemoralis and Notiophilus palustris) which appeared to use a meadow and hedge strip as a dispersal corridor were initially present in the semi-natural source habitat and gradually appeared along the strip over the 9 years following planting (1982 to 1990). Although three other ground beetle species also immigrated to the corridor, they were able to fly, so the linear shape of the habitat was unlikely to be important to them and it could not be confirmed that they originated from the studied source habitat. Twenty-five ground beetle species present in the source habitat showed no tendency to disperse to the corridor. The corridor was established in 1982, consisting of a 1.6 km-long, 10 m-wide meadow strip, along which nine 400 m2 hedge islands were planted as stepping stones. It was attached at one end to an area of old mixed semi-natural habitat (woods, hedge fragments, ponds surrounded by small reeds and wet and dry meadows) and extended into intensive arable farmland. Ground beetles were sampled along the corridor using six pitfall traps 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 replicated study from 1992 to 1996 in an arable area in the Czech Republic (Bryja & Zukal 2000) found that from the third year after planting, two bio-corridors (10 m-wide, planted with trees and shrubs) had more small mammal species and individuals than two adjacent fields or a forest. The bio-corridors had eight small mammal species (supporting both field and forest species) and 128-143 captures compared to five species and 47-68 captures in fields (maize and wheat) and 66 captures in the forest. The mammal community in the forest differed from that of the bio-corridors and fields, where wood mouse Apodemus sylvaticus and common vole Microtus arvalis tended to dominate. During the autumn (from 1994), the wood mouse population peaked in bio-corridors, but few were caught in (bare) fields. The two bio-corridors were planted in 1991, one extended perpendicular to a forested area into an arable field and the second extended from the end of the first bio-corridor further into the crop. They were fenced and ploughed in the first years after planting to allow short-lived weeds to grow in the herb layer. Fifty snap traps were set in a 150 m line in each habitat and left for three nights twice in spring and autumn from 1992 to 1996 and in summer 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 meadow and hedge-island 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 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 replicated study in 2004 in Lower Saxony, Germany (Holzschuh et al. 2009) found that the numbers of cavity nesting wasp (Hymenoptera) species, brood cells and caterpillar-hunting wasp (Eumenidae) brood cells in trap nests, were higher in grass strips connected to forest edges than in trap nests in isolated grass strips. The number of wasp species was significantly higher in connected (2.3 species) than in highly isolated grass strips (0.8), differences were not significant between connected and slightly isolated (1.2) or between slightly and highly isolated strips. Numbers of wasp brood cells were significantly higher in connected (30 brood cells) than slightly (7) and highly isolated grass strips (4), caterpillar-hunting wasps showed the same pattern. Numbers did not differ between strip types for spider-hunting wasps (Sphecidae), species richness of parasitoids or numbers of parasitized brood cells. At each of 12 arable sites, 9-12 traps were placed in three types of 3 m-wide grass strip: ‘connected’ strips connected via a corridor to a forest edge (traps set 200 m from forest), ‘slightly isolated’ strips separated from forest by a cereal field (traps 200 m from forest, no connecting corridor) and ‘highly isolated’ strips 600 m from the nearest forest edge (no connecting corridor). Distances between trap nests were at least 600 m. Trap nests consisted of four plastic tubes filled with common reed Phragmites australis sections (2-10 mm diameter) and were installed at a height of 1.0-1.2 m from April-September 2004.
- van Dijk T.S. (1986) Changes in the carabid fauna of a previously agricultural field during the first twelve years of impoverishing treatments. Netherlands Journal of Zoology, 36, 413-437
- 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.
- Bryja J. & Zukal J. (2000) Small mammal communities in newly planted biocorridors and their surroundings in southern Moravia (Czech Republic). Folia Zoologica, 49, 191-197
- 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.
- Holzschuh A., Steffan-Dewenter I. & Tscharntke T. (2009) Grass strip corridors in agricultural landscapes enhance nest-site colonization by solitary wasps. Ecological Applications, 19, 123-132