Action: Raise water levels in ditches or grassland
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- Seven studies from Denmark, the Netherlands and the UK (two replicated controlled studies and two before-and-after studies) found that raising water levels in ditches or grassland was associated with increased bird numbers, breeding bird numbers, plant species that favour wet conditions, and invertebrate numbers or biomass in agricultural landscapes.
- Two replicated studies from the Netherlands and the UK found that raising water levels resulted in a net loss of plant species and did not affect lapwing foraging rate. A review found three studies reporting that re-wetting soils on old arable fields is not an effective method of reducing nutrient levels and restoring species-rich grassland. A replicated study from the UK found that unflooded pastures contained a high biomass of soil macroinvertebrates of importance to breeding wading birds.
- A controlled, randomized study from the Netherlands found that raising the water level resulted in a more rapid establishment of species typical of wet grassland, than vegetation management. A review of agri-environment schemes from the UK found studies that suggested more expensive agri-environment scheme options for wetland habitats, such as controlling water levels, were more effective at providing good habitat for wading birds than easier-to-implement options.
Wet habitats have been lost from agricultural systems as a result of filling ditches, loss of ponds and removal of water from fields by surface run-off and extensive under-field drainage. These habitats provided important resources for a variety of species and re-wetting may offer an important mechanism for facilitating a reverse in the decline of birds and other farmland biodiversity.
Studies which raise water levels along with a combination of other measures, such as vegetation management, to restore wet grassland are included in the intervention ‘Restore or create traditional water meadows’.
Supporting evidence from individual studies
A replicated trial from 1987 to 1989 on what was once a species-rich wet meadow at the Veenkampen, near Wageningen in the Netherlands (Berendse et al. 1992) found that areas with raised water levels lost plant species overall, but species favouring wet conditions increased. Cuckoo flower Cardamine pratensis, creeping buttercup Ranunculus repens, water foxtail Alopecurus geniculatus and creeping bent grass Agrostis stolonifera increased on wet and intermediately wet plots, but the total number of plant species fell from around 20 to around 15. Dry plots had 14-18 plant species throughout. The area was divided into three compartments. One had water levels 30-70 cm below the soil surface in summer and 5-40 cm below in winter, like the surrounding farmland. A wet and an intermediate compartment had embankments built and water added. In the wet compartment, summer water levels were 10-50 cm below the surface, and winter levels 0-20 cm below the surface. The intermediate compartment was in between wet and dry levels. Other experimental treatments were tested in these compartments. No fertilizer was applied during the experiment, and plots were mown for hay once or twice each year. Plants were monitored annually, in fifty 0.25m2 quadrats/plot.
A controlled, randomized study of a former improved pasture in the Netherlands (Oomes et al. 1996) found that raising the water level resulted in a more rapid establishment of species typical of wet grassland, than vegetation management (cutting and removing hay; cutting, mulching and leaving hay; topsoil removal to 5 cm followed by cutting and removing hay). In 1985, the water level was raised to its former level in one area (1.5-2 ha), the other area was left dry. Plant species composition was recorded annually (20-50 samples/plot).
A 2000 literature review of grassland management practices in the UK (Wakeham-Dawson & Smith 2000) reported that there were numerous studies detailing the success of providing high ditch-water or water table levels in restoring breeding and wintering bird numbers (e.g. Andrews & Rebane 1994, Evans et al. 1995).
Andrews J. & Rebane M. (1994) Farming & Wildlife: A Practical Management Handbook. Sandy: Royal Society for the Protection of Birds.
Evans C., Street S., Benstead P., Cadbury J., Hirons G., Self M. & Wallace H. (1995) Water and sward management for conservation: a case study of the RSPBs West Sedgemoor Reserve. RSPB Conservation Review, 9, 60-72.
A replicated study in 1993-1995 at 17 UK lowland grassland sites (12 with winter flooding introduced in the previous 1-14 years) (Ausden et al. 2001) found that unflooded pastures contained high biomass of soil macroinvertebrates (mainly cranefly (Tipulidae) larvae and earthworms (Lumbricidae)) of importance to breeding wading birds. Conversely, grasslands with a long history of winter flooding had a much lower soil macroinvertebrate biomass, comprising mainly a few semi-aquatic earthworm species.
A replicated study from January-March 2002 of 15 northern lapwing Vanellus vanellus chicks on one grassland site in the Isle of Islay, UK (Devereux et al. 2004) found that raising water levels in the grassland did not affect lapwing foraging rate. Foraging rate increased with decreasing vegetation height and was greater in ditches than on rigs. Soil moisture, however, did not significantly affect foraging rate after sward height and rig versus ditch effects were factored out. The timing of fertilizer application (to promote grass growth) and water level in ditches was manipulated at the field scale, which resulted in a range of soil moisture levels and vegetation heights. Water level was controlled through sluiced canals that ran along field boundaries and in-field ditches. The authors point out that spring 2002 was particularly wet and may have confounded any effect of added soil moisture.
A before-and-after study of grazing marshes in eastern England (Smart & Coutts 2004) found that opening up existing footdrains, creating new ones and reconnecting drains to ditches resulted in an increase in breeding wading bird numbers. Northern lapwing Vanellus vanellus numbers increased from 19 pairs in 1993 to 85 pairs in 2003 and common redshank Tringa totanus rose from four to 63 pairs. Numbers of winter wildfowl also increased over the period and changes in vegetation communities to those more tolerant of inundation occurred. In 1993, water levels were raised by 45 cm. From 1995, approximately 600 m of footdrains were opened/year; from 2000 onwards, approximately 2,000 m of footdrains were opened or added. Grazing intensity was also reduced from 1.5-2 head of cattle to 0.7 head/ ha and fertiliser inputs were stopped.
A before-and-after study at Campfield Marsh RSPB Reserve, Cumbria, England (Lyons 2005) found that five years after water levels were raised in August 1995, breeding common snipe Gallinago gallinago and northern lapwing Vanellus vanellus recolonized the site and that, over the reserve as a whole, breeding Eurasian curlew Numenius arquata densities were 5.5 pairs/km² (one of the highest UK breeding densities). Five fields comprising 23 ha of former cattle-grazed, species-poor perennial rye grass Lolium perenne dominated grassland and arable cropland were restored. Over the five years vegetation also shifted towards target plant communities characteristic of wet grassland.
A replicated controlled study of 32 ditches in arable and pastoral land in 2005 in Leicestershire, UK (Aquilina et al. 2007) (same study as (Defra 2007)) found that bunded ditches, which dammed water, had significantly greater invertebrate biomass than controls (dry weight: 10 g/m² vs 4 g/m²). Invertebrate families other than flies (Diptera) showed a more mixed response to bunding. Ditches were bunded (small dams placed across ditches) and slightly widened in 5-20 m lengths, with equal length control sections approximately 50 m upstream. Five insect emergence traps (0.5 mm mesh, surface area 0.1 m²) were spaced along each section. Samples were collected every two weeks (April-August 2005), invertebrates identified to family and recorded as biomass estimates.
A replicated, controlled (paired) study of wet pasture and drainage ditches in arable and pastoral areas in Leicestershire, UK (Defra 2007) (same study as (Aquilina et al. 2007)) found that wetting-up resulted in higher invertebrate and bird numbers. The following were significantly higher in bunded (dammed ditches) compared to non-bunded ditches: bird visit rates (1.0 vs 0.5 visits/month), emergent aquatic insect biomass (1,400 vs 900 individuals/m²), surface-active flies (Diptera) adults (in arable ditches in 2005; 85-100 vs 60-65/sample) and fly larvae and butterfly/moth (Lepidoptera) larvae (in pastoral ditches in 2006). There was no difference for invertebrates active in the grass layer. Vascular plant species richness was lower and bare ground cover higher in bunded ditches than controls in 2005 due to disturbance. In wet pasture, bird visit rates were significantly higher (livestock: 0.26, livestock-excluded: 0.17 visits) than in control dry plots (livestock: 0.10, livestock-excluded: 0.06). Sampling involved bird observations (45 minutes, 1-2/month; both features), fixed/floating traps for emergence of aquatic insects (ditches), pitfall traps and sweep-netting for terrestrial invertebrates (ditches) and a botanical quadrat survey (0.25-0.5m²; ditches). Data was obtained between April 2005-March 2007; birds all year, other groups spring-summer.
A 2007 review of experimental evidence on how to restore species-rich grassland on old arable fields (Diggelen 2007) found three studies showing that re-wetting soils rich in organic matter works only a little (around 20% less available nitrogen - Oomes 1991, Berendse et al. 1994) or increases nutrients (20% increase in available nitrogen - Eschner & Liste 1995).
Oomes M. J. M. (1991) Effects of groundwater level and the removal of nutrients on the yield of non-fertilized grassland. Acta Oecologia, 12, 461-470.
Berendse F., Oomes M. J. M., Altena H. J. & De Visser W. (1994) A comparative study of nitrogen flows in two similar meadows affected by different groundwater levels. Journal of Applied Ecology, 31, 40-48.
Eschner D. & Liste H. H. (1995) Stoffdynamik wieder zu vernassender Niedermoore. Zeitschrift fur Kurlturtechnik und Landentwickung [Substance dynamics in the fens after rewetting], 36, 113-116.
A replicated, controlled study in 1999-2001 and 2004-2005 in Jutland, Denmark (Kahlert et al. 2007) found that permanent grassland fields under an agri-environment scheme designed to increase water levels had significantly higher numbers of three species of wading bird (northern lapwing Vanellus vanellus, black-tailed godwit Limosa limosa, common redshank Tringa totanus) after the scheme was implemented (2004-2005), compared to numbers before the scheme (1999-2001). However, this was only the case for fields that successfully retained water (40 breeding pairs of northern lapwing before the scheme and 90 after for wet fields vs approximately two pairs before and five after for dry fields). In addition, fields that were dry before the scheme and wet after showed a greater increase in lapwing numbers (280-290% increase) than fields that were wet beforehand (130-170% increase). There were no increases in lapwing numbers on restored grasslands (formerly cropland), whether or not they were under the scheme, or on control fields (i.e. not under the scheme) that failed to retain water. Numbers increased on control fields that retained water, but the numbers found on them were no different from those expected if increases were uniformly distributed across the landscape (i.e. birds did not appear to be selecting the fields preferentially). Eurasian oystercatchers Haematopus ostrolagus did not increase on any field types and the authors note that regional wader numbers were still far lower than in 1978-1988. The scheme involved blocking drainage pipes and ‘rills’ (drainage channels) as well as reducing fertilizer inputs, grazing intensity and restricting when mowing could take place. A total of 615 fields were studied. The four species were surveyed twice during the breeding season (April-May), and the number of each species and their location recorded.
A replicated study in 2005-2006 of 70 fields with wet features at nine lowland pastoral sites in eastern England (Eglington et al. 2008) found that the probability of a field being used by nesting northern lapwing Vanellus vanellus was significantly greater with an increase of foot drain floods. Foot drains are shallow channels used historically for drainage. Foot drain floods are areas where water overtops the foot drain. Fields with foot drain floods held the highest densities of nesting pairs. Nests were more likely to be located within 50 m of foot drain floods and chicks more likely to forage near foot drain floods (in wet mud patches created by receding water). Fields with foot drains, foot drain floods and isolated pools were visited at least once a week (March-July 2005-2006) and the number of lapwing pairs displaying parental behaviour within a 10-min sampling period used as a measure of brood density. Habitat variables and percentage of wet ground were collected around each nest site and the distance measured to the nearest foot drain, pool and flood.
A 2009 literature review of agri-environment schemes in England (Natural England 2009) found studies that suggested more expensive agri-environment scheme options for wetland habitats (such as controlling water levels) were more effective at providing good habitat for wading birds than easier-to-implement options.
- Berendse F., Oomes M.J.M., Altena H.J. & Elberse W.T. (1992) Experiments on the restoration of species-rich meadows in the Netherlands. Biological Conservation, 62, 59-65
- Oomes M.J.M., Olff H. & Altena H.J. (1996) Effects of vegetation management and raising the water table on nutrient dynamics and vegetation change in a wet grassland. Journal of Applied Ecology, 33, 576-588
- Wakeham-Dawson A. & Smith K.W. (2000) Birds and lowland grassland management practices in the UK: an overview. Ecology and Conservation of Lowland Farmland Birds. Spring Conference of the British Ornithologists' Union, 27-28 March 1999, Southampton, UK, 77-88.
- Ausden M., Sutherland W.J. & James R. (2001) The effects of flooding lowland wet grassland on soil macroinvertebrate prey of breeding wading birds. Journal of Applied Ecology, 38, 320-338
- Devereux C.L., McKeever C.U., Benton T.G. & Whittingham M.J. (2004) The effect of sward height and drainage on Common Starlings Sturnus vulgaris and Northern Lapwings Vanellus vanellus foraging in grassland habitats. Ibis, 146, 115-122
- Smart M. & Coutts K. (2004) Footdrain management to enhance habitat for breeding waders on lowland wet grassland at Buckenham and Cantley Marshes, Mid-Yare RSPB Reserve, Norfolk, England. Conservation Evidence, 1, 16-19
- Lyons G. (2005) Botanical monitoring of restored lowland wet grassland at Campfield Marsh RSPB Reserve, Cumbria, England. Conservation Evidence, 2, 43-46
- Aquilina R., Williams P. & Nicolet P. (2007) Effect of wetting-up ditches on emergent insect numbers. Aspects of Applied Biology, 81, 261-262
- Defra (2007) Wetting up farmland for birds and other biodiversity. Defra BD1323 report. Defra.
- Diggelen R.V. (2007) Habitat creation: nature conservation of the future? Aspects of Applied Biology, 82, 1-11
- Kahlert J., Clausen P., Hounisen J. & Petersen I. (2007) Response of breeding waders to agri-environmental schemes may be obscured by effects of existing hydrology and farming history. Journal of Ornithology, 148, 287-293
- Eglington S.M., Gill J.A., Bolton M., Smart M.A., Sutherland W.J. & Watkinson A.R. (2008) Restoration of wet features for breeding waders on lowland grassland. Journal of Applied Ecology, 45, 305-314
- Natural England (2009) Agri-environment schemes in England 2009: A review of results and effectiveness (NE194). Natural England report.