Action: Manage ditches to benefit wildlife
- Five out of a total eight studies from the Netherlands and the UK (including one replicated, controlled paired study and three replicated site comparisons) looking at the effects of managing ditches on biodiversity, found that this intervention resulted in increased invertebrate biomass or abundance, plant species richness, emergent plant cover, amphibian diversity and abundance, bird visit rates and higher numbers of some bird species or positive impacts on some birds in plots with ditches managed under agri-environment schemes.
- One replicated controlled and paired study from the Netherlands found higher plant diversity on ditch banks along unsprayed edges of winter wheat compared to those sprayed with pesticides.
- Three studies from the Netherlands and the UK (including two replicated site comparisons) found that ditch management had negative or no clear effects on some farmland bird species or plants.
Ditch bank biodiversity is declining in agricultural landscapes and so management is required to maintain and increase species diversity. Ditch wildlife has been shown to be affected by agricultural management practices such as mowing and grazing regimes (e.g. van Strien et al. 1989) and by ditch management practices including cleaning/dredging technique, season and frequency (e.g. van Strien et al. 1991, Twisk et al. 2000, 2003).
In the Netherlands, botanical agri-environment schemes to enhance biodiversity are most commonly applied to ditch banks. Farmers are encouraged to follow the recommended management, i.e. low stocking rate, first mowing at the end of June or beginning July, no fertilization and deposition of dredging material at the high end of the ditch bank.
van Strien A.J, van Der Linden, J., Melman, T.C.P & Noordervliet, M. A. W. (1989) Factors affecting the vegetation of ditch banks in peat areas in the western Netherlands. Journal of Applied Ecology, 26, 989-1004.
van Strien A.J., van der Burg T., Rip W.J. & Strucker R.C.W. (1991) Effects of mechanical ditch management on the vegetation of ditch banks in Dutch peat areas. Journal of Applied Ecology, 28, 501– 513
Twisk W., Noordervliet M.A.W. & Ter Keurs W.J. (2000) Effects of ditch management on caddisfly, dragonfly and amphibian larvae in intensively farmed peat areas. Aquatic Ecology, 34, 397-411
Twisk W., Noordervliet M.A.W. & Ter Keurs W.J. (2003) The nature value of the ditch vegetation in peat areas in relation to farm management. Aquatic Ecology, 37, 191–209
Supporting evidence from individual studies
A replicated, controlled paired study in 1991-1992 of ditch banks on arable farms in the Netherlands (de Snoo & van der Poll 1999) found higher plant diversity and more important/rare plant species on ditch banks along unsprayed edges of winter wheat compared to those sprayed with pesticides. Ditch banks next to unsprayed edges of winter wheat had 65 plant species and a floristic value of 2,201 (scoring system based on the importance of different plant species in terms of rarity) compared to those sprayed with pesticides (50 species; floristic value 1,181). There was no significant difference on banks along unsprayed and sprayed edges of sugar beet Beta vulgaris (species: 48 and 41, floristic values: 3,616 and 3,029 respectively) and potato crops (species: 46 and 41, floristic values: 1,961 and 1,864 respectively). Frequency and cover of species and floristic value of vegetation was recorded in two plots on each ditch, one along a sprayed and one an unsprayed edge of sugar beet (seven), potato (eight) and winter wheat (20) fields in June-July.
A replicated controlled study of species sown on ditch banks on six farms in the western peat district of the Netherlands (Blomqvist et al. 2006) found that, overall, there was no significant difference between overall species or species-level germination and establishment, plant survival or reproduction (flowering/seed-setting) under three cutting regimes. However, on high-productivity ditch banks, germination (7% vs 4-5%), establishment (20% vs 7-15%) and reproduction (21-39% vs 15-27%) of many species were higher under ‘conventional management’ than the three cutting treatments. On low-productivity ditch banks, plants tended to have lower survival under ‘conventional management’ (60% vs 70-80%) and higher reproduction under ‘conventional management’ and with the first cut in May (33-40% vs 23-26%). One ditch bank was selected on each farm and was divided into four treatments, each with five replicates: two cuts (July and September), three cuts (June, August, September), two cuts (May, September), ‘conventional management’ (standard cutting and grazing - varied between farms). A mown/artificial gap (15 x 15 cm) was created for each of the nine species in each plot. Approximately 100 seeds were sown of each species in October 2001. Numbers of seedlings and established plants (≥4 cm) were monitored each month until September 2003. Biomass samples were collected from plots (20 x 50 cm) in July 2001 (pre-treatment biomass) and before each mowing event in 2002–2003; dry weights were recorded.
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 drainage ditches in arable and pastoral areas of Leicestershire, UK (Defra 2007) (same study as (Aquilina et al. 2007)) found that that wetting-up ditches resulted in higher invertebrate and bird numbers. The following were significantly greater 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 fly (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 during creation. Sampling involved bird observations (45 minutes, 1-2/month), fixed/floating traps for emerging aquatic insects, pitfall traps and sweep-netting for terrestrial invertebrates and a botanical quadrat (0.25-0.5m²) survey. Data was obtained between April 2005-March 2007; all year for birds and spring-summer for other groups.
A replicated site comparison study from 1999 to 2004 in the Netherlands (Manhoudt et al. 2007) found that ditch management affected plant diversity. Diversity was significantly higher on farms with ecologically managed ditches (mown once in September, cuttings removed to reduce nutrient input) buffered with ≥ 3 m-wide field margin strips (36-65 plant species/400 m2) and organic farms (converted to organic less than 5 years ago: 32 plant species/400 m2, converted more than 5 years ago: 36-52 plant species/400 m2) than conventional farms (26-34 species/400 m2). On ecologically managed farms plant diversity increased significantly over six years (up to 27%), there was a small shift to less common plant species and a decrease in the number of nitrogen rich species and Ellenberg nitrogen-values. There tended to be more nitrogen poor species on ecologically-managed and organic farms compared to conventional farms. Four ecologically managed farms, 18 conventional and 20 organic arable farms were studied. Cutting date varied on conventional and organic farms, but cuttings were never removed, ditches on both farm-types did not have buffer field margin strips. On ecologically managed farms, plant species surveys of 100 m of ditch bank spread over the whole farm were undertaken once a year 1999-2004. On organic (in 2001) and conventional (2003) farms, plant species presence was recorded on 10 x 25 m of ditch bank along a transect (May-June).
A replicated site comparison study of 18 agri-environment scheme-managed and 24 conventionally managed ditches within pasture and perpendicular to eight nature reserve borders in the western peat district of the Netherlands (Maes et al. 2008) found that amphibian diversity and abundance (and emergent plant cover) was significantly higher in agri-environment than conventional ditches. Adult green frog Rana esculenta numbers in conventional ditches declined with distance from reserves; this was not the case in agri-environment scheme ditches. Farmers managing ditches under agri-environment schemes are encouraged to reduce grazing/mowing intensity, reduce fertilizer inputs, and not to deposit mowing cuttings or sediments from ditch cleaning on the ditch banks. Relative amphibian abundance was measured in ditches in April-May and/or May-July 2008 just inside reserves and at four distances (0-700 m) from reserve borders. Three methods were used during each sampling period: five minute counts, 20 dip-net samples and two overnight funnel traps. Habitat variables including percentage cover of aquatic plants were also estimated.
A replicated site comparison (paired) study of ditch banks on six dairy farms in the western peat district of the Netherlands (Blomqvist et al. 2009) found that agri-environment scheme ditch management did not result in increased plant diversity or decreased productivity over 10 years. The total number of plant species on ditch banks under agri-environment scheme management decreased significantly between the periods 1993-1995 (31 species) and 2000-2003 (29 species); numbers of target plant species did not differ (7 species). Productivity on agri-environment scheme ditch banks measured as grass/broadleaved plant ratio increased significantly (1993-1995: 0.37; 2000–2003: 0.44) and Ellenberg nitrogen values increased in four and decreased in two farms (1993-1995: 5.82; 2000-2003: 5.92). Differences between agri-environment scheme and surrounding ditch banks tended to decrease over the study period. Plant diversity data were obtained from agri-environment scheme ditch banks in July-August 1993-1995 and May 2000-2003 (42 repeatedly sampled plots) and non-agri-environment scheme ditch banks surrounding five of the farms (78 plots/year). Five replicate biomass samples were taken from agri-environment scheme ditch banks in mid-May 2000-2002 (9-72 plots) before grazing and mowing. Two productivity measures were also derived from botanical data: grass/broadleaved plant ratios and Ellenberg nitrogen-values.
A replicated site comparison of 2,046, 1 km squares of agricultural land across England in 2005 and 2008 (Davey et al. 2010a) (same study as (Davey et al. 2010b, Davey et al. 2010c)) found that management of hedges and ditches under Entry Level Stewardship did not have clear impacts on farmland bird species. Management had significant positive impacts on five species in at least one region of England, but these effects were often very weak and four of the same species showed negative responses in other regions. The other five ‘hedgerow’ species investigated were never positively associated with boundary management. Generally, effects appeared to be more positive in the north of England.
A large site comparison study of 2,046, 1 km² plots of lowland farmland in England (Davey et al. 2010b) (same study as (Davey et al. 2010a, Davey et al. 2010c)) found that three years after the 2005 introduction of the Countryside Stewardship Scheme and Entry Level Stewardship Scheme, there was no consistent association between the length of ditches managed according to the agri-environment scheme on a plot and farmland bird numbers. There were higher numbers of linnet Carduelis cannabina and reed bunting Emberiza schoeniclus, which are known to nest in ditch bank vegetation, in plots with ditches managed according to the Countryside Stewardship Scheme and Entry Level Stewardship compared to other plots. However, this difference was not observed for other species also expected to benefit from ditch management, including yellowhammer Emberiza citrinella and yellow wagtail Motacilla flava. Between 2005 and 2008, Eurasian skylark Alauda arvensis and grey partridge Perdix perdix declines were greater in plots with lengths of ditch management than other plots. For example, grey partridges showed decreases of 1.3 birds for each 0.08 km of ditch on pastoral farmland. The 2,046 1 km² lowland plots were surveyed in both 2005 and 2008 and classified as arable, pastoral or mixed farmland. Eighty-four percent of plots included some area managed according to Entry Level Stewardship or the Countryside Stewardship Scheme. In both survey years, two surveys were conducted along a 2 km pre-selected transect route through each 1 km² square.
A replicated study in February 2008 across 97, 1 km2 plots in East Anglia, England (Davey et al. 2010c) (part of the same study as (Davey et al. 2010a, Davey et al. 2010b)) found that four farmland birds showed strong positive responses to field boundaries (hedges and ditches) managed under agri-environment schemes. These species were blue tit Parus (Cyanistes) caeruleus, dunnock Prunella modularis, common whitethroat Sylvia communis and yellowhammer Emberiza citrinella. Six other species showed weak or negative responses: Eurasian blackbird Turdus merula, song thrush T. philomelos, Eurasian bullfinch Pyrrhula pyrrhula, long-tailed tit Aegithalos caudatus, winter wren Troglodytes troglodytes, and Eurasian reed bunting Emberiza schoeniclus. The boundaries were classed as either hedges, ditches or hedges and ditches and most were managed under the Entry Level Stewardship scheme.
A replicated study of 24 pastoral ditches in 2008 in the Netherlands (Leng et al. 2011) found that delaying twice yearly mowing dates resulted in higher plant diversity. The highest number of seed-setting species was recorded following mowing on 1 July and 1 September, which was 126% higher than under the conventional regime of mowing on 1 June and 1 August. The effect of mowing date differed between plant species. Species richness was significantly higher and biomass significantly lower on ditches in nature reserves compared to those under long-term agri-environment schemes (>16 years), short-term agri-environment schemes (< 6 years) and conventional management. Plots were mown twice on a unique combination of an early (15th May, 1st June, 15 th June, 1st July) and late date (1st August, 15th August, 1st September, 15th September). Before mowing, presence of species, target species with ripe seeds and biomass was recorded in 16 plots under different biannual mowing treatments within six randomly selected ditches under each of the four management systems: nature reserves, long-term agri-environment schemes, short-term agri-environment schemes and conventional farms.
- de Snoo G.R. & van der Poll R.J. (1999) Effect of herbicide drift on adjacent boundary vegetation. Agriculture, Ecosystems and Environment, 73, 1-6
- Blomqvist M.M., Tamis W. L. M., Bakker J. P. & van der Meijden E. (2006) Seed and (micro)site limitation in ditch banks: Germination, establishment and survival under different management regimes. Journal for Nature Conservation, 14, 16-33
- 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.
- Manhoudt A.G.E., Visser A.J. & de Snoo G.R. (2007) Management regimes and farming practices enhancing plant species richness on ditch banks. Agriculture, Ecosystems & Environment, 119, 353-358
- Maes J., Musters C.J.M. & Snoo G.R.D. (2008) The effect of agri-environment schemes on amphibian diversity and abundance. Biological Conservation, 141, 635-645
- Blomqvist M.M., Tamis W.L.M. & de Snoo G.R. (2009) No improvement of plant biodiversity in ditch banks after a decade of agri-environment schemes. Basic and Applied Ecology, 10, 368-378
- Davey C., Vickery J., Boatman N., Chamberlain D., Parry H. & Siriwardena G. (2010) Regional variation in the efficacy of Entry Level Stewardship in England. Agriculture Ecosystems & Environment, 139, 121-128
- Davey C.M., Vickery J.A., Boatman N.D., Chamberlain D.E., Parry H.R. & Siriwardena G.M. (2010) Assessing the impact of Entry Level Stewardship on lowland farmland birds in England. Ibis, 152, 459-474
- Davey C.M., Vickery J.A., Boatman N.D., Chamberlain D.E. & Siriwardena G.M. (2010) Entry Level Stewardship may enhance bird numbers in boundary habitats. Bird Study, 57, 415-420
- Leng X., Musters C.J.M. & de Snoo G.R. (2011) Effects of mowing date on the opportunities of seed dispersal of ditch bank plant species under different management regimes. Journal for Nature Conservation, 19, 166-174