DEPARTMENT for ENVIRONMENT, FOOD and RURAL AFFAIRS CSG 15Research and Development

Final Project Report(Not to be used for LINK projects)

Two hard copies of this form should be returned to:Research Policy and International Division, Final Reports UnitDEFRA, Area 301Cromwell House, Dean Stanley Street, London, SW1P 3JH.

An electronic version should be e-mailed to resreports@defra.gsi.gov.uk

Project title Enhancing the biodiversity value of arable drainage ditches          

DEFRA project code BD1319

Contractor organisation and location

ADASWoodthorneWergs Road, WolverhamptonWV6 8TQ

Total DEFRA project costs £ 40650

Project start date 01/04/01 Project end date 31/12/02

Executive summary (maximum 2 sides A4)

Ditches and drains in intensively farmed arable land are usually regarded as being of less ecological interest than those of grazed pastures, but the very fact that they are surrounded by intensively managed land means that they may provide the richest habitats in these arable areas. Most arable drainage ditches have the potential to be valuable habitats for a wide range of wildlife, both aquatic and terrestrial. However, in many parts of the country, this potential is not achieved because land managers are not aware of the ecological value of their ditches and regard and manage them purely as a means for safeguarding their crops and land from flooding.

Thus, the overall objective of the project was to assess the feasibility of enhancing the potential of arable drainage ditches for wildlife, without compromising their function as drainage channels. This was achieved by conducting a review of published literature and unpublished reports relating to the management of arable drainage ditches in England and Wales for conservation purposes. In addition, consultations with interested organisations/individuals involved in arable ditch management or research were carried out. Organisations consulted during this study included Defra/RDS, English Nature, Environment Agency, Countryside Agency, Internal Drainage Boards, CCW, CSL, FWAG, NFU, RSPB, Wildlife Trusts, and a selection of farmers were also contacted. The next stage was to use the information gathered during the consultation and review phase to develop criteria for prioritising different ditch types for biodiversity enhancement, and establishment of the requirements for further research. A set of summary documents, providing reference material, recommendations and guidance notes was then produced.

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Projecttitle

Enhancing the biodiversity value of arable drainage ditches          

DEFRAproject code

BD1319

This report outlines the importance of arable drainage ditches to wildlife (plants, invertebrates, fish, amphibians and reptiles, birds and mammals), then goes on to look at ditch management requirements in terms of bankside vegetation cutting and channel dredging. Factors affecting biodiversity are explored, including salinity, water flow, water levels and water quality. The report then looks at methods for enhancing the biodiversity value of arable ditches and the targeting of particular ditches for enhancement. The management aspects are detailed in Appendix 1, in a comprehensive set of guidelines aimed primarily at Project Officers and land managers. There is a separate document (Appendix 2) containing a more concise, user-friendly set of guidelines in leaflet format. This document is designed for Project Officers to hand out to landowners and could also be displayed on the ADAS or Defra websites, or in one of the ADAS environmental newsletters. Appendix 3 details the possibilities for future research, where the suggestions put forward are laid out in the style of a concept note, complete with estimated costings for such research.

CSG 15 (Rev. 6/02) 2

Projecttitle

Enhancing the biodiversity value of arable drainage ditches          

DEFRAproject code

BD1319

Scientific report (maximum 20 sides A4)

Contents

Page1. Introduction 32. The importance of arable drainage ditches to wildlife 43. Ditch management requirements 94. Other factors affecting ditch biodiversity 125. Methods for enhancing the biodiversity of arable drainage ditches 156. Which ditches should be targeted for biodiversity enhancement? 187. Conclusions 208. References 219. Appendix 1 – Guidelines for managing and prioritising ditch types in arable land for biodiversity10. Appendix 2 – Guidelines (leaflet version)11. Appendix 3 – Proposed future research topics

1. INTRODUCTION

In areas dominated by arable agriculture, current management of drainage ditches is usually tailored to ensuring the adjacent land is drained as efficiently as possible. Drainage of arable land starts within sub-surface field drains which feed water into private drainage ditches, usually managed by farmers or their contractors. These ditches tend to dry up in summer and many have been infilled to create larger fields. Those that are left are usually intensively managed, and there is little consideration for their wildlife interest despite the fact that they are often the only refuges for wildlife in some arable landscapes. The bank sides are flailed each year, water is regularly pumped out and, generally, the wildlife value of ditches is given low priority.

Agri-environment schemes in England and Wales attempt to address this issue by providing payments for ditch restoration, sympathetic management and establishing grass margins alongside ditches within certain ‘target areas’. Agri-environment scheme Project Officers, and other individuals and organisations concerned with improving the wildlife value of arable drainage ditches, need to be able to provide appropriate guidance to farmers considering ditch restoration. Within the Countryside Stewardship and other agri-environment schemes there are guidelines for restoring drainage ditches and, within arable areas, buffering them with grass margins. However, there is little guidance available to Project Officers on within-channel management and they tend to limit their advice to bank and field edge management. In some areas, there may now be scope for local initiatives, potentially funded by the Countryside Stewardship Scheme, to enhance the biodiversity value of arable ditches through a combination of water level manipulation, bank side and bank top management. Many of the management prescriptions within ESAs with a high proportion of grazing marsh relate to drainage ditches, including the manipulation of water levels, sediment removal and bank management regimes. From the late 1980’s onwards, MAFF commissioned a number of studies of such ditches within various ESAs. However, publications relating to the manipulation of arable drainage ditch management specifically to benefit wildlife are few in number. Information about the potential benefits to wildlife and ways of resolving any problems with drainage, siltation, etc. is therefore required.

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2. THE IMPORTANCE OF ARABLE DRAINAGE DITCHES TO WILDLIFE

Ditches in intensively farmed arable land are usually regarded as being of less ecological interest than those of grazed pastures but the very fact that they are surrounded by intensively managed land means that they may provide the richest habitats in these arable areas (Newbold et al, 1989).

The principal types of wildlife found in arable drainage ditches are aquatic plants, macro-algae (e.g. Charophytes), aquatic invertebrates, amphibians, fish, certain mammals and water birds. These need different combinations of a variety of features, including water (which will vary in quality, depth and permanence), substrate on the channel bottom (usually clay or peat), plant management / ditch cleaning management, bank sides (varying soil type and steepness of slope) and bank tops (varying in cover and width according to closeness of cultivation line and other management operations).

These plants and animals are dependent on appropriate water management regimes to maintain the different degrees of wetness required or tolerated. There will be situations where high water levels will benefit some species but disadvantage others. Also the rate and timing of fluctuations in water levels can have a major impact on some species.

There are significant differences in structure, and often the amount and quality of water, between ditches and drains in grassland and those in arable land. Grazing not only maintains a diverse sward structure on the bank side and provides areas of bare mud, but high water levels are often maintained to provide drinking water for the livestock and/or ‘wet fences’. By contrast, the banks tend to be steeper in arable situations, and bank side vegetation is managed by cutting, burning, or the use of herbicides instead of by grazing. With infrequent cutting, the development of tall emergent and bank side vegetation will be more rapid prior to cutting than where grazing occurs, with consequently greater shading of submerged and floating plants. Even with more frequent cutting, the steep slope and depth of the banks themselves can cause a significant shading effect. Bare mud may only result from fluctuations in water level or through ditch management operations. Furthermore, in arable drains and ditches there is usually no incentive in terms of ditch function to artificially maintain high water levels in summer, except where water is held back to use for irrigation. Water levels will generally fluctuate quite markedly due to rapid drainage in spring, or due to water extraction for irrigation during the summer (except in the rare cases where sub-surface irrigation is employed). Water quality is often affected by cultivation of adjacent land which can not only cause releases of nitrate but can also produce elevated phosphate levels in suspended solids resulting from erosion and runoff.

2.1 PLANTSThe most interesting ditches botanically are those with a good variety of different types of water plants, including species which grow under water (submerged), float on its surface (floating-leaved) or have their roots submerged but leaves growing above the water (emergent).

The consultation exercise and literature review have indicated that there has not been a comprehensive botanical survey of pumped arable drainage ditches across the country. There is, however, information available on clusters of ditches in various locations which have been surveyed for personal interest and short-term R&D projects, but little of this has been done in recent years, e.g. ditches at Swavesey, Cambridgeshire surveyed 30 years ago and again during the late 1980s (O. Mountford, pers. comm.); ditches in the Humberhead Levels during 1970s and 1980s; ditches in Romney and Walland Marshes; ditches between Whittlesey, Peterborough and Thorney in the early 1980s. Driscoll (1986a and 1986b) looked at the effect agricultural improvements (a change from grassland to arable use) had on the aquatic flora and fauna of dykes in the Norfolk Broads, and Palmer (1986) carried out a similar study in the Pevensey Levels in Sussex, after a change from permanent pasture to cereal farming. The more recent Countryside Surveys (e.g. CS2000) have collected some botanical information relating to drainage ditches within randomly selected 1km squares but there are complexities and costs in extracting the relevant data (C. Barr, pers. comm.). A study took place at Arthur Rickwood between 1988 and 1996 (Milsom et al., submitted), and there are also new surveys being undertaken in three locations within the Cambridgeshire Fens in 2002, surveying both plants and invertebrates (J. Graham, pers. comm.).

Typical arable drainage ditches where water is present for a large proportion of the year have a variety of submerged, floating and emergent plants. Ditches at the Arthur Rickwood Research Station in the Cambridgeshire Fens, for instance, contained as many as 23 aquatic species of which six had submerged growth forms, two were floating and 15 were emergent (Milsom, 1994). These species are mostly abundant in Britain and included water plantain (Alisma plantago aquatica), various rushes (Juncus spp.), purple loosestrife (Lythrum salicaria), spiked water-milfoil (Myriophyllum spicatum), common reed (Phragmites australis), common water crowfoot (Ranunculus aquatilis). In addition, a species with a restricted distribution in Britain was also recorded. This was whorled water-milfoil (Myriophyllum verticillatum).

The distribution of aquatic plant species differs within ditches and over time. For instance, some ditches tend can be dominated by emergent species such as common reed, reed mace (Typha latifolia) and branched bur-reed (Sparganium erectum) whereas others have a greater abundance of floating and submerged species. The floating and submerged species are often more abundant after management by dredging but can disappear completely if the ditch dries out or if they are out-competed and/or overshaded by emergent plants. Emergent species are terrestrial plants adapted to anoxic soils and their populations therefore tend to be more stable and tolerant of slight fluctuations in water level than the floating and submerged species. Apart from common reed, the emergent and riparian species found on ditch banks tend to be restricted to a narrow strip along the margin with the water (Milsom et al, submitted). This is because of the effect of the sharp moisture gradient up the banks of arable ditches.

Interesting differences between the species composition of grassland ditches and arable ditches have been noted in bordering study areas. For instance, a sample of 18 ditches adjacent to the Ouse Washes, Cambridgeshire tended to have reed sweet-grass (Glyceria maxima) as the dominant species, whereas 17 ditches on bordering arable land had virtually none of this species but instead had a great abundance of common reed (Milsom et al., submitted). Common reed is relatively scarce in ditches bordering the Washes, perhaps because it is less tolerant of grazing.

In the Gwent Levels (formerly Monmouthshire Levels) on the edge of the Bristol Channel, the extensive grazing marsh grasslands have been agriculturally improved and/or converted to arable from the 1950s onwards (Scotter et al, 1977; Wade & Edwards, 1980; CCW, 1991). This period of agricultural intensification followed the rebuilding of sea defences, increased frequency of ditch dredging, improved under-drainage and the introduction of herbicides as a means for controlling water weeds. Despite these changes, 55 of the 100 plants species recorded between 1840 (reference was made to previous surveys) and 1976 stayed more or less the same. Certain plants remained widely distributed throughout the ditch system. These included duckweed (Lemna minor) and common reed, both abundant. Gipsywort (Lycopus europaeus), water dock (Rumex hydrolapathum), broad-leaved pondweed (Potamogeton natans) were less abundant species but also survived the period of agricultural intensification. Plants that extended their distribution following more intensive management included frogbit (Hydrocharis morsus-ranae), fan-leaved water crow-foot (Ranunculus circinatus), rigid hornwort (Ceratophyllum demersum), Canadian pondweed (Elodea canadensis) and water fern (Azolla filiculoides), the latter two species both introduced from North America in the early 1900s. Species that have declined in distribution and abundance tended to be those that were always more scarce and most e.g. mare’s-tail (Hippuris vulgaris), greater bladderwort (Utricularia neglecta), lesser water-plantain (Baldellia ranunculoides), narrow-leaved water-plantain (Alisma lanceolatum) and beaked tasselweed (Ruppia maritima) have not been recorded since 1971. Whorled water-milfoil and soft hornwort (Ceratophyllum submersum) have been recorded since 1971 but have become more scarce.

Some notable arable ditches have been found to contain such a diverse and/or nationally scarce aquatic flora that they have been designated as Sites of Special Scientific Interest (SSSI). Examples include Cross Drain in Lincolnshire and Delph Bridge Drain in Cambridgeshire (English Nature, 1991 and 1998). Cross Drain has a rich flora typical of relict fenland, including the locally rare species lesser water plantain (Baldellia ranunculoides) and various-leaved pondweed (Potamogeton gramineus) as well as the nationally scarce species fen pondweed (Potamogeton coloratus) and needle spike-rush (Eleocharis acicularis). Delph Bridge Drain supports the only known British population of fen ragwort (Senecio paludosus).

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If the ditches dry out, such as during very dry years, most of the submerged and floating plants will be lost quite rapidly. This can occur within one season (e.g. Milsom, 1994) especially in peat soils, where such rapid loss of these species may be partly due to release of acid and ochre deposits during a drought period (Gosling and Baker, 1980). Losses of these open water plants also occur through shading by bankside vegetation and through accumulation of plant debris and silt during the period between slubbing operations which, in extreme situations, can also result in drying out. Regular, but sympathetic, management is therefore necessary to maintain the wet conditions suited to the range of plants associated with ditches (Newbold et al, 1989).

2.2 INVERTEBRATESIn most waterbodies, the margins are the most important parts for invertebrates (Kirby, 1992). Marginal invertebrates benefit from gently sloping banks with a broad ‘drawdown’ zone with patches of bare mud. In grazing marsh situations, gently sloping banks and areas of bare mud created by the treading actions of livestock are commonplace. Grazing maintains a diverse sward structure and water levels usually have to be maintained to provide drinking water for the livestock. In arable situations, on the other hand, the banks tend to be steeper, grazing is replaced by cutting, burning or the use of herbicides, and bare mud may only result from fluctuations in water level or through ditch management operations.

In shallow water, the most species-rich invertebrate communities tend to be associated with dense vegetation e.g. tussocky and trailing grasses. In general, there should be a varied structure of vegetation rather than too even a sward. Most common emergent plants support invertebrate communities (Kirby, 1992), some feeding on the plants, others, such as aquatic species, living amongst the stems in shallow water. This vegetation is also used for emerging dragonflies and other invertebrates with aquatic larvae and flying adults (Painter, 1998 and 1999).

All stages of hydroseral succession support interesting invertebrate communities and the best communities tend to occur in sites where there are numerous ditches. This is because extensive networks of ditches safeguard the communities from localised extinctions due to extremes of climate or management operations and allow damaged or managed ditches to be recolonised more easily. The greater the range of different ditch sizes and stages of succession, the greater the range of invertebrates a site will support (Kirby, 1992). Some species of water beetle e.g. Hygrotus confluens and Berosus affinis require very open water with little vegetation (J. Bratton, pers comm). The medicinal leech (Hirudo medicinalis) of which there are about 20 isolated populations of this species remaining in the UK, also requires open water, and is found in Walland Marsh SSSI in Kent/ E. Sussex (B. Banks, pers comm). Other invertebrates, including certain rare water snails e.g. Anisus vorticulus require much more overgrown ditches (Killeen & Willing, 1997).

Flowering plants on the ditch banks and bank tops are also important for invertebrates as sources of nectar for hover-flies, bees, soldier flies and butterflies. In their research at Arthur Rickwood Research Centre, Milsom et al. (in prep), found the distribution of butterflies using ditches was correlated with the species composition of the ditch. They tended to be more abundant in ditches where tall-herb fen had developed and less abundant in ditches dominated by common reed (though several species of moth including wainscot moths are associated with reeds and rushes). In particular, the butterflies appeared to favour ditches with purple loose-strife (Lythrum salicaria) and hemp agrimony (Eupatorium canabium) which provide good nectar sources. The most abundant butterfly species recorded were small/Essex skipper (Thymelicus sylvestris/T. lineola), small white (Pieris rapae), small tortoiseshell (Aglais urticae), peacock (Inachis io) and gatekeeper (Pyronia tithonus).

The fenland ditches at Arthur Rickwood Research Centre were also found to support at least 21 species of dragonflies, including one Red Data Book species, the scarce chaser (Libellula fulva). In a survey of ditches bordering the intensive arable land around the Wash in East Anglia, 130 species of water beetle were found, including four listed in the Red Data Book and a further 35 rated as nationally notable (Foster et al., 1990). TWINSPAN was used to interpret the habitats of the water beetle assemblages in the Wash drainage system. Eight end-groups were established with the commonest species occurring in most. Other species, used as ‘indicator species’, were more selective in their habitat requirements. Factors affecting the communities present included salinity, pH, water depth and water flow, water quality (usually a measure of the concentration of nitrates), abundance of submerged vegetation, abundance of emergent and floating vegetation, and evidence of recent management. The highest quality water beetle assemblages were found in the main IDB drains (and

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rivers). These had the deepest water and all contained a mixture of emergent, floating and submerged plants. The best site was Cross Drain which, at the time of this survey (mid-summer), had 56 species of water beetle (Foster et al, 1990). This drain lies on fen gravels capped by peat. It develops extremely rich vegetation, which is routinely cut twice a year.

Cross Drain supports an exceptional beetle fauna with more than 20% of all British water beetles, including the nationally rare Hydrochus ignicollus and Agabus undulatus which are both typical of relict fen habitat. The quality of the aquatic vegetation and invertebrate fauna in this drain reflects the relatively low nutrient levels and unpolluted status of the water which is exceptional in an arable environment (English Nature, 1991).

The other drains in the ‘high quality’ end-group with good numbers of water beetle species all showed signs of regular management (Foster et al, 1990). The three poorest quality sites in the end-group, on the other hand, lacked any form of management. One of these was a ditch on peat with a floating mat of grass. Another was a drain containing yellow water lily (Nuphar lutea) on a nature reserve.

Another example of an arable drain with an exceptionally rich invertebrate fauna is Mother Drain in Nottinghamshire. This drain also has good water quality and an excellent variety of open water, emergent and bankside plant communities (English Nature, 1993). There are 14 species of dragonflies and damselflies that have been recorded as breeding in this drain, including the black darter (Sympetrum danae) and two of nationally notable status - variable dragonfly (Coenagrion pulchellum) and hairy dragonfly (Brachytron pratense). There are also seven nationally scarce species of water beetle. The banks of the drain also support the rare marsh carpet moth (Perizoma sagittata), whose larval foodplant is common meadow-rue.

It is interesting to note that Cross Drain is fed by private farm ditches overlying peat which, at the time of Foster’s survey in 1986 (Foster et al, 1990), contained shallow water, were choked with vegetation and showed no sign of recent management. These ditches all ran between intensively managed cereal fields, indicating that they can attain a high conservation value even in this environment. This suggests that a combination of intensive management of some ditches/drains and neglect of others is important in retaining the habitats required by a wide diversity of water beetles (and other invertebrates). It is also no doubt significant that Cross Drain and its associated ditches are also fed by groundwater, which is presumably lower in nutrients than that draining directly off the adjacent land.

2.3 FISH, AMPHIBIANS AND REPTILESA variety of fish species are found in ditch systems, including pike (Esox lucius), stickleback (Gasterosteidae) and eel (Anguilla anguilla). Tench (Tinca tinca), bream (Abramis brama) and rudd (Scardinius erythrophthalmus) are all found in drains and ditches containing plenty of aquatic vegetation (Newbold et al, 1989). Other common fish species associated with drains include dace (Leuciscus leuciscus), bleak (Alburnus alburnus), perch (Perca fluviatilis), roach (Rutilus rutilus), zander (Stizostedion lucioperca) and stone loach (Noemacheilus barbatulus) (J. Graham, pers comm). Baston Fen in Lincolnshire is known to support a population of spined loach (Cobitis taenia).

Frogs will use ditches for breeding and feeding but tend to avoid intensively farmed areas (Newbold et al, 1989) and ditches that are dry in the spring since their tadpoles are unable to survive in these situations. Drying out later in the year is a worthwhile fish deterrent, so frogs can benefit from ditches that dry up mid-summer. Great crested newt (Triturus cristatus) tadpoles are very vulnerable to fish predation and only occasionally breed in ditches (Newbold et al, 1989). A number of ditches on arable land on Walland Marsh SSSI do support great crested newt populations. Although none of the populations are particularly large, they do help to maintain extensive newt meta-populations across the marsh, which is an important component of maintaining effective populations of this BAP species. As great crested newts tend to occur in fish-free ditches (e.g. those that occasionally dry out) it is important to manage these ditches appropriately and to avoid joining them up to the wider ditch network. Smooth newts (Triturus vulgaris) are more common and grass snakes (Natrix natrix) frequently hunt in ditches.

2.4 BIRDS

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Birds associated with drainage ditches, such as reed warbler (Acrocephalus scirpaceus), reed bunting (Emberiza schoeniclus) and sedge warbler (A. schoenobaenus) are usually those requiring persistent stands of reeds and tall emergents, and a lack of disturbance. For waders, such as snipe (Gallinago gallinago) a gently sloping bank with a broad drawdown zone offers plenty of damp mud in which to feed. Green sandpiper (Tringa ochropus) can also be found on ditch banks in autumn and winter. Young and overwintering bittern (Botaurus stellaris) are reported to use some reed-fringed ditches in Somerset. The grey heron (Ardea cinerea) is a frequent user of ditches and moorhen (Gallinula chloropus), mute swan (Cygnus olor) and mallard (Anas platyrhynchos) can all be found nesting along well-vegetated ditch banks.

2.5 MAMMALSThe water vole (Arvicola terrestris), a priority Biodiversity Action Plan species, can benefit where there are ditch banks with soil types that allow them to burrow, good bankside cover and moderately deep water. This species used to be common along river banks, streams, ditches, canals, lakes and ponds but has undergone dramatic declines in its population since at least 1900 (UK Water Vole Steering Group, 1997). The main reasons have been direct habitat loss or degradation and, since about the 1950s, predation by American mink (Mustela vison). Surveys of water voles in East Anglia have found good populations in drainage ditches in the Cambridgeshire Fens, Suffolk, Norfolk and Lincolnshire.

Water shrews (Neomys fodiens) inhabit unpolluted ditches, particularly those with water-cress (Rorippa nasturtium-aquaticum). Otters (Lutra lutra) can be found in well vegetated ditch systems, particularly in Wales and south-west England (Newbold et al, 1989).

The drier parts of ditch banks can also act as habitat for ‘terrestrial’ small mammals which are, in turn, an important source of food for foxes and predatory birds, including the barn owl (Tyto alba) and short-eared owl (Aseo flammeus). It is not certain however that they act as corridors, allowing movements that would otherwise not occur. Stands of common reed, managed as for reed and sedge warblers and reed buntings, may also be used by nesting harvest mice (Micromys minutus). Badgers (Meles meles) occasionally make their setts in ditch or drain banks, although this may not always be desirable from the point of view of bank stability and structure.

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3. DITCH MANAGEMENT REQUIREMENTS

Without a permanent presence of water (or at the very least, damp conditions), none of the true wetland plants will survive or become established. Where water is present there is a tendency for emergent plants to progressively colonise open water, compete with submerged vegetation and reduce species diversity. In ditches with shallow water this can be very rapid e.g. within one season. Retention of water and regular management by a combination of vegetation cutting and sediment removal is therefore necessary.

In dry ditches, there is no water to slow down the succession from grassy vegetation to tall herbaceous vegetation and scrub, so flailing to maintain a clear channel is often required more than once a year.

3.1 BANKSIDE VEGETATION CUTTINGAn experiment was carried out at Arthur Rickwood Research Centre between 1989 and 1992 (Milsom, 1994). The effect of bankside vegetation cutting treatments was assessed within two ditches that were responsive to pumping operations. Both ditches had been slubbed out in 1987. Three treatments were used:

i) banks mown twice a year, in March and again in Octoberii) banks mown once a year in Octoberiii) banks mown once every two years in October

Control plots were left unmown throughout the four years of the project.

A Bomford 457 flail mounted on the back of a tractor was used and the flail cut the vegetation until a low uniform sward was produced. The vegetation in the watercourse itself was not cut so the effects of mowing were assessed by measuring changes in a) species richness and b) the species composition of the emergent aquatic flora.

The results were as follows:i) overall there was a loss of species within the ditches during the period of the experimentii) the loss of species was least in plots mown twice a yeariii) the loss of species was greatest in plots that were left unmown for four yearsiv) emergent plant communities were more stable than the floating and submerged plant communities. This was exhibited by losses in floating and submerged species being proportionately much greater than those of emergent species. This may be explained by successional changes following the 1987 slubbing out e.g. shading/competition from emergent species, the accumulation of plant debris and silt and possibly exacerbated by low rainfall resulting in the temporary drying out of the ditches.v) mowing twice a year tended to encourage common reed to grow more vigorously and densely on the banks and within the watercourse itself. This causes impedance of the water flow and shading of the watercourse which in turn leads to a reduction in the diversity of floating and submerged aquatic plants. However, common reed does provide a valuable habitat and cover for nesting birds and game birds.vi) mowing the bankside did not aid the re-establishment of floating or emergent species when water levels were higher.

In a second experiment at Arthur Rickwood between 1992 and 1997, two mowing regimes were compared, using the same equipment as that in the first experiment (Milsom et al, submitted):

i) banks mown twice a year, in November and Marchii) banks mown once every two years, in November

In addition, the ditches in the second experiment were slubbed out in early spring 1993 and again during the winter of 1995/96.

The results of this second experiment indicated that a greater number of floating and submerged species colonised ditches that were mown twice a year than those that were mown once every two years.

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The optimum frequency and timing of cutting bank side vegetation will vary according to the particular conservation objectives, the size of the ditch, and possibly the slope and orientation of the ditch as well. Where an objective is to enhance the colonisation of floating and submerged plants, cutting in late winter-early spring (March) will tend to delay growth of riparian vegetation and reduce shading of the open water, compared to cutting the previous autumn. This refinement is likely to be more important in narrow ditches and those with steeper banks than in those with a wider expanse of open water. In narrower ditches, it may be less important in those orientated north-south, where the water surface receives more sunlight, than those orientated east-west. Plant species diversity within the riparian communities will be maintained better by cutting annually or twice a year (in autumn and again in late winter-early spring) than by cutting only once every two years or not at all. The same regime will also benefit the re-colonisation of floating and submerged species following ditch clearance, compared with less frequent cutting. However, where reeds are dominant and the objective is to provide nesting sites for reed and sedge warblers and reed buntings (see Section 4.4), cutting once every two years will be beneficial. Nevertheless, leaving reed beds uncut for several years can lead to loss of reed cover, and it is advisable to rotate cutting between different sections of ditch bank so that some two year-old reed is available each year.

Cutting of bankside vegetation in the autumn, winter or early spring is much less damaging to wildlife, including invertebrates, than that carried out in the summer. For instance, dragonflies and other insects emerging from watercourses will be adversely effected if vegetation is cut before or during their period of emergence, generally between about May and September depending on species and location within the country (Gibbons, 1986). Cutting during the summer also removes the cover, food and nectar sources of many invertebrates which in turn provide food for breeding birds. Farmers may however not wish to cut in autumn as this could mean that they would compact the soil of their crop (J. Webb, pers comm) and cutting in winter may also remove the immobile larvae, pupae or eggs of some species.

Cutting tall, emergent vegetation including stands of common reed between March and September will damage the nests and kill the young of breeding birds such as reed warbler, reed bunting, sedge warbler. It will also remove caterpillars of moths that inhabit this vegetation. Flailing in July can prevent birds from having second broods, often vital for maintaining their populations. Bank mowing, even where tall vegetation is not present, should also be done outside the nesting season to avoid damage to the nests and young of ground nesting birds. Harvest mice tend also tend to nest from May until the autumn, so cutting during this period should be avoided where they are present (S. Bence, pers. comm.). One of the few species able to survive flailing during the summer is the meadow pipit (Anthus pratensis) which normally fledges its young by mid-May (N. Watts, pers. comm.).

To minimise damage to wildlife, bankside vegetation should be cut on one ditch side only, and sides cut in rotation (e.g. once every 2 – 6 years) to ensure good structural variation e.g. for invertebrate species relying upon standing dead vegetation and seed heads throughout the winter (J. Webb, pers. comm.). Leaving certain parts of the mown zone untouched will create a variety of habitats which is beneficial for many species.

3.2 CHANNEL DREDGINGIn the Gwent Levels, since the 1950s, the drainage authorities historically managed the main drains by dredging the channel with mechanical excavators once every 5.5 years and by spraying with herbicide each year. Before 1936, debris in the channels was removed by hand (Wade & Edwards, 1980). This suppressed the growth of emergent species but it was thought unlikely to be responsible for the disappearance of species such as greater bladderwort which is associated with the early stages of hydroseral succession. The two species that have increased their distribution and are generally found in main drains, fan-leaved water crow-foot (Ranunculus circinatus) and rigid hornwort, may have benefited from more intensive channel management. However, it is possible that the regular use of herbicides may have had an overriding effect in this situation.

In the experiment at Arthur Rickwood Research Centre (1992–1997) there was a net increase in the number of floating and submerged plant species between the summer of 1995 (before dredging) and the summer of 1996 (after dredging). The number of species colonising dredged ditches was also found to be related to their

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distance from the nearest main drain with those closest tending to be more species rich than those furthest from the main drain. It would appear that the main drain acted as a source for floating and submerged species colonising the smaller ditches. Dispersal appeared to diminish in ditches more than about 400m from the main drain, implying that management for floating and submerged plant species should be targeted at those ditches close to species-rich IDB drains (Milsom et al., submitted). However, this criterion may not apply in systems with less clean or clear water where main drains may not represent such a good source of these species.

The frequency with which dredging/silt removal needs to be carried out to maintain drainage function varies according soil type, water flow and farming operations. Siltation tends to be more rapid in peat areas than in clay areas. In the peat moors of the Somerset Levels, dredging is done on a 1–3 year rotation whereas in the Broads a longer rotation (4–10 years) is more normal (McLaren et al, in prep.). In peaty areas, where the ditch bottom is ‘perched’ on a thin layer of clay, care has to be taken when dredging not to disturb the ditch bottom as the water will drain completely if it is damaged. Wherever possible, dredging should be carried out on a long scale rotation using as many smaller plots as possible, though this might not be practical on a working farm (J. Webb, pers. comm.).

To maintain the existing biodiversity value of ditches it is probably most appropriate to either continue with the traditional dredging frequency and/or make the interval between dredges as long as possible (Hand, 1989). Conversely, where the aim is to increase biodiversity within a holding, a variety of dredging frequencies within a holding should result in the development of different habitats and species assemblages. Ideally, all stages of succession should be present in any given stretch, and should be in a relatively close proximity to each other (J. Webb, pers. comm.).

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4. OTHER FACTORS AFFECTING DITCH BIODIVERSITY

4.1 SALINITYIn coastal areas freshwater overlies the saline water table because of the higher density of salt water. In areas where the freshwater table extends to a depth of 2 m or more, most shallow ditches will only contain freshwater with associated plant and animal communities. If, however, drains are deepened to provide more efficient drainage systems, salt water can intrude into the drains. This will alter the balance of plant species in favour of those more tolerant of saline conditions, and in severe cases can kill the existing freshwater communities (Newbold et al, 1989). The opposite can also occur in areas where brackish plant and animal communities have developed. For instance, in their review of the changes in abundance and distribution of aquatic macrophytes in the Gwent Levels, Wade & Edwards (1980) suggested that changes in salinity in the drainage ditches could be at least as important as changes in management. Many of the species that have been lost or reduced in abundance/distribution e.g. soft hornwort and beaked tasselweed are associated with saline water and are dependant on the influx of salty water through old and inefficient tidal outlets. When the seawall was rebuilt in the early 1950s, the tidal outlets were renovated and some tidal doors closed. Aquatic and emergent vegetation often grows slower under a saline influence, and this may reduce the frequency with which vegetation needs to be cleared. Though salinity itself is important, the mechanism by which it impacts the vegetation biodiversity is via competition - only certain species can tolerate higher salinities.

4.2 INTRODUCED PLANT SPECIES Species such as parrots feather (Myriophyllum aquaticum), floating pennywort (Hydrocotyle ranunculoides) and water fern (Azolla filiculoides) can quickly become established in water bodies such as drainage channels and alter their ecology. Problems caused include deoxygenation of the water, death of fish and aquatic invertebrates, smothering of native aquatic plants, choking drainage channels and localised flooding (Leach & Newman, 2000, Plantlife, 2000). Mechanical control methods tend to be ineffective or even increase the problem by spreading fragments of these plants that can regrow. The use of herbicides, such as Diquat, has been recommended (Leach & Newman, 2000) although approval for the use of Diquat has since been withdrawn.

4.3 INTRODUCED ANIMAL SPECIES The American mink (Mustela vison) has become widely distributed through escape and/or release from mink farms since the 1930s (Strachan, 2000). It is now a very serious predator of water voles, particularly in riparian habitats which have suffered degradation from agricultural practices or urban development where water vole populations have already undergone serious losses. However, it has been suggested that mink free areas, which can include networks of drainage ditches running through arable land, can support thriving water vole populations, regardless of habitat quality (Strachan, 2000). This view is not widely supported however.

4.4 CLIMATE CHANGEIt has been suggested that climate change may have been responsible for the increase in the distribution of frogbit in the Gwent Levels (Wade & Edwards, 1980). Germination of this species’ hibernacula is controlled by water temperature - no germination occurs below 100C and the plants only start to float to the water surface as the temperature approaches 200C. However, in addition to air temperature, water temperature is influenced by the surface area : volume ratio and by flow rate, so that changes in these factors could override the effects of climate change. 4.5 WATER FLOW Maintaining the flow of water through ditches by regular management, proper design and periodic pumping will reduce the chances of stagnation and the build up of plants such as duckweed. In ditches where there are extensive floating stands of duckweed, oxygen is depleted within the water and the growth of submerged plants is prevented (Clare & Edwards, 1983). In addition, when the duckweed decays during the winter, it further reduces available oxygen so that many ditches become oxygen deficient or even anaerobic. In these situations, the invertebrate populations become severely impoverished with only blood worms (Tubifex sp.) and midge

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larvae (Chironomus sp.) surviving where the anaerobic conditions persist for several weeks (Clare & Edwards, 1983).

4.6 WATER LEVELSWater levels within ditches are determined by changes in the following factors:

Rainfall/run off within the catchment Distance from the main IDB drain and hence to the IDB pump (if a pumped system) IDB pumping regime Level of the ditch outlet culvert (if present) or bed relative to the water level in the main IDB drain Ditch gradient Capacity of the outlet culvert (if present) The ‘roughness’ (flow resistance) of the ditch channel, due mainly to aquatic and bank side vegetation

In addition to conferring direct benefits to the aquatic plants and invertebrates in the ditches, maintaining high water levels may also help to reduce the amount of certain nutrients (esp. nitrate N) entering the ditches, by preventing or reversing water flow from field to ditch. High water levels also help to maintain bank stability. The maintenance of some water in small fieldside ditches prone to drying out over the summer months, by the installation of cheap shallow (20-30cm high) dams (bunds), will have an extremely positive ecological effect. This will have no deleterious effect on field drainage provided that it does not cause the tile drain outfalls to be submerged. If the superposition of winter flows over the held back water causes the tile drains to be submerged this will impact on the drainage status of the field. The significance of the effect will depend on the duration and amount by which the tile drains are submerged.

Water levels can be controlled much more effectively with modern electric pumps than with diesel pumps. Moreover, the use of radio telemetry by some IDBs, to sense changes in water levels, allows them to start pumping earlier to prevent flooding problems. These developments mean that they are able to risk maintaining higher water levels throughout the year with consequent benefits to wildlife. IDBs with less modern equipment have to take a more cautionary approach and keep water levels lower, especially in the winter.

Tilting weirs or sluices are used by some IDBs to maintain high water levels in the summer for irrigation purposes, and ‘cloughs’ (a form of valve) are used to help retain water whilst allowing it to flow through the system in times of high rainfall. Some farmers use portable pumps to control water levels in their ditches and these pumps could also be used to help maintain flows around a holding where sluices are employed, to avoid problems with stagnation.

4.7 WATER QUALITYThe growth of vegetation is greatly influenced by the supply of nutrients, particularly nitrate and phosphate, in the water it uptakes. The amount and availability of these nutrients during the year can greatly affect species diversity in ditch systems. Pesticides leaching from arable land may also adversely affect invertebrate populations and, indirectly, the fish, amphibians, birds and mammals that feed on them.

In arable situations, the quality of water in ditches and drains is usually influenced by cultivations and fertiliser application. Nitrate levels are often high following autumn cultivations, and phosphate levels can be elevated due to erosion and runoff, particularly where land is ploughed right up to the top of the ditch bank. However, the highest concentrations of nitrate-N tend to occur during the March to April pumping period, following fertiliser applications on the surrounding land. Where water levels are kept higher in summer for irrigation purposes, the nitrate-N concentrations tend to be much lower. Work in the USA has shown that reducing the rate of water drainage from fields during the winter, by maintaining water levels in the ditches higher than normal, can reduce nitrate entry into surface water by as much as 10–20% on moderately well-drained soils. Where such drainage control was employed throughout the year, the reduction in nitrate losses was much greater. Adopting this practice could have a major beneficial impact on water quality throughout the drainage system, particularly in peat soils where lowered water tables through deep drainage, sometimes combined with cultivations, can lead to the release of acids and ochre deposits. In mineral soils, retaining a higher water table might also help to reduce silt inflow during the winter and spring following cultivation, leading to less frequent

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dredging. However, in the case of soils from which sediment losses are large (i.e. depending upon the amount of sediment and its particle size), this may result in blockage of tile drains. Loss of suspended sediment in subsurface flows is most likely to be a major problem on unstable soils with a well developed macropore structure. Whilst the danger of affecting crop growth may be increased by drainage control, it seems likely that a compromise will be possible in many situations, especially where the technology described in the previous section allows a more rapid response to flooding risk. In some systems, irrigation demand may result in water being pumped in from adjacent rivers, resulting in a change in water quality.

In the absence of under-drainage, grass or woodland buffer strips beside the drain can help to improve water quality, both by slowing surface water movement and thus reducing erosion, and by absorbing dissolved nutrients in vegetation growth. Impeding surface water flow also allows a higher proportion of dissolved nitrate to be lost through denitrification, reducing the amount reaching the ditch or drain. Such buffer strips, particularly those incorporating reed beds, may also reduce pesticide and herbicide concentrations in surface runoff and in water percolating through the upper soil layers.

In peat soils, water quality may be severely affected under drought conditions by ochre deposits and by the release of acids. It is also worth noting that, when flooding occurs in mixed farming areas, poor quality water originating from arable areas may cause eutrophication and deplete biodiversity in nearby unimproved grasslands, due not only to dissolved nitrates but also to phosphates contained in deposited silts.

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5. METHODS FOR ENHANCING THE BIODIVERSITY VALUE OF ARABLE DRAINAGE DITCHES

5.1 WATER LEVELS Water levels within ditches are determined by changes in the following factors: rainfall/run off within the catchment; distance from the main IDB drain and hence to the IDB pump; IDB pumping regime; presence of under drains; whether used for irrigation; height of the outlet culvert relative to the water level in the main IDB drain; ditch gradient; and capacity of the outlet culvert. The roughness of the ditch channel due to aquatic and bankside vegetation also influences fluctuations in water level (Rose & Harris, 1994). At Arthur Rickwood, following the drought conditions in the summer of 1990, the IDB reduced the amount of pumping in the early spring and installed sluice boards in the main IDB drain to retain water in the ditch system. This was done primarily for irrigation purposes but also benefited aquatic plants and invertebrates in the ditches (Runham et al, 1994). Maintaining high water levels may also help to reduce the amount of nitrate N entering the ditches (Gilliam et al, 1979 - see below) and can help to maintain bank stability.

Water levels can be controlled much more effectively with modern electric pumps than with diesel pumps. Some IDB drains make use of radio telemetry to sense changes in water levels so they can start pumping earlier to prevent flooding problems. This means they are able to take the risk of maintaining higher water levels throughout the year, with consequent benefits to wildlife. IDBs without this equipment have to take a more cautionary approach, keeping water levels lower, especially in the winter (Newbold, pers. comm.).

Some IDBs have constructed tilting weirs or sluices e.g. in the Crooked Drain, Lincolnshire, to maintain high water levels in the summer for irrigation purposes. In South Yorkshire ‘cloughs’ are used to help retain water (D. Patrick, pers. comm.). In times of high rainfall the cloughs are forced open by water pressure, allowing the water to flow through the system. Elsewhere farmers use portable pumps to control water levels in their ditches. These pumps could also be used to help maintain flows around a holding where sluices are employed to avoid problems with stagnation.

5.2 WATER QUALITYThe growth of vegetation is greatly influenced by the supply of nutrients, particularly nitrate and phosphate, in the water it uptakes. The amount and availability of these nutrients during the year can greatly affect species diversity in ditch systems. Pesticides leaching from arable land may also adversely affect invertebrate populations and, indirectly, the fish, amphibians, birds and mammals that feed on them. In arable situations, the quality of water in ditches and drains is usually influenced by cultivations and fertiliser application. Nitrate levels are often high following autumn cultivations, and phosphate levels can be elevated due to erosion and runoff, particularly where land is ploughed right up to the top of the ditch bank.

The highest concentrations (e.g. 10–17 mg N/l at Arthur Rickwood) of nitrate-N tend to occur during the March to April pumping period, following fertiliser applications on the surrounding agricultural land (Runham et al, 1994). In the summer, when water levels are generally kept higher for irrigation purposes, the nitrate-N concentrations are much lower (e.g. 1 mg N/l at Arthur Rickwood). At Arthur Rickwood the concentrations rarely rose above 2 mg N/l even during the autumn when the land had been cultivated prior to the establishment of the following crop. In an experiment carried out in North Carolina, USA, water level control structures were installed in the ditches to assess whether the amount of NO3- N movement into surface waters could be reduced (Gilliam et al, 1979). The researchers found that controlling the amount of water drainage from fields during the winter can reduce nitrate entry into surface water by as much as 10–20% on moderately well-drained soils which is estimated to be equivalent to 3–8 kg N/ha per year. If drainage control is employed throughout the year, the reduction in nitrate losses is much greater. If this practice was adopted this could have a major beneficial impact on water quality throughout the drainage system. It might also help to reduce silt inflow during the winter and spring following cultivation, leading to less frequent dredging.

5.3 GENERAL BIODIVERSITY WITHIN A MANAGEMENT UNITEnsure that appropriate conditions for a range of communities are present at all times by piecemeal management of ditches in rotation rather than a whole farm approach (Scotter et al., 1977; Hand, 1989; Foster

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et al, 1990; Kirby, 1992; Harpley, 2000). This will ensure that both open water and dense vegetation are present within a farm/management unit.

Cutting vegetation on ditch banks in both winter/early spring and autumn should help reduce loss of bankside species that are otherwise shaded out or out competed by more competitive emergent species. Cutting twice a year (e.g. February and November) is also thought to assist the colonisation of recently dredged ditches by floating and submerged species by reducing shading (Milsom et al., submitted).

Mowing is essential for maintaining the diversity of emergent and riparian species in ditches. However, mowing twice a year and mowing once every two years does not appear to have a marked effect (Milsom et al. submitted). What is probably more appropriate is to balance the needs of vegetation management with those of, for instance, birds, dragonflies, butterflies, etc. For instance, the reed warbler uses the previous season’s reed seed heads for nesting material. This species and the sedge warbler prefer to nest in standing dead reed i.e. reed which has been left uncut in the previous year. The reed bunting requires dykes where the vegetation has not been cut for two or more years (N. Watts, pers. comm.). The best option is to only cut one side of the ditch each year after September (to avoid disturbance to late broods). Particular care must be taken not to cut common reeds along ditch banks next to oilseed rape crops in the summer. These are particularly attractive to reed warblers and many nests are destroyed by the practice of cutting in July after the rape harvest (N. Watts, pers. comm.).

During management operations, leave one side of the ditch/drain unmanaged to retain suitable habitat for recolonisation of the parts of the ditch/drain that have been managed (Kirby, 1992; Harpley, 2000) and to provide habitat and cover for wintering invertebrates, birds and other animals. Aim to leave a fringe of vegetation at the water’s edge, even along those banks that have been cut.

Cut vegetation should not be left in the ditch as it will cause blockages and potential flooding problems, and will enrich the water as it rots. Cuttings should not be left to rot on ditch banks either as this will smother existing vegetation, enrich the soil and result in invasion by weeds. If the water level rises suddenly, cuttings left on the bank can be swept into the ditch, again causing blockages. Instead, the cuttings of stemmy material can be raked into piles on the bank tops where they provide a good habitat for invertebrates (Hand, 1989).

Aim for a variety of dredging frequencies within a holding to increase the types of habitat and species assemblages present (Hand, 1989). Do not dredge all the ditches on a holding at once but in rotation, ensuring that recently dredged ditches are near to undredged ditches that can act as sources for recolonisation.

Establish shallow margins or berms to provide habitat and more opportunities for the colonisation of marginal plants and invertebrates than steep banks (Kirby, 1992; Harpley, 2000), though shallow berms are possibly not appropriate where water voles are known to be present. A shallow slope and/or a berm will provide a gradual transition from wetland and marginal communities through to drier grassland. Where there is a steep slope the transition is much narrower. Some IDBs have started to develop ‘working berms’ along some of their main drains when reprofiling (D. Sisson, pers. comm.). These berms are wide enough to allow access for diggers when management is required. A fringe of tall emergent vegetation is always retained on the opposite bank as wildlife habitat.

Some shade from overhanging shrubs and other vegetation is acceptable. However, the bulk of ditches within a farm/management unit should be open and sunny. Overhanging vegetation and shrubs should not be allowed to develop along previously unshaded ditches (Kirby, 1992). Invasive and exotic plants and animals should be controlled (Strachan, 2000; Leach & Newman, 2000).

The use of 10m set-aside strips or Countryside Stewardship margins should be encouraged to provide potential buffers against spray and fertiliser drift, to extend the terrestrial habitat for small mammals and invertebrates and to help stabilise ditch banks. This management option is appropriate for all ditches no matter how important their drainage function. Set-aside margins also provide useful access for ditch management during the winter when crops might otherwise prevent this.

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6. WHICH DITCHES SHOULD BE TARGETED FOR BIODIVERSITY ENHANCEMENT?

In general, where a ditch is important for drainage then management should primarily be targeted to maintain this function but aiming to carry out the management necessary for drainage in manner that is sensitive to biodiversity. Ditches with little or no drainage function can be managed primarily for their biodiversity value (Hand, 1989). This may include most ditches in nature reserves or other protected areas.

The first step in planning which ditches can be managed for biodiversity enhancement is to rank them according to their drainage importance and then again according to their biodiversity value or potential (surveys should be carried out to identify the presence of notable species to ensure management can be tailored appropriately). The King’s Lynn Consortium of IDBs are compiling a matrix to identify areas of conflicting interest in ditches with both high drainage value and high biodiversity value so that they can balance the management requirements of both (B. Hornigold, pers. comm.).

Even in ditches with high drainage value e.g. IDB main drains, steps can be taken that will enhance the biodiversity interest without compromising their drainage function. At the very least, this could include sensitive timing of cutting i.e. outside the nesting season, or the establishment of 10m set-aside margins to help buffer the ditch against spray or fertiliser drift, etc.

For those ditches with lower drainage management requirements e.g. private farm drains, the hierarchy of ditches in terms of their importance to drainage on an individual holding, together with knowledge about water quality, seasonal water levels, etc. will help determine how the biodiversity management options can best be targeted as follows:

1. For aquatic plants - target ditches close (i.e. within about 800 m) to IDB drains rich in floating and submerged aquatic plant species (Milsom et al, submitted). Once or twice yearly mowing of bankside vegetation should help to retain bankside and aquatic plant diversity (the actual frequency will depend on the species of interest in a particular area).

2. To provide game cover and suitable habitat for nesting birds - stands of common reed could be encouraged in ditches which are less important for rapid water run-off, including dry and redundant ditches. These may also be used for nesting harvest mice. Mowing in alternate years, or every third year, will ensure the retention of stands of dead reed important for some of the characteristic bird species e.g. reed bunting.

3. To expand existing water vole (and water shrew) populations - target ‘mink free’ ditches in the vicinity of known water vole populations and manage appropriately e.g. maintain stable water levels (not extreme fluctuations between summer and winter) and provide a continuous swathe of tall and luxuriant riparian plants. Prevent overshading by keeping shrub and tree growth to a minimum. Humane control of mink using live capture cage traps may be essential at some sites to ensure the long-term survival of water voles (UK Water Vole Steering Group, 1997). Management of vegetation should be carried out during the winter when water voles are underground (Anglian Otters and Rivers Project). Avoid damage to the banksides when de-silting work is carried out.

4. For dragonflies - target ditches with good water quality (i.e. low nutrient and pesticide concentrations) and which retain water throughout the year without sudden fluctuations in water levels (Gibbons, 1986). The best populations are likely to be found in relatively shallow, slightly acid water which is not heavily overshaded by trees or other tall vegetation. Avoid management which results in dominance by very dense stands of common reeds and do not cut emergent vegetation before September, to avoid removing plants used by emerging dragonflies and other invertebrates. The provision of shelter, roosting and feeding habitats from nearby tall vegetation, woodland and grassland will be beneficial for adult dragonflies (Gibbons, 1986).

5. For butterflies - target ditches with good nectar sources, such as purple loose-strife and hemp agrimony and maintain management suited to these species (Milsom et al., in prep). Consider the use of plug plants to introduce nectar sources along other ditch banks to increase the abundance of nectar sources on the holding.

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6. Freshwater snails - are usually more abundant in nutrient rich, calcareous water (Newbold et al, 1989) and some species prefer neglected well vegetated ditches (Killeen & Willing, 1997). Ditches which retain water but are of lower importance for drainage could be targeted to provide suitable habitat for snails by managing on a much longer rotation. When management is carried out, only manage one side of the ditch at a time.

7. Ditches surrounding SSSIs - where the local hydrology is vital for maintaining its interest should be targeted for water level and other sensitive management practices. Many of these situations will already have been covered by Water Level Management Plans but the farmers and other land holders responsible for managing ditches in these areas should be targeted for additional advice on the environmental importance of drainage ditches.

8. Ditches adjacent to very large fields - should be targeted as locations for 10 m set-aside strips and/or Countryside Stewardship margins. These will enhance the existing terrestrial habitat of the ditch banks, help prevent spray/fertiliser contamination and help stabilise ditch banks. Large fields are often indicative of the most intensive management practices where the field boundaries may form the only refuges for wildlife. Here, even dry ditches, will provide a habitat and cover for a wide range of plants, invertebrates and other animals.

9. Ditches adjacent to fields growing crops requiring sub-surface irrigation - could be targeted for raised water level management e.g. using sluices, etc. or by initiating a more benign pumping regime.

10. Ditches, including IDB drains, due for reprofiling - could be targeted for berm creation. Many IDBs are already developing berms, including ‘working berms’ when reprofiling main drains. Farmers could do this when next carrying out dredging/reprofiling operations and then aim to maintain berms with more sensitive management in future.

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7. CONCLUSIONS

Most arable drainage ditches are, or have the potential to be, valuable habitats for a wide range of wildlife, both aquatic and terrestrial. However, in many parts of the country this potential is not achieved because land managers are not aware of the ecological value of drainage ditches and regard and manage them purely as a means for safeguarding their crops and land from flooding.

The literature and individuals consulted during this project have provided at least some indication that management of arable drainage ditches can be manipulated to enhance their biodiversity value without compromising their use for drainage and flood defence. The degree to which that manipulation can occur will be determined by the importance of the ditch for drainage and the interest of the land holder in wildlife. To some extent the latter factor is also governed by financial incentives.

At the very least, management of all drainage ditches should only be carried out during times of the year when least damage to wildlife is likely to occur. For those ditches which are of highest importance to drainage, frequent bankside and watercourse management will continue to be necessary but could be combined with the creation of grassy strips along the field edges, and berms along the bank bottoms as is being done by several IDBs. Farm ditches, which vary in their importance for drainage, could also be managed more imaginatively with plans being drawn up for the whole holding.

Agri-environment schemes, such as Countryside Stewardship, may already include capital payments for regrading ditch banks and the establishment of grassy margins, but now need to incorporate incentives for surveys of existing biodiversity value and ongoing sensitive management. Such options also include rotational management, managing only one side of a ditch at a time and using water level control structures.

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8. REFERENCES

Anglian Otters and Rivers Project: Factsheet No. 2 The Water Vole.

Clare, P. & Edwards, R. W. (1983) The macroinvertebrate fauna of the drainage channels of the Gwent Levels, South Wales. Freshwater Biology (1983) 13, pp 205–225.

Countryside Council for Wales (1991) Nature conservation and physical developments on the Gwent Levels. Cardiff.

Driscoll R.J. (1986a) The effect of changes in land management on the dyke flora at Somerton and Winterton. Trans. Norfolk Norwich Nat. Soc. 27, 33-41.

Driscoll R.J. (1986b) Changes in land management in the Thurne catchment area, Norfolk, between 1973 and 1983 and their effects on the dyke flora and fauna. Proceedings 7th International Symposium on Aquatic Weeds, p87-92.

English Nature (1991) Cross Drain, South Kesteven, Lincolnshire: Site of Special Scientific Interest (SSSI) notified under Section 28 of the Wildlife and Countryside Act 1981.

English Nature (1993) Mother Drain, Misterton, Nottinghamshire: Site of Special Scientific Interest (SSSI) notified under Section 28 of the Wildlife and Countryside Act 1981.

English Nature (1998) Delph Bridge Drain, Cambridgeshire: Site of Special Scientific Interest (SSSI) notified under Section 28 of the Wildlife and Countryside Act 1981.

Foster, G. N., Foster, A. P., Eyre, M. D. & Bilton, D. T. (1990) Classification of water beetle assemblages in arable fenland and ranking of sites in relation to conservation value. Freshwater Biology (1990) 22, pp 343–354.

Gibbons, B. (1986) Dragonflies and Damselflies of Britain and Northern Europe: Country Life Guides. Newnes Country Life Books.

Gilliam, J. W., Skaggs, R. W. & Weed, S. B. (1979) Drainage control to diminish nitrate loss from agricultural fields. J. Environ. Qual. Vol 8, no. 1, 1979.

Gosling L.M. and Baker S.J. (1980) Acidity fluctuations at a Broadland site in Norfolk. Journal of Applied Ecology 17, 479-490.

Hand, K. (1989) A Management Handbook for Drainage Ditches in Arable Land. ADAS/MAFF, Cambridge.

Harpley, J. (2000) Standard Maintenance Operations. King’s Lynn Consortium of Internal Drainage Boards, King’s Lynn.

Killeen, I. J. & Willing, M. J. (1997) Survey of ditches in East Anglia and south-east England for the freshwater snails Segmentina nitida and Anisus vorticulus. English Nature Research Report No. 229, English Nature, Peterborough.

Kirby, P. (1992) Habitat Management for Invertebrates: A practical handbook. RSPB.

Leach, J & Newman, J. (2000) Myriophyllum aquaticum, Crassula helmsii, Hydrocotyle ranunculoides and Azolla filiculoides. In ‘Exotic and Invasive Species - should we be concerned? Proceedings of the 11th Conference of the Institute of Ecology and Environmental Management.

McLaren, R. P., Riding, A. & Lyons-Visser, H. (in prep) The effectiveness of ditch management in ESAs. ADAS Report to the Department of the Environment, Food and Rural Affairs.

Milsom, T. P. (1994) Management of ditches in arable Fenland for wildlife and drainage - Final Report. Prepared on behalf of Land Use Conservation and Countryside Group, MAFF, London.

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Milsom, T. P., Town, S. J., Crowe, J., Sherwood, A. J. & Runham, S. R. (in prep). Ditches as a habitat for butterflies (Lepidoptera) on arable farmland in the Cambridgeshire Fens, UK. Part of a DEFRA funded project BD0405.

Milsom, T. P., Sherwood, A. J., Rose, S. C., Town, S. J. & Runham, S. R. (submitted). Dynamics and management of wetland plant communities in ditches on drained arable farmland in the Fens of eastern England. Submitted to Agriculture, Ecosystems and Environment (October 2002).

Newbold, C., Honnor, J. & Buckley, K. (1989). Nature Conservation and the Management of Drainage Channels. Nature Conservancy Council in conjunction with the Association of Drainage Authorities.

Painter D. (1998) Effects of ditch management patterns on Odonata at Wicken Fen, Cambridgeshire, UK. Biological Conservation 84, 189-195.

Painter D. (1999) Macroinvertebrate distributions and the conservation value of aquatic Coleoptera, Mollusca and Odonata in the ditches of traditionally managed and grazing fen at Wicken Fen, UK. Journal of Applied Ecology 36, 33-48.

Palmer M. (1986) The impact of a change from permanent pasture to cereal farming on the flora and invertebrate fauna of watercourses on the Pevensey Levels, Sussex. Proceedings EWRS/AAB 7 th Symposium on Aquatic Weeds, p233-238.

Plantlife (2000) At war with aliens: Changes needed to protect native plants from invasive species. A report from Plantlife - The wild-plant conservation charity. Plantlife, London.

Rose, S. C. & Harris, G. L. (1994) Hydrological Investigations - Appendix 1 in Milsom, T. P. (1994) Management of ditches in arable Fenland for wildlife and drainage - Final Report. Prepared on behalf of Land Use Conservation and Countryside Group, MAFF, London.

Runham, S.R., Rose, S.C., Milsom, T.P. & Sherwood, A. (1994). Management of fenland ditches for enhancement of wildlife habitats. 2nd International Conference on River Flood Hydraulics, pp 473–481. John Wiley & Sons, Chichester.

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