The small mammals of drainage ditches - the influence of structure

THE SMALL MAMMALS OF DRAINAGE DITCHES - THE INFLUENCE OF STRUCTURE Dr. Martin R. Perrow, Nie Peet & Adrian Jowitt The sight of a barn owl quartering or moving from post to post along the drainage ditches and hedgerows bordering fields and roads is a familiar sight in some areas of the county and obviously it is the small mammals in these habitats that are the attraction. But how many small mammals are in these habitats and are some types better than others? It is this question and the thought that ditches could be managed for small mammals, for their own benefit as well as that of predatory birds that first started us on this project.

To us, there seemed to be several distinet structural types of drainage ditches on the edge of arable fields. Firstly, the classic grassy (typically Holcus lanatus and Poa pratensis) sward on both sides of the ditch; the simple short (1.5m high) trimmed 'box' hedge (often a mixture of hawthorn, field maple and eider) on one side of the ditch and grass on the other (what we call ditch hedges); the unmanaged hedge with some trees (hawthorn with some oak, ash and hazel) dominating both sides of the ditch and the even more unmanaged hedge that is bordered on one side by rough pasture rather than being surrounded by arable crops. All of these ditches are typically about Im deep and 4m wide.

The different types of ditch basically represent a successional continuum from extreme regulär management preserving the dominance of grasses through to a managed selection of hedgerow shrubs and ultimately to an unmanaged Situation where trees are allowed to develop and the understorey Vegetation is often rank and diverse. The aim of the study was to investigate the differences in the small mammal communities of these different types and to try and understand the population dynamics of at least some of the species.

Methods Five lengths (each of 100m) of ditch of each of the four types were selected in and around St. Margaret South Elmham in north Suffolk (TG632285).

Monitoring, using 30 live traps in each replicate length of dyke (a 600 trap effort on each sampling occasion), has so far taken place in the winter of '90 and in the summer and autumn/winter of '91 i.e. pre- and post-harvest of the arable crops. Two thirds of the traps (Longworths and plastic Trip-traps) were positioned in the Standard fashion on the ground but one third (of a new design by Mike Jordan) were placed in the aerial habitat (within the hedge or rank Vegetation usually between 0 . 3 - l m off the ground) of each ditch. All traps were left on pre-bait for 2 - 3 days. Seed baits included wheat and sunflower seeds and commercially available 'parakeet' and 'foreign finch' mix. The latter have been shown to be excellent bait for use in aerial traps, particularly for species such as harvest mouse (Perrow & Jowitt 1992). Traps were also baited for shrews with large numbers of blowfly pupae (around 3

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gallons were used on each sampling occasion) and set to catch. Trapping was then conducted over a three-day period in each length of ditch and all small mammals were individually marked by fur-clipping (Gurnell & Flowerdew, 1991).

Results A total of 818 individual small mammals of seven species were recorded, in 1484 captures, in the following order of abundance; bank vole (Clethrionomys glareolus) (43.6%), wood mouse ( A p o d e m u s sylvaticus) (31.8%), common shrew (Sorex araneus), (16.2%), field vole (Microtus. agrestis) (7%), pygmy shrew (Sorex minutus) (1.2%), harvest mouse (Micromys minutus) (0.1%) and yellow-necked mouse ( A p o d e m u s flavicollis) (0.1%).

The mean number of individuals of each species in each habitat type on each sampling occasion is presented in Table 1.

Discussion The management preferences and changes in abundance of some species in the various habitats are striking. For example, the preferred prey item of barn owl, field vole (Glue, 1974, Bunn et al., 1982), is only numerous in the unmanaged habitat that is bordered on one side by a hay crop (all of which are cut annually). Even then, the numbers of voles caught along the hedgerows, which is a measure of the voles in the hayfield itself, varies dramatically between years (Table 1). Marked population fluctuations are rather typical of this species (Tapper, 1976; Gipps & Alibhai, 1991) (population cycles may be present in the more northerly latitudes of mainland Europe) and this can potentially affect the breeding success of owls.

Furthermore, there are some dramatic seasonal trends in the numbers (and therefore availability) of mammals. For example, in grassy ditches, the total number of animals in the summer in these ditches is very low consisting mainly of bank vole and common shrew. However, when the surrounding crops (mainly wheat and barley) are harvested and the fields are ploughed, wood mice that have summered in these habitats flock into the ditches (nearly 10 to 100m of ditch). This large scale immigration has been reported before (Skrivan & Andera, 1987) and yet there is further evidence that wood mice are capable of persisting in fields throughout the year (Pollard & Relton 1970; Green, 1979). Perhaps the key to the total number of mice moving into the ditches is the extent and depth of winter ploughing. Indeed, while conducting our third sampling in two unmanaged hedges bordering the same large wheat-field we made an Observation that supports this view. One of the hedges, next to which that part of the field has already been ploughed contained 8 individual mice while the other, where the field had not yet been ploughed, contained only a single individual.

Cayford et al. (1992) of the RSPB's barn owl project, present some evidence that owls switch their attentions from hayfields and the like (which

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Mean number of individual mammals captured per ditch on the three sampling occasions, where; a) grass ditch b) ditch hedge c) unmanaged and d) unmanaged with adjacent grass pasture

species

Nov. '90

July '91

Oct. '91

common shrew a) b) c) d)

2.4 2.0 2.7 2.5

1.8 3.0 3.0 4.2

1.8 1.8 2.0 1.8

pygmy shrew a) b) c) d)

0.4 0.2 0.0 0.0

0.0 0.4 0.4 0.2

0.0 0.4 0.2 0.0

bank vole a) b) c) d)

2.0 7.4 10.3 12.0

1.6 5.4 7.8 8.8

3.6 4.8 9.6 9.4

field vole a) b) c) d)

0.8 1.6 0.7 13.5

0.0 0.8 0.0 0.2

0.8 1.2 0.4 0.4

wood mouse a) b) c) d)

9.2 4.4 10.6 4.0

0.4 0.6 0.4 0.4

9.6 5.4 7.0 6.6

total mammals a) b) c) d)

15.0 15.6 24.3 32.0

3.8 10.2 11.6 13.8

15.8 13.6 19.2 18.4

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have been harvested some time before) to these ditch habitats at this time. As the preferred (a combination of size and catchability) field vole are not available, then wood mice, the most abundant species, become the chief prey item. This is too late to influence the breeding success of the birds, but may be very important in maintaining post-breeding adults and dispersing immature birds in the winter. Reinforced by the number of owls that starve to death at this time, the availability of mammal prey is often considered to be the crucial factor in determining the overall success of barn owl (Taylor, 1989) although the role of other factors is currently under debate (Percival, 1991).

The influx of mice into other ditch types is not as marked and we think that this has little to do with the choice of the mice, which would select the more complex habitats with a wide ränge of foodstuffs (berries and tree seeds etc.) (see Flowerdew, 1991), but rather a case of who is already occupying the habitat. In the hedgerow habitats (particularly the unmanaged) this is bank vole, the numbers of which fluctuate little between seasons and years suggesting that these habitats already contain all the animals that can cope with. However, there is some scope for coexistance as mice appear to be much more able to exploit the aerial habitat of the hedgerows than bank vole (over 30% of mouse captures were off the ground compared to 5% of Bank vole) (see Pollard & Relton 1970). Nevertheless, the majority of mice avoid the high densities of bank voles, as reported by Flowerdew et al. (1977) and move elsewhere.

Of the other species, common shrew also shows stability between seasons and years. In summer there are higher densities in the more structured ditches especially those with adjacent grassland. This is presumably related to the absolute density of ground invertebrates being higher in these habitats.

In winter, after the high mortality of post-breeding adults in particular (Churchfield, 1991), the number of shrews per ditch tends to be lower.

However, the density whatever the ditch type is virtually constant (usually 2 or 3). Therefore, more shrews have died where numbers of shrews were highest (i.e. mortality is density-dependent) in the summer. Furthermore, even though the number of individual shrews appears constant, the number of captures is 40% higher in the unmanaged habitats compared to grass ditches and ditch hedges. This suggests that territories are smaller in unmanaged habitats leading to a greater encounter rate of a shrew with live traps and a greater probability of being captured. Therefore, shrew 'usage' may be higher in unmanaged habitats and perhaps these habitats offer a better chance of survival.

In general, the total number of animals per ditch increases with structural complexity and in winter after a summer of recruitment within the ditch perhaps supplemented by an immigration of wood mouse or field vole (in a good year) into the habitat, the best ditches can have over 20 animals using a 100m section. However, the most productive habitats for small mammals are not the best foraging habitat for barn owl, as owls find it difficult to penetrate very dense Vegetation to get at the animals underneath. This has led to habitats being managed to provide easier hunting for owls by cutting rank Vegetation (Johnson, 1991). However, if refuges are not provided then the mammals may be severely depleted by owl predation. A compromise should

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be Struck and the example of the ditch hedges may be a good one to follow. In these habitats, the grass bank provides good hunting habitat for the owls whereas the hedges provide cover for the mammals. In the current study, bank voles were much less likely to venture out of the hedges, (66% of captures within the hedge) than all other small mammals (between 25-40% of captures within the hedge). This may explain why bank vole never features very highly in the diet of ditch-hunting barn owls in the region (Cayford, unpubl. data) even though the species is the most abundant small mammal in these habitats. Also, the preference of bank vole for dense cover (Alibhai & Gipps, 1991); which is reflected in the preponderence of voles in field borders (Jefferies et al., 1973), hedgerows (Pollard & Relton, 1970) and dry stone walls (Healing, 1980) rather than in open fields in arable habitats; tends to account for the unimportance of bank vole as prey for barn owl in Britain as a whole (Bunn etal., 1982; Glue, 1974). The exception to thisis Ireland (Smal, 1987) where field vole and common shrew are absent and perhaps bank voles may move out into the available open habitat. Conversely, barn owl was often found in mixed habitats with plenty of woodland in the sites sampled (perhaps linked to the lack of tawny owl in Ireland) where they were perhaps more likely to encounter bank voles.

From a mammal conservation viewpoint, unmanaged hedges may be better habitat for the more uncommon species. The single female yellownecked mouse captured was always associated with unmanaged hedges although she ranged widely between ditches (over 100m in a couple of days) suggesting that large amounts of suitable hedgerow may be needed. Also, where there is rank Vegetation in association with the hedges, harvest mouse may be present. Results from other work (Perrow & Jordan, unpubl. data) suggests that cereal crops in modern agricultural land are generally unsuitable for harvest mouse. This seems to be partly related to the harvest of these crops, which appears to be getting earlier and earlier under today's highly intensive regimes, during the peak of the harvest mouse's breeding season.

Combined with potential losses during harvest and the lack of suitable habitat to migrate to, it is easy to see how populations become fragmented and decline to very low levels, making this species in particular, worthy of conservation interest.

Conclusions 1 different types of ditch contain markedly different small mammal communities, that vary in space and time 2 suitable ditch refuges, as breeding and/or wintering habitat, appear to be essential in maintaining populations of some species of small mammal (and their predators) in arable land 3 once the relationships between the mammals and their ditch habitat are fully understood, suitable ditch habitat for small mammals may be managed and even created relatively easily.

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8 Suffolk Natural History, Vol. 28 Acknowledgements We are particularly grateful to Mr. John Walpole and Mr. Aldridge on whose land this study was undertaken and to Dr. John Cayford for useful discus sion.

References Alibhai, S. K. & Gipps, J. H. W. (1991) Bank vole Clethrionomysgla In: The Handbook of British Mammals, 3rd edition. Blackwell Publications, Oxford, England.

Bunn, D. S., Warburton, A. B. & Wilson, R. D. S. (1982) The Barn O & A. D. Poyser, London, England.

Cayford, J. T., Perrow, M. R. & Peet, N. B. (1992) Foraging behaviour o radio-tagged barn owls (Tyto alba) in relation to seasonal changes in th distribution and density of small mammal prey. Proceedings of th World Conference on birds of prey, Berlin, Germany (in press Churchfield, J. S. (1991) Common Shrew Sorex araneus. In: The H of British Mammals, 3rd edition. Blackwell Scientific Publications, ford, England.

Flowerdew, J. R. (1991) Wood mouse Apodemus sylvaticus. In: Th book of British Mammals, 3rd edition. Blackwell Scientific Publica Oxford, England.

Flowerdew, J. R., Hall, S. J. G. & Clevedon Brown, J. (1977) Small roden their habitats and the effects of flooding at Wicken Fen, Cambridgeshire.

Journal of Zoology (London), 182, 323.

Gipps, J. H. W. & Alibhai, S. K. (1991) Field vole Microtus agrestis Handbook of British Mammals, 3rd edition. Blackwell Scientific tions, Oxford, England.

Glue, D. E. (1974) Food of the barn owl in Britain and Ireland. Bird stu 21, 200.

Green, R. (1979) The ecology of wood mice (Apodemussylvaticus) o farmland. Journal of Zoology (London), 188, 357.

Gurnell, J. & Flowerdew, J. R. (1990) Live trapping of small mammals practical guide. Occasional Publications ofthe Mammal Soci edition.

Healing, T. D. (1980) The dispersion of bank voles (Clethrionomys glareolus) and wood mice (Apodemus sylvaticus) in dry sto Journal of Zoology (London), 191, 406.

Jefferies, D. J., Stainsby, B. & French M. C. (1973) The ecology of sm mammals in arablefieldsdrilled with winter wheat and the increase in their dieldrin and mercury residues. Journal of Zoology (London), 17 Johnson, P. N. (1991) Barn Owls in North Norfolk. Norfolk Bird & Ma Report 1990 91.

Percival, S. (1991) Population trends in British barn owls - a review of som possible causes. British Wildlife, 2(3), 131.

Perrow, M. R. & Jowitt, A. J. D. (1992) The small mammal Community beds of Common reed (Phragmites australis), with special refer Harvest mouse (Micromys minutus). Mammal Review (in pre Trans. Suffolk Nat. Soc. 28 (1992)

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Pollard, E. & Relton, R. (1970) Hedges V: A study of small mammals in hedges and cultivated fields. Journal of Applied Ecology, 7, 549.

Skrivan, P. & A n d e r a , M. (1987) Small mammal communities of the stream corridors in agrocenosis. Ekologia (CSSR), 6(2), 129.

Smal, C. M. (1987) The diet of Barn Owl Tyto alba in southern Ireland, with reference to a recently introduced prey species - the Bank Vole Clethrionomys glareolous, 34, 113.

Tapper, S. C. (1976) Population fluctuations of field voles (Microtus): a background to the problems involved in predicting vole plagues. Mammal Review, 6, 93.

Taylor, I. R. (1989) The Barn Owl. Shire Natural History Series, no. 42, Aylesbury, England.

Dr Martin R. Perrow, Nie Peet & Adrian Jowitt, E C O N Ecological Consultancy, School of Biological Sciences, University of East Anglia, Norwich, N R 4 7TJ

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Plate 10: Harvest Mouse (Micromys minutus), one of the rarer species of small mammal associated with drainage ditches. (p. 3).