Mammal Rev. 1993, Volume 23. Nos 314. 121-1 26. Printed in Great Britain.

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20,000 - 15,000 - 10.000 - 5,OOo - Fig. 2 Changes in the annual rate of loss of

The impact of land-use change on voles and raptors

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MARTYN L. GORMAN & PETER REYNOLDS* University of Aberdeen. Department of Zoology, Culterty Field Station. Newburgh, Ellon AB41 O M , Scotland

INTRODUCTION Voles are common animals and they form an important part of the diet of a variety of raptors. For example, Shorteared Owls Asioj'ammeus depend upon them, to a greater or lesser extent, throughout much of Europe (Fig. 1). One might predict. therefore, that any changes in land use that reduce vole populations, or destroy the favoured hunting grounds of raptors, will lead to a reduction in raptor populations (O'Connor & Shrubb, 1986). For example, in the British Isles the increasing loss of hedgerows that has continued unabated since the 1930s (Fig. 2) has coincided with a major decline in populations of Barn Owls Tyro a h , a species that hunts preferentially in such linear habitats. In 1932 there were some 25 OOO barn owls in Britain, but by 1982 these had been reduced to around 8000 (Shawyer, 1987).

Until quite recently the Orkney islands, lying off the north coast of mainland Scotland. had not been subject to intensive agriculture and they retained large areas of moorland, wetlands and seminatural habitats. However, over the last few decades increasingly more land has been devoted to agriculture

YO Microtus in winter diet of Short-eared Owls

Fig. 1 The contribution of voles (Microfus sp.) to the winter diet of Short-eared Owls in different parts of Europe. (Uttendorfer, 1952; Martin & Saint- Girons, 1953; Schmidt & Szlivka. 1968; Data from Glue, 1977).

" Britain France Germany Yugoslavia

122 M. L. Gormon & P. Reynolds

% Land under agriculture

loo 1

" 1

1825 1850 1875 1900 1925 1950 1975 2000 Date

Changes in the percentage of the land area of Orkney used for agriculture during the period 1833-1990. Data from Berry (1985) and the Scottish Office (1990).

(Fig. 3). Thus, on Orkney Mainland at least 40% of the moorland and over 15% of the wetland present in 1940 had been lost by 1985. Throughout the 1980s increasing concern was being expressed by both voluntary and statutory conservation bodies about the loss and fragmentation of seminatural habitats in Orkney. In particularthere were fears forthe future of the nationally important populations of raptors that breed in Orkney; Hen Harriers Circus cyaneus, ground-nesting Kestrels Falco tinnunculus and Short-eared Owls. In this paper we look, in some detail, at the relationship between land use and populations of voles and raptors in the Orkney islands.

The Orkney Vole Microtusarvafisorcudenscodensis is the only rodent that is active by day in Orkney and thus is potentially of central importance 10 the future of the raptor populations. The vole is also of great importance in its own right as an endemic subspecies that is replaced in other parts of the British isles by the Field Vole Microtus agreslis. Orkney Voles were probably introduced to the islands by early settlers and vole remains are to be found in the lowest levelsof middens of the Neolithic village at Skara Brae, dated at around 5500 BP. Orkney Voles are big animals, with large males weighing up to 90 g compared to 35 g for Continental Microtus arsalis.

In this paper our specific objectives are: to determine the importance of voles in the diets of breeding raptors to measure population densities of voles in a variety of habitats to determine whether raptors concentrate their hunting in particular habitats and if so whether they select those habitats that support most voles.

THE DIET OF BREEDING RAPTORS We have investigated the importance of voles to breeding raptors by observing what prey items they bring to their nests (Table I). In the 1988 breeding season all three species of raptors included voles in their diet, but not equally so. The Shorteared Owls clearly specialised on voles and these accounted for all of the observed daytime nest deliveries. Kestrels were rather less dependent on voles (69% of deliveries) and Hen Harriers least so with voles forming only 15% of prey items.

Table 1 The percentage composition of prey items brought to the Hen Harrier Kestrel Shon-eared Owl

Voles Rats Rabbits Shrews Birds

I5 69 100 5 0 0

28 0 0 0 12 0

52 18 0

. - nests of Kestrels ( I nest), Hen Harriers (5 nests) and Short-eared Owls (19 nests) in 1988

No. of prey items 67 49 32

Land-list. change: impact on voles and raprors 123

Habitat

Fig. 4 Vole densities in different habitats during August and September. The data are averages for the years 1988 to 1990 inclusive.

Old peat cuttings Fenceline grass

Rough grass Abandoned reseed

Wet heath Calluna moorland

Blanket bog Eriophorum moor

Reseeds Barley

Turnips

0 100 200 300 400 500 Voles per hectare

. HABITAT SELECTION B Y VOLES We have collected density data from 14different habitats on the island of Mainland using grids of snap- traps, or trap lines in linear habitats. Each habitat was trapped for 5 days, 4 times per year, for 2 years. Densities were estimated from an analysis of the decrease in catch per unit effort (Krebs, 1989).

Although voles were present in a wide variety of habitats, there were marked differences in average density from one habitat to the next (Fig. 4). Maximum densities of 300-500 voles per hectare were found in old peat cuttings, rough grassland and in linear habitats such as fence lines, ditches and road side verges. At the other extreme, voles were completely absent from agricultural crops, including reseeded grassland which would appear to be ideal habitat. Within agricultural areas voles were confined to the rough grass habitats along ditches and fence lines and there they were present at very high densities. We have confirmed by radiotelemetry that voles rarely, if ever, left the fence lines in order to forage in the adjacent fields.

There is historical evidence to suggest that up to the 1940s voles were present in a range of crops. Millais (1904) mentions their Occurrence in rich grass and clover fields and ‘doing here and there considerable damage to crops’. Hewson ( 1948) also found voles in both pasture and arable fields during the early 1940s. In parts of Europe Microrus arvafis continue to frequent agricultural habitats. In Czechoslovakia, for example, Gaisler el af (1967) noted that the species was abundant in agricultural land while Wojciechowska (1969) refers to the vole as the most common rodent in agricultural landscapes of western Poland. Extensive cultivated fields support high densities of Common Voles in Finland (Mikkola & Sulkava, 1969). The marked absence of voles from agricultural areas in Orkney may be due to the particularly intensive nature of land management. including high stocking rates and extensive areas devoted to silage production.

HABITAT SELECTION BY HUNTING RAPTORS In this paper we have chosen to present data on habitat selection by Shorteared Owls since this species is clearly a vole specialist. We test the prediction that hunting birds will concentrate their hunting activities in those habitats which support most voles. To do so we canied out an intensive study at Backatown on Orkney Mainland. The study site consisted of an ‘island’ of natural habitats surrounded by a sea of agricultural land, mainly reseeded grassland. The habitat island supported a single breeding pair of Shorteared Owls

In order to determine whether or not there was selection of particular habitats for hunting, the male owl was observed continuously for a total of 39.6 hours, in bouts of 30-390 minutes, spread over a period of 15 days. During the period of observation, the male spent 27% of his time hunting and the

124 M. L. Gorman & P. Reynolds

Voles per hectare Preference index 140

120

100

80

60

40

20

0

Fig. 5

A 0 C D E F G H

The densities of voles in different habitats within the home range of a pair of Short-eared Owls, together with habitat preferences for hunting owls: linear habitats rough wet grass old peat cuttings mixed moorland abandoned reseed Cuffunn moorland reseed barley

A B C D E F G H

female remained continuously on the nest. All the detailed movements of the male were transcribed onto a large scale map of the area and his extreme locations were used to define the home range as a minimum convex polygon. This home range contained 10 main habitat types whose areas were measured from aerial photographs. Vole densities were measured in each of the major habitats.

Habitat preference or avoidance by the hunting owl was determined by means of a simple log- transformed index of habitat preference for habitat i (Duncan, 1983):

in which Pi is the preference index for habitat i, U, is the percentage of time spent hunting in habitat i and Ai is the percentage of the owl’s range covered by habitat i.

With this log-transformed index, valuesgreaterthan 0.3 imply preferencefora particularhabitat and values less than 0.3 indicate avoidance.

Preference indices for the hunting owl together with vole densities in the different habitats are shown in Fig. 5. Clearly. the owl selectively hunted linear habitats, rough wet grassland and old peat cuttings, while mixed moorland was hunted in proportion to its area. and agricultural crops such as barley and reseed were avoided. It is also apparent from Fig. 5 that those habitats which were preferentially hunted by the male owl were also those which supported the highest densities of voles.

CONSERVATION OF VOLES AND RAPTORS Finally, we would like to briefly address the problem as to how both voles and raptors might be best conserved in the contemporary Orcadian landscape.

Based on our density measurements in different habitats, tlie current population of Orkney Voles is still likely to be substantial, with a seasonal minimum and maximum of around 1 and 4 million respectively for the whole archipelago. However, voles are now confined to seminatural habitats, having abandoned agricultural areas sometime in the last 50 years, probably in response to increasing agricultural intensification and changes in cropping patterns. Since 1936 the area under agriculture has increased, at the expense of natural habitats, From 37% to 8 1 %. It is likely, therefore, that since the war there has been a very substantial reduction in vole populations together with an increase in the degree of population fragmentation and isolation. To give just one specific example of how populations may have changed, it is likely that on Orkney Mainland over 100,OOO voles have been lost from what were once moorland habitats.

It is clear that the population of Orkney Voles is now substantially less than it once was. Given the importance of voles in the diets of raptors, is there any evidence of an associated decline in their own fortunes?

hnd-use change: impact on voles and raptors 125

The breeding population of Kestrel has certainly declined in abundance over recent decades; for example one area of west Mainland that supported 19 pairs in the 1950s now has less than half that number. There is no good evidence for a similar decline in the size of the population of Hen Hamen in the last 40 years. However, it must be noted that breeding productivity. as measured by the mean numbers of young fledged per nest has declined from around 1.3 in the 1950s to less than 0.9 in the 1980’s. Unfortunately the available dataon past numbers of Short-eared Owls in Orkney are inadequate for any analysis of changes in population size or productivity.

If one accepts that the Orkney Vole should be conserved, both in its own right as a unique island subspecies of Microtus amah and as a prey item for avian predators, precisely what measures should be implemented so as to achieve this objective? The vole resource is currently widely dispersed in numerous fragments of seminatural habitat, many of which are very small. In view of this, existing site- specific conservation measures such as Site of Special Scientific Interest designation under the Countryside and Wildlife Act ( I 98 I ) would seem to be both inappropriate and unworkable. Clearly what is required is a more holistic, wider countryside approach to land use and conservation, as embodied in the concept of Environmentally Sensitive Farming (RSPB, 1991). in which less intensive land management tailored specifically to the needs of environmental conservation is encouraged.

Specifically, all remaining areas of seminatural vegetation should be retained, including both optimal and suboptimal vole habitats. Areas of optimum vole habitat, such as rough grassland, tend to behighly localisedandlimited in total areasoconservationoftheseareasinisolationmaybeinsufficient tosecure the long termconservationofvolepopulations.Theadditionalretentionof suboptimal habitats will increase the potential reservoir of voles and thus reduce the chances of local extinction.

Within the intensively managed agricultural land that now dominates Orkney, voles are very largely confined to linear habitats consisting of rough-grass-tall-herb vegetation associated with fence lines, ditches and road verges. These habitat comdors typically support very high densities of resident voles and thus function as linear reserves that serve to maintain vole populations within the intensively managed agricultural landscape. In addition, these corridors maintain contact between vole populations living in otherwise isolated fragments of natural habitats. These linear features may well, therefore, be important for enhancing gene flow and in allowing recolonisation of habitat patches following local extinction.

Within the context of environmentally sensitive farming, linear habitats provide good opportunities for positive conservation management. In particular, the extension and enhancement of existing linear features and the creation of new ones tocreate a network of comdors with maximum connectivity may be beneficial to the conservation of both voles and raptors.

Encouragement of less intensive methods of agricultural production and a return to more mixed farming systems may also be beneficial in allowing the return of voles to agricultural land. This could be achieved by reductions in grazing intensity, particularly on areas of seminatural habitats such as rough grassland and moorland, together with an increase in hay making at the expense of silage. All such measures would of course require financial incentives of the type already embodied in existing Environmentally Sensitive Areas (RSPB, 199 1 ).

Many of the larger remaining areas of moorland habitat are owned or leased by the Royal Society for the Protection of Birds in recognition of their importance as breeding areas for raptors. Judicious and localised habitat manipulation on these Reserves, particularly those with large areas dominated by Calluna vulgaris, should perhaps be considered with the objective of increasing vole populations. Calluna moorland typically supports very low densities of voles and these could be considerably enhanced by the local establishment of areas of rough, ungrazed grassland. For example, linear comdors of rough grass could be sown during the creation of moorland firebitaks, thus serving a dual function.

126 M . L. Gorman & P . Reynolds

if implemented, such measures should ensure the continued existence of the Orkney Vole and of the nptors that feed upon it, thus perpetuating an association that began over 5000 years ago with the amval of the first animals.

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