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Bioscience Horizons Advance Access originally published online on April 17, 2009
Bioscience Horizons 2009 2(2):155-163; doi:10.1093/biohorizons/hzp018
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© 2009 The Author(s)
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Study of small mammal populations within two Barn owl corridors at Folly Farm

Alex Keene*

School of Science and the Environment, Bath Spa University, Newton St Loe, Bath BA2 9BN, UK

* Corresponding author: 62 Ashgrove, Peasedown St John, Bath BA2 8EF, UK. Tel: +44 1761 420094. Email: alex.thewalkingcamera{at}virgin.net

Supervisor: Ian Todd, School of Science and the Environment, Bath Spa University, Newton St Loe, Bath BA2 9BN, UK.


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 Author Biography 
 
This study examines small mammal populations present within Barn owl corridors on Folly Farm, an Avon Wildlife Trust Reserve located near the village of Bishop Sutton in Bath and North East Somerset. Two corridors were chosen, the primary difference between the two sites being only one has undergone management (grazing). The focus of this study was the Microtus agrestis (Short-tailed field vole) population, the most frequently taken prey item by Barn owls. Apodemus sylvaticus (Wood mice) and Sorex araneus (Common shrew) populations are also discussed as they are frequently taken. Using Longworth live traps, 600 trap-nights data were collected from three sessions in November 2006, February and March 2007. Although M. agrestis was the most abundant species in both corridors, they were more prevalent in the un-grazed corridor (comprising 19 of the 31 individuals). In the corridor that had undergone management, fewer M. agrestis were captured (eight), although a higher species diversity and richness was recorded. Unusually for a grassland habitat, there were nearly as many A. sylvaticus as there were M. agrestis (seven compared with eight) in the grazed corridor. Some small mammal species not usually found in grassland habitats were captured; explanations for these seemingly anomalous results are discussed. Differences in population characteristics between the two corridors are discussed including: sex ratio, weights, seasonal variation and age structure. Pellet analysis from the nearby pair of Barn owls showed that they were preferentially hunting M. agrestis; the pellet data largely mirrored findings of the trapping data.

Key words: Short-tailed field vole, small mammal, Barn owl corridor, pellet, Longworth trap


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 Author Biography 
 
Barn Owls
The Barn owl, Tyto alba, has suffered in recent decades as a result of intensive agriculture,1 although it is suggested that climatic deterioration may also be a significant cause.2 Between 1932 and 1998, Barn owl populations declined in Britain from 12 000 pairs to <3000.3 A census carried out between 1995 and 1997 indicated that the decline has halted if not reversed in some areas.4 Unfortunately, their plight was once again highlighted in 2006, with national breeding failure rates as high as 70% (Andre Fournier, personal communication).

Barn owls are opportunists and will hunt over open countryside.5 Their ideal habitat is rough grassland with a thick sward and deep litter layer supporting high densities of small mammals. In an average year, a breeding pair of Barn owls may consume as many as 5000 small mammals,6 with predominance of Microtus agrestis.7 Changes in small mammal abundance and species composition can have serious effects on the breeding success of Barn owls.7 Barn owls cast an average two pellets a day and the structure of small mammal communities can be deduced from the comparative frequency of different species in the pellet sample. The Mammal Society National Owl Pellet Survey showed that M. agrestis make up the bulk of prey.8

Small Mammals
Changes in farming practice and the decline of other suitable small mammal habitats, as discussed by Harris et al.,9 have led to a decrease in many small mammal species, particularly M. agrestis.3 This has led to a variety of studies focused on both seasonal and annual cycles within populations at a single location10–12 and nationally.13 Previous small mammal surveys have been conducted locally looking specifically at populations present in Barn owl feeding areas.14, 15

This study looked at the overall composition of small mammal fauna within two corridors, three species were of particular interest: M. agrestis, Apodemus sylvaticus and Sorex araneus. Found in rough grassland throughout the UK mainland,9 the population dynamics of M. agrestis has been studied in some detail.16 Densities can range from 1/ha in mixed farmland9 up to 250/ha in rough grassland,17 with home ranges as large as 1.4 ha.18 A. sylvaticus is also highly mobile with similar home ranges, 0.26–1.77 ha in farmland.18 It is widely distributed in woodland, field and hedge habitats, but also occurs in sand dunes, gardens and urban areas.19 Densities of 0.5–17.5/ha are typical on farmland.16 S. araneus are also found in a wide variety of habitats, including grassland, arable and woodland. They occupy relatively small home ranges, often less than 0.1 ha20 with densities of 5–69/ha.

Populations of some small mammals fluctuate. The causes of cyclic population changes, and the role that predators play, have been largely unresolved, despite high volumes of literature.21 Specialist hunters, such as Barn owls, are particularly reliant on small mammal populations, especially M. agrestis. Populations of these predators increase when voles are abundant and contract when numbers are low.22

Numerous studies have tried to determine what influences small mammal community composition. Gause23 demonstrated both empirically and theoretically that coexistence of competing species is only possible if each have different, specialized abilities. Several authors have attempted to find correlations between plant and animal species richness and diversity; some have shown correlations,24 others have not.25 Rozenweig and Winakur26 found that spatial variations in the density of some species are responses to spatial characteristics of their environment. Dueser and Shugart27 both found that simple measurements of density and height of vegetation within different strata provided definite correlations with the densities of species present.

A structurally diverse habitat is likely to contain a larger variety of food sources and nesting sites, providing niches for predatory species as well as protection for prey.28, 29 If mobility allows then mammals have been shown to distribute themselves between habitats in relation to habitat quality.30 The most important factors being food availability, food quality, predation risk and competition.31


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 Author Biography 
 
Study Area
Avon Wildlife Trust has owned Folly Farm since 1987. It is located near Bishop Sutton in the Chew Valley (Grid reference ST607604). Much of the farm has escaped modern, intensive practices and is traditionally managed with the primary aim of protecting and enhancing its wildlife value.32 Over 28 ha of the farm are designated as a biological Site of Special Scientific Interest (SSSI).

Six Barn owl corridors were instated in 1999/2000. The pair of owls present on Folly Farm failed to breed in 2006 for the first time since monitoring began in 1992 (Andre Fournier, personal communication). There is no strict cutting/grazing rotation in place within the corridors; those with stock fencing are grazed, others are mown, some are even cut by hand. Management is accessed annually and is dependent on stock and labour availability. Until now, no small mammal survey work has been conducted in the corridors.

Time and resources made it impossible to survey all six corridors and therefore two were chosen, the primary difference between them being only one has undergone management. Site 1, the ‘un-grazed’ corridor, has received no management since being fenced off at the end of 1999, other than having ragwort pulled and thistles topped when infestations occur. The result of this is a thick, deep sward and relatively high species diversity. The best NVC match is MG1b, an Arrhenathetum community, typical of un-grazed grassland33 on neglected pastures and meadows. There are two Lime trees, Tilia cordata, within the corridor. Site 2, the ‘grazed’ corridor, has been grazed twice, first in August 2000 and again in 2004. This site is botanically less interesting, the sward less dense and vegetation height substantially lower. NVC results produced a classification of MG10/MG9. These plant communities occur mainly in fields and associated boundaries, and are often associated with poorly drained permanent pasture.34 There are two veteran Oaks, Quercus robur, present; the most southerly contains a Barn owl box.

Trapping
Trapping was undertaken using Longworth live-capture small mammal traps.35 Both corridors were trapped simultaneously during 3 and 4 days, sessions in the mid-November 2006, early February and late March 2007. The traps were prepared with hay bedding, mixed birdseed and blowfly puparia for bait. Due to the linear structure of the habitats, and their effectiveness with assessing community structure,36 traps were laid in transects. Twenty-five trapping points were marked at 5 m intervals within each corridor; the Longworth traps were then located ± 1/2 m from the mark. On Site 2, the transect was located at the southern end of the corridor to avoid a regularly used footpath. Traps were deliberately placed in positions that indicated the presence of, or would be favoured by, small mammals. All field signs were noted and located by relevant trap number.

There were 200 trap-nights in each session, a total of 600 trap-nights for the whole programme. The traps were laid and set on the afternoon of the first day and checked in the morning and evening of the next 3 days, then checked and removed on the morning of the fourth day.

All small mammals caught, except shrews, were identified to species, weighed to the nearest gram using a 50 x 0.2 g Salter spring balance, aged, sexed and breeding condition noted. All species captured were assigned to one of the following three age classes: adult, subadult or juvenile, according to Gurnell and Flowerdew.37 New captures were given a unique fur clip. All mammals were released at the point of capture. All shrews caught were identified to species, weighed and released immediately. Environmental data were recorded every time the traps were checked, giving an account of weather during the previous 12 h.

Vegetation of both corridors was largely homogenous throughout; therefore, detailed floral surveys around each trap were deemed unnecessary. Instead physiognomic vegetation classification was used; a popular method of habitat classification in researching habitat selection and utilization.38 Vegetation height was measured above the entrance to each trap and recorded as short (<10 cm), medium (10–50 cm) or long (>50 cm).

Owl Pellets
Owl pellets were collected from below the Barn owl roost on Folly Farm in May and June 2006. The roost was located in the SSSI grassland ~500 m from the survey sites, making both corridors well within hunting range of the owls. The pellets were soaked in warm soapy water, teased apart and all mammals remain cleaned and laid aside. Identification was carried out on skulls and lower jaw bones using a x10 hand lens. Species were keyed out according to Yalden and Morris.39


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 Author Biography 
 
Trapping
A total of 62 small mammals were captured over the 600 trap-nights (summarized in Table 1). One Sorex minutus was found dead and one A. sylvaticus escaped before being processed. The vast majority of captures were recorded at dawn and therefore captured overnight. The two captures during the day were both S. araneus, one during the November session and the other in March. Both were adults and recorded in the un-grazed corridor. During all three sessions, there were no captures on the first day and highest number of captures were made on the final day. Small mammal catches were relatively evenly distributed across the un-grazed corridor, whereas the grazed corridor had a more patchy distribution pattern, with a clear lack of captures in the first seven traps. There are distinctly different patterns of seasonal population change in the M. agrestis and A. sylvaticus populations. Numbers peak in November in the case of M. agrestis and February in the case of A. sylvaticus and Sorex sp. (Fig. 1).


Figure 1
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  		Figure 1. Seasonal change in population structure of all species captured. Total number of individuals captured in November (Figure 1), in February (Figure 1) and in March (Figure 1).

 


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  		Table 1. Total number of captures and individuals caught during the survey period, where individuals are first capture of an animal (i.e. no visible fur clip)

 
Sex ratio and reproductive condition
The sex of shrews was not assessed. The overall sex ratio of all other species caught in November and February was close to 1:1; however, by March, the ratio was skewed in favour of males (1.8:1) (Table 2). Between November and February, all captures were non-breeding (condition of shrews was not assessed), all males having abdominal/small testes and all females imperforate. In March, all male captures were still non-breeding except for two A. sylvaticus caught in the un-grazed corridor, recorded as having testes large and scrotal, and all females were still non-breeding.


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  		Table 2. Number of male and female captures, of all species, in both corridors

 
Age structure and weights
Juvenile mice were only captured in November, when they made up 14% of the population (Fig. 2). At this time, the rest of the population was made up of a nearly even mix of adults and subadults. By February, the majority of the population is adult, 73%. The adult proportion has reached 100% by March. In Fig. 3, mean weights of S. minutus remained unchanged between February and March. A. sylvaticus and S. araneus weights remained unchanged between November and February but increased in March. The mean weight of M. agrestis fell, although not significantly, from November to February (t = 1.54, df = 20, p < 0.138). The mean weight of M. agrestis rose significantly between February and March (t = –3.56, df = 13, p < 0.003).


Figure 2
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  		Figure 2. Age classes of all mammals caught in both corridors. Whole bar is total population, Figure 2 is the number of adults, Figure 2 the subadults and Figure 2 the juveniles.

 


Figure 3
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  		Figure 3. Mean weight of individuals in all three sessions. The mean weight of M. agrestis (Figure 3), A. sylvaticus (Figure 3), S. araneus (Figure 3) and S. minutus (Figure 3).

 
Species
The total number of small mammals present (Table 3) was not significantly higher in the un-grazed corridor (X2 = 2.88, df = 1, p ≤ 1). The proportion of M. agrestis was significantly higher in that corridor (X2 = 4.48, df = 1, p ≤ 0.05). The species diversity was higher in the grazed corridor (Simpson's index of diversity,40 1 – D = 0.8) than in the un-grazed (0.6). Five different species were recorded in the un-grazed corridor, the majority were M. agrestis (19) followed by S. araneus (6) then A. sylvaticus (4). Six species were recorded in the grazed corridor; the highest occurrence was still that of M. agrestis (eight) then A. sylvaticus (seven). Micromys minutus was only recorded in the un-grazed corridor, and Clethrionomys glareolus and Neomys fodiens were only recorded in the grazed corridor.


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  		Table 3. Species composition, richness and diversity (Simpson's index) in the grazed and un-grazed corridors

 
Movement of individuals
Ten small mammals were recaptured, eight of these in the un-grazed corridor. Seven were M. agrestis and three A. sylvaticus. The mean distance travelled was 17.5 m in the un-grazed corridor, higher than that recorded in the grazed corridor (5 m). The mean distance travelled by M. agrestis was higher, 18.6 m, than A. sylvaticus, 6.7 m. These figures will however be skewed by the movement of an individual M. agrestis in the un-grazed corridor, travelling 65 m in a 24 h period. Only one long-term movement was recorded, between capture on the 9th February and recapture on the 25th March the M. agrestis moved only 15 m. There was no recorded movement between the two corridors, a minimum distance of ~150 m.

Effect of vegetation height on trapping success
The proportion of mammals caught in the short and long grass is different between corridors (Fig. 4). More mammals were captured in the medium length vegetation (10–50 cm) in both corridors but probably this is an artefact of the greater availability of traps within this length. In fact, in the un-grazed corridor, a preference for long vegetation and an avoidance of short vegetation was shown. Although the differences were more subtle, the opposite was recorded in the grazed corridor, with more mammals preferring short vegetation. Figure 5 shows the un-grazed corridor to have the highest capture rates of M. agrestis in traps placed under long vegetation, with 13% of traps making a capture. A. sylvaticus and Sorex sp. both showed a slight preference for the medium length vegetation. In the grazed corridor, these trends are reversed for all three species. For example, the long vegetation records the lowest capture rates for M. agrestis.


Figure 4
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  		Figure 4. Vegetation length in which mammals were captured on both sites. L, long (>50 cm); M, medium (10–50 cm); S, short (<10 cm). Whole bar represents the number of traps placed in different vegetation heights, area shaded (Figure 4) is the total number of captures within that length and the figure in white is the percentage of captures within each length.

 


Figure 5
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  		Figure 5. Vegetation length in which different species were captured on both sites. Bar represents the percentage of captures made within available traps at each different vegetation height, in both corridors.

 
Owl Pellets
Table 4 shows the results of the pellet analysis alongside a summary of the trapping data in Table 1 and the results from the Mammal Society's National Survey (1993–2005).8 The Barn owl pellets recovered in May contained an estimated 114 individual mammals and pellets collected in June contained 55; a total of 169. One hundred of these were identifiable to species. M. agrestis formed the majority of prey species followed by A. sylvaticus and then Sorex spp. The rank of species in all three data sets is the same with two exceptions: A. sylvaticus and Mus domesticus are ranked lower in the Mammal Society results than in the data sets from Folly Farm.


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  		Table 4. Percentage of species trapped in both corridors, in owl pellets from Folly Farm and in pellets analysed in The Mammal Society National Owl Pellet Survey8

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 Author Biography 
 
Microtus agrestis in particular, A. sylvaticus and Sorex species were the most common small mammals captured at Folly Farm. This was expected, as Barn owl corridors should provide suitable habitat for these species.9, 41, 42 The single C. glareolus capture was unexpected as they are rarely found in grassland,43 although studies have shown those resident in nearby hedgerows or woodland to enter field margins.44, 45 The Micromys minutus capture in the un-grazed corridor was also unusual as field margins are not considered suitable habitat. Bence et al.46 did suggest that, given suitable growth, they could support M. minutus. The most unusual capture was a single N. fodiens in the grazed corridor, a species generally found along watercourses. In 2004, a survey of the adjacent watercourses47 recorded no indicators of their presence on the farm, making the find even more unusual. Tew10 has recorded them in hedgerows and suggested that linear features were vital corridors between preferred habitats. The presence of streams running either end of the corridor suggests a transient capture.

The majority of small mammals were captured at dawn. There were two exceptions, both S. araneus, caught in the un-grazed corridor. This is little surprise as A. slyvaticus and M. agrestis are mainly nocturnal, whereas Sorex species can be active night and day throughout the year.16 During all three trapping sessions, no captures were made on the first day. There is no obvious reason for this other than the small mammals being ‘trap-shy’. This reluctance to enter ‘new objects’, particularly by Microtus species, is often found.35 Three nights of trapping is considered sufficient to allow trap-shy mammals to accept the presence of traps48 and therefore give an accurate picture of the population.

In arable landscapes, seasonal variations in A. sylvaticus densities have been shown to vary dramatically.49 The results in this study agree with Harris et al.;9 although there is seasonal variation, with a winter peak, these cycles are not as marked in marginal habitats such as grassland. Similar trends were seen for the Sorex species, although these figures should be examined with caution, the February peak could purely be a reflection of the increased number of traps without ‘shrew-holes’ (an unavoidable consequence of having to borrow traps at short notice). M. agrestis show an opposite seasonal change in population with numbers dropping off in February and increasing again by March. As the March increase could not be down to breeding, it must be a result of immigration from an adjacent habitat or possibly an inaccurate assessment of the population in February, with a percentage of M. agrestis alluding capture. Low numbers meant no trends could be seen for the remaining species. If these seasonal fluctuations are accurate, the peak in A. sylvaticus numbers coinciding with a trough in M. agrestis numbers could benefit Barn owls. Meek et al.50 indicated that Barn owls are able to easily switch to A. sylvaticus, if present, when M. agrestis numbers are low.

In March, there is a clear increase in the proportion of males within the population. It is recognized that such seasonal changes occur, with males being more numerous over winter and spring.51 Krebs and Davies52 identify two main mechanisms contributing to this seasonal variation, differential recruitment and differential survival rates. Of these two, it is likely that differential recruitment is the dominant of the two mechanisms as the increases coincide with the onset of the breeding season. It is also possible that the number of males has not increased; those present may simply be more active. During the breeding season, males are often noted as having increased activity and larger ranges.16

The overall trend of increase in mean weight will be largely reflective of changes in age structure, as the proportion of adults increases so does mean weight. However, the only long-term recapture (M. agrestis) did show an increase of 2 g between February and March, indicating that individual weights had also increased.

M. agrestis were found to be the most abundant species in both corridors, and this mirrors the findings of Wilkinson15 in rough grassland at nearby Weston Moor Nature reserve. Other studies by Lambin et al.17 also show relatively high abundances within this habitat. The proportion of M. agrestis within the total population was significantly greater in the un-grazed corridor; this was unexpected. The Barn Owl Trust guidance42 suggests that low-intensity grazing/cutting should be carried out at least every 2 years to maximize vole numbers. Even after 7 years with no cutting/grazing routine, there is still a higher population of M. agrestis in this corridor. The grazed corridor, undergoing a grazing routine every 2/3 years, should have higher vole numbers;53 in fact, the opposite is the case.

The grazed corridor supports a higher species richness and diversity than the un-grazed one. Higher numbers of more generalist species, in this case A. sylvaticus, were found in the grazed corridor. These findings are similar to those of Trump14 and Tattersal et al53 who were looking at field boundaries and set aside, respectively. The generalist nature of A. sylvaticus make it able to adapt to change, i.e. grazing,3 and they are less dependent on ground cover than voles.49 Another factor may be that they are able to travel considerable distances, so may not inhabit the area in which they are trapped.43 Increased species richness and diversity in this corridor could be reflecting its diverse surroundings of woodland, streams, farm buildings and veteran trees.

The use of transects and the low number of recaptures make the use of population estimates flawed in this study. Although the Peterson-Lincoln method of population estimation will prove unreliable, it is interesting to speculate as this allows the population to be compared with documented densities. At their peak in February, M. agrestis densities reach 128/ha in the un-grazed corridor and 48/ha in the grazed corridor. Although slightly on the low side, they do fall within most expected densities for grasslands,9, 17 and mixed farmland.54

The small mammals captured in this study appear to be fairly sedentary. Despite recorded home ranges of up to 1000 m2, 18 the mean distance travelled by M. agrestis was only 18.8 m and the maximum being 65 m. Even more surprising was the 6.7 m mean distance covered by A. sylvaticus whose home ranges on farmland often exceed 1 ha (10 000 m2). This study did not extend to the breeding season, and Corbett and Harris16 noted that home ranges can remain small over winter until the onset of sexual maturity in spring. Similar changes in movement patterns were recorded by Trump14 with few A. sylvaticus moving more than 10 m in the winter months, but recorded movements of 30–50 m by May.

The distribution patterns of small mammals in both corridors are different. The higher floral species richness and deeper thatch layer in the un-grazed corridor may provide a wider range of food resources, microhabitats and increased protection from avian hunters explaining the even distribution. The distribution pattern in the grazed corridor is more patchy. The peak in capture rates in some traps could be explained by their location under the Oak tree, Q. robur. Interestingly, in the un-grazed corridor, the Lime trees, Tilia cordata, appeared to have no effect on capture success. There was also an absence of captures in traps near a footpath in the grazed corridor. The presence of the footpath could in part explain this; however, there was also found to be a healthy population of weasels, Mustella nivalis which could have increased local predation pressure on the small mammal population.

In the un-grazed corridor, as vegetation height increases so do the number of captures. This pattern might be expected. Hamback et al.31 showed that, especially over the winter months, increased vegetation height led to increased vole activity. They indicated that when selecting over-wintering habitat vegetation height was important to reduce predation and freezing risk. Looking at the individual species, it is clear that this trend is confined to M. agrestis, the other species appear to avoid long vegetation. There are no perceivable trends in the grazed corridor.

The pellet data from Folly Farm are broadly similar to that published from the Mammal Society's National Survey.8 The Folly Farm pellets show M. agrestis to be the most popular prey item, then a preference for A. sylvaticus over S. araneus, the opposite was the case in the national survey. This may be reflective of the relatively high proportion of A. sylvaticus seen in the trapping data, particularly in the grazed corridor. No M. minutus or N. fodiens were found in the pellet samples, indicating that the low capture numbers in the Longworth traps accurately depict the population. Optimal Foraging Theory52 would suggest that a low number of less preferential species, such as S. minutus, indicates that the proportion of more profitable species is high.

There is a similar species ranking found between the trapping results and the pellet data. Similarly, Bonvicino and Bezerra55 showed that regurgitated pellets from Barn owls were a good source of information for assessing species richness. They found strong correlations, when assessing total population, despite stressing the potential for bias in numbers due to the preferential feeding habits of such species.

There is little information on the accessibility of prey items to Barn owls. There is an expected preference for M. agrestis in this study; a species that is more abundant and dominant in the un-grazed corridor. What is unknown is to what degree the high population, in the deep thatch layer, is offset by their increased inaccessibility. Barn owl morphology does suggest that their legs and talons should enable them to penetrate dense grassland1. Lack56 stated that sward heights of at least 25 cm were ideal for Barn owl foraging. Meek et al.50 suggested that there is an optimal corridor width if Barn owls are to hunt at maximum efficiency. This width is one where the greatest number of prey can be intercepted on a single fly over. Flying most commonly at a height of 3 m when hunting and having an effective hearing range of 50° indicates that this margin width would need to exceed 7 m. Although not as wide as the grazed corridor, the un-grazed corridor is up to 55 m wide and no <20 m wide at any point.

This study is based on the premise that the strongest influence on community structure within these two corridors is habitat quality, the main influence on this being past and present management. Without a long-term monitoring programme and no specific historical data, the accuracy of this statement, and the degree to which un-measured external influences play a role on populations, has to remain un-quantifiable. Although M. agrestis was the most abundant species in both corridors, they were particularly dominant in the un-grazed corridor. The total population is low, despite the peak falling within the expected range. The un-grazed corridor has not received any management over the past 8 years and still supports a relatively healthy M. agrestis-dominated community of small mammals. The data presented in this paper suggest that the current recommended rotation of 2/3 years is unnecessary. The apparent M. agrestis densities in the grazed corridor are surprisingly low. The trapping results, NVC classifications and sward condition suggest that this corridor has been over-grazed. The use of a closely monitored system of cattle grazing would be recommended.42 If not financially or practically possible, the corridors could be mechanically topped to a height of no <20 cm. Although a valuable data set, the potential strength of this study lies in it becoming baseline data for future monitoring. It is suggested that a long-term monitoring programme be set up to give a more accurate picture of populations present within all the corridors at Folly Farm.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
Author Biography 
 

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   Author Biography 
Top
 Abstract
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 Methods
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 Discussion
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 Author Biography 
 
    Alex Keene is currently studying Environmental Biology at Bath Spa University. On completion of the course he will be seeking work involving both the practical and educational side of environmental/conservation management. Alex has been working as a freelance photographer for the past ten years and is keen to integrate this into his new career at some point in the future.
Submitted on 30 September 2008; accepted on 12 February 2009


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