The diet of field voles Microtus agrestis at low population density in upland Britain

Transcription

1 Acta Theriologica 50 (4): , PL ISSN The diet of field voles Microtus agrestis at low population density in upland Britain Philip WHEELER* Wheeler P The diet of field voles Microtus agrestis at low population density in upland Britain. Acta Theriologica 50: Studies on field voles Microtus agrestis Linnaeus, 1758 in lowland grasslands have shown them to be unselective grazers. The diet of the field vole in upland Britain was investigated using feeding trials with four of the dominant British upland monocots, Molinia caerulea, Nardus stricta, Deschampsia flexuosa and Eriophorum vaginatum. The suitability of faecal analysis was assessed and then used to analyse the diet of wild voles from faecal samples. Percentages of plant species in the faeces were compared to percentages on the ground in sites dominated by Molinia caerulea, Eriophorum vaginatum, Nardus stricta and Calluna vulgaris. Significant preferences for the grass Deschampsia flexuosa were observed in feeding trials and in the wild while the sedge Eriophorum vaginatum was avoided in both. There was no clear preference for Molinia caerulea and Nardus stricta. Preference for plant species was related to palatability and nutrient content. The low nutrient conditions in British uplands mean that voles that live in these environments must be selective feeders to maximise nutrient intake. University of Manchester, School of Biological Sciences, Oxford Rd Manchester, M13 9PT, United Kingdom Key words: Microtus agrestis, diet selection, faecal analysis, cafeteria tests Introduction Studies on the field vole Microtus agrestis Linnaeus 1758 are numerous but have focussed on areas where field voles are abundant, not necessarily those that hold the majority of the population. In Britain, they have typically been investigated in lowland grasslands, or more recently in plantation clearfells. This is despite suggestions that the majority of British field voles live in upland areas (Harris et al. 1995) where conditions are significantly harsher than in lowlands. Uplands are defined as areas above enclosed farmland, which in Britain is usually land over 300 m above sea level. Field voles are an integral part of the British small mammal fauna and form the major food source for many mammals and birds (Dyczkowski and Yalden 1998), several of which are themselves upland species. This study presents the first analysis of field vole diet from upland areas. * Present address: University of Hull, Scarborough Campus, Filey Rd, Scarborough, YO11 3AZ, United Kingdom, p.wheeler@hull.ac.uk. [483]

2 484 P. Wheeler The aim was to identify the general diet from a wide area, rather than fine details from a small number of sites. Field voles are usually regarded as non-selective graminivores (Faber and Ma 1986). They eat mostly grasses but seem to show little selection within this group; dicotyledonous plants will be taken if sufficiently abundant (Evans 1973). Some studies have suggested selection for specific grasses, though factors governing preferences are unclear (Chitty et al. 1968, Hjalten et al. 1996). Whether an organism is selective in its food intake will depend on a variety of factors including the quality of food, the relative abundance of different food types and the energy requirements and foraging efficiency of the animal. Animals that feed on abundant, low-energy food are expected to show little or no selection. However, if there is a large difference between the net energy supplied by two food sources, selection is more likely (Krebs and Davies 1993). Given that field voles live on low-energy, highly abundant food, they might be expected to be non-selective in their diet. Consequently most studies have produced little or no evidence of food selection in the wild (for example Hansson 1971, Evans 1973, Ferns 1979). However, the majority of dietary studies have been carried out in lowland habitats, unrepresentative of the upland areas inhabited by the majority of voles. Here there are fewer available food plants, and several die back in the winter, so that for long periods the potential food supply is restricted. The main plants available to voles in much of upland Britain are the grasses Nardus stricta, Deschampsia flexuosa and Molinia caerulea and the sedge Eriophorum vaginatum. Other grasses such as Festuca ovina, Agrostis tenuis and Agrostis canina are also available in smaller quantities. The main dicotyledons available in British uplands are woody shrubs such as bilberry Vaccinium myrtillus, crowberry Empetrum nigrum and heather Calluna vulgaris and it seems unlikely that voles would choose to eat these. Material and methods Study site The study was carried out in the Peak District National Park, which covers km 2 of the south Pennine hills in Northern England. The north of the Park (the North or High Peak to N, 1 35 to 2 10 W) reaches almost 700 m above sea level and contains extensive upland moorland habitats. The area consists mainly of acid grasslands, heather moor and blanket bog. Dominant species are the grasses Molinia caerulea and Nardus stricta, the sedge Eriophorum vaginatum, and the shrubs Calluna vulgaris, Empetrum nigrum and Vaccinium myrtillus. Feeding trials Field voles were caught in the wild at two upland grassland sites in the Peak District during the summer of 2001 using Longworth small mammal traps. Only adult males were retained in order to avoid taking females in the early stages of pregnancy. Animals were kept in accordance with UK Home Office regulations. The animals were placed in holding cages with a water dish, food and a nest box containing cotton wool bedding. In order to standardise the trials and make sure that only the plant species of interest were involved, each animal was fed on chopped carrots and sunflower seeds

3 Diet of upland voles 485 for three days. These were easily distinguishable in the droppings. After this time, 20 g wet mass of each of two plant species of interest (A and B) were placed in the cage at equal distance from the nest box. Field voles eat less than their body mass in grass per day ( Ranson 1934, Phillipson et al. 1983) and 20 g of each was regarded as sufficient such that an individual would not be forced to consume the less preferred food plant. Six trials were carried out between the following pairs of plant species: (1) Nardus stricta and Deschampsia flexuosa, (2) Nardus stricta and Molinia caerulea, (3) Nardus stricta and Eriophorum vaginatum, (4) Deschampsia flexuosa and Molinia caerulea, (5) Deschampsia flexuosa and Eriophorum vaginatum, (6) Molinia caerulea and Eriophorum vaginatum. Five animals were used per trial, each in its own cage. Trial food was left for 24 hours after which the remaining uneaten plant material was removed, washed and dried. The cage was cleaned and all droppings collected. Another 20 g of each of the plant species were then placed in the cage with their positions reversed (species A on the right and B on the left). Animals were removed from the cage and weighed at this time to ensure significant weight loss was not occurring as Hansson (1971) described field vole feeding trials where animals consistently lost mass over a period of days and then died. In the event, only two animals lost considerable amounts of weight. These were returned to the wild and the trial restarted with new individuals. The procedure was repeated for three consecutive days. Droppings from the first day of each trial were discarded as they were potentially contaminated with carrot and sunflower seed. Droppings from the second and third days were stored in 70% ethanol and retained for analysis. They were washed through a 200 then a 100 sieve in order to break them up and remove fragments too small for analysis (Hansson 1970). Material remaining in the 100 sieve was retained and plated on to a mm slide. The sieving separated the plant epidermal fragments sufficiently for analysis. It was not thought necessary to perform lengthy clearing and staining used in some previous analyses (Williams 1962, Hansson 1971) since the plant species to be identified were few and readily distinguishable from one another (Wheeler 2002). The first 100 identifiable plant epidermal fragments on each slide were counted. Uneaten plant material from each trial was dried for 48 hours and weighed. In order to estimate the percentage of the initial dry-mass of plant material eaten in the course of the trial, several 20 g lots of each species were dried in the same way and weighed. The means of the dry masses were calculated and used as an estimate of the initial dry mass of the plants used in the trial. At the end of each trial the animals were marked by fur clipping, to ensure they were not re-used, and returned to the wild. In total 30 voles were used (5 in each of the six trials). Percentages of each plant species consumed were calculated and used as an indicator of diet preference. They were then compared to the proportions of each species found in the faeces to assess the effectiveness of faecal analysis as an indicator of food consumed. Diet of wild voles The diet of voles in the wild was assessed by analysing faecal matter and stomach contents. Faeces were collected from the four main High Peak habitats: (1) Molinia caerulea-dominated grassland (hereafter referred to as Molinia habitats ), (2) Eriophorum vaginatum blanket bog ( Eriophorum habitats ), (3) Nardus stricta-dominated grassland ( Nardus habitats ), (4) Calluna vulgaris moorland ( Calluna habitats ). Habitats are referred to by the genus name of their dominant plant species in order to distinguish them from the species themselves, which are referred to using the abbreviated genus and full species name. Twelve sites were surveyed in each of the four habitat types between late winter 2000 and summer Four sites were surveyed in each habitat (totalling 16 sites) in winter and spring In the summer and autumn of 2000 these sixteen sites were surveyed again. Throughout summer and autumn 2000 and 2001 eight more sites were surveyed in each habitat (making a total of 48 sites). In each site a m square was surveyed by restricted random sampling using a 1 m 2 quadrat placed by randomly generated co-ordinates in each m square of the area. This produced 25 survey quadrats for each site. Each quadrat was searched for fresh field vole droppings and the percentage cover of the plant species present was estimated. Droppings from each quadrat

4 486 P. Wheeler were stored in 70% ethanol and analysed separately as described previously. Percentages of plants identified from each set of droppings were averaged over each site, giving 12 replicates for each of the four habitats assessed. The percentage cover of the main plant species in each quadrat was also measured. In total, 246 dropping samples were analysed. Of these, 48 were from Calluna sites, 54 from Eriophorum, 72 from Nardus, and 72 from Molinia. Between two and five dropping samples were analysed from each site. The data from samples from each site were combined for statistical analysis, producing four diet records from each of the four habitats in winter and twelve in each from the summer, making 16 winter and 48 summer samples. As before, the first 100 identifiable plant epidermis fragments were counted, but since a much larger number of species were present in the faeces, not all fragments were recognised. It was therefore necessary to distinguish between fragments that were simply too small or unclear to be identified and those which were potentially identifiable but unrecognisable from the reference collection. The former were not included, but the latter were classified as unknown. Data were therefore collated as the percentage of identifiable fragments of each species in the faeces. Data were analysed using compositional analysis (Aitchison 1986) with log ratios calculated according to Aebischer et al. (1993): d XY = ln(x U /Y U ) ln(x A /Y A ) where X U and Y U represent the percentage of species X and Y in the faeces ( use ) and X A and Y A are the percentages cover of the two species measured on the ground ( availability ). d XY is then an index of selection for species X over Y. This was calculated separately for all combinations of species in each site. Evidence of overall significant selection for plant species was assessed using Wilk s Lambda test (Manly et al. 2002). Individual preferences between species were then determined by t-tests using the mean and standard error of d XY (Aebischer et al. 1993). Results Feeding trials There was no significant mass loss in the course of any of the trials, though mass loss in animals in the N. stricta E. vaginatum trial approached significance (General Linear Model: n = 5, p = 0.067). Water content varied between the species used in the trials. D. flexuosa had the highest water content (69%) followed by N. stricta (60%), M. caerulea (47%) and E. vaginatum (29%). All four Table 1. Mean (± SE) dry and wet mass of plants consumed by field voles from the Peak District National Park (northern England) in feeding trials and significance of preferences revealed by a one sample t-test. + denotes preference for species A over B and for B over A ( + = p < 0.1, + + / = p < 0.05, = p < 0.01, ns = not significant, n = 5 trials for each species combination). Plants Wet mass (g) Dry mass (g) Species A Species B A B Significance of difference A B Significance of difference N. stricta D. flexuosa 10.9 ± ± ± ± 0.2 ns N. stricta M. caerulea 10.5 ± ± 0.6 ns 4.2 ± ± 0.3 N. stricta E. vaginatum 13.9 ± ± ± ± D. flexuosa M. caerulea 7.7 ± ± 1.1 ns 2.4 ± ± 0.6 ns D. flexuosa E. vaginatum 18.2 ± ± ± ± M. caerulea E. vaginatum 12.7 ± ± ± ±

5 Diet of upland voles 487 plant species were at least partly consumed in the trials (Table 1) but no species was totally consumed. A one-sample t-test of the hypothesis that the percentage of plant A consumed was not equal to 50% was carried out on the data in order to assess whether one plant was significantly preferred (Table 1). On the basis of the dry masses consumed, there were significant preferences for M. caerulea over N. stricta, and both D. flexuosa and M. caerulea over E. vaginatum. Preference for N. stricta over E. vaginatum approached significance at the 5% level (Table 1). There was no significant preference for D. flexuosa over N. stricta or D. flexuosa over M. caerulea. An analysis of wet masses consumed showed the same significant preference in the trials of D. flexuosa and M. caerulea with E. vaginatum, and lack of preference in the D. flexuosa M. caerulea trial. However, there were in addition, highly significant preferences for D. flexuosa over N. stricta and N. stricta over E. vaginatum, though there was no significant preference shown in the N. stricta M. caerulea trial. Comparison between plants consumed and faecal analysis The amount of each plant consumed was compared to the amount in the faeces using a general linear model analysis of wet and dry masses, controlling for plant. There were highly significant overall Table 2. Relationship between wet mass eaten and prevalence in faeces of field voles, for the four plants used in the feeding trials by linear regression, n = 15 for each species, n = 30 overall. B is the slope of the regression line. Species B r 2 p D. flexuosa < M. caerulea < N. stricta E. vaginatum ns Overall < relationships between percentage wet and dry mass consumed and prevalence in the faeces though the correlation was closer for wet mass than dry mass (df = 7, p < 0.001, r 2 = and df = 7, p < 0.003, r 2 = respectively). This shows that, at least in these trials, faecal analysis can be used to determine consumption. The relationship is independent of the plant in question (p = for wet masses, p = for dry masses). Separate regressions of wet mass against percentage in faeces show that wet mass of each plant consumed is significantly correlated with prevalence in the faeces for three of the four species (Table 2). Diet analysis from the wild Data taken in the pairs of sites surveyed in both winter and summer were analysed using a multivariate analysis of variance (MANOVA). This showed there was no significant effect of season on consumption of any of the main plant species (n = 4 pairs of sites for each habitat, p > 0.05 for all) though mosses were consumed significantly more in the winter than in summer (df = 1, F = 5.89, p = 0.021). Shannon s index of diversity was calculated for the diet of field voles in each of the habitats in summer and winter (Table 3). Diversity of food plants consumed did

6 488 P. Wheeler Table 3. Shannon s diversity index (H ) for plant species in faeces of field voles from four habitats in winter and summer and variance of H. Differences were not significant in any case (t-test) following Magurran (1988). n = 4 for each habitat in each season. Habitat Winter Summer H Var H H Var H Significance of difference Molinia ns Eriophorum ns Nardus ns Calluna ns Overall ns Table 4. Significance of difference between selection exhibited by field voles for main plant species measured in faeces and on ground in each habitat by paired t-test of mean log-ratio differences. n = 12 for each habitat, ns = not significant, + = significantly more in faeces than ground p < 0.01, + + = significantly more in faeces than ground p < 0.001, = significantly less in faeces than ground p < More stringent p values have been used in order to compensate for multiple t-tests. Molinia habitat, Wilks Lambda 0.531, p = D. flexuosa M. caerulea N. stricta J. effusus Agrostis spp. M. caerulea ns N. stricta + + J. effusus ns Agrostis spp ns ns Moss spp ns ns ns Eriophorum habitat, Wilks Lambda 0.076, p < D. flexuosa E. vaginatum C. vulgaris E. nigrum V. myrtillus E. vaginatum + + C. vulgaris E. nigrum V. myrtillus ns + + Moss spp ns + + ns Nardus habitat, Wilks Lambda 0.385, p = D. flexuosa N. stricta Agrostis spp. F. ovina V. myrtillus N. stricta ns Agrostis spp. + ns F. ovina ns V. myrtillus ns Moss spp ns ns Calluna habitat, Wilks Lambda 0.184, p < D. flexuosa N. stricta Agrostis spp. C. vulgaris V. myrtillus N. stricta + + Agrostis spp. + + ns C. vulgaris + ns ns V. myrtillus + + ns ns ns Moss spp. + + ns ns ns ns

7 Diet of upland voles 489 not differ significantly between winter and summer in any habitat. The highest diversity of winter food plants was in Eriophorum habitats and the lowest in Nardus habitats. In the summer, the highest diversity was found in Molinia habitats and lowest in Calluna. In the light of these results, further analyses were carried out with the winter and summer data combined. Data from each habitat type were analysed separately, with the six most prevalent plant species in each habitat being used in the analysis to avoid the large number of zero values that would otherwise be present, and could affect the analysis. In contrast to several previous studies, significant preferences were observed in all habitats (Table 4). Moreover, selection was observed between monocotyledons in all habitats, with D. flexuosa being consistently preferred throughout (though not significantly preferred to M. caerulea or N. stricta in Molinia or Nardus habitats respectively). D. flexuosa was clearly more prevalent in the diet than on the ground in all habitats, and was preferred over other species (Table 5). The difference was particularly marked in Eriophorum and Calluna habitats, and here D. flexuosa was significantly preferred to other habitat constituents. There was also considerably less E. vaginatum in the diet than present on the ground. In all habitats dicotyledonous plants were avoided and formed relatively minor constituents of the diet. Table 5. Percentage of plant species in faeces of field voles and measured on the ground in upland sites where faeces were collected. The category Other Dicot includes minor constituents of diet and ground cover that were clearly dicots but were not always identifiable to species, while the category Other refers to plant fragments that could not be identified. Values are rounded to nearest percent resulting in column totals not always summing to 100. Habitat Species Molinia Eriophorum Nardus Calluna Diet Ground Diet Ground Diet Ground Diet Ground Molinia caerulea Nardus stricta Deschampsia flexuosa Juncus effusus Juncus squarrosus Agrostis spp Eriophorum angustifolium Eriophorum vaginatum Festuca ovina Calluna vulgaris Empetrum nigrum Vaccinium myrtillus Other Dicot Pteridium aquilinum Moss spp Other Bare

8 490 P. Wheeler Discussion Feeding trials In deciding whether to be selective, and if so, which foods to select, the decision rules that animals use are based on a number of factors. Nutrient content, digestibility, and handling time all play a part. Nitrogen content is a rough indicator of protein content of a grass, and hence one measure of how nutritious it is. Kirkham (2001) shows that N. stricta and E. vaginatum collected from upland sites in August have similar nitrogen contents (about 1.5% of shoot dry mass) and Molinia about 1.8%. D. flexuosa varies in its nutrient content depending on light environment with those in high light environments having higher nutrient levels (Scurfield 1954). Plants in this study were from high light environments. Scurfield (1954) quotes the nitrogen content of such plants of D. flexuosa as being relatively constant through the season at around 2.2% much higher than the other three species. It is relevant to note here that the only trial where animals consistently lost mass was that involving E. vaginatum and N. stricta the two species with lowest percentages of nitrogen. Of the plants used in the feeding trials, N. stricta has very high silica content. Its nutrient content varies considerably through the season being highest in June and July (Chadwick 1960). The high silica content, however, may make it difficult or costly to ingest and digest, and may explain why it is less favoured in feeding trials with D. flexuosa, which is much more palatable. D. flexuosa is probably preferred to other plants in the trial because of its relatively high palatability and nitrogen content. In terms of wet mass consumed, it was eaten as readily as M. caerulea, an equally palatable grass. D. flexuosa contains the most moisture of all of the plants used in the feeding trials. The fact that it is not preferred over M. caerulea when wet masses are compared may be because voles are limited by the volume of material they can consume. Diet in the wild In the wild, the diet of voles is clearly more variable than in feeding trials. However, animals mostly restrict themselves to a small number of species. The faeces analysed in this study showed that voles were consuming mainly grasses. The main dicots available through most of the sites studied were woody dwarf shrubs, and although these were taken in small numbers, they are usually avoided. This may be due to a combination of unpalatability and inappropriate nature of the growth form as has been suggested in the past (Hjalten et al. 1996). Diet varied depending on the habitat of origin of the faeces, but the one consistent trend was the preference for D. flexuosa. This was least evident in Molinia- -dominated habitats, where there was no preference for D. flexuosa over M. caerulea. This agrees with the results of the feeding trials, which showed no significant preference for D. flexuosa over M. caerulea. M. caerulea was only present in large quantities in habitats that it itself dominated. Since few species

9 Diet of upland voles 491 other than D. flexuosa were present in these areas, there was little opportunity for measuring selection for M. caerulea over other plants in the wild. Preference for N. stricta was only observed in the feeding trial with E. vaginatum, but since these two plants rarely occur together in the wild, there are no comparative data from faecal analysis. Data from faecal analysis agree with those from the feeding trials except in Molinia habitats, where preference for M. caerulea is evident over N. stricta, though in the feeding trials there is none. N. stricta was avoided in feeding trials and in the wild when compared to D. flexuosa. However, there was no evidence of aversion compared to D. flexuosa in Nardus habitats, possibly because N. stricta dominates large areas, and costs of searching for the favoured species become an important consideration. In areas where N. stricta dominates specifically avoiding N. stricta in order to search for D. flexuosa may become inefficient. It is interesting that the diet appears to change little between seasons since, particularly in Molinia habitats, there is considerable seasonal change in the nature of fresh plant material available. The M. caerulea recorded in the faeces is probably a combination of dormant shoots, which remain green within tussocks, and dead material. The fact that M. caerulea is still consumed during the winter shows that Molinia grasslands are not as inhospitable to voles in the winter as they might appear. This is corroborated by results from studies of vole habitat selection (Wheeler 2002). Voles have previously been reported to consume more mosses in the winter than summer (Evans 1973, Turchin and Batzli 2001), and although the percentages of mosses in the faeces were small (around 2% overall), the significant seasonal differences in their presence supported these previous results. Conclusion When field vole diet has been studied in the past there has been little evidence for selection. However, most of these studies have been carried out on lowland populations that do not represent the majority of British field voles. It is clear from this study that field voles in upland Britain do show some selection in their food intake, and that D. flexuosa is usually favoured, both in feeding trials and in the wild. The sedge, E. vaginatum is also avoided, despite the fact that it dominates large areas of the uplands. Preference for D. flexuosa in habitats not dominated by grasses is so clear that it could feasibly govern habitat selection, and may result in competitive interactions with other selective upland grazers such as sheep. Our management of the uplands must therefore incorporate the fact that field voles, as important parts of that ecosystem, may be affected by subtle floristic changes, as well as broad habitat differences, in the makeup of their surroundings. Acknowledgements: I would like to thank Dr D. W. Yalden for advice with this work and Dr L. Hansson and two anonymous referees for comments on the manuscript. The work complied with UK Home Office regulations on animal husbandry. Funding was provided by the Samuel Gratrix trust.

10 492 P. Wheeler References Aebischer N. J., Robertson P. A. and Kenward R. E Compositional analysis of habitat use from animal radio-tracking data. Ecology 74: Aitchison J The statistical analysis of compositional data. Chapman and Hall, New York: Chadwick M. J Nardus stricta L. Journal of Ecology 48: Chitty D., Pimentel D. and Krebs C. J Food supply of overwintered voles. Journal of Animal Ecology 37: Dyczkowski J. and Yalden D. W An estimate of the impact of predators on the British field vole (Microtus agrestis) population. Mammal Review 28: Evans D. M Seasonal variations in the body composition and nutrition of the vole Microtus agrestis. Journal of Animal Ecology 42: Faber J. and Ma W Observations on seasonal dynamics in diet consumption of the field vole Microtus agrestis with some methodological remarks. Acta Theriologica 31: Ferns P. N Growth, reproduction and residency in a declining population of Microtus agrestis. Journal of Animal Ecology 48: Hansson L Methods of morphological diet micro-analysis in small rodents. Oikos 21: Hansson L Habitat, food and population dynamics of the field vole Microtus agrestis in south Sweden. Viltrevy 8: Harris S., Morris P., Wray S. and Yalden D A review of British mammals: Population estimates and conservation status of British mammals other than cetaceans. JNCC, Peterborough: Hjalten J., Danell K. and Ericson L Food selection by two vole species in relation to plant growth strategies and plant chemistry. Oikos 76: Kirkham F. W Nitrogen uptake and nutrient limitation in six hill moorland species in relation to atmospheric nitrogen deposition in England and Wales. Journal of Ecology 89: Krebs J. R. and Davies N. B An Introduction to Behavioural Ecology. Blackwell Science, Oxford: Magurran A Ecological Diversity and its Measurement. Croom Helm, London: Manly B. F. J., Macdonald L. L., Thomas D. L., McDonald T. L. and Erickson W. P Resource selection by animals: Statistical design and analysis for field studies. Kluwer Academic, London: Phillipson J., Sarrazincomans M. and Stomatopoulos C Food-consumption by Microtus agrestis and the unsuitability of fecal analysis for the determination of food preference. Acta Theriologica 28: Ranson R. M The field vole (Microtus) as a laboratory animal. Journal of Animal Ecology 3: Scurfield G Deschampsia flexuosa (L.) Trin. Journal of Ecology 42: Turchin P. and Batzli O. G Availability of food and the population dynamics of arvicoline rodents. Ecology 82: Wheeler P The distribution of mammals across the upland landscape. PhD thesis, University of Manchester, Manchester: Williams O A technique for studying microtine food habits. Journal of Mammalogy 43: Received 13 October 2003, accepted 20 January Associate Editors were Leszek Rychlik and Magdalena Niedzia³kowska.