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The impact of feral mink Mustela vison on water voles Arvicola terrestris in the North Yorkshire Moors National Park
Biological Conservation 51 (1990) 49-62
The Impact of Feral Mink Mustela vison on Water Voles Arvicola terrestris in the North Yorkshire Moors National Park Gordon L. Woodroffe, John H. Lawton* Department of Biology, University of York, Heslington, York YO1 5DD, UK
& Walter L. Davidson Howe Gate, Danby, North Yorkshire YO21 2NG, UK (Received 19 January 1989; revised version received 11 April 1989; accepted 12 April 1989)
ABSTRACT Th& study explores mink-water vole interactions on five o f the principal rivers in the North Yorkshire Moors National Park, in an area where mink are still spreading. It uses a variety o f techniques--tracking (field signs), trapping and radio-tracking--at a range of 38 sites that differed in average water vole and mink abundances over two )'ears, in conjunction with a longer term fouryear study at one site on the river Esk under active colonisation by mink. The overall conclusion is unavoidable. Mink pose a serious threat to the long-term survival o f water vole populations, at least in the North Yorkshire Moors study area.
INTRODUCTION Alien predators often have devastating effects on indigenous prey species (Carlquist, 1965; Pimm, 1987; Ebenhard, 1988). Not unreasonably, the * Present address: Centre of Population Biology, Department of Pure and Applied Biology, Imperial College, Silwood Park, Ascot, Berkshire SL5 7PY, UK. 49 Biol. Conserv. 0006-3207/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
50
Gordon L. Woodroffe, John H. Lawton, Walter L. Davidson
impact of feral mink Mustela vison Schreb on populations of native birds, mammals and fish along British waterways has been strongly debated (Lever, 1978; Linn & Chanin, 1978a,b). But with the exception of some water birds (Smith, 1988) there is very little evidence to show whether mink do, or do not, threaten any of their British prey. One species that may be particularly vulnerable to mink is the water vole Arvicola terrestris (L.) (Stoddart, 1977). Adult voles appear close to the optimum prey-size of adult mink, and are not agile enough to escape either on land, or in the water. Nor are their burrow systems small enough to exclude mink (personal observations). Anecdotal evidence points to an adverse effect of mink on water vole numbers in Britain (D. Jefferies, pers. comm.), with the specific suggestion from the River Wye Conservation Group that mink caused local extinction of water voles on the river in 1987 (D. Shaw, pers. comm.). Unfortunately, there is no hard evidence to support these claims. This study explores mink-water vole interactions on five of the principal rivers in the North Yorkshire Moors National Park, in an area where mink are still spreading. It uses a variety of techniques--tracking (field signs), trapping and radio-tracking--at a range of sites that differed in average water vole and mink abundances over two years, in conjunction with a longer term four-year study at one site under active colonisation by mink. STUDY AREA Thirty-eight study sites, covering distances of 100-350m per site, were selected on five of the principal rivers in, or just outside, the North Yorkshire Moors National P a r k - - t h e Esk, Dove, Seven, Seph and Rye (Fig. 1). These were split as follows: (1) River Esk: nine study sites on the 21 km of river stretching from Sleights to Castleton. (2) Rivers Seph and Rye: two sites were chosen on the river Seph before this river joins the Rye some 2-5 km downstream. Seven sites were surveyed on the 11.3 km stretch of the Rye down to Helmsley. (3) River Dove: nine sites on the 11 km of river from Low Mill down to Kirby Mills. (4) River Seven: eleven sites were chosen on the 16 km of river below its source in Rosedale down to Sinnington. METHODS
Indices of activity Water voles leave well-established runs along the m u d and sand flats at the sides of the rivers. Both they and mink have highly characteristic tracks
51
Mink predation on water voles
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Gordon L. Woodroffe, John 1-1. Lawton, Walter L. Davidson
(Lawrence & Brown, 1974). At the outset of the study, track identification was verified for us in the field by Dr R. W. Brown. These tracks were used to provide indices of activity by both species over all sites. Most sites were visited monthly during 1985-86 and every mud and sand fiat on both sides of the river at each site was searched, and the number of footprints identified and counted (water voles are indifferent to these occasional levels of disturbance). By measuring the length of each mud and sand fiat, track data were converted into the number of tracks per metre per site, to yield an index of activity for each species. The vast majority of suitable, soft muddy and sandy substrates consisted of thin strips along the edges of the river, or small bars towards the middle. Measuring length takes no account of variation in width or shape of these areas, but this simplification greatly speeded up recording (essential in a large-scale survey) and more accurate methods were not justified by the inevitably crude nature of the data on tracks themselves. Moreover, variation in the length of soft banks available at each site was much greater than variation in the width or shape of each bank.
Statistical analysis of activity index data The short-term effects of mink on water vole activity were analysed by listing all the activity indices from all the rivers and sites where water vole and mink activity had been observed and grouping them into the following classes: (A) Mink initially absent; water vole activity index within one month prior to the appearance of mink activity at a site; (B) Water vole activity index in the presence of mink activity at the same site; if mink were present for several weeks the mean water vole activity index was calculated for that period; (C) Water vole activity index within one month after the disappearance of mink activity from the site. A total of 47 sets of data were obtained where information was available for all three categories (A, B, and C), within a sufficiently short span of time, from the same site. Suitable triplets of data were distributed throughout the year. As a control, triplets of water vole indices which had not been subjected to any mink activity were also identified in the data; the middle values (B') in each triplet were then compared with the indices recorded up to one m o n t h before (A') and one m o n t h after (C'). Thirty-one sets of control data were available. Again suitable triplets were distributed throughout the year. These data were then analysed by taking the difference between pairs of indices of activity from each site: (B-A), (B-C), (C-A) (or B'-A' etc., for the
M i n k predation on water voles
53
controls). Under the null hypothesis that mink activity does not influence water vole activity, the expected mean values of (B-A), (B-C) and (C-A) (or B'-A' etc.) are zero. Doing the test in this way factors out big differences in the absolute values of indices of activity between sites. Mean values were tested using a t-test against an expected mean value of zero (Bailey, 1984).
Trapping and marking water voles Traps were based on Stoddart's design (Stoddart, 1970). They were made from 20g aluminium 2 x 1 cm wire mesh, with a nest box, door and triggering mechanism all made from 20 g aluminium. The nest box and run were clipped together and held in place by a spring. Nest boxes were filled with clean, dry straw and baited with mashed carrot or chopped apples. Traps were not prebaited but usually laid in the late afternoon or early evening and inspected twice a day. This reduced exposure of trapped animals and, hence, trap mortality. Numbered Monel metal ear tags (National Band and Tag Company, Kentucky, USA) were used to mark water voles. Eight water voles were radio-tracked on the river Esk at site 4. Animals were fitted with collar transmitters (Kenward, 1987). Tags consisted of a 2-5 g epoxy potted transmitter mounted on a radio cable nylon tie round the water vole's neck, and fastened through a non-release action clip. Tags were supplied by B I O T R A C K (Stoborough Croft, Dorset). Animals were located by a three element Yagi directional aerial (BIOTRACK), and a portable 150kHz band-width M57 radio receiver (supplied by Mariner Radar, Lowestoft, Suffolk).
RESULTS It proved impossible to catch water voles at the four initial trapping sites (19 and 28, R. Seven; 30, R. Seph and 16, R. Dove) although water vole activity was regularly recorded. However, as soon as traps were moved to sites showing high indices of activity and which also had water vole latrines, animals were readily caught (Woodroffe, 1988). The presence or absence of latrines at a site appeared to be the principal factor demarcating success or failure when trapping. It led us to grade sites into two categories: (1) Core sites which had permanent breeding water vole populations. These sites were marked with latrines, and animals were regularly caught in traps. (2) Peripheral sites which showed regular water vole activity but where latrines were never found and, despite intensive and extensive trapping, animals were never caught. Detailed observations (Woodroffe, 1988) suggested that
54
Gordon L. Woodroffe, John H. Lawton, Walter L. Davidson TABLE 1 Water Vole Sites River
Total sites by site number
Water vole core sites by site number
Peripheral sites by site number
Esk Dove Seven Seph/Rye
1-9 10-18 19-29 30-38
3, 4, 5, 6, 7, 8, 9 16, 18 ---
2 15, 17 19, 21, 23, 25, 27, 28 30, 31, 32, 35, 36, 37
All remaining sites, although searched regularly, never revealed any evidence of the presence of water voles (i.e. no tracks, no latrines, no sightings of animals and none trapped).
peripheral sites were not used for breeding, and appeared to be only visited by water voles. In addition, there was a third group of sites which never yielded any signs of water vole activity, and at which animals were never caught in traps. The study sites are classified in Table l; cumulative water vole and mink activity indices for 1985-86 for each site are in Table 2. Core sites were only found on the Esk and Dove and, as shown in Table 1 and Fig. 2, these rivers had the lowest overall mink activity during 1985-86. Most striking in the activity data (Table 2) are the high mink activity indices at peripheral sites and sites without water voles, compared with core sites. The three regular trapping sites (16 on the Dove, 4 and 6 on the Esk--all core sites) had the lowest mink activity indices of all the rivers during 1985-86.
Analysis of activity indices Short-term changes in water vole activity prior to, during and after the appearance of mink at a site are shown in Fig. 3(a) with control data in Fig. 3(b). There is a significant rise in the water vole activity in the presence of mink (A-B, t = 2.73, P<0-01), followed by a significant fall (B-C, t = 2.47, P < 0.01) back to levels that are lower than initial levels, but not significantly different from them (A-C, t = 1-57, P > 0 " I ) . There are no significant differences between any of the control comparisons (A'-B', B'-C' and A'-C') lacking mink. These analyses of short-term changes in water vole activity suggest that when mink enter a water vole colony, vole activity markedly increases, and subsides again when mink leave. These short-term responses presumably reflect changes in vole behaviour, not numbers. Indeed the paired
Mink predation on water voles
55
TABLE 2 C u m u l a t i v e W a t e r Vole a n d M i n k Activity Indices (AI) 1985-86
Site number
Water vole A I (tracks m - 1 month- x)
Mink A I (tracks m - l month- l)
70 70 18 8 La L L 96 L
1 0 0 0 ---0"1 --
Core sites 3 4 5 6 7 8 9 16 18
Peripheral sites 2 15 17 19 21 23 25 27 28 30 31 32 35 36 37 Sites without water voles 1 10 11 12 13 14 20 22 24 26 29 33 34 38 a
61 11 31 9 25 9 0-5 23 17 31 16 2 18 6 14
0"1 1 1 5 1"5 6 3-5 0" 1 6 7"5 6-5 7"1 4'5 2 15
0
0'3 0.2 0.1 0-5 5 4 1.5 0.3 8 7 1'1 5 10 0'3
0 0 0 0 0 0 0 0 0 0 0 0 0
L, latrines present: no activity index was m e a s u r e d at these sites.
Gordon L. Woodroffe, John H. Lawton, Walter L. Davidson
56
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M i n k predation on water voles
(a)
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Fig. 3. (a) Mean water vole activity indices on all study rivers. A, up to one month prior to mink entering the colony; B, during the presence of mink; C, up to one month after disappearance of mink (means and 95% confidence limits). (b) Control data; triplets of samples, similar to those in 3(a) but without mink.
comparison tests are deliberately designed to eliminate major differences in overall levels of water vole activity (and hence numbers), between sites. Longer term effects of mink on water voles, presumably reflecting change in vole numbers, were therefore analysed using the cumulative totals of the water vole and mink activity indices at six monthly intervals for sites with complete data on the four study river systems during 1985 and 1986. In this way activity indices were included which covered all the possible ranges of
Gordon L. Woodroffe, John H. Lawton, Walter L. Davidson
58
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~ 30
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Fig. 4. Water vole activity indices per site, over six months, as a function of mink activity indices over the same time period. The equation of the fitted line is y = 4 7 " 1 - 1"52x Ft,~s = 9.96, P < 0-01.
mink activity, from sites with mink more or less continuously present to sites where mink activity was never found. Analysed in this way, there is a significant negative correlation between long-term mink activity and water vole activity at each site (F1,15 = 9.96, P < 0.01) (Fig. 4) which suggests that water vole numbers are either being reduced by mink predation, or that in the presence of mink, water voles in the longer term disperse to safer areas. TABLE 3 Summary of Radio-Tracking Data at Site 4, River Esk
Date tagged
29 June 1987 14 July 1987 17 July 1987 14 October 1987 5 November 1987 10 November 1987 3 August 1988 7 August 1988
Sex Weight (g)
M M F M M M F M
295 250 200 180 180 180 175 170
Wavelength
257 236 224 275 209 238 305 325
Days Maximum tracked observed linear range (m) 3 8 72 121 42 48 2 40mins
218 243 135 130 148 189 30 30
Notes
Disappeared Disappeared Died in burrow Killed by mink Died in burrow Predated in burrow Killed by mink Killed by mink
59
M i n k predation on water voles
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Radio-tracking and the decline of water voles at site 4 Eight water voles were radio-tracked at site 4 on the Esk during 1987 and 1988 (Table 3). Two males tagged in June and July 1987 disappeared within days. We were unable to retrap or locate them over 3 km or river and as mink tracks appeared at this site in July 1987 we suspect that mink were responsible. Thereafter mink tracks were recorded monthly (but because of shortage of time these tracks were not quantified as activity indices: Fig. 5(a)), with mink being caught in water vole traps in August, October and November (Fig. 5(b)). In February 1988 we obtained the first direct evidence of mink taking water voles when the only surviving tagged water vole was found freshly killed by a mink. Mink activity continued throughout 1988 (Fig. 5(a)). Water voles became increasingly difficult to catch, although five mink were caught during this period and two radio-tagged water voles were killed by mink (both kills were observed directly). After these kills no further voles were caught at site 4 (270 trap-nights in September and 100 in October) and all signs disappeared.
DISCUSSION Feral mink slowly extended their distribution within the North Yorkshire Moors National Park during the study period (1984-88), allowing us to monitor changes in the short-term activity of water voles as mink entered and left water vole colonies; we were also able to assess the longer term impact of mink by comparing sites with different average levels of mink activity, and to follow the demise of a vole colony as mink established a resident population. The study highlights four aspects of the interaction between mink and water voles. First, when mink enter a water vole colony after a period of absence, vole activity, revealed by tracking, markedly increases in the short term, and subsides once mink leave. The simplest explanation is that voles move about more when mink are present in the area, presumably because they are agitated, or alarmed. (This interpretation is supported by Clark's (1981) observation on 'panicking' water voles as mink entered the colony.) In the longer term, however, there is a significant inverse correlation between average mink activity and average water vole activity across 38 sites on five rivers in the National Park, again revealed by tracking. The inference here is that in the longer run, mink depress water vole numbers. This interpretation is strongly supported by two other pieces of evidence. Site 4 on the river Esk held a thriving water vole colony in 1985, and lacked mink. Then between 1987 and 1988, mink activity on this stretch of river steadily
M i n k predation on water voles
61
increased (indices of mink activity based on tracking rose and mink were increasingly caught in water vole traps). During this time numbers of water voles declined, until the colony finally disappeared (no animals caught in 300 trap-nights) in October 1988. Last, but not least, 5 out of 8 radio-tagged voles were known or strongly suspected to have been killed by mink in this colony, during the period of increasing mink activity and declining water vole abundance. There is good evidence to show that water voles are particularly vulnerable to mink predation (Jeppsson, 1987; Stoddart, 1977) not least because of mink's ability to hunt both on land and in the water. For example, when radio-tagged water vole 325 was killed, the mink first searched all the burrow entrances, and then located the vole seemingly well camouflaged in dense cover. The water vole was killed when it dived into the river; its usual escape mechanism, diving into the water and kicking up a mud cloud, being completely ineffective. More generally, once mink are resident on stretches of river, regular foraging alongside the waterway will virtually ensure that they locate all water vole colonies, and may exterminate water voles from large stretches of British rivers. Both theoretical considerations (Holt, 1977; Jeffries & Lawton, 1984) and empirical observations (Carlquist, 1965; Churcher & Lawton, 1987; Pimm, 1987; Ebenhard, 1988) show that polyphagous predators, sustained by a large variety of alternative prey, may wreak havoc with particularly vulnerable species, eliminating them entirely from certain localities or ecosystems. In North America mink exterminated muskrats Ondarata zibethicus from some locations (Errington, 1946). At the present time, water voles and mink coexist, both on rivers in the North Yorkshire Moors and elsewhere (Birks, 1986). Voles may persist providing mink are not equally abundant throughout a water course. Keepering by water bailiffs and game keepers may be an important factor preventing mink from occupying all suitable habitats along a river and may unwittingly provide water voles with temporarily safe breeding sites. But by reducing population size and fragmenting water vole colonies mink pose a serious long-term threat to the survival of water voles on British rivers.
ACKNOWLEDGEMENTS We would like to thank all the landowners who allowed us to work on their stretches of river. Thanks are also due to the Mammal Conservation Trust who awarded one of us (G.L.W.) a generous grant to purchase radiotracking equipment. Extra water vole traps were loaned to us by Dr G. Triggs, Liverpool Polytechnic, to whom we are most grateful.
62
Gordon L. Woodroffe, John H. Lawton, Walter L. Davidson REFERENCES
Bailey, N. T. J. (1984). Statistical Methods in Biology. Hodder & Stoughton, London. Birks, J. D. S. (1986). Mink. Mammal Society, London. Carlquist, S. (1965). Island Life. New York Natural History Press, New York. Churcher, P. B. & Lawton, J. H. (1987). Predation by domestic cats in an English village. J. Zool., Lond., 212, 439-55. Clark, M. (1981). Mammal Watching. Severn House, London. Ebenhard, T. (1988). Introduced birds and mammals and their ecological effects. Swed. Wildl. Res., 13, 1-107. Errington, P. L. (1946). Predation and vertebrate populations. Quart. Rev. Biol., 21, 144-77, 221-45. Holt, R. D. (1977). Predation, apparent competition, and the structure of prey communities. Theoret. Pop. Biol., 12, 197-229. Jeffries, M. J. & Lawton, J. H. (1984). Enemy free space and the structure of biological communities. Biol. J. Linn. Soc., 23, 269-86. Jeppsson, B. (1987). Behavioural ecology of the water vole (Arvicola terrestris) and its implications to theories of microtine ecology. PhD. dissertation, University of Lund. Kenward, R. (1987). Wildlife Radio Tagging. Academic Press, London. Lawrence, M. J. & Brown. R. W. (1974). Mammals of Britain. Their Tracks, Trails and Signs. Blandford, London. Lever, C. (1978). The not so innocuous mink. New Scient., 78, 812-14. Linn, I. J. & Chanin, P. W. R. (1978a). Are mink really pests in Britain. New Scient., 77, 560-2. Linn, I. J. & Chanin, P. W. R. (1978b). More on the mink 'menace'. New Scient., 80, 38-40. Pimm, S. L. (1987). The snake that ate Guam. Trends in Ecology & Evolution, 2, 293-5. Smith, R. (1988). Mink and waterway birds. BTO News, 156, 10. Stoddart, D. M. (1970). Individual range, dispersion and dispersal in a population of water voles (Arvicola terrestris (L.)). J. Anim. EcoL, 39, 403-25. Stoddart, D. M. (1977). Water vole. In Handbook of British Mammals, ed. G. B. Corbet & H. N. Southern. Blackwelis Scientific Publications, Oxford, pp. 196-204. Woodroffe, G. L. (1988). Ecology of riverside mammals in the North Yorkshire Moors National Park. M.Phil. thesis, University of York.
The Impact of Feral Mink Mustela vison on Water Voles Arvicola terrestris in the North Yorkshire Moors National Park Gordon L. Woodroffe, John H. Lawton* Department of Biology, University of York, Heslington, York YO1 5DD, UK
& Walter L. Davidson Howe Gate, Danby, North Yorkshire YO21 2NG, UK (Received 19 January 1989; revised version received 11 April 1989; accepted 12 April 1989)
ABSTRACT Th& study explores mink-water vole interactions on five o f the principal rivers in the North Yorkshire Moors National Park, in an area where mink are still spreading. It uses a variety o f techniques--tracking (field signs), trapping and radio-tracking--at a range of 38 sites that differed in average water vole and mink abundances over two )'ears, in conjunction with a longer term fouryear study at one site on the river Esk under active colonisation by mink. The overall conclusion is unavoidable. Mink pose a serious threat to the long-term survival o f water vole populations, at least in the North Yorkshire Moors study area.
INTRODUCTION Alien predators often have devastating effects on indigenous prey species (Carlquist, 1965; Pimm, 1987; Ebenhard, 1988). Not unreasonably, the * Present address: Centre of Population Biology, Department of Pure and Applied Biology, Imperial College, Silwood Park, Ascot, Berkshire SL5 7PY, UK. 49 Biol. Conserv. 0006-3207/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
50
Gordon L. Woodroffe, John H. Lawton, Walter L. Davidson
impact of feral mink Mustela vison Schreb on populations of native birds, mammals and fish along British waterways has been strongly debated (Lever, 1978; Linn & Chanin, 1978a,b). But with the exception of some water birds (Smith, 1988) there is very little evidence to show whether mink do, or do not, threaten any of their British prey. One species that may be particularly vulnerable to mink is the water vole Arvicola terrestris (L.) (Stoddart, 1977). Adult voles appear close to the optimum prey-size of adult mink, and are not agile enough to escape either on land, or in the water. Nor are their burrow systems small enough to exclude mink (personal observations). Anecdotal evidence points to an adverse effect of mink on water vole numbers in Britain (D. Jefferies, pers. comm.), with the specific suggestion from the River Wye Conservation Group that mink caused local extinction of water voles on the river in 1987 (D. Shaw, pers. comm.). Unfortunately, there is no hard evidence to support these claims. This study explores mink-water vole interactions on five of the principal rivers in the North Yorkshire Moors National Park, in an area where mink are still spreading. It uses a variety of techniques--tracking (field signs), trapping and radio-tracking--at a range of sites that differed in average water vole and mink abundances over two years, in conjunction with a longer term four-year study at one site under active colonisation by mink. STUDY AREA Thirty-eight study sites, covering distances of 100-350m per site, were selected on five of the principal rivers in, or just outside, the North Yorkshire Moors National P a r k - - t h e Esk, Dove, Seven, Seph and Rye (Fig. 1). These were split as follows: (1) River Esk: nine study sites on the 21 km of river stretching from Sleights to Castleton. (2) Rivers Seph and Rye: two sites were chosen on the river Seph before this river joins the Rye some 2-5 km downstream. Seven sites were surveyed on the 11.3 km stretch of the Rye down to Helmsley. (3) River Dove: nine sites on the 11 km of river from Low Mill down to Kirby Mills. (4) River Seven: eleven sites were chosen on the 16 km of river below its source in Rosedale down to Sinnington. METHODS
Indices of activity Water voles leave well-established runs along the m u d and sand flats at the sides of the rivers. Both they and mink have highly characteristic tracks
51
Mink predation on water voles
0
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52
Gordon L. Woodroffe, John 1-1. Lawton, Walter L. Davidson
(Lawrence & Brown, 1974). At the outset of the study, track identification was verified for us in the field by Dr R. W. Brown. These tracks were used to provide indices of activity by both species over all sites. Most sites were visited monthly during 1985-86 and every mud and sand fiat on both sides of the river at each site was searched, and the number of footprints identified and counted (water voles are indifferent to these occasional levels of disturbance). By measuring the length of each mud and sand fiat, track data were converted into the number of tracks per metre per site, to yield an index of activity for each species. The vast majority of suitable, soft muddy and sandy substrates consisted of thin strips along the edges of the river, or small bars towards the middle. Measuring length takes no account of variation in width or shape of these areas, but this simplification greatly speeded up recording (essential in a large-scale survey) and more accurate methods were not justified by the inevitably crude nature of the data on tracks themselves. Moreover, variation in the length of soft banks available at each site was much greater than variation in the width or shape of each bank.
Statistical analysis of activity index data The short-term effects of mink on water vole activity were analysed by listing all the activity indices from all the rivers and sites where water vole and mink activity had been observed and grouping them into the following classes: (A) Mink initially absent; water vole activity index within one month prior to the appearance of mink activity at a site; (B) Water vole activity index in the presence of mink activity at the same site; if mink were present for several weeks the mean water vole activity index was calculated for that period; (C) Water vole activity index within one month after the disappearance of mink activity from the site. A total of 47 sets of data were obtained where information was available for all three categories (A, B, and C), within a sufficiently short span of time, from the same site. Suitable triplets of data were distributed throughout the year. As a control, triplets of water vole indices which had not been subjected to any mink activity were also identified in the data; the middle values (B') in each triplet were then compared with the indices recorded up to one m o n t h before (A') and one m o n t h after (C'). Thirty-one sets of control data were available. Again suitable triplets were distributed throughout the year. These data were then analysed by taking the difference between pairs of indices of activity from each site: (B-A), (B-C), (C-A) (or B'-A' etc., for the
M i n k predation on water voles
53
controls). Under the null hypothesis that mink activity does not influence water vole activity, the expected mean values of (B-A), (B-C) and (C-A) (or B'-A' etc.) are zero. Doing the test in this way factors out big differences in the absolute values of indices of activity between sites. Mean values were tested using a t-test against an expected mean value of zero (Bailey, 1984).
Trapping and marking water voles Traps were based on Stoddart's design (Stoddart, 1970). They were made from 20g aluminium 2 x 1 cm wire mesh, with a nest box, door and triggering mechanism all made from 20 g aluminium. The nest box and run were clipped together and held in place by a spring. Nest boxes were filled with clean, dry straw and baited with mashed carrot or chopped apples. Traps were not prebaited but usually laid in the late afternoon or early evening and inspected twice a day. This reduced exposure of trapped animals and, hence, trap mortality. Numbered Monel metal ear tags (National Band and Tag Company, Kentucky, USA) were used to mark water voles. Eight water voles were radio-tracked on the river Esk at site 4. Animals were fitted with collar transmitters (Kenward, 1987). Tags consisted of a 2-5 g epoxy potted transmitter mounted on a radio cable nylon tie round the water vole's neck, and fastened through a non-release action clip. Tags were supplied by B I O T R A C K (Stoborough Croft, Dorset). Animals were located by a three element Yagi directional aerial (BIOTRACK), and a portable 150kHz band-width M57 radio receiver (supplied by Mariner Radar, Lowestoft, Suffolk).
RESULTS It proved impossible to catch water voles at the four initial trapping sites (19 and 28, R. Seven; 30, R. Seph and 16, R. Dove) although water vole activity was regularly recorded. However, as soon as traps were moved to sites showing high indices of activity and which also had water vole latrines, animals were readily caught (Woodroffe, 1988). The presence or absence of latrines at a site appeared to be the principal factor demarcating success or failure when trapping. It led us to grade sites into two categories: (1) Core sites which had permanent breeding water vole populations. These sites were marked with latrines, and animals were regularly caught in traps. (2) Peripheral sites which showed regular water vole activity but where latrines were never found and, despite intensive and extensive trapping, animals were never caught. Detailed observations (Woodroffe, 1988) suggested that
54
Gordon L. Woodroffe, John H. Lawton, Walter L. Davidson TABLE 1 Water Vole Sites River
Total sites by site number
Water vole core sites by site number
Peripheral sites by site number
Esk Dove Seven Seph/Rye
1-9 10-18 19-29 30-38
3, 4, 5, 6, 7, 8, 9 16, 18 ---
2 15, 17 19, 21, 23, 25, 27, 28 30, 31, 32, 35, 36, 37
All remaining sites, although searched regularly, never revealed any evidence of the presence of water voles (i.e. no tracks, no latrines, no sightings of animals and none trapped).
peripheral sites were not used for breeding, and appeared to be only visited by water voles. In addition, there was a third group of sites which never yielded any signs of water vole activity, and at which animals were never caught in traps. The study sites are classified in Table l; cumulative water vole and mink activity indices for 1985-86 for each site are in Table 2. Core sites were only found on the Esk and Dove and, as shown in Table 1 and Fig. 2, these rivers had the lowest overall mink activity during 1985-86. Most striking in the activity data (Table 2) are the high mink activity indices at peripheral sites and sites without water voles, compared with core sites. The three regular trapping sites (16 on the Dove, 4 and 6 on the Esk--all core sites) had the lowest mink activity indices of all the rivers during 1985-86.
Analysis of activity indices Short-term changes in water vole activity prior to, during and after the appearance of mink at a site are shown in Fig. 3(a) with control data in Fig. 3(b). There is a significant rise in the water vole activity in the presence of mink (A-B, t = 2.73, P<0-01), followed by a significant fall (B-C, t = 2.47, P < 0.01) back to levels that are lower than initial levels, but not significantly different from them (A-C, t = 1-57, P > 0 " I ) . There are no significant differences between any of the control comparisons (A'-B', B'-C' and A'-C') lacking mink. These analyses of short-term changes in water vole activity suggest that when mink enter a water vole colony, vole activity markedly increases, and subsides again when mink leave. These short-term responses presumably reflect changes in vole behaviour, not numbers. Indeed the paired
Mink predation on water voles
55
TABLE 2 C u m u l a t i v e W a t e r Vole a n d M i n k Activity Indices (AI) 1985-86
Site number
Water vole A I (tracks m - 1 month- x)
Mink A I (tracks m - l month- l)
70 70 18 8 La L L 96 L
1 0 0 0 ---0"1 --
Core sites 3 4 5 6 7 8 9 16 18
Peripheral sites 2 15 17 19 21 23 25 27 28 30 31 32 35 36 37 Sites without water voles 1 10 11 12 13 14 20 22 24 26 29 33 34 38 a
61 11 31 9 25 9 0-5 23 17 31 16 2 18 6 14
0"1 1 1 5 1"5 6 3-5 0" 1 6 7"5 6-5 7"1 4'5 2 15
0
0'3 0.2 0.1 0-5 5 4 1.5 0.3 8 7 1'1 5 10 0'3
0 0 0 0 0 0 0 0 0 0 0 0 0
L, latrines present: no activity index was m e a s u r e d at these sites.
Gordon L. Woodroffe, John H. Lawton, Walter L. Davidson
56
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57
M i n k predation on water voles
(a)
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Fig. 3. (a) Mean water vole activity indices on all study rivers. A, up to one month prior to mink entering the colony; B, during the presence of mink; C, up to one month after disappearance of mink (means and 95% confidence limits). (b) Control data; triplets of samples, similar to those in 3(a) but without mink.
comparison tests are deliberately designed to eliminate major differences in overall levels of water vole activity (and hence numbers), between sites. Longer term effects of mink on water voles, presumably reflecting change in vole numbers, were therefore analysed using the cumulative totals of the water vole and mink activity indices at six monthly intervals for sites with complete data on the four study river systems during 1985 and 1986. In this way activity indices were included which covered all the possible ranges of
Gordon L. Woodroffe, John H. Lawton, Walter L. Davidson
58
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~ 30
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MINK ACTIVITY INDEX
Fig. 4. Water vole activity indices per site, over six months, as a function of mink activity indices over the same time period. The equation of the fitted line is y = 4 7 " 1 - 1"52x Ft,~s = 9.96, P < 0-01.
mink activity, from sites with mink more or less continuously present to sites where mink activity was never found. Analysed in this way, there is a significant negative correlation between long-term mink activity and water vole activity at each site (F1,15 = 9.96, P < 0.01) (Fig. 4) which suggests that water vole numbers are either being reduced by mink predation, or that in the presence of mink, water voles in the longer term disperse to safer areas. TABLE 3 Summary of Radio-Tracking Data at Site 4, River Esk
Date tagged
29 June 1987 14 July 1987 17 July 1987 14 October 1987 5 November 1987 10 November 1987 3 August 1988 7 August 1988
Sex Weight (g)
M M F M M M F M
295 250 200 180 180 180 175 170
Wavelength
257 236 224 275 209 238 305 325
Days Maximum tracked observed linear range (m) 3 8 72 121 42 48 2 40mins
218 243 135 130 148 189 30 30
Notes
Disappeared Disappeared Died in burrow Killed by mink Died in burrow Predated in burrow Killed by mink Killed by mink
59
M i n k predation on water voles
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60
Gordon L. Woodroffe, John H. Lawton, Walter L. Davidson
Radio-tracking and the decline of water voles at site 4 Eight water voles were radio-tracked at site 4 on the Esk during 1987 and 1988 (Table 3). Two males tagged in June and July 1987 disappeared within days. We were unable to retrap or locate them over 3 km or river and as mink tracks appeared at this site in July 1987 we suspect that mink were responsible. Thereafter mink tracks were recorded monthly (but because of shortage of time these tracks were not quantified as activity indices: Fig. 5(a)), with mink being caught in water vole traps in August, October and November (Fig. 5(b)). In February 1988 we obtained the first direct evidence of mink taking water voles when the only surviving tagged water vole was found freshly killed by a mink. Mink activity continued throughout 1988 (Fig. 5(a)). Water voles became increasingly difficult to catch, although five mink were caught during this period and two radio-tagged water voles were killed by mink (both kills were observed directly). After these kills no further voles were caught at site 4 (270 trap-nights in September and 100 in October) and all signs disappeared.
DISCUSSION Feral mink slowly extended their distribution within the North Yorkshire Moors National Park during the study period (1984-88), allowing us to monitor changes in the short-term activity of water voles as mink entered and left water vole colonies; we were also able to assess the longer term impact of mink by comparing sites with different average levels of mink activity, and to follow the demise of a vole colony as mink established a resident population. The study highlights four aspects of the interaction between mink and water voles. First, when mink enter a water vole colony after a period of absence, vole activity, revealed by tracking, markedly increases in the short term, and subsides once mink leave. The simplest explanation is that voles move about more when mink are present in the area, presumably because they are agitated, or alarmed. (This interpretation is supported by Clark's (1981) observation on 'panicking' water voles as mink entered the colony.) In the longer term, however, there is a significant inverse correlation between average mink activity and average water vole activity across 38 sites on five rivers in the National Park, again revealed by tracking. The inference here is that in the longer run, mink depress water vole numbers. This interpretation is strongly supported by two other pieces of evidence. Site 4 on the river Esk held a thriving water vole colony in 1985, and lacked mink. Then between 1987 and 1988, mink activity on this stretch of river steadily
M i n k predation on water voles
61
increased (indices of mink activity based on tracking rose and mink were increasingly caught in water vole traps). During this time numbers of water voles declined, until the colony finally disappeared (no animals caught in 300 trap-nights) in October 1988. Last, but not least, 5 out of 8 radio-tagged voles were known or strongly suspected to have been killed by mink in this colony, during the period of increasing mink activity and declining water vole abundance. There is good evidence to show that water voles are particularly vulnerable to mink predation (Jeppsson, 1987; Stoddart, 1977) not least because of mink's ability to hunt both on land and in the water. For example, when radio-tagged water vole 325 was killed, the mink first searched all the burrow entrances, and then located the vole seemingly well camouflaged in dense cover. The water vole was killed when it dived into the river; its usual escape mechanism, diving into the water and kicking up a mud cloud, being completely ineffective. More generally, once mink are resident on stretches of river, regular foraging alongside the waterway will virtually ensure that they locate all water vole colonies, and may exterminate water voles from large stretches of British rivers. Both theoretical considerations (Holt, 1977; Jeffries & Lawton, 1984) and empirical observations (Carlquist, 1965; Churcher & Lawton, 1987; Pimm, 1987; Ebenhard, 1988) show that polyphagous predators, sustained by a large variety of alternative prey, may wreak havoc with particularly vulnerable species, eliminating them entirely from certain localities or ecosystems. In North America mink exterminated muskrats Ondarata zibethicus from some locations (Errington, 1946). At the present time, water voles and mink coexist, both on rivers in the North Yorkshire Moors and elsewhere (Birks, 1986). Voles may persist providing mink are not equally abundant throughout a water course. Keepering by water bailiffs and game keepers may be an important factor preventing mink from occupying all suitable habitats along a river and may unwittingly provide water voles with temporarily safe breeding sites. But by reducing population size and fragmenting water vole colonies mink pose a serious long-term threat to the survival of water voles on British rivers.
ACKNOWLEDGEMENTS We would like to thank all the landowners who allowed us to work on their stretches of river. Thanks are also due to the Mammal Conservation Trust who awarded one of us (G.L.W.) a generous grant to purchase radiotracking equipment. Extra water vole traps were loaned to us by Dr G. Triggs, Liverpool Polytechnic, to whom we are most grateful.
62
Gordon L. Woodroffe, John H. Lawton, Walter L. Davidson REFERENCES
Bailey, N. T. J. (1984). Statistical Methods in Biology. Hodder & Stoughton, London. Birks, J. D. S. (1986). Mink. Mammal Society, London. Carlquist, S. (1965). Island Life. New York Natural History Press, New York. Churcher, P. B. & Lawton, J. H. (1987). Predation by domestic cats in an English village. J. Zool., Lond., 212, 439-55. Clark, M. (1981). Mammal Watching. Severn House, London. Ebenhard, T. (1988). Introduced birds and mammals and their ecological effects. Swed. Wildl. Res., 13, 1-107. Errington, P. L. (1946). Predation and vertebrate populations. Quart. Rev. Biol., 21, 144-77, 221-45. Holt, R. D. (1977). Predation, apparent competition, and the structure of prey communities. Theoret. Pop. Biol., 12, 197-229. Jeffries, M. J. & Lawton, J. H. (1984). Enemy free space and the structure of biological communities. Biol. J. Linn. Soc., 23, 269-86. Jeppsson, B. (1987). Behavioural ecology of the water vole (Arvicola terrestris) and its implications to theories of microtine ecology. PhD. dissertation, University of Lund. Kenward, R. (1987). Wildlife Radio Tagging. Academic Press, London. Lawrence, M. J. & Brown. R. W. (1974). Mammals of Britain. Their Tracks, Trails and Signs. Blandford, London. Lever, C. (1978). The not so innocuous mink. New Scient., 78, 812-14. Linn, I. J. & Chanin, P. W. R. (1978a). Are mink really pests in Britain. New Scient., 77, 560-2. Linn, I. J. & Chanin, P. W. R. (1978b). More on the mink 'menace'. New Scient., 80, 38-40. Pimm, S. L. (1987). The snake that ate Guam. Trends in Ecology & Evolution, 2, 293-5. Smith, R. (1988). Mink and waterway birds. BTO News, 156, 10. Stoddart, D. M. (1970). Individual range, dispersion and dispersal in a population of water voles (Arvicola terrestris (L.)). J. Anim. EcoL, 39, 403-25. Stoddart, D. M. (1977). Water vole. In Handbook of British Mammals, ed. G. B. Corbet & H. N. Southern. Blackwelis Scientific Publications, Oxford, pp. 196-204. Woodroffe, G. L. (1988). Ecology of riverside mammals in the North Yorkshire Moors National Park. M.Phil. thesis, University of York.