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Reduction in numbers of the wild rabbit (Oryctolagus cuniculus) due to changes in agricultural practices and land use
CROP PROTECTION Vol. 6 October 1987, 347-351
Reduction in n u m b e r s of the wild rabbit (Oryctolagus cuniculus) due to changes in agricultural practices and land use B. BOAG
6 Park Road, Invergowrie, Dundee DD2 5,41-1, Scotland
ABSTRACT. Rabbits were collected each month over a 9-year period from a farm in Eastern Scotland. Parts of the study area of c. 500ha were improved over the 9-year period and the impact of these changes on the numbers and distribution of rabbits collected was monitored. The removal of favoured habitats significantly reduced rabbit numbers in the more intensively farmed areas and reduced the chances of these areas being recolonized. It is suggested that, because of its long-term benefits, habitat manipulation should be considered when devising rabbit control programmes. The patchy distribution of the resurgence of rabbit numbers since myxomatosis in Great Britain may be explained in part by the rate with which land improvements have been introduced in different parts of the country. KEYWORDS: Rabbit; Oryctolagus cuniculus; control; habitat; manipulation
Introduction
Rabbits were one of the major pests of agricultural crops in Great Britain before myxomatosis reduced numbers by an estimated 99.9°70 (Thompson, 1953; Armour and Thompson, 1955). In spite of the use of a range of conventional control measures, e.g. gassing, snaring and shooting (Buckley, 1935; Lockley, 1940; Southern, 1948; Thompson and Armour, 1951, 1953; Phillips, 1955a, b) and the persistence of myxomatosis, it has been estimated that, in Great Britain, rabbits have attained at least 20°70 of their numbers prior to myxomatosis and that in 1986 they will cause between £90000000 and £120000000 of damage to agricultural crops (Mills, 1986). The reason for the marked increase in rabbit numbers in some areas and not in others (Lloyd, 1970; Trout, Tapper and Harradine, 1986) is not understood but weaker attenuated strains of the myxoma virus (Fenner and Chapple, 1965) and increased resistance (Ross and Sanders, 1984) have been reported. Another factor which has not been investigated, but which could have influenced rabbit populations, is the changes in agricultural practices and land use that have occurred since 1955. This paper reports the findings of a 9-year study into the impact of the introduction of modern agricultural practices and land improvement on rabbit populations. Materials and m e t h o d s
Rabbits were shot monthly from January 1977 until 0261-2194lS7/0510347-05$03.00 © 1987 Butterworth & Co (Publishers)Ltd
December 1985 from Littleton Farm, Inchture, Perthshire. The study area was 150-300m above sea level and extended to approximately 500ha (Boag, 1986). The soil type on the farm belonged predominantly to two different soil associations, both of which were classified as a loam (Laing, Robertson and Birse, 1974). A wide range of vegetation types were present in 1977 (Figure 1) and changes due to improvements in agricultural practices and land use were mapped on a 1:100000 scale Ordnance Survey map. The area from which rabbits were shot and the commensurate hunting effort increased from 1977 until 1981 when both remained constant for the duration of the investigation. Hunting was confined to 2 - 3 h before dusk, irrespective of season, and consequently variability in numbers due to the rabbits' circadian rhythm was reduced (Kolb, 1986). Initially, in 1977, the area from which rabbits were shot was restricted to mainly arable land but subsequently this area was enlarged to encompass tracts of permanent pasture and heather moorland (Figure 2). The distribution of all the rabbits shot and the warrens they inhabited were recorded using North/South and East/West co-ordinates obtained from a 50m grid which was superimposed over the ordnance map (Boag, 1986). The analysis of the distribution of the rabbits shot during 1981 compared with those observed indicated that there were no significant differences between both sets of data. To investigate how representative the distribution of rabbits shot during the study was of the
Habitat manipulation and the control of rabbits
348
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500m FIGURE 1. Vegetation and land used in the study area in 1977 and agricultural improvements. K e y : [ ~ , gorse, broom and heather;[~-'], woodland;]---I, arable land and permanent pasture; A, quarry; B, quarries which had rabbit warrens and which were filled in; C, fences removed and not replaced; D, fence removed and replaced by rabbit fence; E, new stock-proof fencing; F, wall removed and replaced by stock-proof fencing; G, farm track removed; H, heather moor/rough grazing which was improved to permanent pasture/long-term grass Icy; I, gorse/whins removed and reseeded with grass; J, loch.
observed population, the map co-ordinates of all rabbit sightings were also recorded during 1981. From 1981 onwards the total study area was examined using the same route each month and this allowed the distribution of the rabbits to be statistically analysed to determine significant changes in rabbit numbers in areas I, II, III and IV (Figure 2). Results The major changes in vegetation type and land improvements between 1977 and 1985 can be seen by comparing Figures 1 and 2. In areas I and II (Figure 2) fields were made larger by removing fences, a quarry was filled in, a farm track ploughed up and areas of gorse and broom removed and reseeded with grass. In area III, the large area around the loch, which was originally mainly heather (Figure 1), was slowly improved by increasing the grazing pressure and in 1981 the area was sub-divided into six smaller fields of CROP PROTECTION Vol. 6 October 1987
c. 15 ha (Figure 2). Some of these were ploughed and reseeded with a perennial ryegrass (Lolium perenne L.)/ white clover (Trifolium repens L.) mixture. The larger area to the north of the study area was not improved. The land improvements and the new agricultural practices, e.g. the inwintering of the flock of sheep, resulted in the stocking rate being increased from 500 ewes in 1977 to 1000 ewes in 1985, while comparable figures for cattle numbers were 170 and 250. The median Ordnance Survey co-ordinates plus one standard deviation in the north/south and east/west direction (in metres) for shot rabbits was No. 282339 (563 m) (427 m) while comparable figures for observed were No. 284339 (486 m) (465 m). The changes in land use reduced the amount of cover available for rabbits to live in and this was reflected in the relative numbers collected each year (Tables 1 and 2). In 1977, 143 rabbits were shot from the vicinity of 38 warrens in area I (Figure 2) but the number of rabbits decreased until in 1982 only two
B. BOAG
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FK;URE 2. Vegetation and land used in the study area in 1985, agricultural improvements and the areas from which rabbits were collected in 1977, 1978-1979, 1980 and 1981-1985. Key: (see Figure 1 for alphabetic symbols); I, area from which rabbits were collected in 1977. II, extension of area from which rabbits were collected in 1978-79. III, extension of area from which rabbits were collected in 1980. IV, extension of area from which rabbits were collected 1981-85.
were collected from around these same warrens. Similar marked reductions in rabbit numbers were observed from the warrens where rabbits were first shot in 1978 and 1979 (area II, Figure 2) but not from those from 1980 onwards. Some of the new warrens opened up by rabbits from 1980 to 1985 were in areas I and II and consequently, to investigate the spatial distribution of the rabbits in the study area, it was necessary to tabulate the rabbits shot in the different areas (Table 2). The results showed that rabbit numbers in areas I and II were significantly reduced from 55 in 1981 to only 5 in 1985, a 91% reduction. In areas III and IV, where land improvements were less marked and plenty of cover remained, rabbit numbers were not reduced to the same extent. Discussion
Rabbit numbers have increased in Great Britain since
1955, in spite of the persistence of myxomatosis and the use of conventional control measures (Anonymous, 1984; Trout et al., 1986). New control measures have either been of limited use, e.g. acoustic scaring device~ (Wilson and McKillop, 1986), warren ripping (Parker, Myers and Caskey, 1976) or, although successful in other countries, have not yet been introduced into Great Britain, e.g. poisoning (Rowley, 1968; Richards, Hampson and Sleeman, 1986). However, the importance of the rabbits' habitat, especially vegetative cover and soil conditions conducive for the construction ot burrows (Parker et al., 1976; Caughley, 19771 Wheeler, King and Robinson, 1981; Anonymous, 1984; Kolb, 1985) and the manipulation of these factors to control rabbit numbers has received little attention. Previous investigations into the effectiveness of control measures have generally been of short duration but the protracted nature of the present study has shown the long-term benefits of habitat destrucCROP PROTECTION
Vol. 6 October 198~
Habitat manipulation and the control of rabbits
350
TABLE 1. The relationship between the number of rabbits shot each year and the warrens from which they were collected Year (and number of new warrens from which rabbits were collected for that year) Year in which rabbits were shot
1977 (38)
1978 (7)
1979 (8)
1980 (54)
1981 (78)
1982 (6)
1983 (21)
1984 (4)
1985 (2)
1977 1978 1979 1980 1981 1982 1983 1984 1985
143 50 55 39 22 2 2 4 2
9 3 2 2 0 0 0 0
12 3 6 0 0 0 0
136 81 42 39 35 48
137 35 53 42 53
7 1 2 3
40 16 18
7 3
3
TABLE 2. Number of rabbits shot each year in areas I, II, III and IV* Year Areas
1 9 7 7 1 9 7 8 1 9 7 9 1 9 8 0 1981 1 9 8 2 1 9 8 3 1 9 8 4 1985
I II III IV
143 0 0 0
53 6 0 0
62 8 0 0
45 i0 125 0
51 4 156 37
11 1 60 14
9 0 81 45
22
5 0 98 27
Total
143
59
70
180
248
86
135
106
130
11 0 75
* See Figure 2 for areas I, II, III and IV
tion. In the present study area the reduction of the amount of gorse, broom and heather, the filling-in of old quarries and the removal of walls and erection of rabbit-proof fencing markedly reduced the areas available to, or favoured by, rabbits. After these improvements had been introduced (mainly between 1977 and 1980) the numbers of rabbits collected in the improved areas decreased and remained low. Although these results were based on the number of rabbits shot and not on a direct assessment of the rabbits present, the numbers of rabbits collected throughout 1981 were in close agreement with the numbers observed in the different parts of the study area and rabbit sightings have been shown to give a reasonable estimate of rabbit numbers (Myers, 1957). The number of rabbits collected from area I decreased between 1977 and 1985 and the results of the comparison of the numbers of the rabbits collected from areas I and II from 1981 onwards, when the whole of the study area was regularly examined, indicated a continuing decline in rabbit numbers in these areas whereas those in areas III and IV remained relatively constant. It is suggested that the main reason for rabbit populations remaining low in areas I and II between 1982 and 1985 was the lack of adequate suitable habitats which rabbits could recolonize. The very small amount of re-colonization which did occur was generally easily controlled by an annual winter gassing of the few new occupied burrows. There was no evidence of any of the rabbits in the study area living above ground, all disturbed animals immediately running to nearby burrows. Rabbit numbers remained high in areas III and IV, due to the large reservoir of rabbits inhabiting the heather CROP PROTECTION Vol. 6 October 1987
area to the north of the study area, and to scattered distribution of small patches of land which could not be cultivated or farmed intensively and where rabbits were left undisturbed. The conclusions which can be drawn from the present long-term study demonstrated that improvements associated with modern agricultural practices can have long-term beneficial effects and, when possible, should be borne in mind by farmers when considering rabbit control programmes. However, the recent suggestion that the trend towards more intensive agricultural practices should be reversed to reduce agricultural surpluses in the European Common Market and to conserve the countryside, could also lead to a more rapid increase in rabbit populations.
Acknowledgements I am grateful to Mr H. A. McLaren for permission to collect rabbits from the study area and to Dr P. B. Topham for statistical analysis of the results.
References ANONYMOUS (1984). The Wild Rabbit. ADAS/MAFF Advisory Leaflet 534. 9 pp. Alnwick, Northumberland: Ministry of Agriculture, Fisheries and Food (Publications). ARMOUR, C. J. AND THOMSON, H. V. (1955). Spread of myxomatosisin the first outbreak in Great Britain. Annals of Applied Biology 43, 511-518. BOAG,B. (1985). The incidenceofhelminth parasitesfromthe wild rabbit Oryctolagus cuniculus (L.) in eastern Scotland.Journal of Helminthology 59, 61-69. BOAG,B. (1986). Observationson the localiseddistributionof wild rabbits (Oryctolagus cuniculus) with non agouti coat colouring. Journal of Zoology 210, 640-642. BUCKLEY,M. F. H. (1935). Report on a solution of the rabbit problem: Cyanidegassing. VeterinaryJournal 91, 210-215. CAUGHLEY, G. (1977). Analysis of Vertebrate Populations, p. 200. Chichester: John Wiley& Sons. FENNER, F. AND CHAPPLE, P. J. (1965). Evolutionarychanges in myxoma virus in Britain. An examination of 222 naturally occurring strains obtainedfrom 80 countiesduring the period October-November 1962.Journal of Hygiene 63, 175-185. KOLB,H. H. (1985). The burrow structure of the European rabbit ( Oryctolagus cuniculus L.). Journal of Zoology 206, 253-262. KOLB,H. H. (1986). Circadianactivityin the wild rabbit (Oryctolagus cuniculus). Mammal Review 16, 145-150.
B. BOAG LAING, D., ROBERTSON, J. S. AND BIRSE, E. L. (1974). Soils of
the Country round Perth, Arbroath and Dundee with an Account of the Vegetation. Edinburgh: HMSO. 328 pp. LLOYD, H. G. (1970). Postmyxomatosis rabbit populations in England and Wales. EPPO Publication Series A 589 197-215. LOCKLEY, R. M. (1940). Some experiments in rabbit control. Nature 18, 767-769. MILLS, S. (1986). Rabbits breed a growing controversy. New Scientist 109, 50-54. MYERS, K. (1957). Some observations on the use of sight counts in estimating populations of the wild rabbit, Oryctolagus cuniculus (L.). CSIRO Wildlife Research 2, 170-172. PARKER,B. S., MYERS,K. ANDCASKEY,R. L. (1976). An attempt at rabbit control by warren ripping in semi and western New South Wales. Journal of Applied Ecology 13, 353-367. PARKER, B. S., HALL, L. S., MYERS, K. AND FULLAGER, P. J. (1976). The distribution of rabbit warrens at Mitchell, Queensland, in relation to soil and vegetation characteristics. Australian Wildlife Research 3, 129-148. PHILLIPS, W. M. (1955a). The effect of commercial trapping on rabbit populations. Annals of Applied Biology 4:], 247-257. PHILLIPS, W. M. (1955b). An experiment in rabbit control. Annals of Applied Biology 43, 258-264. RICHARDS, C. G. J., HAMPSON, S. J. AND SLEEMAN, P. D. (1986). The potential for rational use of anticoagulant baits within rabbit control programmes. Mammal Review 16, 199-200. ROSS, J. AND SANDERS,M. F. (1984). The development of genetic resistance to myxomatosis in wild rabbits in Britain. Journal of Hygiene 699 105-111. ROWLEY, I. (1968). Studies on the resurgence of rabbit populations
351
after poisoning. CSIRO Wildlife Research 13, 59-69. SOUTHERN,H. N. (1948). The persistence of hydrogen cyanide in rabbit burrows. Annals of Applied Biology 35, 331-346. THOMPSON, H. V. (1953). The rabbit disease: Myxomatosis. Annals of Applied Biology 41,358-366. THOMeSON, H. V. AND ARMOUR, C. J. (1951). Control of the European rabbit (Oryctolagus cuniculus L.). An experiment to compare the efficiency of gin trapping, ferreting and cyanide gassing. Annals of Applied Biology 38, 464-474. THOMPSON, H. V. AND ARMOUR, C. J. (1953). Power gassing of rabbits. Agriculture (London) 60, 383-386. TROUT, R. C., TAPPER, S. C. AND HARRADINE,J. (1986). Recent trends in the rabbit population in Britain. Mammal Review 169 117-123. WHEELER, S. H., KING, D. R. AND ROBINSON, M. H. (1981). Habitat and warren utilization by the European rabbit Oryctolagus cuniculus (L.) as determined by radio tracking. Australian Wildlife Research 8, 581-588. WILSON, C. J. AND McKILLOe, I. G. (1986). An acoustic scaring device tested against European rabbits. Wildlife Society Bulletin 14, 409-411.
Received 11 December 1986 Revised 11 March 1987 Accepted 23 March 1987
CROP PROTECTION Vol. 6 October 1987
Reduction in n u m b e r s of the wild rabbit (Oryctolagus cuniculus) due to changes in agricultural practices and land use B. BOAG
6 Park Road, Invergowrie, Dundee DD2 5,41-1, Scotland
ABSTRACT. Rabbits were collected each month over a 9-year period from a farm in Eastern Scotland. Parts of the study area of c. 500ha were improved over the 9-year period and the impact of these changes on the numbers and distribution of rabbits collected was monitored. The removal of favoured habitats significantly reduced rabbit numbers in the more intensively farmed areas and reduced the chances of these areas being recolonized. It is suggested that, because of its long-term benefits, habitat manipulation should be considered when devising rabbit control programmes. The patchy distribution of the resurgence of rabbit numbers since myxomatosis in Great Britain may be explained in part by the rate with which land improvements have been introduced in different parts of the country. KEYWORDS: Rabbit; Oryctolagus cuniculus; control; habitat; manipulation
Introduction
Rabbits were one of the major pests of agricultural crops in Great Britain before myxomatosis reduced numbers by an estimated 99.9°70 (Thompson, 1953; Armour and Thompson, 1955). In spite of the use of a range of conventional control measures, e.g. gassing, snaring and shooting (Buckley, 1935; Lockley, 1940; Southern, 1948; Thompson and Armour, 1951, 1953; Phillips, 1955a, b) and the persistence of myxomatosis, it has been estimated that, in Great Britain, rabbits have attained at least 20°70 of their numbers prior to myxomatosis and that in 1986 they will cause between £90000000 and £120000000 of damage to agricultural crops (Mills, 1986). The reason for the marked increase in rabbit numbers in some areas and not in others (Lloyd, 1970; Trout, Tapper and Harradine, 1986) is not understood but weaker attenuated strains of the myxoma virus (Fenner and Chapple, 1965) and increased resistance (Ross and Sanders, 1984) have been reported. Another factor which has not been investigated, but which could have influenced rabbit populations, is the changes in agricultural practices and land use that have occurred since 1955. This paper reports the findings of a 9-year study into the impact of the introduction of modern agricultural practices and land improvement on rabbit populations. Materials and m e t h o d s
Rabbits were shot monthly from January 1977 until 0261-2194lS7/0510347-05$03.00 © 1987 Butterworth & Co (Publishers)Ltd
December 1985 from Littleton Farm, Inchture, Perthshire. The study area was 150-300m above sea level and extended to approximately 500ha (Boag, 1986). The soil type on the farm belonged predominantly to two different soil associations, both of which were classified as a loam (Laing, Robertson and Birse, 1974). A wide range of vegetation types were present in 1977 (Figure 1) and changes due to improvements in agricultural practices and land use were mapped on a 1:100000 scale Ordnance Survey map. The area from which rabbits were shot and the commensurate hunting effort increased from 1977 until 1981 when both remained constant for the duration of the investigation. Hunting was confined to 2 - 3 h before dusk, irrespective of season, and consequently variability in numbers due to the rabbits' circadian rhythm was reduced (Kolb, 1986). Initially, in 1977, the area from which rabbits were shot was restricted to mainly arable land but subsequently this area was enlarged to encompass tracts of permanent pasture and heather moorland (Figure 2). The distribution of all the rabbits shot and the warrens they inhabited were recorded using North/South and East/West co-ordinates obtained from a 50m grid which was superimposed over the ordnance map (Boag, 1986). The analysis of the distribution of the rabbits shot during 1981 compared with those observed indicated that there were no significant differences between both sets of data. To investigate how representative the distribution of rabbits shot during the study was of the
Habitat manipulation and the control of rabbits
348
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500m FIGURE 1. Vegetation and land used in the study area in 1977 and agricultural improvements. K e y : [ ~ , gorse, broom and heather;[~-'], woodland;]---I, arable land and permanent pasture; A, quarry; B, quarries which had rabbit warrens and which were filled in; C, fences removed and not replaced; D, fence removed and replaced by rabbit fence; E, new stock-proof fencing; F, wall removed and replaced by stock-proof fencing; G, farm track removed; H, heather moor/rough grazing which was improved to permanent pasture/long-term grass Icy; I, gorse/whins removed and reseeded with grass; J, loch.
observed population, the map co-ordinates of all rabbit sightings were also recorded during 1981. From 1981 onwards the total study area was examined using the same route each month and this allowed the distribution of the rabbits to be statistically analysed to determine significant changes in rabbit numbers in areas I, II, III and IV (Figure 2). Results The major changes in vegetation type and land improvements between 1977 and 1985 can be seen by comparing Figures 1 and 2. In areas I and II (Figure 2) fields were made larger by removing fences, a quarry was filled in, a farm track ploughed up and areas of gorse and broom removed and reseeded with grass. In area III, the large area around the loch, which was originally mainly heather (Figure 1), was slowly improved by increasing the grazing pressure and in 1981 the area was sub-divided into six smaller fields of CROP PROTECTION Vol. 6 October 1987
c. 15 ha (Figure 2). Some of these were ploughed and reseeded with a perennial ryegrass (Lolium perenne L.)/ white clover (Trifolium repens L.) mixture. The larger area to the north of the study area was not improved. The land improvements and the new agricultural practices, e.g. the inwintering of the flock of sheep, resulted in the stocking rate being increased from 500 ewes in 1977 to 1000 ewes in 1985, while comparable figures for cattle numbers were 170 and 250. The median Ordnance Survey co-ordinates plus one standard deviation in the north/south and east/west direction (in metres) for shot rabbits was No. 282339 (563 m) (427 m) while comparable figures for observed were No. 284339 (486 m) (465 m). The changes in land use reduced the amount of cover available for rabbits to live in and this was reflected in the relative numbers collected each year (Tables 1 and 2). In 1977, 143 rabbits were shot from the vicinity of 38 warrens in area I (Figure 2) but the number of rabbits decreased until in 1982 only two
B. BOAG
349
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FK;URE 2. Vegetation and land used in the study area in 1985, agricultural improvements and the areas from which rabbits were collected in 1977, 1978-1979, 1980 and 1981-1985. Key: (see Figure 1 for alphabetic symbols); I, area from which rabbits were collected in 1977. II, extension of area from which rabbits were collected in 1978-79. III, extension of area from which rabbits were collected in 1980. IV, extension of area from which rabbits were collected 1981-85.
were collected from around these same warrens. Similar marked reductions in rabbit numbers were observed from the warrens where rabbits were first shot in 1978 and 1979 (area II, Figure 2) but not from those from 1980 onwards. Some of the new warrens opened up by rabbits from 1980 to 1985 were in areas I and II and consequently, to investigate the spatial distribution of the rabbits in the study area, it was necessary to tabulate the rabbits shot in the different areas (Table 2). The results showed that rabbit numbers in areas I and II were significantly reduced from 55 in 1981 to only 5 in 1985, a 91% reduction. In areas III and IV, where land improvements were less marked and plenty of cover remained, rabbit numbers were not reduced to the same extent. Discussion
Rabbit numbers have increased in Great Britain since
1955, in spite of the persistence of myxomatosis and the use of conventional control measures (Anonymous, 1984; Trout et al., 1986). New control measures have either been of limited use, e.g. acoustic scaring device~ (Wilson and McKillop, 1986), warren ripping (Parker, Myers and Caskey, 1976) or, although successful in other countries, have not yet been introduced into Great Britain, e.g. poisoning (Rowley, 1968; Richards, Hampson and Sleeman, 1986). However, the importance of the rabbits' habitat, especially vegetative cover and soil conditions conducive for the construction ot burrows (Parker et al., 1976; Caughley, 19771 Wheeler, King and Robinson, 1981; Anonymous, 1984; Kolb, 1985) and the manipulation of these factors to control rabbit numbers has received little attention. Previous investigations into the effectiveness of control measures have generally been of short duration but the protracted nature of the present study has shown the long-term benefits of habitat destrucCROP PROTECTION
Vol. 6 October 198~
Habitat manipulation and the control of rabbits
350
TABLE 1. The relationship between the number of rabbits shot each year and the warrens from which they were collected Year (and number of new warrens from which rabbits were collected for that year) Year in which rabbits were shot
1977 (38)
1978 (7)
1979 (8)
1980 (54)
1981 (78)
1982 (6)
1983 (21)
1984 (4)
1985 (2)
1977 1978 1979 1980 1981 1982 1983 1984 1985
143 50 55 39 22 2 2 4 2
9 3 2 2 0 0 0 0
12 3 6 0 0 0 0
136 81 42 39 35 48
137 35 53 42 53
7 1 2 3
40 16 18
7 3
3
TABLE 2. Number of rabbits shot each year in areas I, II, III and IV* Year Areas
1 9 7 7 1 9 7 8 1 9 7 9 1 9 8 0 1981 1 9 8 2 1 9 8 3 1 9 8 4 1985
I II III IV
143 0 0 0
53 6 0 0
62 8 0 0
45 i0 125 0
51 4 156 37
11 1 60 14
9 0 81 45
22
5 0 98 27
Total
143
59
70
180
248
86
135
106
130
11 0 75
* See Figure 2 for areas I, II, III and IV
tion. In the present study area the reduction of the amount of gorse, broom and heather, the filling-in of old quarries and the removal of walls and erection of rabbit-proof fencing markedly reduced the areas available to, or favoured by, rabbits. After these improvements had been introduced (mainly between 1977 and 1980) the numbers of rabbits collected in the improved areas decreased and remained low. Although these results were based on the number of rabbits shot and not on a direct assessment of the rabbits present, the numbers of rabbits collected throughout 1981 were in close agreement with the numbers observed in the different parts of the study area and rabbit sightings have been shown to give a reasonable estimate of rabbit numbers (Myers, 1957). The number of rabbits collected from area I decreased between 1977 and 1985 and the results of the comparison of the numbers of the rabbits collected from areas I and II from 1981 onwards, when the whole of the study area was regularly examined, indicated a continuing decline in rabbit numbers in these areas whereas those in areas III and IV remained relatively constant. It is suggested that the main reason for rabbit populations remaining low in areas I and II between 1982 and 1985 was the lack of adequate suitable habitats which rabbits could recolonize. The very small amount of re-colonization which did occur was generally easily controlled by an annual winter gassing of the few new occupied burrows. There was no evidence of any of the rabbits in the study area living above ground, all disturbed animals immediately running to nearby burrows. Rabbit numbers remained high in areas III and IV, due to the large reservoir of rabbits inhabiting the heather CROP PROTECTION Vol. 6 October 1987
area to the north of the study area, and to scattered distribution of small patches of land which could not be cultivated or farmed intensively and where rabbits were left undisturbed. The conclusions which can be drawn from the present long-term study demonstrated that improvements associated with modern agricultural practices can have long-term beneficial effects and, when possible, should be borne in mind by farmers when considering rabbit control programmes. However, the recent suggestion that the trend towards more intensive agricultural practices should be reversed to reduce agricultural surpluses in the European Common Market and to conserve the countryside, could also lead to a more rapid increase in rabbit populations.
Acknowledgements I am grateful to Mr H. A. McLaren for permission to collect rabbits from the study area and to Dr P. B. Topham for statistical analysis of the results.
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Received 11 December 1986 Revised 11 March 1987 Accepted 23 March 1987
CROP PROTECTION Vol. 6 October 1987