The status and conservation of the butterfly Plebejus argus L. (Lepidoptera: Lycaenidae) in North West Britain

Biological Conservation 33 (1985) 29-51

The Status and Conservation of the Butterfly Plebejus argus L. (Lepidoptera: Lyeaenidae) in North West Britain C. D. Thomas* School of Animal Biology, University College of North Wales, Bangor, Gwynedd, Great Britain

ABSTRACT A survey was made of the declining butterfly, Plebejus argus L., in North West Britain in 1983. Twenty-six colonies were found on limestone grassland, many of them large. Twenty colonies were found on heathland, but most were relatively small. One large colony that inhabits mossland was rediscovered. P. argus occupies early successional habitat at low altitude, predominantly on sheltered southerly ./'acing slopes. Through biotope loss and.Jragmentation and declining traditional management, suitable habitat is created only sporadically in modern Britain. Some of these habitats (especially on heathland) are short lived. Thus, man), colonies are transient. As P. argus Jbrms closed colonies and rarely.flies .far, the decline appears to be a compound e[.Ject of reduced amounts of suitable habitat and inability to colonise them: the distances are often too great. Active conservation is required to check the decline of P. argus, especially habitat management and also introduction to suitable habitat non' beyond the dispersal range of P. argus. Recommendations are made.

INTRODUCTION The silver studded blue Plebejus argus L. is a local British butterfly and one o f the most rapidly declining (for comparison with other species, see H e a t h et al., 1984). P. argus inhabits lowland heaths, mosses (the biotope * Present address: Department of Zoology, University of Texas, Austin, Texas, 78712, USA. 29 Biol. Conserv. 0006-3207/85/$03.30 © ElsevierApplied Science Publishers Ltd, England, 1985. Printed in Great Britain

30

C. D. Thomas

moss, or mossland, is formed by hydrosere succession and resembles very wet or boggy heathland), calcareous grassland and sand dunes, many of which have been lost to cultivation, and to a lesser extent to forestry and urban development. But, even within surviving areas of these biotopes, P. argus is locally distributed and appears to have declined faster than the biotopes themselves. Several local races of P. argus have been described in Britain, associated with particular biotopes (Tutt, 1908; de Worms, 1949; Dennis, 1972, 1977). Although these may better be thought of as regional, clinal and inter-population variations in appearance with relatively minor ecological differentiation, nonetheless, they do make P. argus of particular conservation interest in Britain (C. D. Thomas, 1983; in press). One of these, P. argus masseyi Tutt, is extinct and chalk forms in southern England are now either very rare or also extinct. P. argus is still quite frequently encountered on surviving heaths in Surrey, Hampshire and Dorset, but elsewhere colonies are rare. Despite this decline, and the concern it has generated (Sutton, 1981, 1983), no ecological information has been available on which to base conservation efforts (Heath et al., 1984). To draw up a conservation programme for a declining butterfly it is useful to know: (i) the distribution and sizes of colonies and the rate of decline; (ii) what constitutes suitable habitat; (iii) how this can be maintained and/or created; (iv) the minimum area able to support a population at any one time; (v) the length of time for which a particular location can support a population; and (vi) over what distance the species is capable of colonising new habitat. The aims were to furnish as much of the above information as possible on P. argus in North Wales and North West England. This survey encompassed the entire distribution of P. argus caernensis Thompson, which is the most distinct extant British form and is endemic to limestone grassland in North Wales (Dennis, 1972, 1977). It also included the last surviving moss population in Britain. METHODS Distribution

Past localities of P. argus within 100 km grid squares 23, 25, 33, 34 and 35 were compiled from records held at the Biological Records Centre (BRC),

Status of Plebejus argus

31

from University College of North Wales Entomological Records, and from local entomologists. A survey for adults was conducted from June to August 1983, searching all localities with post 1940 records. Most other localities with older records were also covered, as were many other sites with potentially suitable habitat, but less intensively. Site/habitat details, weather conditions and adult sightings were recorded on a standard form during each visit.

Estimates of adult population size Absolute estimates of numbers Mark-release-recapture estimates of population size were made at six sites. Adults were caught throughout each locality, marked individually and immediately released at the point of capture. On each such occasion, record was made of the number mark, sex, condition (see below) and position of capture. Adult numbers were estimated using Craig's (1953) frequency of capture method. Recapture frequencies were compared to two mathematical model distributions: a zero-truncated Poisson model (Craig, 1953) which assumes random sampling and that any individual has a similar chance of being caught, and a truncated geometric model (Eberhardt, 1969), which assumes certain individuals are more likely to be caught than others. In 28 comparisons, the Poisson model fitted better, in 4 the geometric model fitted better, and in 3 they were equally good. As none of the four frequencies of capture that fitted the geometric model better was significantly different from the Poisson model distribution, the latter has been used throughout to calculate population estimates and their standard errors. Relative estimates There was insufficient time to estimate the absolute numbers of adults present at most localities, so the relative sizes were estimated using J. A. Thomas' (1983a) survey method. This involves measuring the area covered by a colony and assessing the population density in it by conducting transect walks (following the guidelines of Pollard, 1977). A population index is calculated (area times density). This was strongly correlated with absolute population size (using Craig's method) over the population size range of 4 to 3000 individuals (log Index = 0.949 log Craig Population E s t i m a t e - 0.229; r =0.952, n = 12, p < 0.001).

32

C. D. Thomas

For localities for which there was insufficient time to make Craig estimates, the adult numbers present-could be estimated by substituting the Index in the above equation. The main adult flight period lasts several weeks, during which numbers increase, then decline (Fig. 1). Not all Population Indices can be obtained on the same day, or at the same stage of emergence, so they are adjusted to the value expected on the day of peak adult numbers. This is done by reference to sites where the population is monitored throughout the season by fixed transect walks; comparing calender date, average wing wear (4--mint to 1--tatty) and sex ratio (log male/female ratio) (Fig. 1). From these an equivalent date is estimated. The peak flight adjustment is calculated as the peak numbers at the reference site divided by the numbers at the reference site on the equivalent date (J. A. Thomas, 1983a). To test the accuracy of peak flight period adjustments, ten comparisons were made between two limestone reference localities (using one to calculate the adjustment at the other). The ratio of the larger to smaller estimate (i.e. the error) was less than 1-4 in seven cases (maximum 2.4).

I1

LIMESTONE

HEA

S

\t

..........

_

1: 4

°ti | I

1.0

1'0

~

0.5

0'5

To ¢¢ -0.5

0

-0.5 25

30 JUNE

5

10 JULY

15

30 JUNE

10

20 JULY

30

10 AUGUST

Fig. 1. Changes in numbers of (a) adults, (b) wing wear and (c) sex ratio of Plebe~us argus with date (in 1983), at three localities in North Wales: limestone--Great Orme (closed circles) & Rhyd-y-Foel (+); Heathland--South Stack Range.

33

Status of Plebejus argus TABLE 1 Size Categories of P. argus Colonies used in this Study

Size category

Relative population index on day of peak numbers

Predicted number of adults on day of peak numbers

I

> 7 000

2 3 4 5 6

1 500-7 000 175-1 500 20-175 < 20 single sighting

> 20 000 4 000-20 000 400-4000 40-400 < 40 --

Averaging the limestone reference localities (as in Fig. 1) should further reduce this error. This correspondence is satisfactory since such errors are dwarfed by the range of P. argus colony sizes, which cover four orders of magnitude. Limestone populations emerged earlier than heathland ones, so adjustments are calculated using the reference localities of the same biotope. The population estimate is not perfect, so colonies have subsequently been lumped into five size categories (Table 1; category 6 is probably stray individuals rather than small breeding pockets).

RESULTS Definition of a colony Plebejus argus populations usually occupy discrete areas, but in some localities colonies were hard to define, where breeding pockets were close together. Since P. argus was found to be so immobile (see below) most of these can be considered to be separate (if not genetically isolated). For the practical purposes of categorisation and conservation I have adopted a definition of groups of individuals separated by at least 100m. Occasionally, predominantly on the Great Orme, where populations were practically continuous and very large, division has been based on natural barriers (e.g. patches of scrub) and areas of minimum population density, across which adult movements are restricted but not absent.

34

C. D. Thomas

o

ou

•

f~f

.~,

), •

o ~

•

•

-

Status of Plebejus argus

35

Distribution and status

In the British Isles, P. argus occurs in England, Wales and the Channel Isles, but is absent from Ireland, and from early this century also from Scotland. Two distinct races were restricted to Britain: P. argus masseyi on the mosses of Westmorland and Lancashire, until about 1942 when it became extinct, and P. argus caernensis which still occurs on limestone grassland in North Wales. Figure 2 gives the known distribution of P. argus in Britain in three date classes. It shows an especially marked decline north of the Thames and also in the South West and Kent. Up to 1945, P. arguswas recorded in 218

/

I

o o

•

EXISTING 1983

0 EXTINCT

0

~0

Oo

>

P

•

o 0

%o •

0 0

0 0

i

Fig. 3.

Past and present distribution of Plebejus argus in North West Britain.

36

C. D. Thomas

10 km grid squares, which declined to 170 (78 ~o) between 1945 and 1972, and to 96 since 1972 (44Y/oo of pre 1945 levels). This considerably underestimates the real decline as early records are far from complete (some are too inaccurate to map) and new records in the later two date classes probably only very rarely represent colonisation (in view of the sedentary nature of P. argus). Under the assumption of no colonisation o f new 10 km squares (except where introductions were made), 56 ~o of squares that have ever been occupied were recorded between 1945 and 1972, and 32 ~o since 1972. This is a more realistic estimate of the decline in range; a two-thirds reduction. Even so, it is probably an underestimate. The maps take no account of declines within 10 km squares: m a n y 10 km squares contained more than one colony that has become extinct and most squares that still contain at least one population have nonetheless lost others. Only one Scottish record is mapped, although undoubtedly there were more breeding in the 19th century, as other colonies are either hard to locate or are considered possible errors (Thompson, 1980). The past and present distribution of P. a r g u s in North West Britain is given in Fig. 3. On limestone, ten colonies were found on the Great Orme and 16 in the Dulas valley, to the latter of which P. a r g u s was originally introduced (at Rhyd-y-Foel) in 1942 (Marchant, 1956) (Table 2). Many colonies were very large (23 ~o category 1 or 2). In contrast, only one heathland colony ( 5 ~ ) , South Stack Range Royal Society for the Protection o f Birds reserve, was in category 1 or 2. New colonies were TABLE 2

The Relative Sizes of P. argus Colonies in North West Britain in 1983 Size category

Predicted adult numbers on peak day

Number of colonies Limestone Gt. Orme

Dulas V.

Heathland

Moss

I

> 20 000

3

0

1

0

2 3 4 5 6

4 000-20 000 400~ 000 40-400 <40 single sighting

1 3 3 0 I

2 6 5 3 1

0 8 9 or 10 1 or 2 8

I 0 0 0 0

10

16

20

I

Total (excluding category 6)

Status of Plebejus argus

37

found along the southern coast of Holy Island and to the north of Holyhead Mountain. The still fairly strong (category 3) Prees Heath colony is the only surviving Midlands population. Its daughter colony at Marford (to which P. argus was introduced) is still small. A large population inhabiting mossland was rediscovered, having apparently been unrecorded since about 1950 (Richardson, 1951).

Life cycle Eggs are laid on or under short vegetation containing larval foodplants, and most importantly at the margins between vegetation and open ground. On limestone in North Wales, they are laid on or under Helianthemum canum (L.) Baumg., H, chamaecistus Miller, Lotus corniculatus L., Medicago lupulina L., Thymus drucei Ronn., Cotoneaster microphyllus Lindley, Calluna vulgaris (L.) Hull, and occasionally others. On heathland, butterflies laid mostly on or under C. vulgaris, Erica cinerea L., E. tetralix L., Ulex europaeus L., U. gallii Planchon, L. corniculatus, Sarothamnus scoparius (L.) Koch and Genista anglica L. Moss butterflies laid on C. vulgaris, E. cinerea and E. tetralix. Eggs remain dormant over winter, for about eight months, and hatch in late March or April. Larvae accept most if not all the above as foodplants provided they have tender new growth, especially flowers, buds, or soft new leaves or stems. At least from the third instar (of five) wild larvae are constantly attended by Lasius alienus Forster and L. niger L. ants which consume secretions from the nectar (Newcomer's) and other glands. Larvae often rest and subsequently may pupate inside ants' nests. Pupae are also highly attractive to ants and are constantly attended (whether in ants' nests or not) by workers which frequently enclose them in earth cells. Adults may be on the wing from the last week of May (at the Great Orme) through to the end of September (at some heathland sites). The precise date varies from year to year and from site to site (C. D. Thomas, 1983, in press). In 1983, limestone populations in North Wales peaked in the first two weeks of July. South Stack Range (heathland) peaked some two weeks later. Males start to emerge several days before females (hence change in sex ratio during the season; Fig. I c). Females mate shortly after emergence, and probably rarely do so more than once. Adults live for an average of approximately three days, though some may live for several weeks. Further details are given in C. D. Thomas (1983, in press).

38

C. D. Thomas

TABLE 3 Steepness of Slopes at P. argus Localities in North West Britain. (Number of sites and sub-sites are given)

Slope in degrees

Limestone Heathland

0-5

6-10

11-15

16-20

21-25

26-30

31-35

36-40

>40

3 40

2 7

5 5

8 1

I1 0

21 1

15 0

7 0

9 0

Habitat characteristics Limestone Most P. argus caernensis colonies were on steep south/south west facing

slopes (Tables 3 and 4) below 200 m altitude. Ten of the 26 limestone colonies were either partly or wholly in quarries (as are most other limestone colonies, on Portland Bill, Dorset). These may be favoured because they are sheltered and provide broken ground, and hence have many vegetation/bare ground margins. There was a positive correlation between the percentage bare ground at limestone localities and the butterfly population density (y = 1.34x + 8.1 ; r = 0.41, n = 34, p < 0.02). Whilst the presence of some bare ground is favourable, very heavy grazing seems detrimental. At Rhyd-y-Foel, populations used to be practically continuous in 1971/73 (Dennis, 1972, 1977), but are now restricted to under half their former distribution. They are still present in broken 5-6 cm tall vegetation, but are absent from turf shorter than 2 cm. P. argus is also absent from tall thick (10-40 cm) vegetation where there is no bare ground, despite an abundance of larval foodplants. Several TABLE 4 Aspects of P. argus Localities in North West Britain. (Number of sites and sub-sites are given)

Aspect in degrees

Limestone Heathland

030-090

090-150

150-210

210-270

270-330

330~930

0 1

9 3

27 1.4

26 4

1 0

0 2

Status o f Piebejus argus

39

TABLE 5 Grazing Animals Present on Limestone P. argus Localities in North Wales

None

Number of sites and sub-sites

4

Rabbit only

12

Rabbit plus Sheep

Sheep and goats

Cattle

14

20

1

colonies in the Dulas valley are separated by such overgrown habitat even though it contains H. chamaecistus, L. corniculatus, T. drucei, U. europaeus, C. vulgaris and E. cinerea. Thus, intermediate grazing intensity appears to be most favourable ( 3 ~ c m broken turf is probably most suitable). Sheltered crags and quarries provide ideal habitat since succession is slow on bare rock and parts are inaccessible and, hence, cannot be easily overgrazed. Grazing is provided mainly by rabbits, sheep and goats (Table 5). Heathland Most colonies occur on shallow slopes or flat ground (Table 3). Thus, the aspect is of less significance, though southerly facing slopes are again favoured (Table 4). All colonies are below 150m altitude. As on limestone, population density tends to be higher where there is a high percentage of bare ground (y = 0.32x + 4.77; r = 0-41, n = 51, p < 0-01), but is not correlated with foodplant cover. Eggs are laid on or under'young heathland plants growing adjacent to bare ground. These conditions are most frequently found in recently disturbed or burned heath (Table 6). Disturbed heath is more frequently used than recent burns, perhaps because disturbed heath takes longer to TABLE 6 Habitat Condition at Heathland P. argus Localities in North West Britain. (Number of sites and sub-sites are given)

Existing Extinct

Recently burned

Recently disturbed

Grazed heath

Wet heath + runnels

Sparse mature

Closed mature

9 0

27 0

2 0

!5 0

7 2

0 1

40

C. D. Thomas

recover to maturity, providing time for P. argus to colonise and breed on it. Disturbed heath typically occurs along paths, disused tracks and in abandoned quarries. Sparse mature heath rarely supported populations except when close to other larger colonies (it is the commonest heath type surviving on Anglesey and Holy Island). Many pockets of population exist in damp hollows, where P. argus seemed slightly less particular as to the maturity of foodplants, whilst still favouring disturbed paths, short heath and wet runnels. In hollows with runnels even mature heath does not become completely closed. Population density was low or absent in mature wet heath, but never present in mature dry heath. Mossland One moss colony persists in Britain, at 10m above sea level. P. argus occupies an early successional stage where heathers (C. vulgaris, E. cinerea and E. tetralix) grow out of living bryophytes and lichens. Eggs are laid on short heather plants (< 3 cm tall) adjacent to open lichen or bryophyte-covered ground. Succession to domination by grass tussocks (Molinia caerulea (L.) Moench.) and shrubs (Myriea gale L.) is prevented by sheep and cattle grazing. P. argus is absent from tall M. caerulea and M. gale dominated vegetation nearby, although some heather persists, P. argus is also absent from more heavily grazed land. Highest population density occurred where there was approximately 1 5 ~ bare moss/lichen, 2 ~ other vegetation less than 4 cm tall, 10 ~ between 4 and 6 cm, 20 ~o 7 to 14 cm, and 53 ~ taller than this. Most eggs were found in patches with up to 80 bare or less than 4cm, especially on relatively dry hummocks. Formerly, other moss colonies may have been maintained by burning and peat cutting.

Colony lifespans and minimum areas Limestone P. argus caernensis has remained at fairly consistently high abundance on the southwest facing slope of the Great Orme since early this century, and probably long before (Appendix 2 in C. D. Thomas, 1983). Sheep and goat grazing have been continuous during this period and the habitat still appears ideal for the butterfly. Under the same management regime, it seems likely that P. argus can persist indefinitely at this locality. In the Dulas valley, on commercially managed land, there has been

Status of Plebejus argus

41

considerable population flux in the last decade. From a comparison of population strength in 1 ha grids in 1971/73 (Dennis, 1977) and 1983, it appears that total population strength has changed rather little since then, but that there has been an approximately 10~o flux in spatial distribution every year. This gives an average estimate of 6 or 7 years for habitat to continue to support a P. argus population at the same strength (C. D. Thomas, 1983). However, as at the Great Orme, some localities have supported colonies for much longer (41 years at the locality where P. argus was first introduced). Several colonies occupied less than 0.5ha, but only one (tiny population) less than 0.1 ha (Table 7). About 0-3 ha is probably the normal minimum unless the habitat is ideal. TABLE 7

Areas of Limestone P. argus Colonies in North Wales Area in hectares Number of colonies

< 0.1 1

0.1 4).45 9

0.5 1.45 6

1.5-8.0 8

> 8.0 1

Heathland

On heathland, P. argus usually occupies short-lived pioneer habitat. From correlations between numbers of P. argus recorded on different sections of transect walks in different years, it appears that suitable habitat becomes unsuitable within 5 to 10 years (the time for there to be no correlation between walk section totals; C. D. Thomas, 1983). The real situation is more complex because P. argus uses both burned (lifetime normally under 5 years) and disturbed areas (lifetime perhaps 10 years or more). Whatever the exact values of habitat lifespan, it is normally short. On heathland, P. argus survives through regular colonisation of newly created pioneer habitat, rather than persisting in any one location. Only large heaths support populations, even though colonies regularly breed on 0.5 ha or less of suitable habitat (Table 8). A 'safe' minimum may be 0.5 ha on a particular heath. At present less than 2 ~ of the South Stack Cliffs/Holyhead Mt/Breakwater Quarries heath in the north of Holy Island supports P. argus. Two per cent of a 25-ha heath is this minimum 0"5 ha. Thus, only very large heaths are likely to provide sufficient continuity of habitat, and hence support P. argus indefinitely unless deliberate conservation measures are taken. Few remaining heaths are this large.

42

C. D. Thomas

TABLE 8 Areas of Heathland P. argus Breeding Pockets, and Areas of Heaths that Still Support One or More P. argus Colonies in North West Britain. (Areas in hectares) Areas of breeding pockets Number of breeding pockets

< 0.1

0-1-0-45

0.5-1.45

I-5 -8-0

> 8.0

3

18

8

6

1

Total areas of heathland Number of heaths

<5 0

5-20 5

20-50 0

> 50 2

Mossland

This colony has been known to exist in its present location since 1939, and was sufficiently abundant in 1950 for a 'very large series' to be taken (Richardson, 1951). With sufficient grazing, as has occurred in the past and continues (N. Sykes, pers. comm.), the population may be able to persist indefinitely. However, if grazing ceases it would probably become extinct in a few years. Other former moss habitats, such as post peat cutting or fire regrowth, would be suitable for a shorter period. Most of the population was in 0.8 ha. The total area was 4 ha.

Adult mobility and colonisation Mark-release-recapture experiments were conducted to determine whether colonies had an open or closed population structure. Provided deaths and emergences are small, progressive population estimates become increasingly accurate (errors decrease with increasing number of recaptures) and eventually stabilize if the population is closed. But, if it is open, estimates will rise throughout the day as marked individuals emigrate and/or flesh ones immigrate (see J. A. Thomas, 1983b). P. argus population estimates invariably stabilize once sufficient numbers of captures have been made (Fig. 4). The time taken to reach a stable level depended on the size of the population being marked. The largest took 6 or 7 h of marking whereas the smallest gave reasonable estimates after 2 h. Hence, populations are closed and emigration and immigration are relatively minor. This is confirmed as no movements were detected between two sub-sites of one colony (95 ~ probability that 10 ~o daily migration would have been detected) despite being separated by only 80m of open limestone grassland. Eighty nine per cent of movements by marked individuals were

Status of Piebejus argus

43

375"

1 300

2OO

u~ +1

100 100

UJ

2OO t"-

(n uJ

5O

150

Z 0 ,.J

100,

0 a.

50,< r,, 0

50-

7OO0

4030-

5OOO

2010.

3OOO

O-

20001

2

3

4

5

HOURS FROM

Fig. 4.

1

2

3

4

5

6

7

8

START OF EXPERIMENT

Population estimates made throughout the day against time.

of less than 20 m and none was greater than 50 m. Butterflies recaptured two days later had moved no further. However, 10 individuals were seen away from colonies, although the furthest of these distances was only 350m (mean less than 200m). Colonisation has been recorded for limestone forms following introduction to the Dulas valley. From a source colony introduced in 1942 (Marchant, 1956) colonies have been found up to 1.2 km to the north, and 2.2 km to the west (Hughes, 1956, 1960; Dennis, 1972, 1977; this study). It

44

C. D. Thomas

is hard to date exactly when each colony was founded, but P. argus colonised the Dulas valley from a source point at roughly 1 km per decade, crossing up to 1.4 km of unsuitable habitat (but not in one 'hop'). P. argus was not capable ofcolonising the Dulas valley from the Great Orme, despite there being a huge population on the Great Orme for many years. A distance of 13km provided an insurmountable barrier to colonisation (in fact there used to be smaller colonies 9 km away). During the same period, and since, apparently suitable habitat at Graig Fawr and Ochr Foel has not been colonised from the Great Orme (27 km), or in the past 41 years from the Dulas valley (15 km). Similarly, heathland P. argus failed to colonise Cors Erddreiniog, 21 km from the nearest Holy Island colonies, in an unknown period, or Marford from Prees Heath (28km) in 10 years or more. Both Cors Erddreiniog and Marford now support introduced P. argus populations.

DISCUSSION Reasons for decline

Throughout Britain, P. argus has declined. The principal reason for this is loss of biotopes to agricultural 'improvement', forestry and urban development (Blackwood & Tubbs, 1970; Gimmingham, 1972; Smith, 1980). The decline has been particularly apparent north of the Thames, perhaps because there was less suitable biotope to start with, and because biotope loss has been particularly great in the Midlands. Near the surviving Midlands colony (10 km grid squares: 33/42, 33/43, 33/52, 33/53) 23 (5 9~o) of the place names on the 1:50000OS map incorporate the word 'heath', and others refer to 'sand' and 'mosses'. Now there is scarcely any heathland left in those 400 km 2, and less than 10 ha at Prees Heath. In Merseyside, only 0.6 ~ of the original heath and moss cover is left (C. J. Edwards, pers. comm.). However, P. argus biotopes are still much more widespread than the butterfly. P. argus requires suitably grazed (limestone, moss) or other pioneer habitat (heath, former moss localities). Decline has been particularly noticeable on heaths and mosses, the reasons for which are probably twofold: (1) Traditional management (furze cutting, peat digging for fuel, intentional rotational burning to produce fresh forage) would

Status of Plebejus argus

(2)

45

have provided continuity of suitable habitat 100 years ago. The first two are hardly ever still practised, and the last less regularly (usually accidental). Present human activities (trampling and quarrying) do create suitable habitat, but they are sporadic and local in nature. The reduction in area covered by heaths and mosses, and their increasing remoteness from one another, results in insurmountable barriers to dispersal. Small isolated heaths (of less than, say, 25 ha) may provide the required habitat for many years through disturbance and burning. But occasionally, if left to chance, none of the vegetation exists in pioneer condition and the butterfly is eliminated. This is increasingly likely to occur in the absence of traditional management. However suitable the heath or moss becomes in the future, it is very unlikely to be recolonised if the nearest P. argus colony is over 5 km away. Even distances of less than I km may prove too far if neighbouring colonies are small. Thus, it is possible for P. argus to be eliminated from areas which continuously contain some suitable habitat if colonisation distances are too great. This problem may even arise within some large heaths.

Conversely, on limestone on the Great Orme, where land use remains much as it has for the past hundred years or more, P. argus thrives in undiminished numbers. Because limestone habitat can often support a population for many years, colonies are able to persist on smaller areas of biotope. Most remaining unimproved limestone grassland is on very steep hillsides, so further biotope loss is not likely to be very significant. The only major threat to P. argus on limestone in North Wales is reduction in grazing. Conservation If this decline is to be checked, active conservation is required. The problems facing P. argus fall into three main categories: loss of biotope, lack of suitable biotope management and failure of P. argus to colonise new habitat. Biotopes can be protected by purchase of nature reserves, and less effectively by agreement with private owners. Remedial management can be applied on nature reserves, and occasionally again by agreement with private owners. Colonisation failure can be alleviated by

46

c. D. Thomas

deliberate management to minimise distances between old and new habitat, and by transporting butterflies to habitat beyond P. argus' colonisation range. On limestone, the emphasis lies with management as further biotope loss does not pose a major threat. It may actually be possible to increase numbers through introductions. Management is also the major feature in the conservation of heathland P. argus, but introductions and purchase of reserves m a y play a greater role. Only large heaths are likely to support P. argus indefinitely if left to chance. However, given deliberate management to provide habitat continuity, there is no reason why P. argus could not persist even on small heaths. All three conservation measures are important if the future ofmossland P. argus is to be assured. Reserves

P. argus is fairly well represented on nature reserves in North Wales (Table 9), so management of existing populations should take precedence over purchase of more, especially since the record of extinctions from reserves is not much better than on privately owned land (22 % versus 31 ~ loss). The representation of P. argus on National Nature Reserves and County Naturalist Trust reserves is poor. Top priority should be given to purchasing the sole surviving moss colony. Protection of large areas o f heathland should also be given a high priority, since it is on these that long-term survival is most probable. Prees Heath, the last surviving Midlands colony, is in imminent danger of being lost to agricultural

TABLE 9

The Representation of P. argus colonies on Nature Reserves in North West Britain in 1983. (Scores of 0.5 denote that only part of that colony is in a particular category) National Nature Reserve

Limestone Heathland Moss Extinct

County Naturalist Trust

RSPB

0.5

Local Nature Reserve

Private

17 13-5 1 14

Ia

5.5

9.5 1

I

(?1)

3

a Colony introduced in 1982.

Status of Plebejus argus

47

'development'. Conservation on a reserve is probably the only way to save this population.

Habitat requirements and maintenance Although P. argus occupies several biotopes in Britain and uses many larval foodplants, all colonies share certain characteristics. They occur on flat or southerly facing ground at low altitude, predominantly in southern Britain. Eggs are laid where the vegetation is shorter than average and along vegetation/bare ground margins. Larvae specialise on feeding on tender meristomatic or reproductive tissues, especially of young plants, and they are tended by ants. All colonies are supported by early successional habitat, in which there is an abundance of young plant growth and a warm microclimate (C. D. Thomas, 1983; in press). However, the means by which these conditions are created and maintained differ in the different biotopes. Limestone P. argus requires medium intensity grazing (2-7cm vegetation with

broken turf). This can be maintained by continuous grazing (as on the Great Orme) or by a rotational grazing regime (as proposed by Morris, 1971, for other insects) provided new habitat is situated very close to old and that continuity is provided on sheltered southerly facing slopes. Grazing cannot be left to rabbits with confidence because numbers (and hence grazing intensity) fluctuate in response to myxomatosis rather than to the needs of P. argus. Myxomatosis was prevalent in North Wales in 1983. Overgrazing (rarely) and undergrazing both lead to the elimination of P. argus, although its foodplants are more tolerant (at least in the short term). In the absence of grazing, coarse grasses and scrub encroach, eliminating the bare ground essential to P. argus" needs. Loss of P. argus from most or all of the chalk in southern Britain, but not from limestone in North Wales or Portland Bill (Dorset), suggests that the level of management (grazing) may be less critical on limestone. Perhaps this is because limestone more readily forms scars and crags (naturally or through quarrying) which can neither be overgrazed easily, nor become overgrown quickly if left ungrazed. Heathland P. argus requires a continuous supply of pioneer heath habitat within

colonisation range. This may be provided by burning, disturbance,

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cutting and/or grazing. Confident recommendations cannot be made and further research is needed to determine which of these best meets the needs of P. argus. Disturbance is perhaps the only assured way of creating suitable habitat on heaths (given present knowledge). For example, recolonisation by heathland plants of bare quarry floors, of sandy ground after removal of an aerodrome runway, after military activities, of disused tracks, and of ground trampled by tourists have all created suitable breeding conditions. The best way to create P. argus habitat may be to remove patches or strips of mature heath and some of' the top soil with a bulldozer. Rotavation would probably suffice on very sandy soils. This should not cause undue alarm as most of the best heathland localities for P. argus left in North West Britain have at one time had their vegetation and topsoil almost completely removed. Bakker e t a ! . (1983) found in Holland that heathland re-establishes well once disturbed ground is left to regenerate. Burning is a good method of rejuvenating heathland and can produce suitable P. argus habitat, but is less frequently occupied than disturbed heath (Table 6). This may be due to colonisation failure as this habitat is suitable for a shorter period. Experiments are needed to determine the effects of burning at different times of year, and also of cutting at different times of year. Sutton (1983) in one small trial found that cutting in autumn produced a regrowth dominated by grasses, but spring cutting looked similar to fire regrowth. Bakker et al. (1983) found that dunging by livestock favoured grasses. Continuous grazing along much of the Holy Island coast appears to have eliminated heathland in favour of grasses. Mossland

Grazing maintains open early successional habitat, suitable for P. argus, at the moss locality. Adjacent overgrown areas could probably also be rendered suitable by burning off the mature unpalatable Molinia caerulea foliage, and thereafter maintained by grazing. Formerly, at other moss localities, peat cutting, burning and grazing may all have been important (Wright, 1942). Introductions

In 1942, 90 limestone P. argus from the Great Orme were released at Rhyd-y-Foel (Marchant, 1956). Since then 16 colonies have been

Status of Plebejus argus

49

founded, and after 40 years (in 1983) there were about 30 000 adults flying on the day of peak numbers. The total emergence can be expected to be 2 to 3 times this amount, representing a 650 to 1000 fold increase. In 1978, five freshly paired females were transported from Prees Heath to Marford in North East Wales (G. Wotherspoon, pers. comm.). The colony still survived in 1983, though at rather low numbers (peak, 40400). In 1982, 20 adult P. argus were transported from Holy Island (Trearddur Bay) to Cors Erddreiniog in Anglesey, and up to 17 were seen at one time in 1983 (L. T. Colley, pers. comm.). This is likely to represent an increase in numbers. These illustrate that suitable habitat continues to exist or be created beyond the colonisation range of P. argus. As a major reason that these habitats are not exploited naturally is because of recent fragmentation of biotopes and changes in management, introductions appear to be a legitimate (and inexpensive) means of conserving P. argus. Four further trial introductions were made in North Wales in 1983, two onto heathland and two onto limestone (C. D. Thomas, 1983). The highest priority should be given to the re-establishment of moss butterflies in their former haunts in Westmorland, where there now appears to be some suitable habitat (further details and suggested localities are given in C. D. Thomas, 1983). Butterflies should only be introduced tolocalities beyond the normal colonisation range of P. argus, using similar (i.e. same biotope) or nearby large populations for the source of colonists, and they should only be released on apparently suitable habitat. The success or failure of introductions should be monitored. It is vital that exact details of any release are lodged with the Biological Records Centre. The Nature Conservancy Council should be consulted first. ACKNOWLEDGEMENTS I am grateful to P. T. Harding (BRC, Monks Wood), M. J. Morgan (UCNW, Bangor) and many others who kindly provided records of P. argus. L. T. Colley (NCC) and G. Wotherspoon supplied me with unpublished details of P. argus introductions. The Royal Society for the Protection of Birds allowed me to work at South Stack. Dr J. A. Thomas helped with calculation of population estimates, and Dr J. M. Cherrett supervised the project. This work was carried out whilst supported by a NERC studentship.

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C. D. Thomas REFERENCES

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Thomas, C. D. (in press). Specialisations and polyphagy of Plebejus argus (Lepidoptera: Lycaenidae) in North Wales. Ecol. Ent. Thomas, J. A. (1983a). A quick method of estimating butterfly numbers during surveys. Biol. Conserv., 16, 195-211. Thomas, J. A. (1983b). The ecology and conservation of Lysandra bellargus (Lepidoptera: Lycaenidae) in Britain. J. appl. Ecol., 20, 59-83. Thompson, G. (1980). The butter[lies of Scotland: A natural history. London, Croom Helm. Tutt, J. W. (1908). British Lepidoptera. London, Swan Sonnerschein. Wright, A. E. (1942). Plebejus argus Linnaeus, race masseyi (Tutt), in North Lancashire and South Westmorland. Entomologist, 75, 7-13.