The Ecology and Conservation of the Heath Fritillary Butterfly, Mellicta athalia. III. Population Dynamics and the Effect of Habitat Management M. S. Warren The Journal of Applied Ecology, Vol. 24, No. 2. (Aug., 1987), pp. 499-513. Stable URL: http://links.jstor.org/sici?sici=0021-8901%28198708%2924%3A2%3C499%3ATEACOT%3E2.0.CO%3B2-%23 The Journal of Applied Ecology is currently published by British Ecological Society.
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Journal of Applied Ecology (1987) 24, 499-5 13
THE ECOLOGY AND CONSERVATION O F THE
HEATH FRITILLARY BUTTERFLY, MELLICTA ATHALIA.
111. POPULATION DYNAMICS AND THE EFFECT O F HABITAT MANAGEMENT BY M. S. WARREN*
Nature Conservancy Council, Institute of Terrestrial Ecology,
Furzebrook Research Station, Wareham, Dorset BH20 5 A S
SUMMARY (1) The size of forty-three M. ntl~nliapopulations in woodland and grassland habitats was monitored in 1980-84 in Kent and in 1980-85 in S.W. England. (2) Egg mortality was estimated to be consistently < 10% and pupal mortality on a site in S.W. England was c. 50%).(No estimate was made of larval mortality due to sampling difficulties.) (3) In woodland habitats in Kent, M. ntlzalin population size varied according to the time of coppicing or clearance. Populations usually reached a peak 2-3 years after cutting and thereafter declined rapidly. All colonies in vigorous coppice became extinct by the fifth year after cutting and in conifer plantations by the ninth year. This was related to the rate of regrowth in each habitat. On the Blean Woods National Nature Reserve, a large increase in population size was recorded after an extensive conservation programme was introduced. (4) In S.W. England, a few colonies became extinct when their habitat became too shaded. The large fluctuations in population size on one unmanaged grassland site were thought to be related to the weather during April and May when the post-diapause larvae were developing. On two grassland sites managed deliberately for M. ntl~nlia,the fluctuations were far smaller and large populations were maintained. ( 5 ) The population dynamics of M. nthalin and the effects of habitat management are discussed. The decline of M. ntlznlin in Britain is related to the decline of coppicing as the major form of woodland management.
INTRODUCTION There has recently been a concerted effort by several conservation organizations to safeguard the remaining British populations of the heath fritillary, Mellicta athalia Rott., by establishing nature reserves (e.g. Warren 1984a). However, the first two reserves that were established to conserve this species were initially unsuccessful and, by the time of the first national survey in 1980, contained only a few, probably vagrant, individuals (Warren, Thomas &Thomas 1984). The studies of Warren (1987a, b) have shown that M. athalia is an extremely sedentary butterfly with very precise habitat requirements, yet its breeding habitats are often ephemeral and need regular management. These attributes make it a difficult species to maintain on nature reserves, particularly if the reserves are small or isolated. Nevertheless, experience with this and other butterflies has shown that it is possible to maintain populations on nature reserves, provided these are large enough and correctly managed.
* Present address: Nature Conservancy Council, Foxhold House, Crookham Common, Newbury, Berks RG15 8EL.
500
Population dynamics of M. athalia
The following study was therefore conducted in 1980-85, primarily to examine natural population fluctuations and the effect of incidental or deliberate habitat management. From this, recommendations have been made about the practical conservation of M . athalia and conclusions have been drawn about the factors causing its decline in Britain. Because of the sensitive nature of the colony localities, the sites are referred to by the system of coding described by Warren (1987a). METHODS
Estimation of egg, larcal and pupal mortality The survival of eggs was determined by regular visits to marked batches, which were located either by watching ovipositing females or by searching. The number of eggs in each batch was recorded during each visit until they had successfully hatched. Larval survival was not studied as the larvae are hard to locate in the field and are difficult to census accurately, except in their sixth instar. On a few sites, the rate of parasitism was determined from samples of pre- and post-diapause larvae which were subsequently reared in captivity. Wild pupae are also hard to locate and mortality was estimated by placing cage-reared pupae in simulated natural conditions at site S.W.1, Cornwall. Larvae were reared in captivity and were encouraged to pupate beneath dead leaves which they commonly use in the field. As soon as they had pupated, they were transferred to marked locations scattered throughout the main breeding area. The pupae were visited subsequently every 3-4 days to keep an accurate record of mortality.
Estimation of adult population size Absolute estimates Population size was estimated on a number of sites by mark-recapture techniques as described by Warren (1987b). Following tests on numerous sets of data using the frequency of capture method for daily estimates, a Poisson model was found to give the most reliable estimates of population size. It has thus been used throughout this study, though there may be some confusion with the results of previous reports by Warren, Thomas & Thomas (1981, 1984) where the geometric model was adopted after an examination of fewer data sets. Relative estimates The relative size of all forty-three known British colonies was estimated during most years in 1980-85 using Method 1 or 2 of Warren, Thomas & Thomas (1984). These procedures express the estimated population at the peak flight period as the number of sightings per site (Method 1) or the number of sightings per hour x area (Method 2) and are far less time-consuming than marking experiments. The absolute estimates obtained in selected colonies were closely correlated with relative estimates of population size obtained on the same day, whether using Method 1 (r = 0.892, n = 8, P < 0,001) or Method 2 (v = 0.896, n = 9, P
base, were found. Small mammals are known to be a major predator of other lepidopterous pupae (Dempster 1971 ; Frank 1967) and the palatability of six M. athalia pupae was, therefore, tested in captivity on two species that were common at the site: the wood mouse, Apodemus sylvaticus, and the bank vole, Clethrionymus glareolus. Both species quickly found and ate the pupae, leaving remains similar to those found commonly in the field. Each year, a small proportion of the pupae was discovered with either a jagged gash along their full length or a small jagged hole. The former was similar to the pupal damage caused by carabid beetles, and the latter to damage by staphylinid beetles that have been described by Frank (1967). A few pupae turned black or became crumpled and eventually were covered with mould. These may have been damaged initially by an unknown predator and may have subsequently become infected with fungus, but the true cause of death was unknown. Four of the pupae placed in the field during 1984 soon turned dark brown and two were placed in small boxes for further observation. A large, wingless parasite emerged from one pupa and was identified as a Gelis sp. (Ichneumonidae). About twelve small (winged) parasites emerged from the other pupae and were identified as Pteromalus sp. (Pteromalidae). Both species of parasite emerged through small, round holes in the side of the pupal case. All the pupae that were parasitized were obtained from a sample of twenty sixth-instar larvae collected directly from the field and reared in captivity until pupation. The remaining sixteen pupae placed in the field during 1984, and all those used in 1982 and 1983, were reared in captivity from third-instar larvae collected the previous autumn. None of these pupae developed parasites and may have been collected too early to be attacked by them. Thus, the true rate of pupal parasitism was not determined during 1982 and 1983 but, in 1984, was probably about 20%. Consequently, the pupal mortalities indicated in Table 2 are likely to be somewhat lower than those that occurred under completely natural conditions. Population dynamics Woodland colonies in Kent The size of all M. athalia populations varied greatly during 1980-84, and many colonies became extinct while others flourished (Figs 1 , 2 & 4). Fluctuations in population size appeared to be most rapid in Kent where fifteen of the tweity-six colonies known in 1980 had died out by 1984 (Figs 1 and 2). These extinctions were offset to some extent by the establishment of ten new colonies. The fluctuations in the size of the Kent colonies appear
S ~ t ecode
2j
K.5
, 9 7 , ! , 0
1
F 1975-76
1980
1981
1982
1983
K.6
1984
Year
FIG. I . Annual changes in M. athalia colony size (note log, scale) in vigorous coppice habitats,
Kent, 198@84. Arrows indicate timing ofcutting. (N.B. No accurate data were obtained in 1981.)
to have been largely independent of each other and show no consistent pattern from year to year. In every year between 1980 and 1984, some colonies increased while others decreased and a few became extinct. This seems to be true of colonies occurring in all types of habitat, such as vigorous coppice (i.e. well-stocked and fast growing coppice), poor coppice (i.e. poorly stocked and/or slow growing) and coniferous plantations (see Figs 1 and 2). It is, therefore, unlikely that weather plays an important role in determining the annual fluctuations or, indeed, the success or failure of each population. In addition, migration rates are much too low to account for these changes (Warren 1987b). The factor that appears to be most important in determining the fluctuations in the size of the Kent colonies is the length of time since the habitat was either cut for coppice or
Population dynamics o f M . athalia ( a ) Poor copplce
1
( b ) Conifer plantations / h i g h - f o r e s t edge
1980
1981
1982 Year
1983
1984
FIG. 2. Annual changes in M. athalia colony size (note log. scale) in (a) poor coppice, and (b) conifer plantations with deciduous high-forest edge habitats, Kent, 1980-84. Arrows indicate timing of cutting or clearance. (N.B. N o accurate data were obtained in 1981.)
cleared for conifer planting (indicated by an arrow in Figs 1 & 2). Figure 3 shows the trends in mean population size when these are standardized to the same time of cutting or clearance. Despite the large variation in the area of habitat occupied by each colony, the trends after cutting invariably followed the same pattern. In all types of habitat, the populations reached a peak 2 4 years after cutting, and thereafter numbers declined at a
:
/
I a i Vgorous copplce I 6 s ~ t e Is
Number of years a f t e r c u t t ~ n g
Number of years after cutting
FIG. 3. Annual changes in M. athalia mean colony size (note log. scale) following cutting or clearance in three different types ofwoodland habitat, Kent, 1980-84. (N.B. Sites K.5 and 27 have been excluded from the means for vigorous coppice as they have been managed repeatedly since initial clearance. The greater size of the peak population recorded in poor coppice is not of special significance as this was produced by one exceptionally large colony.)
rate that varied according to the precise nature of the habitat. In vigorous coppice, numbers declined extremely rapidly after the fourth year and all the colonies had become extinct by the sixth year after cutting (Fig. 3a). Populations in poor coppice and conifer plantations declined far more slowly after their peaks and a few still remained at low densities 10-1 1 years after cutting (Fig. 3b, c). The colonization of newly cleared or coppiced habitats in Kent occurred mostly in the first summer after cutting, but there were important exceptions. Colonization at sites 7 and 37 was not recorded until 2 and 3 years, respectively, after cutting. In both cases, numbers subsequently built up rapidly indicating that the habitat was highly suitable. There was evidence that colonization was also delayed at sites 23 and 36, although the lack of data for 1981 makes this less obvious in Figs 1 and 2. In addition, a number of other potentially suitable habitats were created within the Blean Woods complex during the study period but were never colonized. In all the above cases, the sites were within 6001000 m of the nearest known colony and interconnecting rides were sometimes present. Site K.5, the Blean Woods National Nature Reserve, was managed regularly throughout the study period. Here a new programme of conservation management was introduced in 1979, specifically to encourage M. athalia, and new areas of 1-3 ha were cut or coppiced every year subsequently. In 1980, the peak population was estimated to be less than twenty adults (based on the sightings ofjust one or two individuals) but, after a delay of a few years, this rose rapidly to around 600 adults in 1984 (Fig. 1). This increase is undoubtedly attributable directly to the increased management on the reserve, which now
506
Population dynamics oj'M. athalia
supports one of the largest colonies in Britain (an estimated 1100 adults at peak in 1985, representing a total emergence in the region of 3000 adults).
Woodland and grassland colonies in S. W. England The colonies monitored in S.W. England generally showed smaller fluctuations in population size than those in Kent, although two became extinct during the study period and a new one was established (cf. Fig. 4 and Figs 1 and 2). Most of the sites were in predominantly grassland habitats and, as with woodland habitats, the time since clearance or conifer planting appears to have been an important factor determining the annual fluctuations on at least two sites. The colony in an abandoned hay meadow at site S.W.3eventually became extinct in 198 1, 5 years after the area was planted with conifers. The colony at S.W.4 occurred along a disused railway line, but the grassy habitat soon became overgrown with scrub and M. atlzalia became extinct about 10 years after the line was abandoned. Soon after these colonies died out, a new one became established in a clear-felled woodland area at site S.W.6 (Devon), probably from adults emigrating from the colony at site S.W.5 which was about 2.5 km away. This new woodland habitat was similar to those in Kent and was planted with conifers immediately after clearance in 198 1-82. After reaching a peak in 1984, the colony has started to decline and is expected to become extinct within the next few years. The causes of fluctuations in population size at the remaining three sites in S.W. England were thought to be a combination of factors and the explanations are rather more complex. At site S.W.5 (Devon), the grassland vegetation was not managed between 1980 and 1985 but the site was planted with conifers, some of which were removed regularly each winter for sale as Christmas trees. The latter prevented the usual succession that follows conifer planting and effectively maintained open, sunny conditions. Nevertheless, the population still fluctuated enormously and very nearly became extinct in 1983 when the estimated peak population was only five adults. The rapid decline from an estimated peak population of nearly 500 the previous year coincided with a long period of unusually cold, wet, spring weather. M. atlzalia post-diapause larvae spend much of the daytime basking and Warren (1987a) has shown that poor weather during this critical period slows larval development and delays the adult flight period. The fluctuations recorded in population size, plotted in Fig. 5, show a close relationship to the changes in mean maximum temperature during April and May-the main period of larval development. It seems, therefore, that poor spring weather not only delays adult M. athalia emergence but may also lead to a reduction in the adult population on unmanaged grassland habitats. The temperature at other potentially critical times of year, such as the main oviposition period in June and during pre-diapause larval development in July and August, showed no obvious relationship with population changes. At site S.W. 1 (Cornwall), the changes in population size may have been influenced by spring temperature in a similar manner but there was an additional effect produced by deliberate habitat management for the conservation of M. atl?alia (Fig. 6). The indices of adult abundance recorded on the two monitoring transects (A and B) fluctuated according to spring temperature in 1980-82 when the level of management was comparatively small but, when the management was more extensive in 1982-85, it seemed to have an overriding effect on the yearly changes. For example, despite the poor spring weather of 1983 (which nearly caused the extinction of the colony in unmanaged grassland at site S.W.5), the adult index on Transect A actually increased, probably in response to the extensive scrub clearance that occurred in 1981-83. Numbers recorded on
A
1
..
v
1
S t e code
S.W.4 (Grassland / dlsused
t u,
S.W.5 (Unmanaged grassland / Xmas trees)
S.W. 6 (Cleared woodland 1980
981
982
983
9 8 4
985
Year
FIG.4. Annual changes in M. athalia colony size (note log. scale) in various habitats. S.W. England. 1980-85. Arrows indicate time of conifer planting, clearance or management. (N.B. No data are shown for the colonies in Somerset as most were not discovered until 1984.)
Transect B decreased slightly in 1983, but the effect of the poor spring weather appeared to be ameliorated by the regime of annual autumn cutting which was started in this area during 1982. On site S.W.2, which was deliberately managed for conservation from 1982 onwards, the population fluctuations were also small. The net effect of management on both these sites seems to have been to reduce the adverse effect of poor spring weather and effectively to decrease the amplitude of the annual fluctuations in population size. Even though data are available for only five annual fluctuations (calculated as log. ratio N,/Ni- I), the changes in population size show a highly significant positive correlation (r = 0.978, P 5-10 years, extinction is probably inevitable. Thus, the major factor causing the decline of M. utlzaliu during the last 150-200 years is thought to be the decline of coppicing as a major form of woodland management. This has effectively put a stop to the supply of new habitats on most of its former sites, even though many have continued to remain as woodland. The recent declines of several other British butterflies, notably the woodland fritillaries, are thought to be directly attributable to the decline in coppicing (e.g. Heath, Pollard & Thomas 1984; Thomas 1984, Warren 1984b). Until the mid-nineteenth century, most of the woods in lowland Britain were managed by coppicing, but from then on this form of management declined rapidly (Peterken 198 1; Rackham 1976). By the time of the first accurate national census in 1905, the proportion of woodland actively coppiced had already fallen to 21 O/O (Peterken 1981). Since then, the proportion of coppice has declined drastically and during the last official census of 197982 it had fallen to
