The Ecology and Conservation of the Heath Fritillary Butterfly, Mellicta athalia. II. Adult Population Structure and Mobility M. S. Warren The Journal of Applied Ecology, Vol. 24, No. 2. (Aug., 1987), pp. 483-498. Stable URL: http://links.jstor.org/sici?sici=0021-8901%28198708%2924%3A2%3C483%3ATEACOT%3E2.0.CO%3B2-8 The Journal of Applied Ecology is currently published by British Ecological Society.
Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/about/terms.html. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/journals/briteco.html. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission.
The JSTOR Archive is a trusted digital repository providing for long-term preservation and access to leading academic journals and scholarly literature from around the world. The Archive is supported by libraries, scholarly societies, publishers, and foundations. It is an initiative of JSTOR, a not-for-profit organization with a mission to help the scholarly community take advantage of advances in technology. For more information regarding JSTOR, please contact
[email protected].
http://www.jstor.org Sun Jun 24 07:16:42 2007
Journal of Applied Ecology (1987) 24, 483498
THE ECOLOGY AND CONSERVATION OF THE
HEATH FRITILLARY BUTTERFLY, MELLICTA ATHALIA
11. ADULT POPULATION STRUCTURE A N D MOBILITY BY M. S. WARREN* Nature Conservancy Council, Institute of Terrestrial Ecology,
Furzebrook Research Station, Wareham, Dorset BH20 5 A S
SUMMARY (1) The population structure and mobility of the endangered butterfly Mellicta aihalia was examined by mark-recapture experiments at several sites in S.W. England and Kent. (2) The majority of individuals of both sexes remained within small, discrete areas and the presence of even small belts of unsuitable vegetation resulted in the formation of distinct subpopulations or colonies. (3) Adults were extremely sedentary, and at two small study sites the mean daily range within a day was 30-33 m for females and 46-83 m for males. In three larger study sites the mean range over sampling periods of up to 20 days was 84-214 m, but there was no consistent difference between the sexes. (4) The mean distance moved by females was significantly greater for recaptures over 1 or more days than recaptures during the same day. The results indicated that females eventually covered similar distances to males, but took longer to do so. (5) Males appeared to become rather less mobile with age while females became more mobile. (6) Low levels of migration were regularly observed between colonies, over distances of up to 1 km. The minimum mean percentage of the population moving from one colony to another in three woodland study sites in Kent was estimated to be 1.7%)for males and 1.3%)for females. (7) Adult residence time was generally similar in males and females, and varied from 2.35 to 10.78 days. There was some evidence that the shorter residence times were linked to habitat deterioration or high population density, but this was not proven. (8) The implications of these findings for the population dynamics and conservation of M. athalia are discussed. The proportion of the population caught during the markrecapture experiments is used to examine the potential impact of collectors.
INTRODUCTION The heath fritillary butterfly, Mellicta athalia Rott., is an endangered species in Britain and has been the subject of a detailed ecological study since 1980 (Warren 1987a, b). It forms well-defined, discrete colonies and a preliminary study suggested that it might be relatively immobile (Warren, Thomas &Thomas 1984). However, most of its habitats are short-lived and, in order to survive, colonies often have to move as conditions become unsuitable. This combination of attributes would make the species particularly vulnerable to local extinctions, especially in the fragmented landscape of modern lowland Britain. The mobility of M. athalia, and its ability to colonize new areas, are therefore crucial factors to consider when planning for its conservation. This study examines its mobility * Present address: Nature Conservancy Council, Foxhold House, Crookham Common, Newbury, Berks RG15 8EL.
484
Population structure of M . athalia
both within and between colonies and relates the findings to the practical conservation of the remaining populations. Site names follow the coding described by Warren (1987a). METHODS Mark-recapture estimates of adult population size Population size was estimated by marking and recapturing adults along irregular routes through each site. Each adult caught was marked individually using a Staedtler felt-tip pen (Lumocolor 3 13 Permanent) and immediately released at the point of capture. After each capture a record was made of sex, number of mark, and position of capture. The degree of wing-wear was also recorded using the following classification: (1) freshly emerged; (2) good condition, no visible wing damage; (3) noticeable wear of scales from wings or body, or slight wing damage; (4) extensive wear of scales or wing damage. When estimates were required from a single day of marking, population parameters were estimated by Craig's (1953) frequency of capture method. Two mathematical distribution models were then used to estimate total population size: the zero-truncated Poisson model of Craig (1953) and the truncated geometric model described by Eberhardt (1969). Chi-squared tests on numerous sets of data showed that the Poisson model was most consistent at fitting the observed data, when the sexes were analysed both separately and together. It also gave the most realistic estimates when compared with other methods such as the simple Lincoln index. The Poisson model has consequently been used for all the frequency of capture estimates. (There may, therefore, 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.) In most cases, too few females were recaptured to permit a separate analysis, and the estimates had to be calculated by combining the data for the sexes. When recapture data were obtained over several days at the same site, population size was estimated using the method of Jolly (1965). Average residence rates (which include losses due to emigration as well as deaths) were calculated from recapture duration decay plots, following the method of Watt et al. (1977). These were converted to residence time (days) using the formula of Cook, Brower & Croze (1967). Total population size was calculated by plotting the daily estimates obtained from the Jolly analysis. The estimates for the start and end of the marking period were extrapolated linearly to the known start and finishing dates (respectively) of the flight period, as recorded on the butterfly monitoring transect. The area under the subsequent curve was then multiplied by the mean daily loss rate (1 -mean residence rate, after Watt et al. 1977). Adult movement Adult niohility ~vithincolonies The position of each adult captured or recaptured during the mark-recapture experiments was plotted on a map of each study site. The following mobility parameters were then calculated for each recaptured individual (after Scott 1975): 4 , minimum straight-line movement in metres between captures i and (i+ 1); time in days between captures i and (i+ 1); t,, D, sum of d,s for each individual (minimum distance moved);
D,,,,, T, R,
maximum D recorded in the population; sum of tis for each individual (number of days between first and last capture), distance in metres between two furthest capture points for each individual (minimum range); R,,,,, maximum range recorded within each population. Means of the above parameters were calculated for each sex and each age group, by using the average wear-rating between captures. The sample size of d,and ti is the total number of recaptures, and the sample size of D, T and R is the number of individuals recaptured. Scott's (1975) parameters for velocity (ci=cl,jti) have been omitted as the mean distance moved (di) did not increase steadily with the time interval between recaptures ( t , ) . Consequently, the values calculated for this parameter were largely a function of the mean value of ti during each marking experiment, and not a meaningful measure of velocity. Adult migration between colonies The number of adults migrating between colonies was assessed during 1982 by regularly marking and recapturing butterflies in three colonies in the Blean Woods complex, Kent. If: n,,, MI,= estimated total number of individuals in population a and b, respectively, during the marking period; V,/, = number of individuals initially marked in colony a and subsequently recaptured in colony h; M(,, M h = total number of individuals initially marked in colony a and b, respectively; then the proportion of individuals caught in population b that were originally marked in a is:
The estimated total number of inarked individuals moving from a to b over the marking period is:
The proportion of individuals in population a that were inarked is:
M,, n,, Therefore, a crude minimum estimate of the total number of individuals (marked and unmarked) moving from population a to b during the marking period is:
The figures obtained from the above formula represent minimum estimates of the rate of migration between colonies as no allowance has been made for the death rate of migrating individuals. The inclusion of this factor is extremely complicated when considering movements over a series of sampling dates and has not been attempted in most previous methods for calculating rates of migration (e.g. Arnason 1972, 1973; Iwao 1963). However, the estimates obtained for the intercolony movement of M , athalia were based on the regular sampling of each study colony and the longest gap between samples was only 2 days. The proportion of migrating individuals dying over these short periods is thought to be small but their omission in the above formula undoubtedly leads to the
486
Population structure of M . athalia (a
Moles
FIG.1. M. athalin same-day movements at sites S.W.2 & 3, Cornwall, 4 June-5 July 1980. Dots represent position of first capture and lines indicate each movement recorded. Dots without lines represent individuals recaptured within 30 m.
underestimation of the true rates of migration. The formula will also underestimate migration because individuals marked earlier in the flight period will have a greater chance of migrating (compared with those marked later), and at that time the proportion of marked to unmarked individuals was lower than given by M,/n,. From a conservation viewpoint, this underestimation is of minor importance, for it is the ability of even a few individuals to reach a new site and start a new colony that is of prime interest. The method also assumes that migration is more or less constant throughout the flight period, and that the number of individuals migrating and returning undetected is negligible. RESULTS Patterns o f movement ~vitlzinand between colonies Examples of patterns of adult movement recorded during the mark-recapture experiments are shown in Figs 1 and 2. These diagrams show that for both sexes the
( a ) Moles
I
( b ) Females
I
FIG. 2. M. athalia movements at sites K.24, 26 & 28 in the Blean Woods, Kent, 2-19 July 1982. Dots represent position of first capture and lines indicate each movement recorded. Dots without lines represent individuals recaptured within 30 m. (N.B. For clarity, only a quarter of the male movements within sites K.26 & 28 are illustrated.)
488
Population structure of M . athalia
majority of individuals remained within fairly small, discrete areas within which they appeared to move freely. The presence of even small belts of unsuitable vegetation between such areas seemed to restrict the free movement of individuals and led to the formation of subpopulations or even distinct colonies. For example, at sites S.W.2 and 3, M , athalia occurred on two grassland areas on opposite hillsides. These sites had unimpeded views of each other, but were separated by a 100 m wide valley that contained a conifer plantation with trees 3 4 m tall. On 5 days, adults were marked and released on one hillside in the morning and then on the other side in the afternoon. The results show clearly that, although there was a very small exchange ofadults, this plantation provided a substantial barrier to movement and that each hillside contained a separate population (cf. definition in Warren 1987a). In the Blean Woods, Kent, M. athalia occurs in numerous colonies scattered throughout three large blocks of woodland (Warren, Thomas &Thomas 1984). Here, the definition of some colonies was more difficult as many occurred close together and a greater proportion of adults moved between some adjacent colonies. The pattern of movement within and between three of these colonies was examined during 1982, and the results are shown on Fig. 2. Each of these colonies occurred within an area of recently TABLE1. Adult M. athalia same-day movement data at selected sites, 1980 Sites S.W.2 & 3 (Cornwall) Males Females
Movement parameter Number of individuals marked Number of illdividuals recaptured Total number of recaptures Mean T (days) Mean r, (days) R m a x (m) Mean R (m) Mean D (m) Max D (m) Mean d, (m)
158 43 56 0.05 0.04 135 46 57 325 44
71 13 14 0.06 0.05 125 32 33 125 31
Site K . l (Kent) Males Females 57 26 42 0.13 0.08 220 83 108 300 70
27 11 12 0.12 0.1 1 80 30 30 80 29
TABLE2. Adult M. atllulia movement data over 1 or more days at selected sites, 1982-84
Movement parameter
Site S.W.l Sites K.24, 26, 28 Sites K. 1, 2 & 4 Sites K . l , 2 & 4 (Cornwall) 1982 (Kent) 1982 (Kent) 1983 (Kent) 1984 Males Females Males Females Males Females Males Females
Number of individuals marked 1023 Number of individuals recaptured 339 Total number of recaptures 491 Mean wear-rating 2.2 4.6 Mean T (days) Mean r, (days) 3.2 580 R m a x (m) Mean R (m) 110 Mean D (m) 130 Max D (m) 650 Mean d, (m) 90
218 29 30 2.4 4.7 4.5 350 119 123 350 119
524 259 247 94 585 148 2.4 2.8 4.4 2.9 1.6 1.8 1160 1000 118 89 157 99 1970 1000 66 63
90 57 147
35 15 20
151 86 201
-
-
-
5.6 1.8 750 214 328 1590 107
3.5 2.6 720 135 142 720 106
4.1 1.8 1120 131 197 2240 84
87 42 67 -
3.6 2.3 580 84 89 580 56
2 300 +I
1 All wear-ratings
t
Wear-ratings
1.0-2.0
Wear-ratings
2.5-4.0
( a i Males
Interval between coptures ( t i ) (doys)
FIG.3. Mean distance moved (d,) at different time intervals between captures for M. ati~nliaadults with different wear-ratings at sites K.24, 26 & 28, Blean Woods, Kent, 2-19 July 1982.
cleared woodland (one, K.26, replanted with conifers and two in newly cut coppice) and was isolated to some extent by shady, intervening woodland. Although the vast majority of individuals (98.6%) were recaptured within the same discrete area, a few were recorded to have moved between colonies-a linear distance of nearly 1 km. This movement is very different from the trivial movement observed within each colony and is best considered as migration. Movenzent parameters The results obtained during 1980 for same-day movements at sites S.W.2 & 3, Cornwall, and site K.l, Kent, show that the mean distance moved between capture (di) and the mean velocity (v;) is far smaller for females than for males (Table 1). This supports independent observations on adult behaviour which have shown that females spend less time in flight than males (Warren 1985). However, the mean daily range of adults (R) was small for both sexes (30-33 m for females; 46-83 m for males), indicating that M. uthuliu is an extremely sedentary species. Movement parameters have been calculated for three larger areas over periods of up to 20 days between captures (Table 2). Here, the maximum recorded range (R,,,,,) of individuals was far greater (350-1 160 m compared with 80-220 m at the smaller sites), but the mean range of adults (R) was still small (84-214 m). The mean distance moved between captures (d;) was also small (59-1 19 m) but there was no consistent difference between the means of each sex. The mean distance moved (d,) at different time intervals between capture (t;) has been plotted on Figs 3 and 4. These plots show that the mean distance moved (d;) by males remains more or less constant with time (t;), but for females it increases considerably over longer time intervals between captures. For the longest time intervals between captures (6-20 days), the mean distance moved by females was actually greater than by males, indicating that females eventually fly as far as males (or probably
490
Population ctructure of M. athalia
further) but take longer to d o so. Table 3 shows that the mean distance moved by females recaptured after one or more days was significantly greater than by those recaptured on the same day both at site S.W.l and at sites K.24, 26 and 28 ( P < 0 . 0 5 and P 300 adults. However, for collecting to have a major impact even on vulnerable sites, it would have to be maintained for several days at peak season to deplete the population severely. Even then, the mark-recapture data indicate that only 25-45" of the total adult emergence is alive on the peak day. The marking experiments also showed that only a small proportion of the total adults caught was female (usually about 30% of all captures) even though the sex ratio was thought to be approximately 1 : 1. This is probably because females spend most of their active lives basking or resting and are far less easily seen than males who spend most of their active lives in flight. Thus, it would be much more difficult for collectors to deplete female populations and thereby reduce the reproductive capacity of M. athalia colonies. Also, M . athalia seems to have large powers of reproduction and, if the habitat is suitable, should quickly regain any losses that are caused in this way. The conclusion of these experiments is, therefore, that collecting could have a significant effect, but only on small populations. Most M. athalia colonies occupy fairly small, discrete areas but the results described above suggest that these colonies are not vulnerable if they contain large populations. There is some evidence that at least one small colony may have been affected recently by collectors (see Warren, Thomas & Thomas 1984) but overall it is considered that collecting has had a very minor effect on the decline of M. athalia in Briatin. The real cause of its decline is thought to be the large changes in land management, particularly the decline of coppicing which has led to the increased shading of its traditional woodland habitats. This topic is discussed in detail by Warren (1987b). REFERENCES Arnason, A. N. (1972). Parameter estimates from mark-recapture experiments on two populations subject to migration and death. Reseclrclzes in Population Ecologj,, 13, 97-1 13. Arnason, A. N. (1973). The estimation of population size, migration rates and survival in a stratified population. Researches in Populatiorl Ecologj,, 15, 1-8. Cook, L. M., Brower, L. P. & Croze, H. J. (1967). The accuracy of a population estimation from mult~ple recapture data. Journal of Animcll Ecology, 36, 57-60. Craig, C. C. (1953). On the utilization of marked specimens in estllnating populations of flying insects. Biometrika, 40, 170-1 76.
498
Population structure o f M . athalia
Eberhardt, L. L. (1969). Population estimates from recapture frequenc~es.Journal of Wildlife Management, 33, 28-39. Frohawk, F. W. (1934). British Butterflies. Ward Locke, London. Iwao, S. (1963). On a method for estimating the rate of population interchange between two areas. Researches in Populatiorl Ecology, 5, 44-50. Jolly, G. M. (1965). Explicit estimates from capture-recapture data with both death and immigrationstochastic model. Biometrika, 52, 225-247. Murphy, D. D., Launer, A. E. & Ehrlich, P. R. (1983). The role of adult feeding in egg production and population dynamics of the checkerspot butterfly. E u p h y d r p s editha. Oecologicl. 56, 257-263. Scott, J. A. (1973). Lifespan of butterflies. Journal of Research or1 the Lepidoptera, 12, 135-144. Scott, J. A. (1975). Flight patterns among eleven species of diurnal Lepidoptera. Ecology, 56, 136771377, Thomas, J. A. (1984). The conservation of butterflies in temperate countries: past efforts and lessons for the future. The Biology of Butterflies (Ed. by R. I. Vane-Wright & P. R. Ackery), pp. 333-353. Symposium of the Royal Entomological Society of London. No. 11. Academic Press. London. Warren, M. S . (1985). The ecologj, and conserc,cltion o f the heati~,fiitillc~ry buttetf7~3,Mellicta athalia. Confidential report to the Nature Conservancy Council (unpublished). Warren, M. S. (1987a). The ecology and conservation of the heath fritillary butterfly. Mellictcl atlialia. I. Host selection and phenology. Journcll of Applied Ecology, 24,467-482. Warren, M. S. (1987b). The ecology and conservation of the heath fritillary butterfly, Mellicta clthalia. 111.
Population dynamics and the effect of habitat m,anagement. Journcll of Applied Ecology, 24, 499-513.
Warren, M. S., Thomas, C. D. & Thomas, J. A. (1981). The Heath Fririllary: Survey and Consewation Report.
Joint Committee for the Conservation of British Insects. London (unpublished). Warren, M. S., Thomas, C. D. &Thomas, J. A. (1984). The status of the heath fritillary butterfly Mellicta athalia Rott., in Britain. Biological Conservatiorl. 29, 287-305. Watt, W. B., Chew, F. S., Snyder, L. R. G., Watt, A. G. & Rothschild, D. E. (1977). Population structure ofpierid butterflies. I. Numbers and movements of some montane Colias species. Oecologia, 27, 1-22.
(Received 18 August 1986; reuision received 19 January 1987)
