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Keywords:

  • Burnet moth;
  • emigration;
  • host plant density;
  • immigration;
  • mark–release–recapture;
  • metapopulation

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Abstract 1. Migration into local populations may increase the likelihood of persistence but emigration may decrease the persistence of small and isolated populations. The dispersal behaviour of a day-flying moth Zygaena filipendulae was examined to determine whether emigration is correlated positively or negatively with population size and host plant density.

2. A mark–release–recapture study showed that most moths moved small distances (< 40 m on average) and only 6% of movements were > 100 m.

3. Twenty-five individuals moved between populations, a measured exchange rate of 8%. Moths were more likely to move between patches that were close together and they moved to relatively large patches.

4. The fraction of residents increased with increasing population size in the patch and increasing host plant cover. Relatively high proportions of individuals left small patches with small moth populations.

5. Moths released in grassland lacking Lotus corniculatus (the host plant) tended to leave the area and biased their movement towards host plant areas, whereas those released within an area containing L. corniculatus tended to stay in that area.

6. Biased movement away from small populations and areas of low host plant density (normally with low population density) was found. This migration-mediated Allee effect is likely to decrease patch occupancy in metapopulations, the opposite of the rescue effect. The effects on metapopulation persistence are not known.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The rescue effect (Brown & Kodric-Brown, 1977) is recognised widely as an important aspect of metapopulation dynamics (Stacey et al., 1997; Hanski, 1999). Migration into small populations may prevent them from becoming extinct, due to demographic (Brown & Kodric-Brown, 1977; Lande, 1993; Nunney & Campbell, 1993) and genetic (Lande, 1995; Lynch et al., 1995) rescue effects, and patches of habitat may be recolonised immediately after extinction, before extinction is noticed (Russell et al., 1995). It is also recognised that migration may threaten populations, especially if they are small and isolated (Thomas & Hanski, 1997; Thomas et al., 1998, 1999). If emigration is too great from a patch of habitat, individuals that remain may not be able to reproduce fast enough to sustain the loss. Therefore, a very high rate of migration might reduce patch occupancy and accelerate rather than delay metapopulation extinction (Hanski & Zhang, 1993). Whether the beneficial or deleterious effects of migration predominate in highly fragmented landscapes remains an open question.

If the deleterious effects of migration are important in highly fragmented populations, the emigration rates (but not immigration rates) would be greatest for small populations. This could arise if patches were small and isolated (as has already been found; Hill et al., 1996; Kuussaari et al., 1996; Sutcliffe et al., 1997; Baguette et al., 2000), either with random movement patterns or if individuals emigrate disproportionately from areas with small populations and low habitat quality (the two are often correlated in nature).

In the work reported here, a mark–release–recapture study of a day-flying moth Zygaena filipendulae was carried out, and adult moths were also released experimentally in areas with and without host plants. The study was designed to determine whether emigration is correlated positively or negatively with population size and host plant density.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Study species

The six-spot burnet moth Zygaena filipendulae is widespread in Europe, absent only in Portugal and south-western Spain. It is the commonest and most widely distributed zygaenid in Britain, ranging from southern England to the Outer Hebrides (Heath & Emmet, 1985). Larvae feed on Lotus corniculatus L. and have also been recorded on Lotus uliginosus L. in Britain (Heath & Emmet, 1985). In the study area (North Wales), Z. filipendulae has one generation per year, with its flight period in July and August. The moth is most abundant in unimproved or semi-improved grassland with a high cover of its host plant (L. corniculatus) and low levels of grazing or mowing (Gutiérrez et al., 2001). This species has aposematic larvae that feed on top of the vegetation, and conspicuous puparia on exposed stalks.

Although larvae feed only on L. corniculatus in the study area, it is common for Z. filipendulae to oviposit on plant species other than its host plant. Eggs were found on non-host plants on average 14 cm above ground and in places with tall vegetation (Gutiérrez et al., 2001).

Study area and habitat characteristics

The fieldwork was carried out during 1998–1999 at Bryn Pydew-Llangwstenin (SH815795, SH820795, SH820800; Ordnance Survey, 1994; Fig. 1) in North Wales, U.K. (53°18′N, 3°50′W). In this area, Z. filipendulae shows a fragmented distribution, with highest densities in areas with tall vegetation and high cover of L. corniculatus. The study system consisted of a cluster of six habitat patches (Fig. 1) and was ≈ 700 m away from the nearest populated area. Patches were defined as separate if they were at least 75 m apart, except for one pair of patches (B and C in Fig. 1) that were regarded as distinct, although separated by only 40 m, because they were separated by tall trees that provided an additional barrier to migration.

image

Figure 1.  The location in Britain (shown by the arrow) and map of the mark–release–recapture study area with locations and sizes of the six habitat patches (A–F) occupied by Zygaena filipendulae populations. *Location of the two release areas (20 × 20 m squares).

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Patches A and B consisted of unmanaged, semi-improved grassland with tall vegetation, and patch C was an abandoned quarry on a steep slope, with remnant limestone grassland. This patch was separated from patches D and E by 200 m of semi-improved grassland, where Z. filipendulae was absent. Patches D and E were two fragments of ancient limestone grassland, separated from patch F by 100 m of dense woodland. Finally, patch F was a complex of ancient limestone grassland and abandoned quarries, with bands of woodland and dense scrub dividing areas of clearings. It was surrounded by woodland and improved grassland.

Habitat characteristics (area, connectivity, host plant density) were recorded for each patch in the study system. Patch area (ha) was measured from maps, and the connectivity (Si) of patch i was calculated using the following index (Hanski, 1999):

  • image

where dij is the distance between the centres of patch i and j (km), and Nj is the estimated moth population size in patch j (see below for more details about estimates of population size). The exponent α sets the extent to which immigration probability declines with distance; a value of three was chosen to reflect the similar dispersal distances recorded for Z. filipendulae to those recorded for Plebejus argus, given the comparable dispersal distances of these species (Gutiérrez et al., 2001).

The presence of the host plant was recorded in 50 × 50 m British National Grid cells throughout the mark–release–recapture area. The total amount of host plant (L. corniculatus) in each habitat patch was estimated as the length (cm) covered by L. corniculatus in a 10-m transect. To ensure an even distribution of samples, transects were placed randomly in each of the 50 × 50 m grid cells (up to four transects per grid cell) covering the entire patch, so the number of transects per patch was proportional to the patch area (range from four in patch D to 36 in patch F). The host plant cover in each habitat patch was estimated as the average values of all transects within the patch.

Mark–release–recapture study

The mark–release–recapture study took place on 13 days between 24 July and 11 August 1998, corresponding to the early and peak flight period. Every patch within the study area was visited by one field worker once a day during good weather. Every unmarked moth was given a unique mark and released immediately at its position of capture. Marks consisted of a code of coloured dots applied using a permanent pen to the dorsal wing surface. The date, time, location on a map, sex, and behaviour were recorded at each capture.

Daily population sizes were estimated using the Jolly–Seber method (Southwood, 1978; Blower et al., 1981). Population size in each patch was calculated as the geometric mean of daily population estimates. To check the accuracy of population estimates in each patch, the sum of the partial estimates was calculated and compared with the population estimate for the mark–release–recapture area as a whole. The interest here was to obtain the relative population size in different patches (which vary over two orders of magnitude), so the results and conclusions should be robust for any alternative statistical method of estimating population sizes.

Dispersal distances between successive recaptures were assessed for males and females separately. Two approaches were used to analyse the migration patterns between habitat patches in the study system. Firstly, the presence and absence of recorded transfers between all pairs of patches (30 possible cases) were analysed by logistic regression. The distance between patches, area, population size, and host plant cover of both donor and receiver patches were added in a forward stepwise selection procedure as independent variables (Norusis, 1993; Trexler & Travis, 1993). Secondly, the fraction of individuals emigrated from and immigrated to each patch, and the fraction of residents in a given patch were analysed. The number of residents in patch i (Ri) was the number of individuals marked in patch i and recaptured in patch i, the number of emigrants (Ei) was the number of individuals marked in patch i and recaptured in patches other than patch i, and the number of immigrants (Ii) was the number of individuals recaptured in patch i that had been marked previously in other patches. Thus, fractions were calculated as follows (Sutcliffe et al., 1997): resident fractioni = Ri/Ri + Ei + Ii, emigrant fractioni = Ei/Ri + Ei, immigrant fractioni = Ii/RiIi. Recaptures on the day of marking were not included in the analyses to avoid underestimating dispersal (Gall, 1984).

Multiple regression analyses were performed with both absolute number and fraction of residents, emigrants, and immigrants in each habitat patch as dependent variables, and area, connectivity, population size, population density (population size/area), and host plant density of the patch as independent variables. Independent variables were log10 transformed in all analyses.

Release experiments in habitat and non-habitat areas

To quantify the effect of host plant density on adult dispersal, moths were released in two areas, which differed in the presence and absence of the host plant. The experiments were conducted in summer 1999, in an unmanaged, semi-improved grassland of ≈ 0.6 ha with tall vegetation and surrounded by tall trees, which included patch B of the mark–release–recapture study area (see Fig. 1 for location).

The experiments consisted of releasing adult moths at two points within the same grassland, one in the centre of a 20 × 20 m square containing host plants, the other in a square 60 m away (release points were 80 m apart) where the host plant was absent (Fig. 1). For both release areas, nectar resources were sampled in each of the four 10 × 10 m squares, by counting all inflorescences of nectar species in a 10 × 2 m transect within each 10 m square. Nectar species were those used by moths in the mark–release–recapture study. Vegetation height was measured at the centre of each 10 m square. The host plant was extremely patchy, with all host plants located in the northern part of the grassland. Nectar resources showed an opposite pattern, with a greater abundance of flowers where the host plant was absent. Vegetation height was similar throughout the site (Table 1).

Table 1.   Vegetation height and density of nectar resources in both squares (20 × 20 m) where moths were released within habitat patch B. Data are shown as average ± SE of four 10-m transects per square.
 Host plant presentHost plant absent
Vegetation height (cm)140.0 ± 9.0148.0 ± 9.0
Density of nectar resources (number of inflorescences/20m2)
Centaurea nigra3.8 ± 1.50.3 ± 0.2
Cirsium arvensis0.3 ± 0.230.0 ± 13.6
Umbeliferae spp.11.3 ± 3.920.0 ± 3.7
Rubus spp.0.0 ± 0.030.0 ± 4.5

Adults were collected in the afternoon (≈ 7 h before release) from a population located ≈ 4 km away (SH770785; Ordnance Survey, 1994). Moths were given a unique mark and released at 00.00 hours, when these diurnal moths were completely inactive. Only moths classed as new or relatively new were released.

The day after the release, the grassland (which included patch B) was searched systematically, together with the nearest part of patch C (covering ≈ 0.8 ha and a maximum distance of 350 m from the release points), between 13.00 and 16.00 hours (1.5 h in each patch, B and C). All Z. filipendulae observed were netted and the number of the individual and location on a map were recorded. Experiments were conducted for males (10 and 11 July, 16 and 17 July) and females (28 and 29 July, 31 July and 1 August) separately.

To analyse the effect of the host plant on dispersal behaviour of the moth, the distances moved for insects released in the presence and absence of the host plant were compared. The patterns of movements were also compared using three categories: (a) residents, individuals that stayed in the 20 × 20 m square where they were released; (b) emigrants to other areas of habitat (distinguishing between those moving elsewhere in patch B and those emigrating to patch C); (c) individuals moving to non-habitat areas, where there was no host plant.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Mark–release–recapture study

The habitat characteristics of each of the six study patches are summarised in Table 2. Estimated population sizes ranged from less than five individuals in patch E to over 300 individuals in patch A. There was no significant correlation between population size and area of the patch. Estimated adult population size for the mark–release–recapture area as a whole was exactly the same as the sum of the six separate estimates (geometric mean for the whole area = 781).

Table 2.   Characteristics of the six habitat patches in the mark–release–recapture patch network. Patch connectivity was a function of distance to, and the population size in, each of the other habitat patches (Hanski, 1999).
PatchArea (ha)ConnectivityPopulation sizeLotus corniculatus cover (%)Number of marked mothsProportion of recapture
A0.5198.53375.02180.59
B0.2368.71329.8810.63
C3.0359.010214.91320.33
D0.1236.0251.8300.60
E0.3321.0410.6190.47
F1.0152.118210.61410.52

A total of 621 individuals was marked during the mark–release–recapture experiment. Three hundred and twenty-two (52%) individuals were recaptured a total of 627 times on different days, with only 6% of movements > 100 m. Males moved a distance of 34 ± 59 m (mean ± SD, n = 470; Fig. 2) between successive capture events; females moved a mean distance of 30 ± 42 m (n = 157), with no significant differences between sexes (Mann–Whitney test; Fig. 2). The maximum recorded distances were 533 m for males and 271 m for females; the larger value for males may simply reflect the larger sample size, given that the mean distances moved were not significantly different.

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Figure 2.  Frequency of recaptures of Zygaena filipendulae with distance for males (470 recaptures) and females (157 recaptures).

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Direct measures of dispersal ability have some disadvantages because marking and handling individuals may affect their mobility (Morton, 1984). Though it is not possible to know whether marking affected moth dispersal, this technique has been used successfully in other diurnal lepidopteran species (Kuussaari et al., 1996; Lewis et al., 1997; Schultz, 1997). All individuals received the same level of handling when they were marked: the interest here was to obtain relative rates of movements by individuals marked in different patches more than absolute levels of movement.

Pattern of migration among habitat patches

Twenty-five individuals (17 males and eight females) moved between populations, a measured exchange rate of 8%. No individuals were observed to move between more than two patches. Movements of individuals between patches are shown in Fig. 3. Males and females did not differ in the proportion of individuals recaptured in the same patch compared with a different patch (χ2 = 0.02, d.f. = 1, NS), so data were pooled for subsequent analyses.

image

Figure 3.  Distribution of adults of Zygaena filipendulae (• 10-m grid cell records) and observed movements between populations at the mark–release–recapture patch network. The numbers next to the arrows indicate the number of transfer events between population pairs. The number within each dotted shape indicates the number of same-site recaptures.

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Presence or absence of movement was recorded for each possible pair-wise combination of patches (30 possible cases). The logistic model classified 73.3% of cases correctly. The probability of movement between patches increased with decreasing distance between patches and with increasing area of the receiver patch (Table 3). The area of the donor patch was not significant in the analysis. A statistical problem could arise in this analysis because for each pair of patches the measures of the independent variables are counted twice (e.g. distance between patches A and B is the same as distance between B and A). Non-independent data could result in a lack of power of the statistical test and these results should be interpreted with caution.

Table 3.   Logistic regression analyses of movements of Zygaena filipendulae between habitat patches in the study system. The model classified 73.3% of movements correctly. n1 = number of pairs of patches where a transfer occurred, n2 = number of pairs of patches where no transfer occurred. The log-likelihood ratio χ2 test (−2 ln LR) was used to evaluate the contribution of each parameter; if the decrease in log-likelihood is significant when a parameter is removed from the model, it indicates that this parameter significantly explains variation in the dependent variable. The coefficient indicates that as the variable increases in value (or decreases when it is negative) so does the likelihood of the event occurring.
Significant variablesModel if term removed
Coefficient−2 ln LRPn1n2
log10 distance−2.974.730.0301020
log10 area (receiver)1.884.850.028  
Model χ2=9.40, d.f.=2, P<0.01

The fraction of residents increased mainly with increasing population size but also with increasing cover of L. corniculatus in the patch (overall r2 = 0.99, F = 140.04, d.f. = 2,3, P = 0.001; population size r2 = 0.97, P = 0.001; L. corniculatus cover r2 = 0.02, P < 0.05). The fraction of moths that left a patch (emigrants) was related negatively to area and population size of the patch (overall r2 = 0.97, F = 26.02, d.f. = 2,3, P < 0.05; area r2 = 0.83, P < 0.05; population size r2 = 0.14, P < 0.05) but no connectivity or host plant density effects were found. The same was true when population density was added to the model, with emigration fraction decreasing with increasing area and population density (overall r2 = 0.97, F = 26.02, d.f. = 2,3, P < 0.05; area r2 = 0.83, P < 0.01; population density r2 = 0.14, P < 0.05). The fraction of immigrants (moths arriving in a patch) declined with increasing density (r2 = 0.92, F = 22.66, d.f. = 1,4, P < 0.01) but was not related significantly to any other independent variable analysed (area, connectivity, population size, or host plant density). This is explained most plausibly by the high emigration from the same patches, which results in a higher fraction of individuals in the patches being immigrants.

This was borne out by an analysis of absolute numbers (not fractions) of migrants. More emigrants came from small patches (r2 = 0.69, F = 8.82, d.f. = 1,4, P < 0.05) and fewer immigrants arrived in small patches (r2 = 0.93, F = 50.53, d.f. = 1,4, P < 0.01). On the other hand, immigrants could more likely be missed in patches supporting high population density, leading to the observed results.

Dispersal behaviour in habitat and non-habitat areas

Of the 122 males and 200 females released during the experiments, 47.5% (males) and 30% (females) were recaptured the day after they were released. Females released in the absence of the host plant moved longer distances than those released in the presence of the host plant (Mann–Whitney, Z = −4.07, P < 0.01, n = 60; Fig. 4). The same trend was observed in males although the results were weaker than for females (Mann–Whitney, Z = −2.01, P < 0.05, n = 58; Fig. 4).

image

Figure 4.  The frequency distribution of distances moved by moths released in areas with and without host plant for (a) males and (b) females.

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Females released in the absence of the host plant tended to leave the area where they were released and biased their movement towards host plant areas (Table 4). Females released in the presence of the host plant tended to stay in this area. If they did move, however, it was to another area containing the host plant, either within the same grassland or in another patch of grassland (χ2 = 30.22, d.f. = 3, P < 0.001; Table 4). The same trend was observed in males although this result was not significant (χ2 = 3.19, d.f. = 3, P = NS; Table 4).

Table 4.   Summary of movements by moths released in the presence or absence of the host plant. Data represent the proportion of recaptured individuals. Residents are individuals that stayed in the same area where they were released, emigrants are those that moved to different areas. n1 = number of released individuals, n2= number of recaptured individuals.
Release arean1n2ResidentsEmigrants
Habitat in release grasslandHabitat in neighbouring grasslandNon- habitat areas
Males
With host plant61270.670.150.030.15
Without host plant61310.490.190.160.16
Females
With host plant100450.770.090.070.07
Without host plant100150.130.130.740.00

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Patterns of movement from the mark–release–recapture study support the hypothesis that small populations suffer from disproportionate emigration, an Allee effect (reduced growth rate at low population size or density; Allee et al., 1949) that has the potential to contribute to the extinction of small populations in highly fragmented landscapes.

The mark–release–recapture revealed higher numbers of emigrants (fraction and actual numbers) leaving small patches than immigrants entering them. Individuals emigrated differentially from small populations and stayed in large populations (patch area and population size were not correlated). Both of these results imply that emigration could be a threat to small populations (Thomas et al., 1999). The mark–release–recapture was more ambiguous with respect to host plant density, revealing a positive effect of host density on residence but no significant effect on emigration. To explore the effects of host plants more directly, the release experiment was carried out. This revealed a strong effect of the presence of host plants, with individuals moving shorter distances in the presence of hosts and being less likely to emigrate. Because the release experiment was conducted in only one site, other site- specific characteristics such as openness of the surrounding landscape and nectar resources could contribute to the observed differences in leaving the areas (Kuussaari et al., 1996. The release grassland was surrounded uniformly by tall trees so openness was apparently similar at both release points. Although there were some differences in nectar availability (more nectar where hosts were absent), it is unlikely that these were responsible for the high emigration rate from the release site without host plants; however a positive feedback between host plant presence and moth density may have been responsible for some of the effects, and perhaps for the entire result for males (Brussard et al., 1974; Lawrence, 1987; Daily et al., 1991). Equal numbers of individuals were released at each release location but an increased initial emigration rate from the sites without host plants would have reduced the density in this area. Subsequent emigrants may have been responding to both moth density and host plant density (Brown & Ehrlich, 1980; Kuussaari et al., 1998). Increased emigration rates from low-density populations have been reported in various groups, including Lepidoptera (Gilbert & Singer, 1973; Brown & Ehrlich, 1980; Thomas & Singer, 1987; Hansson, 1991; Kuussaari et al., 1996). Like the present study, most of these studies were conducted on aposematic and distasteful species that have gregarious larvae; aggregation may lead to increased fitness in these species (Stamp, 1980; Fitzgerald, 1993).

The results suggest that the presence of conspecifics and host plant density are likely to be just as important to the migration rate of Z. filipendulae as patch geometry, at least over the range of patch areas studied. Nonetheless, the effects of area and population size usually act in concert; patch D, which had the smallest area and contained low numbers of individuals, showed the highest emigration fraction and no immigrants were detected in this patch. In relatively isolated habitat patches with low or no immigration, small populations are more likely to go extinct as a result of their high emigration rates (Thomas & Hanski, 1997; Thomas et al., 1999). Furthermore, even when isolated populations can tolerate emigration losses, depression of local population size by emigration would make the populations more susceptible to other causes of extinction (Allee effect; Kuussaari et al., 1998; Hanski, 1999; inbreeding effect; Saccheri et al., 1998). Adaptive attraction to conspecifics (and emigration from very small populations) could be due to the need to find mates (Lawrence, 1987), avoid inbreeding (Johnson & Gaines, 1990), and, in Z. filipendulae and other aposematic species, share the costs of educating potential predators (Sillén-Tullberg & Leimar, 1988; Bowers, 1993). Zygaena filipendulae, like most of the other species where migration rates have been shown to be higher from small, low density sites (above references), is aposematic and distasteful to predators at all life states (Stamp, 1980; Heath & Emmet, 1985; Young, 1997); the adult is bright black and red, with conspicuous yellow and black larvae that occur at high density (eggs are laid in clusters and contain alkaloids). Further studies are required to assess the effects of population size and density on emigration by cryptic species that are palatable to predators.

Nonetheless, attraction to high quality habitat (which often contains the highest population density), the need to find mates, and avoidance of inbreeding are potentially widespread across both aposematic and non-aposematic species of animals (Johnson & Gaines, 1990; Hansson, 1991). Individual selection may often favour behavioural responses that result in emigration from very small and low-density populations, regardless of whether or not this maximises the fraction of habitat patches occupied in a metapopulation (Johnson & Gaines, 1990; Ray et al., 1991).

This and other studies of dispersal (e.g. Kuussaari et al., 1996; Conradt et al., 2000) show that behavioural responses are likely to be important determinants of dispersal in metapopulations. Existing data are sufficient to suggest that Allee effects due to migration (away from small populations) could be as important as rescue effects in the spatial dynamics of insects in fragmented landscapes. Open questions are whether increased realism in the assumptions made about dispersal will (a) substantially alter patterns of patch occupancy and persistence predicted by metapopulation models, and (b) affect management decisions for endangered species.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We would like to thank Mark Lineham for his valuable comments and collaboration in the field. The research of Rosa Menéndez and David Gutiérrez was supported by a Marie Curie research training grant from the Commission of the European Communities (contract nos ERBFMBICT972440 and ERBFMICT961523 respectively). We thank the North Wales Wildlife Trust, Conwy County Borough Council and other landowners who allowed access to their land.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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Accepted 23 August 2001