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Dispersal dynamics of burnet moths (Insecta, Lepidoptera, Zygaenidae) among isolated limestone grasslands in southern Germany with special reference to nature conservation aspects
Detailed mapping of burnet moth (Zygaena spp.) habitats in an area covering 30 square km in the Swabian Alb hill country near Blaubeuren, southern Germany, conducted in 1993-1994 revealed, that suitable habitats for the species occur in isolated patches, which are only 0,14 up to 3,2ha in size, and are dispersed throughout the study area. The habitat patches support dry limestone grasslands (Mesobromion) in various stages of succession, and are home of 90% of the endangered butterfly species recorded in the study area. An analysis of agricultural maps of the same area for the years 1821, 1937-1950, and 1991 documents the loss of 95% of dry limestone grasslands until 1821, and the fragmentation of the remaining 5%. The impact of decreasing population sizes and habitat fragmentation on lepidopteran populations is unknown, but it is unlikely that populations of most species can remain viable without constant exchange of individuals among habitat fragments. The diurnal burnet moths are ideal suited to studying dispersal of individuals among isolated populations, because of their fidelity to dry limestone grasslands, their endangered status, and the ease of which they can be captured and marked. In order to gain sights into population structure and the potential exchange of individuals among these isolated populations, on an area covering 10 square km more than 2.000 burnet moths comprising 8 species (Zygaena filipendulae [n=829], Z. carniolica [n=274], Z. viciae [n=580], Z. loti [n=281], Z. transalpina [n=97], Z. lonicerae [n=16], Z. purpuralis and Z. minos [n= 37 (both species together)]) were captured, individually marked, released, and recaptured in two-day intervals over their entire flight periods. The number and distances of dispersal flights, population structure, and information about their ecology and biology, were analyzed. The results reveal, that burnet moths are able to disperse over distances of 1- to 3 km; the maximum flight distance of 3,3km was recorded for a male of Z. filipendulae. Dispersal rates among fragmented habitats were shown to decrease with distance. Striking differences in dispersal abilities were observed among species, but different sexes of the same species showed no differences. The highest numbers of dispersal flights were recorded for Z. carniolica (7,6%) and Z. filipendulae (6,5%), and slightly lower for Z. loti (3,9%). In spite of a large number of marked individuals, only a few dispersal flights were recorded for Z. viciae (1,6%). The young age of burnet moths recorded in new habitats, and direct observations of matings in new populations support the suspected ability of dispersers to contribute to the gene pool of neighbouring populations. Calculations of the size of each population based on the Jolly-Seber method indicate that most of the isolated populations are too small to represent a minimum viable population. Hence, the survival of these populations can be explained only by their contribution to a large meta-population with frequent exchanges of individuals among the sub-populations. Based on this study, conservation management recommendations are presented. These represent a compromise between conservation and land use for protection of endangered species on dry limestone grasslands on the southern Swabian Alb region. These recommendations are based on the necessity of individual exchanges between fragmented habitats and incorporate a habitat network system comprised of small stepping stones, larger limestone grassland reserves, and abandoned quarries, which in the past have recovered to form valuable habitats if left undisturbed. The future integration of quarries after closure would allow the formation of a longitudinal habitat network along the southern rim of the Swabian Alb in order to stabilize the habitat network on a larger spatial scale.