Loss and fragmentation of suitable habitats are the main causes of decline in many species (Caughley 1994). In this context, the establishment of suitable habitat networks is a key issue in biological conservation. Metapopulation biology is a conceptual framework for the processes allowing the persistence of species within such networks (Gilpin & Hanski 1991; Hanski & Gilpin 1997). Metapopulations are groups of local populations exchanging individuals across unsuitable habitats. Each population has its own probability of extinction and (re)colonization. The dynamics of each local population within the metapopulation will depend on (i) local processes (birth and death) at low migration rate and (ii) regional processes (immigration and emigration) at high migration rate (Hill, Thomas & Lewis 1996). Migration is really the ‘glue’ between local populations within a network of suitable habitats (Hansson 1991). Metapopulation structures have been found in many organisms: plants (Ouborg 1993), mammals (Verboom, Lankester & Metz 1991), birds (Verboom et al. 1991), amphibians (Sjögren 1991) and insects, including mainly butterflies (Thomas & Harrison 1992; Thomas, Thomas & Warren 1992; Thomas & Jones 1993; Baguette & Nève 1994; Hanski, Kuussaari & Nieminen 1994; Hanski & Kuussaari 1995; Nève et al. 1996; Lewis et al. 1997).
Several theoretical models have been developed in order to make quantitative predictions on metapopulation persistence (Akçakaya 1994; Hanski & Thomas 1994; Hanski 1994). Such models, parameterised with field data, have been tested successfully on butterfly metapopulations (Hanski & Kuussaari 1995; Thomas & Hanski 1997). Because models test the influence of the spatial arrangement of suitable habitats on metapopulation survival, this approach can provide guidelines on landscape management for the long-term persistence of threatened species. However, as the models are parameterised with species-specific data, the question remains about the applicability of such predictions from one species to others. This question is crucial: it is often impossible to perform detailed analyses of population viability on every threatened species living in a given habitat.
In this study we addressed the question of the generalization of population viability analysis by comparing migration of different butterfly species in the same habitat network. We compared (i) the population spatial structure and (ii) the migration between local populations, in three butterfly specialist species living in chalk grassland at the same time. As we assumed that adult size was related to maximal migration distance, we selected one small-, one medium- and one large-sized species. As migration is the process connecting local and regional population dynamics, we investigated to what extent populations of each species were connected by exchanges of individuals within the same network of suitable habitat patches. We also compared how the migration rates of the different species depended on physical factors of the landscape (patch area, distance among patches). Metapopulation dynamics models have so far used exclusively negative-exponential functions to describe the distribution of migration distances. However, Hill, Thomas & Lewis (1996) have shown that an inverse-power function could fit the empirical distribution of migration distances better than the negative-exponential function. We compared the use of these two functions on different species, in order to find the best model describing the migration distance distribution between sites. Finally, we investigated whether or not a population viability analysis based on a given species could be generalized to the others.