Alpine Parnassius butterfly dispersal:: Effects of landscape and population size

We used mark-recapture methods to estimate the number of Parnssius smintheus (Papilionidae) butterflies moving among 20 alpine meadows separated by varying amounts of forest along the east slope of the Rocky Mountains in Alberta, Canada. We combined generalized additive models and generalized linear models to estimate the effects of intervening habitat type and of population size on butterfly movement. By incorporating habitat-specific distances between patches, we were better able to estimate movement compared to a strictly isolation-by-distance model. Our analysis estimated that butterflies move readily through open meadow but that forests are twice as resistant to butterfly movement. Butterflies also tended to stay at sites with high numbers of butterflies, but readily emigrate from sites with small populations. We showed that P. smintheus are highly restricted in their movement at even a fine spatial scale, a pattern reflected in concurrent studies of population genetic structure. As an example of the utility of our approach, we used these statistical models, in combination with aerial photographs of the same area taken in 1952, to estimate the degree to which landscape change over a 43-year interval has reduced movement of butterflies among subpopulations. At these sites, alpine meadow habitat has declined in area by 78%, whereas the estimated effect of fragmentation has been to reduce butterfly movement by 41%.