Direct estimates of metapopulation capacity from dispersal show high interannual variability, but little effect of recent forest encroachment on network persistence

Abstract

CONTEXT: Habitat loss and isolation are leading threats for biodiversity. Alpine habitat throughout the world is being lost and fragmented by forest encroachment. Previous analysis of forest encroachment from 1952 to 1993 in a population network along Jumpingpound Ridge, Alberta, Canada showed a 78% loss in meadow area, resulting in an estimated 41% reduction in dispersal of the alpine butterfly Parnassius smintheus. OBJECTIVES: Here, we pursue three questions. First, has forest encroachment continued since 1993? Second, if so, has it affected butterfly dispersal? And third, have any changes altered estimates of metapopulation persistence of this species? METHODS: The area of each meadow and distance between each pair of meadows were determined in GIS using aerial photographs taken in 1952, 1993, 1999, 2008, and 2012. Mark-recapture (1995–2015) and landscape data were used to estimate annual butterfly dispersal and network persistence. RESULTS: Alpine meadows along Jumpingpound Ridge continued to be fragmented by encroaching forest showing a decrease in area and increase in the amount of forest between them. Annual dispersal parameters varied over the study, but the only consistent response to forest encroachment was an increase in male dispersal distance within meadow habitat. Estimates of metapopulation capacity also varied greatly among years due to variability in dispersal. The network was predicted to persist based on nearly all estimates. CONCLUSIONS: Metapopulation capacity is a potentially powerful tool for conservation, but interannual variability in dispersal hampers its utility under realistic conditions and could result in erroneous conclusions regarding metapopulation persistence. The 7 km long network of 18 subpopulations of Parnassius smintheus along the Jumpingpound Ridge and Cox Hill is currently at little risk of extinction due to forest encroachment.