The montane butterfly Euphydryas gillettii Barnes is distributed sparsely through the northern Rocky Mountains in metapopulations along riparian corridors. The Nature Conservancy Global Rank for this species is G3 (very rare or local throughout its range). To understand the status of this species better, especially in relation to vegetative changes, I examined the rate of gain and loss of colonies and the reasons for extipation. To do so, during 2002-2006 I repeated a 1981-1984 survey of 14 sites where the butterfly was known to occur; I also monitored two of the sites nearly every year from 1981 to 2009. Surveys consisted of counting egg masses and adults at each site late in or soon after the flight period for a direct comparison of these numbers and of total brood estimates. I compared changes in vegetation at these sites visually and, at the two more intensely monitored sites, by height measurements.

Results/Conclusions

The repeated surveys revealed that seven of the 14 populations had disappeared from the 1980s to the 2000s, with the populations at the lowest elevations and lowest latitudes being at greatest risk. The habitats at these sites became drier, and the vegetation transformed over the 20-year period. One closely monitored population showed a slow annual decline that correlated with a change in the vegetation, in part through post-fire succession and in part through drying out. The population at the second closely monitored site first disappeared, but then a new colony reestablished 16 years later and grew rapidly. This site is a protected, moist meadow in which the vegetation appears not to have changed. Despite searching for them, I have found no other new colonies. The results show isolated populations disappearing over the past 25 years, especially when associated with vegetational changes through succession or drought (presumed from climate change). Reestablishment through natural dispersal and rapid growth in an unaltered habitat was also observed at a single site. The hypothesis is supported that a warming climate has led to loss of populations in warmer habitats within the species' native range.