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A recent increase in ultraviolet B radiation is one hypothesis advanced to explain suspected or documented declines of the boreal toad (Bufo boreas Baird and Girard, 1852) across much of the western USA, where some experi- ments have shown ambient UV-B can reduce embryo survival. We examined B. boreas occupancy relative to daily UV-B dose at 172 potential breeding sites in Glacier National Park, Montana, to assess whether UV-B limits the distri- bution of toads. Dose estimates were based on ground-level UV-B data and the effects of elevation, local topographic and vegetative features, and attenuation in the water column. We also examined temporal trends in surface UV-B and spring snowpack to determine whether populations are likely to have experienced increased UV-B exposure in recent decades. We found no support for the hypothesis that UV-B limits the distribution of populations in the park, even when we analyzed high-elevation ponds separately. Instead, toads were more likely to breed in water bodies with higher estimated UV-B doses. The lack of a detectable trend in surface UV-B since 1979, combined with earlier snow melt in the region and increasing forest density at high elevations, suggests B. boreas embryos and larvae likely have not experienced increased UV-B. Résumé : L'augmentation récente de le rayonnement ultraviolet B est l'une des hypothèses avancées pour expliquer les déclins appréhendés ou observés du crapaud de l'ouest (Bufo boreas Baird et Girard, 1852) dans presque tout l'ouest des É.-U.; certaines expériences y ont d'ailleurs démontré que l'UV-B ambiant peut réduire la survie des embryons. Nous avons déterminé l'occupation du territoire par B. boreas en fonction des doses journalières d'UV-B à 172 sites potentiels de reproduction dans le parc national Glacier, Montana, afin d'évaluer si l'UV-B limite la répartition des crapauds. Les estimations du dosage ont été basées sur des mesures d'UV-B au niveau du sol et sur les effets de

Distribution of boreal toad populations in relation to estimated UV-B dose in Glacier National Park, Montana, USA