Many of the recent, widespread declines and disappearances of amphibian populations have taken place in seemingly undisturbed, montane habitats. The question of whether the observed patterns differ from those expected from natural population dynamics is the subject of an ongoing controversy with important implications for conservation. We examined this issue for the Monteverde region of Costa Rica's Cordillera de Tilaran, where a multi-species population crash in 1987 led to the disappearance of the endemic golden toad (Bufo periglenes) and many other species. Focusing on long-term studies of other amphibian assemblages, we developed probabilistic null models for the number of disappearances. Tests of these models at Monteverde suggest that the patterns observed there are highly improbable in the context of normal demographic variability. Twenty species of frogs and toads (40% of the anuran fauna) were missing throughout our 1990-1994 surveys of a 30-km(2) area. Not all organisms in this area had declined accordingly: the relative frequency of absences was much greater for anurans than for breeding birds. Nevertheless, anuran habitats, most of which are protected within the Monteverde Cloud Forest Preserve, seemed unchanged, and none of the breeding-bird species known to be sensitive to deforestation was missing. Thus, only factors other than direct, obvious human impacts can explain the amphibian declines. Consistent with our tests of null models, analyses of recent population trends do not support the hypothesis that the 1987 crash was an extreme fluctuation from which populations are recovering. Surviving species for which baseline data are available-stream-breeding glass frogs (Hyalinobatrachium fleischmanni and Centrolenella prosoblepon) and a pond-breeding treefrog (Hyla pseudopuma)-remained far less abundant than they were before the crash and showed no increase during 1990-1994. We documented an increase only for one terrestrial-breeding rain frog (Eleutherodactylus diastema).
