FLUCTUATION IN A ROCKY-MOUNTAIN POPULATION OF SALAMANDERS - ANTHROPOGENIC ACIDIFICATION OR NATURAL VARIATION

We monitored the demographics of the salamander Ambystoma tigrinum nebulosum as part of a community-wide study on the effects of acidification in sub-alpine (elevation 3600 m) ponds in central Colorado. A decline in A. t. nebulosum at this site from 1982 to 1988 has been hypothesized to result from embryonic mortality during a pulse of acidity that accompanies snowmelt in spring. Since 1988 we have monitored salamander population size, reproduction, and recruitment, and compared survival and individual growth rates among ponds which differ five-fold in acid-neutralizing capacity (ANC). Larval recruitment fluctuated dramatically from year to year, but we found no evidence that survival of either embryos or larvae was affected by pH. In each year, the pulse of snowmelt acidification was either benign, or occurred earlier than egg deposition. During 1990 and 1991 we followed individual egg masses and found that embryonic survival was high in all ponds and did not differ between high and low ANC ponds. In permanent ponds, survival of first-year larvae was high and independent of pH. Within cohorts, second-year larvae were actually larger in ponds with low ANC than in high ANC ponds. The largest source of mortality in the population between 1988 and 1991 was related to pond drying; about half of all hatchlings that survived to July perished in autumn when many of the ponds dried. Our results suggest that fluctuation in hydroperiod is as viable a working hypothesis as snowmelt acidification for the recent decline in this population. Our water chemistry data show that many ponds at Mexican Cut remain poorly buffered and that this acid-sensitive watershed should continue to be closely monitored.