Long-term data suggest a severe decline in recruitment of the American pika on Niwot Ridge, Boulder County, Colorado

Climate-mediated range shifts have been documented for many species, but details of the demographic response to climate—useful for inferring causation—are often lacking. We used data from the Niwot Ridge Long Term Ecological Research site (NWT LTER, Boulder County, Colorado) to explore demographic trends in the American pika, a species that has disappeared from portions of its range. Pikas were captured, weighed, and classified by stage from 1981 to 1990, in 2004, and from 2008 to 2020. Parturition dates were estimated for each juvenile, using a growth curve developed for this population. Trends in pika recruitment were estimated using generalized linear models to test for a decline in recruitment with year and with a metric of warm-season temperature (growing degree-days) calculated from NWT LTER sensor data. Our metric of recruitment (the annual number of juveniles per capture) fell by over 50 percent during the study, declining inversely with year and with growing degree-days. This decline was not confounded by changes in the phenology of pika parturition or trapping dates, both of which remained stable throughout the study. Together, these results suggest that recruitment is in decline with warming temperatures on Niwot Ridge, providing the first evidence of support for recent projections of climate-mediated pika losses in nearby Rocky Mountain National Park.