As global temperatures rise, species unable to adapt will either have to shift to higher elevations or disperse to higher latitudes. Those unable to relocate, like island populations living at high-elevation mountain-tops are likely to be severely stressed. Since 1992, we have been monitoring a population of Yarrow’s spiny lizard, Sceloporus jarrovii, at the northern extent of its range in the Chiricahua Mountains, a Madrean sky island in southeastern Arizona. Over this period, we have annually captured, marked, and measured all adult (>56 mm SVL) lizards occupying a 0.5 km section in a dry streambed at 1700 m elevation. Here we report on changes in the population size and demographics since 2000. In concert with NOAA weather data for the area, the population transformation presents a unique opportunity to understand the impact of climate change on a species barred from dispersal to higher latitudes by inhospitable barriers. The impacts also extend to an associated community of prey-dependent predators that include endangered rattlesnakes and numerous predatory birds (Mexican jays, Cooper’s hawks, etc.) and mammals (skunks, foxes, coatimundi, and bears).
Results/Conclusions
The population of adult lizards steadily declined over the 18 year observation period, going from an initial population of 124 adults to only 32. The decline was steady, with the exception of two apparently El Niño event-related dips, and significantly negatively correlated with temperature change. Higher summer and fall temperatures in the previous year correlated with the decline in population size (r = -0.53 , p = 0.024 and r = -0.54, p = 0.026, respectively). Historically, recruitment as low as 25% recruitment from juvenile to adult stages was sufficient to sustain the population. Juvenile recruitment is influenced by individual variation as a function of body size and at the population level by the total number of reproductive females. Female size did not vary significantly throughout the period, suggesting potentially consistent per capita reproductive output, although increasing temperature may constrain this in these viviparous lizards. The decline in the number of adult females from 65 to 16 would also result in a serious decrease in reproductive output at the population level. Our study illuminates an unfortunate opportunity to study, in real time, the impacts of local extinctions of a species unable to adapt or disperse in the face of climate change.