Alpine species are extremely vulnerable to climate change-induced extinctions due to their physiological and geographic constraints. Scientists have already documented climate change-generated population extirpations and distributional shifts for numerous alpine plant and mammal species, such as the American Pika (Ochotona princeps). However, few studies have investigated the specific climatic drivers that cause these local species’ extinction at lower alpine levels. Using models and surveys of 69 American Pika population sites, Erb et al. (2011) analyzed pika distribution change throughout the Southern Rocky Mountains by assessing the effects of landscape, microhabitat, and climatic factors on pika persistence. Elevation, maximum summer temperature, annual precipitation, and habitat characteristics with potential climate-buffering effects (talus depth, porosity of rocks, soil moisture, and rock type) acted as the predictor variables. The authors found that only 4 of the 69 pika populations were extirpated in the Southern Rockies. However, these four sites revealed that climatic factors, rather than habitat features, determined pika persistence. Additionally, these 4 sites were among the driest pika habitats in the region; they lacked a sub-talus water source and experienced a smaller mean annual precipitation in comparison to other pika sites. These results suggest that water, in the form of precipitation and sub-surface moisture, was the primary driver of pika distribution patterns in the study region. Therefore, increased drying trends could put American Pika populations at risk to climate change-induced extinction.—Megan Smith
Erb, L.P., Ray, C., Guralnick, R., 2011. On the generality of a climate-mediated shift in the distribution of the American pika (Ochotona princeps). Ecology 92:9, 1730-1735.
The authors conducted their research at 69 sites historically occupied by pikas in the Southern Rocky Mountains of southern Wyoming, Colorado, and New Mexico. The sites were defined as those with recorded pika presence before 1980—the year when climate change became prominent in datasets. The dates of the historical records varied from 1872 to 1979, and the sites were also chosen based on geographic accuracy. The data were collected from 800 historical records of pika presence found in museum records, literature sources, and georeferenced museum specimens. Climate data for each site was accumulated between 1908 and 2007. Site elevation varied from 2703 to 4340 m and the most common vegetation at the sites consisted of alpine forbs, grasses, willow, conifers, and aspen.
The authors sent out crews to each of the 69 sites to assess current pika occupancy. The crews searched for signs of pika presence by detecting individual organisms through sight and sound and by identifying fresh pika food stores (haypiles). If signs of pika occupancy were not found, crews returned 3–5 months later and searched the sites extensively within a 3 kilometer (km) radius. A minimum of 0.5 hours was spent per hectare searching talus for signs of pika presence. While at each site, the crews collected data on microhabitat features. A map of the 69 sites historically occupied by the American pika, differentiated by recent occupancy status, was constructed.
Erb et al. then compared models of pika persistence that incorporated elevation, maximum summer temperature, annual precipitation, and site characteristics with potential climate buttering effects (rock type, talus depth, porosity of individual rocks, and evidence of persistent soil moisture beneath the talus) to assess landscape, microhabitat, and climate characteristics as possible drivers of pika population extirpation. The authors’ model comparisons were guided by pika persistence hypotheses and results from previous studies. The models represented the following hypotheses: 1) pikas persist in locations where the least change in climate (temperature and precipitation) has occurred; 2) pikas persist in areas where the climate has predominantly been wet and cool; 3) pikas persist in sites where they have been exposed to the least climatic variability; 4) pikas persist in locations with the deepest talus, most porous and insulating rock, and where water or ice persist under the talus; 5) pikas persist at higher elevation locations. A table displaying the hypotheses and the candidate model covariates was constructed.
The authors found that only 4 of the 69 pika population sites in the Southern Rockies lacked recent signs of O. princeps in 2008. However, the pattern of these extirpation sites reveal that pika persistence was best explained by water, in the form of mean precipitation and the persistence of moisture under the talus. These four sites were the driest of the 69 sites within the Southern Rockies. The mean annual precipitation across all sites between 1908 and 2007 was 884 mm and the mean annual precipitation across extirpation sites was 593 mm. The four sites also lacked a sub-talus water source. Overall, these results support the authors’ second and fourth hypotheses. A graph displaying observed and predicted pika occupancy as a function of local mean precipitation, change in precipitation, and presence of sub-talus water was constructed, as was a figure comparing observed pika occupancy with mean annual precipitation between 1908 and 2007.
The extirpation locations did not experience significant climate variability. There was very little change in precipitation levels at the extirpation locations since 1980. Therefore, summer maximum temperatures, change in summer maximum temperatures, and variation in both summer maximum and annual precipitation did not support pika persistence. Additionally, elevation, talus depth, and rock porosity did not predict pika persistence. A figure comparing observed pika occupancy with change in mean annual precipitation between the years of 1908 and 1979 and 1980 and 2007 was constructed.
In contrast, Erb et al. found that since 1980, maximum summer temperatures across all sites have averaged 0.48°C warmer than they were from 1908 to 1979. Changes in maximum temperature varied among sites (–1.2°C to +2.5°C), as did changes in annual precipitation (–80 mm to +203 mm, –8.0% to +24.1%). The overall trend in the study region demonstrated an increase in annual precipitation across all sites (+46 mm, +5.6%). These results demonstrate that the overall pika population sites experienced climatic change even though the 4 extirpation sites did not.
Although low precipitation and sub-talus moisture drove pika extinction in the Southern Rockies, pika population sites experiencing a decrease in annual precipitation were not more vulnerable to extirpation. This may seem contradictory with the authors’ previous findings; however, the sites that experienced a decrease in precipitation since 1980 were previously among the wettest locations within the study region. Although these 13 drying sites did not significantly differ in mean post 1980 precipitation levels from the other 56 sites, it would nevertheless be important to monitor their moisture levels since continued drying trends could place these pika populations at risk to extirpation in the near future.
Additionally, though the four extirpation sites were the driest within the study region, pika occupancy was detected at these locations within the past century. Erb et al. proposed that these sites may have been marginal habitats that required immigration from adjacent populations to maintain their own populations. Therefore, these locations only supported pika populations when climatic conditions facilitated their re-colonization by individuals from nearby sites. Each extinction site experienced a year in which annual precipitation was above the site’s upper 99% Confidence Interval 1–4 years before the site’s recorded pika presence. This suggests that variable high precipitation conditions facilitated pika dispersal.
The authors also suggested that the dry extirpation sites were unable to support pika populations because they lacked snow cover. The low precipitation levels recorded at these sites resulted in the low accumulation of snow cover. Since snow cover insulates pika populations from extreme cold weather events, the low levels of snow were inefficient to protect the pika populations from low alpine temperatures. Therefore, projected declines in snowpack throughout the western United States indicate that pika habitats in regions like the Southern Rockies may soon experience drier conditions, placing further pika populations at risk to climate change induced extinction.