Sierra Nature Notes, Volume 3, December 2003

Persistance of pikas
Continued, page 2 of 5

Pikas at Craters of the Moon and Lava Beds National Monuments (hereafter, “Craters” and “Lava Beds”) occurred historically at elevations lower than predicted by the monuments’ latitude, when compared with the latitude- elevation relationship among historic pika sites in the Great Basin (fig. 3). Pikas do not usually persist at low elevations (and consequently, high temperatures), and many of the lowest-elevation populations in the Great Basin have recently become extirpated, including seven recorded from 1925 to 1941 (see fig. 3). For these reasons I sought to determine whether pika populations that had been noted historically in Craters and Lava Beds have continued to persist. If pikas had persisted, then I also sought to explore potential mechanisms that have allowed them to persist in such apparently harsh conditions.

Figure 3. Persistence of pika populations during the 20th century in the intermountain West, at different elevations and latitudes. Open circles represent sites in the interior Great Basin where pikas remain extant, and closed circles represent pika populations that became extirpated in the late 20th century. Open squares represent Craters of the Moon and Lava Beds National Monuments, where pikas were recorded during the mid to late 20th century. The solid line represents the relationship between elevation and latitude among the 25 sites in the interior Great Basin at which pikas were previously recorded, and dotted lines represent 95% confidence intervals around the solid line.

Study sites
Craters consists of 29,000 hectares (71,659 acres) of volcanic craters, cones, 2,000- to 15,000-year-old lava flows, caves, and fissures at the interface of the Snake River Plain and the southeast edge of the high, mountainous region of central Idaho (see fig. 2). Elevations in the monument ranged from 1,590-1,990 m 5,217-6,529 ft) at the time of sampling (1995), but the November 2000 expansion of the monument incorporated areas into the monument as low as 1,280 m (4,200 ft). Lava Beds occurs in northeastern California on the north flank of the Medicine Lake shield volcano that erupted 17 times between 800 and 12,800 calendar years ago (Donnelly-Nolan et al. 1990; see fig. 2). The volcano covers about 2,000 sq km (772 sq mi) and lies about 50 km (31 mi) east-northeast of Mt. Shasta in the southern Cascade Range. The monument’s 18,850 ha (46,578 acres) occupy about 10% of the area of the volcano, and encompass cinder cones, spatter cones, and over 440 lava tube caves at elevations between 1,230 and 1,650 m (4,036 and 5,414 ft). Pikas are one of the more charismatic mammal species in the monuments, and are more frequently heard than seen. They are one of six lagomorph and 48 mammal species known from Craters, and one of three lagomorph and 53 mammal species known from Lava Beds.

To provide a comparison of a low-elevation area with extensive potential pika habitat that was geographically closer to the monuments than the interior Great Basin sites, I also sampled three locations in the Hell’s Half Acre lava flow in south-central Idaho from 17–19 July (see fig. 2). We chose this site because it has extensive amounts of talus-like habitat, much of which occurs at large distances from primary roads; the site also has a similar range of elevations to Craters, but experiences different management. These factors played the most important roles in determining persistence of pika populations in the interior Great Basin during the 20th century. Because historic records of pikas in the vicinity of Hell’s Half Acre do not exist, it would be difficult to ascribe a cause to the absence of pikas there, if we could not detect them. Ideally, other low-elevation sites outside but near either monument having historic records of pikas would have been preferable, but we were not aware of any such sites. In associated research, I also re-sampled populations of pikas recorded between 1916 and 1990 at 25 sites ranging in minimum elevation from 1,680–3,139 m (5,512–10,300 ft) throughout the interior Great Basin in summer from 1994 to 1999 (fig. 2).

Methods
I re-visited locations in Craters from 14–17 July 1995 and in Lava Beds from 22–24 July 1995 where pikas had been observed in previous decades, and I sampled sites at Hell’s Half Acre from 17–19 July 1995. Sampling occurred on lava formations for three days at each site (i.e., Lava Beds, Craters, and Hell’s Half Acre), totaling between 15.5 and 18 hours of censuses per site. I chose specific sampling locations in the monuments based upon presence of precise historic records, relative accessibility, and the desire to sample broadly within each monument. Sampling at interior Great Basin sites occurred on taluses for 8 hours per site in summer between 1994 and 1999, or longer (up to 20 hr) if I could not detect pikas at the site.

During slow-walking transect surveys through lava formations, I recorded locations of pika sign (e.g., sightings, calls, and fresh hay pile sightings) using a handheld global positioning system unit without differential correction (precision ± 100 m [328 ft]). I used standardized recording criteria to avoid counting multiple types of evidence from the same individual. I also made observations on the natural history of pikas sighted within the monuments.

To compare climatic conditions at pika sites in the interior Great Basin and in the monuments, I used PRISM data (Oregon Climate Service, Corvallis) that interpolate values between climate stations across the region, and account for factors such as elevation and aspect. These estimated climatic values represent averages from the years 1961–1990, at a resolution of 4 km (2.4 mi). I compared annual precipitation and averages of the maximum daily temperatures for the months of June, July, and August among sites in the interior Great Basin where pikas have been extirpated recently, sites in the Great Basin where they remain extant, and the three volcanic sites (Craters, Lava Beds, and Hell’s Half Acre) adjacent to the Great Basin.

Results

Persistence
In Lava Beds, I detected a minimum of 10 pikas (6 sightings, =4 calling individuals) from 9 sites, out of 16 sites visited (table 1). Pikas were detected at five of nine sites very near to where they were documented in monument records (1960–1991), and at four of eight sites more distant from historic locations. In Craters, I detected a minimum of 27 pikas (8 sightings, =18 calling individuals, and one active hay pile) at 8 of 12 sites visited. Pikas were detected at four of five historic locations (and an inactive hay pile was found at the fifth location), and at four of seven sites slightly more distant from historic locations. No pikas were detected at any of seven locations searched within Hell’s Half Acre.

Climatic analyses
Loss of pika populations at study sites in the interior Great Basin occurred at sites that were on average 20% drier and 8–10% warmer than those at which populations persisted (table 2, page 28). However, the Craters and Lava Beds Monuments, where pikas persist, experience climates that are an estimated 18–24% drier annually and 5–11% warmer during the hottest months of the year than climates at areas of even extirpated pika populations in the interior Great Basin (table 2). Hell’s Half Acre, from which pikas are not known in recent times, received an estimated average of 22–28% less precipitation annually and experienced temperatures 3–5% hotter than Craters and Lava Beds.

Natural history
Other mammals observed in Craters lava fields included chipmunks, yellow-bellied marmots, and golden-mantled ground squirrels. From my observations in Craters, pikas apparently use different parts of the volcanic landscape than chipmunks and squirrels, at least during summer. Whereas ground squirrels and chipmunks are more frequently found on flatter areas with less complex relief (usually pahoehoe or short aa lava formations or areas with extensive sagebrush vegetation), pikas appear to frequent lava tubes, caves, and valley trenches 2–5 m (6.6–16.4 ft) deep. I observed several mountain cottontails along margins of lava flows in Lava Beds, but did not observe them well within the lava flow, where pikas were often seen. Although other mammals were less plentiful at Hell’s Half Acre, birds were relatively more abundant.

Although pikas in the monuments dedicated significant amounts of time to vigilance, numerous individuals were less responsive to the presence of nearby humans than were pikas at sites in the interior Great Basin. Whereas I could never approach pikas to a distance less than 13–15 m (42.6–48.2 ft) in the interior Great Basin, I came within 20 cm (8 in) of stepping on one at Craters. Furthermore, one individual on the Devil’s Orchard Trail seemed so habituated to humans that it remained above the lava surface for 5–8 minutes when a group of about 25 relatively boisterous visitors approached it to within 10 m (33 ft).

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