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The American pika (Ochotona princeps) is a temperature-sensitive lagomorph reported to be in decline in warmer sites in California, Nevada and portions of Utah. Talus is used for denning and retreat habitat by the species. Climate envelope modeling and climate projections suggest the species' distribution will retract in coming decades-but other studies suggest pikas may be resilient in the face of warming by taking advantage of talus as a thermal refuge from warming air temperatures. We investigated the thermal environment of mid to low elevation talus habitats in the northern Sierra Nevada between 2010 and 2012 using automated temperature loggers placed generally 0.5 to 1 m below the talus surface. We found temperatures within talus are rarely challenging to pikas-even in taluses well below the inhabited elevational range of pikas. Occurrence of temperature extremes within talus was only weakly correlated with elevation, and exhibited substantial variation between talus patches. Temperatures deeper in talus than we were able to probe but that pikas can likely reach are certain to be even more stable and less physiologically challenging. Despite buffered temperatures in the subsurface talus environment, we observed multiple instances of pika-accessible, previously-inhabited talus patches that did not support pikas in our surveys. Summer daily maximum air temperatures at these taluses averaged more than 2°C warmer than occupied taluses, and taluses that pikas occupied in some years but not in others were intermediate in temperature. Sites with no evidence of past pika occupancy averaged warmest of all. We suggest aboveground air and surface temperatures, rather than temperatures within talus, pose a greater challenge to pika persistence, through effects on foraging and dispersal. Our results indicate that the thermal refuge provided by talus is likely to be necessary and beneficial to American pikas, but sufficient only to partially offset the ongoing impacts of warming ambient temperatures on waning pika distribution.
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Within-talus temperatures are not limiting for pikas in the
northern Sierra Nevada, California, USA
DaviD H. WrigHt* anD JosepH a. e. steWart
California Dept. of Fish and Wildlife, 1701 Nimbus Rd. Suite A, Rancho Cordova, CA
95670, USA (DHW)
University of California- Santa Cruz, Santa Cruz, CA 95064, USA (JAES)
*Correspondent: dwrighteco@protonmail.com
The American pika (Ochotona princeps) is a temperature-sensitive lagomorph
reported to be in decline in warmer sites in California, Nevada and portions of
Utah. Talus is used for denning and retreat habitat by the species. Climate envelope
modeling and climate projections suggest the species’ distribution will retract in
coming decades—but other studies suggest pikas may be resilient in the face of
warming by taking advantage of talus as a thermal refuge from warming air tem-
peratures. We investigated the thermal environment of mid to low elevation talus
habitats in the northern Sierra Nevada between 2010 and 2012 using automated
temperature loggers placed generally 0.5 to 1 m below the talus surface. We found
temperatures within talus are rarely challenging to pikas—even in taluses well be-
low the inhabited elevational range of pikas. Occurrence of temperature extremes
within talus was only weakly correlated with elevation, and exhibited substantial
variation between talus patches. Temperatures deeper in talus than we were able
to probe but that pikas can likely reach are certain to be even more stable and
less physiologically challenging. Despite buffered temperatures in the subsurface
talus environment, we observed multiple instances of pika-accessible, previously-
inhabited talus patches that did not support pikas in our surveys. Summer daily
maximum air temperatures at these taluses averaged more than 2°C warmer than
occupied taluses, and taluses that pikas occupied in some years but not in others
were intermediate in temperature. Sites with no evidence of past pika occupancy
averaged warmest of all. We suggest aboveground air and surface temperatures,
rather than temperatures within talus, pose a greater challenge to pika persistence,
through effects on foraging and dispersal. Our results indicate that the thermal
refuge provided by talus is likely to be necessary and benecial to American pikas,
but sufcient only to partially offset the ongoing impacts of warming ambient
temperatures on waning pika distribution.
Key words: American pika, climate warming, distribution, extirpation, microcli-
mate, talus, temperature logger
_______________________________________________________________________
California Fish and Game 104(4): 180-195; 2018
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WITHIN-TALUS TEMPERATURES ARE NOT LIMITING FOR PIKAS
Conserving biodiversity in the face of climate change requires that we know how
species will respond over the coming decades and centuries. Accurate predictions inform
smart conservation planning, such as identifying species and populations vulnerable and
not vulnerable, emphasizing the conservation of corridors important for climate-mediated
range shifts, and protecting refugial habitat areas projected to remain suitable for taxa of
concern as climate change proceeds.
The American pika (Ochotona princeps) has emerged as a model organism for
investigating the impact of climate change on animal population viability and distribution
(Galbreath et al. 2009, Beever et al. 2010, 2011, Guralnick et al. 2011, Stewart et al. 2015,
Wilkening et al. 2015a, Castillo et al. 2016, Mathewson et al. 2017). Pikas are small lago-
morphs of the family Ochotonidae, with a single living genus. Two species occur in North
America, with only the American pika within the continental USA (see https://nrm.dfg.
ca.gov/FileHandler.ashx?DocumentID=2359&inline=1, Smith and Weston 1990 for species
accounts). Montane habitat use and cold adaptation are widespread in the genus (Leach et
al. 2015, Yang et al. 2008). In response to a state listing petition, which presented climate
change as a primary threat (Wolf et al. 2007), the California Department of Fish and Wildlife
in 2007 and in 2013 reviewed the status of pikas in California and recommended against
listing, but indicated monitoring and more information were needed (CDFW 2013). The
pika is currently included by the State as a Species of Greatest Conservation Need.
Much of American pikas’ sensitivity to global warming appears to be due to direct
thermal-physiological limitations. Thermal metabolic experiments and thermal models have
indicated that the species’ susceptibility to mortality is due to a poor ability to shed excess
body heat (MacArthur and Wang 1973, 1974, Moyer-Horner et al. 2015). Death at tempera-
tures 25.5 – 30° C has been observed in small studies (Smith 1974, n=2 pikas; MacArthur
and Wang 1973, n=2 pikas). Field observations have shown pika hours of activity are re-
stricted by warmer ambient temperatures commonly experienced by the animal, particularly
at lower elevation sites or during the peak of summer (MacArthur and Wang 1974, Smith
1974, Henry et al. 2012, Otto et al. 2015, Moyer-Horner et al. 2015). Sta and O’Connor
(2015), studying American pikas in southeastern British Columbia, Canada, found foraging
activity decreased by 3% for each 1°C increase in aboveground air temperature, declining
to near inactivity at 20°C. Wilkening et al. (2015b) found summertime stress metabolite
concentrations were greater in fresh pika scat from an area that experienced both higher
within-talus temperatures in summer and more extreme within-talus cold conditions during
the preceding winter. Increasing temperatures or declining snowpack due to local effects
of global warming have widely been implicated in shrinking pika distribution in the North
American Great Basin and California (Beever et al. 2013, 2016, Nichols et al. 2017, Stewart
and Wright 2012, Stewart et al. 2015, 2017, Wilkening et al. 2011).
We investigated the ability of subsurface talus temperatures to predict pika occu-
pancy and persistence patterns in mid to low elevation talus patches in the northern Sierra
Nevada. Millar and others (Millar and Westfall 2010, Millar et al. 2014b, Smith et al. 2016)
have suggested pikas may be resilient to local climate warming because they have a thermal
refuge in cool taluses, allowing them to thermoregulate behaviorally. Stable thermal refuge
in talus also was suggested as the reason for pika persistence in seemingly anomalous, low-
elevation lava habitats (Rodhouse et al. 2017). Millar et al. (2014a) offered an interesting
counterpoint, suggesting Great Basin taluses are less likely to be strong thermal refugia and
expecting continuing pika range contraction there. Mathewson et al. (2017) built a mecha-
CALIFORNIA FISH AND GAME Vol. 104, No. 4
182
nistic model of pika response to their various thermal microenvironments and estimated the
thermal refuge provided by talus would protect against pika population loss at 8 to 19% of
sites where extirpation would otherwise be expected—conversely, talus would not protect
against pika loss at 81 to 92% of sites.
Materials and Methods
We sampled in 46 talus habitats across the northern Sierra Nevada extending
from Yosemite National Park to central Sierra County (37.7 – 39.6° latitude) and ranging
in elevation from 1208 to 2933 m (Figure 1). Fifteen were sites we visited in studies of
historical and other pika locations (Stewart and Wright 2012, Stewart et al. 2015, Stewart
et al. 2017), 31 others we selected for accessibility, apparent suitability for pikas in terms
of rock size, depth and amount of talus, and for adequate representation of taluses at a
variety of elevations (Figure 2). In order to focus on the “hot zone” of potentially tenuous
pika viability near the lower edge of its elevation range, we worked relatively low in the
elevational range of the pika (which extends to well above 4000 m in the Sierra Nevada:
Millar et al. 2010, Stewart et al. 2015). We included sites across a range of elevations in
areas like the Yosemite Valley/Merced River drainage, Mariposa County, and the Lovers
Leap/Horsetail Falls area, El Dorado County, that have abundant talus throughout both
low and higher elevations. Unlike many authors, because we wanted to determine whether
within-talus temperatures at low elevation sites were limiting the distribution of pikas, we
included sites lower in elevation and warmer than the occupied range.
Temperature loggers (Lascar “EasyLog USB”), programmed to record hourly
and housed in waterproof aluminum cases, were wired to a surface rock and lowered into
the talus—typically a depth of 0.5—1 m. Often talus is deeper than this, but it was rarely
feasible to reach lower depths. We took care that loggers would not be exposed to direct
sunlight from any angle. We recorded logger locations on a handheld Global Positioning
System (GPS) unit and thoroughly photographed each site of placement to enable relocation
of the site for retrieval. Each talus patch received one logger, placed where fecal pellets were
abundant relative to other areas sampled within the talus—or, at sites without pika evidence,
at locations that resembled pika-preferred microhabitat in our experience, based on rock size,
talus depth, and rock niche geometry. Temperatures have been shown to vary within talus
patches in other studies (Millar et al. 2014b, Rodhouse et al. 2017, Wilkening et al. 2011);
however examining multiple locations per site was beyond the scope of this study. Instead
we used consistent, unbiased sampling at many sites to maximize power in detecting trends
across locations. Each logger remained in place and recording for approximately one year,
depending on site revisit schedule. At many locations loggers were replaced upon retrieval
with another logger and recording continued for another year.
We surveyed for pika following the methods of Stewart and Wright (2012, also
Stewart et al. 2015) upon each occasion of logger placement or retrieval. We searched for
both current sign and relict sign (old pika fecal pellets can remain for decades on rocks
or in soil or duff collecting in pockets in the talus: Nichols 2010, Stewart et al. 2017). For
analysis, each site was categorized as having current pika sign (pikas present), old pika sign
only (buried pellets, old surface pellets), no pika sign, or as “marginal”. We dened marginal
sites as those that had current pika sign in at least one year of survey but only old sign in at
least one other year. Current pika sign included visual conrmation, auditory conrmation,
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WITHIN-TALUS TEMPERATURES ARE NOT LIMITING FOR PIKAS
Figure 1.Map showing locations of 46 sites (black dots) in northern California where temperature loggers
were placed within talus that appeared appropriate for pika. Lake Tahoe is at north center of the gure; California
county boundaries are also shown.
green haypiles, or fresh scat (pikas give distinctive calls, stockpile harvested vegetation in
piles for winter consumption, and have distinctive scat: Smith and Weston 1990, Elbroch
2003). We commonly detected more than one type of current sign when pikas were present.
Pikas have widely been reported to be highly detectable (Beever et al. 2008, 2011, Rodhouse
et al. 2010, Hall et al. 2016).
CALIFORNIA FISH AND GAME Vol. 104, No. 4
184
Logger data were downloaded using Lascar software and analyzed using JMP 9.0.2
statistical software (SAS Institute, Inc.). Because dates of placement and retrieval varied
between sites and years, logger records covered differing durations of the summer months.
For this reason, we report exceedances of warm and cold temperature thresholds as a ratio:
the number of hourly records equal to or exceeding a given threshold, divided by the total
number of hours recorded during the warm or cold season. We dened the warm season
as June, July, August and September (“JJAS”); and the cold season as December, January,
February and March (“DJFM”). We denote these ratios as R20+, R24+, R(-2.5)-, and R(-
5)- (≥ 20°C, ≥ 24°C, ≤ -2.5°C, and ≤ -5°C, respectively). We chose exceedance thresholds
based on values investigated previously in the literature as potentially signicant to pika
(Beever et al. 2010, Moyer-Horner et al. 2015, Yandow et al. 2015, Wilkening et al 2011).
Ratio values were Box-Cox transformed before analysis if needed to correct non-normal
distribution, adding 0.0001 to all values to prevent errors due to transforming zero values.
To check whether warm exceedances were biased by time of placement or retrieval
of loggers, we assessed the correlation of Julian day of placement (start of record) and
retrieval (end) with elevation, latitude, and with the most sensitive talus warm exceedance
response variable: R20+. Elevation, for example, might reasonably be expected to interfere
with timing of logger placement in a non-random way through delay of eldwork by linger-
ing snowpack at high elevations. If any factor related to temperatures was biasing logger
timing we would expect a relationship between timing and R20+.
Elevation (m)
1000 1200 1400 1600 1800 2000 2200 2400 2600 2800
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
PikaStatus
Present
Marginal
OldSign
NoSign
Figure 2.—Warm temperature exceedances (ratio of within-talus hours ≥ 20°C to all hours June 1 through September
30 in the record [R20+]), versus elevation, in meters. Symbol color and shape shows pika population status. R20+
was not strongly related to elevation (Spearman ρ = -0.22, n = 44, P = 0.15). Sites with few or no temperatures
reaching 20°C occurred at all elevations. Pika-occupied (blue diamonds), marginal (intermittently occupied: green
triangles) and absent (open triangles and circles) sites overlapped greatly, with many unoccupied and formerly
occupied sites similar to occupied sites in within-talus 20°C temperature exceedances.
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WITHIN-TALUS TEMPERATURES ARE NOT LIMITING FOR PIKAS
We used the Basin Characterization Model (BCM: Flint et al. 2013) data to esti-
mate aboveground climatic conditions at sites. The BCM dataset is an interpolated, spatially
continuous, downscaled climate dataset for the watersheds of California (270-m resolution).
In addition to minimum and maximum temperature and precipitation, BCM has a variety of
useful variables such as estimated water balance and snowpack, and provides past, recent,
and future estimates of all climate variables for each 270-m cell.
In multi-variable analyses, we excluded moderately to highly correlated variables
(nonparametric Spearman |ρ| > 0.5) from occurring together in the same model. We followed
Vittinghoff and McCulloch (2007) in limiting the complexity of our statistical models to
prevent overtting. Model comparisons were performed using AICC , and we used ΔAICc
≤ 2 as an indication of a well-tting model (Anderson et al. 2000, Burnham and Anderson
2002).
results
Within-talus temperature records were obtained from 46 sites (Figure 1, Appendix
I). Full cold-season records were obtained at all sites, but we excluded two sites from analy-
ses of warm temperature exceedences because battery failure curtailed their warm season
records to fewer than 70 days.
Day of the year of logger placement correlated differently in different years with
elevation, the correlation being negative in 2010 (Spearman rank test, P = 0.02, n = 14),
non-signicant in 2011 (P = 0.16, n = 45), and positive in 2012 (P = 0.02, n = 13). Day of
logger retrieval was negatively correlated with elevation in 2012 (P = 0.03, n = 45) and
non-signicant in other years (2011, P = 0.35, n = 14; 2013, P = 0.08, n = 13).
To check whether these effects might bias our assessment of warm season tem-
perature exceedances, we examined effects of start and end day of the record on the most
sensitive temperature metric, R20+: the ratio of within-talus hourly temperature records ≥
20ºC to total hourly records during the warm season (transformed to approximate normality).
Because elevation and latitude affect temperatures, we also included these variables and
year in an all-combinations model comparison to evaluate variable importance (Burnham
and Anderson 2002). The best model included elevation and year, only, explaining about
18% of the variance in transformed R20+ (Table 1). Eleven models had ΔAICc ≤ 2 (Table
1); of these, none of the top 5 (ΔAICc < 1.4) included start or end day. Variable importance
among the 11 models with ΔAICc ≤ 2 highlighted year and elevation (relative importance
0.75 and 0.72, respectively) and latitude (0.45), followed by end day and start day (0.23 and
0.15, respectively). The best single-factor models were year of survey (ΔAICc = 0.81), eleva-
tion (ΔAICc = 1.92), and latitude (ΔAICc = 2.37), followed distantly by start day (ΔAICc =
5.99) and end day (ΔAICc = 7.87, which trailed the no-variable model ΔAICc of 7.06). We
concluded that start and end dates had effects on R20+, but these effects were small relative
to effects due to elevation and year.
Acute warm temperature extremes.—Acute warm temperature extremes within
talus were uncommon. Only two of 44 sites experienced within-talus temperatures equal to
or above 28°C, and both had ≤12 hourly exceedences of this threshold in any sampled year.
Old pika sign was detected at one of these sites, and the other was currently occupied. More
than two-thirds of sites (32 of 44: 73%) experienced ≤3 hours of within-talus temperature
≥ 24°C in any year of sampling.
CALIFORNIA FISH AND GAME Vol. 104, No. 4
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Model Rank Model: ∆AICc Weight
Year Elevation Latitude Start Day End Day
_______ _______ _______ _______ _______
1 0.00 0.103
2 0.73 0.072
3 0.81 0.069
4 1.09 0.060
5 1.29 0.054
6 1.41 0.051
7 1.43 0.050
8 1.47 0.049
9 1.56 0.047
10 1.79 0.042
11 1.92 0.040
table 1.—Best models (∆AICc 2) explaining within-talus warm temperature exceedances (≥20ºC: R20+,
transformed to approximate normality). Dots indicate variables included in each model.
Acute cold temperature extremes.—Extremely cold temperatures were also uncom-
mon within talus. Only two sites of 46 experienced temperatures at or below -10°C in any
year, each for fewer than 10 hours in a year. Pikas were present at one of these sites, and
we detected old pika sign at the other.
Temperatures between -5° and -10° C were more common, with 25 sites (54%)
experiencing ≥10 hours in at least one year of recording. These sites ranged widely in
elevation, from 1278 – 2933 m. Other studies have associated within-talus temperatures
substantially below 0°C or having wide uctuations below 0°C with lack of snow cover
(Kreuzer and Huntly 2003, Millar et al. 2014b, Beever et al. 2010, 2011).
Comparing a heavy and a light snow winter.—California’s water year 2010 –
2011 was generally wetter than average, while 2011 – 2012 was dry. Snowpack during
March 2011 averaged about 3 times the snowpack during March 2012, as estimated by
BCM for the 13 sites we sampled in both years (mean ± standard deviation across sites:
1180 ± 173 mm in 2011, 370 ± 75 mm in 2012). Within-talus temperature loggers during
2010 – 2011 showed fewer warm temperature extremes in the warm season, and longer
periods of winter temperatures hovering around 0°C as is typical under an insulating
snow layer. There were fewer cold exceedences under this heavy snowpack: 0.7% vs.
3.1% of cold season hours ≤ -5° C in 2010-2011 vs. 2011-2012 (Wilcoxon signed ranks
test, Z = 3.56, P = 0.0004); and 2.6% vs. 23.4% of hours ≤ -2.5° C (Z = 3.83, P=0.0001).
Patterns in 2011 – 2012 within-talus temperatures.—Here we restricted across-
sites analysis to summer 2011 through summer 2012 (n = 44 sites), to control for weather
differences between years. Variation among sites in within-talus temperature exceedances
was considerable, and exceedances did not explain pika status.
Warm temperature exceedances in the talus hourly records were only very weakly
negatively correlated with elevation (R20+ vs. elevation, Spearman ρ = -0.22, P = 0.15;
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Fall 2018 187
WITHIN-TALUS TEMPERATURES ARE NOT LIMITING FOR PIKAS
R24+ vs. elevation, ρ = -0.15, P = 0.33; n = 44). Correlation of talus warm temperature
exceedances with BCM estimates of summer daily maximum air temperatures at each site
(Jun through Sep, 2001 – 2010 averages) were still rather weak (R20+ vs. Tmax, ρ = 0.30, P
= 0.05; R24+ vs. Tmax, ρ = 0.19, P = 0.22, due to great variation in within-talus temperatures
that was not strongly tied to ambient air temperature: the well-known buffering effect of
talus. For example, thirteen sites ranging from 1641 to 2530 m elevation logged zero hours
≥20°C within talus. There was substantial variation across elevations in the ratio of hours
≥ 20°C (Figure 2). Pika-occupied and pika-absent sites overlapped extensively in ≥20°C
exceedances within talus (Figure 2).
Low temperatures within talus during 2011 – 2012 were also resistant to simple
categorization. There was a signicant but high-variance positive correlation of cold ex-
ceedances with elevation (R(-5)- vs. elevation, ρ = 0.55, n = 45, P = 0.0001; Figure 3). Of
45 sites, 16 had zero hourly records ≤ -5°C, and these sites ranged broadly in elevation:
1208 – 2530 m. Two sites with many hours ≤ -5°C were at modest elevations (1792 and
2156 m), one of which seemed topographically likely to be subject to cold air drainage.
Patterns in R(-2.5)- were similar (R(-2.5)- vs elevation, ρ = 0.62, P < 0.0001). Correlation
of within-talus cold exceedances with BCM’s 2001 – 2010 average winter Tmin (average
daily minimum temperature DJFM) gave correlations of -0.30 (for both R(-5)- and R(-2.5),
n = 45, P = 0.05, ). We doubt -2.5°C is acutely challenging to pikas, which routinely live for
months at approximately 0° C, during snow cover, but subzero temperatures within talus
may increase long-term winter energy demand (cf. Otto et al. 2015 Figure 1, MacArthur
and Wang 1973 Figure 3).
1200 1400 1600 1800 2000 2200 2400 2600
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
PikaStatus
Present
Marginal
OldSign
NoSign
Figure 3.—Cold temperature exceedances (R(-5)-: the ratio of within-talus hours ≤ -5°C to all recorded hours
December 1 through March 30, versus elevation. Symbol color and shape shows pika population status. R(-5)- was
positively related to elevation (Spearman ρ = 0.55, n = 45, P = 0.0001). Sites with few or no temperatures ≤ -5°C
occurred at all elevations. Pika-occupied (blue diamonds), marginal (intermittently occupied: green triangles) and
absent (open triangles and circles) sites overlapped greatly, with many unoccupied and formerly occupied sites
similar to occupied sites in within-talus -5°C temperature exceedances.
CALIFORNIA FISH AND GAME Vol. 104, No. 4
188
Comparisons in relation to pika status at sites.—Pika status was negatively as-
sociated with warm aboveground temperatures and positively with elevation. Sites where
pika were present averaged higher elevation and cooler aboveground than marginal sites,
sites with old sign, or sites lacking any sign, in that order (Figure 4). Model comparison of
ordinal logistic regression on factors related to pika status, in which pika status was ranked
no sign < old sign < marginal < present, showed aboveground average warm season daily
maximum temperature (Tmax) to be the best single predictor examined (effect likelihood
ratio chi-square 21.4, 1 df, P < 0.0001). Elevation and mean annual temperature also per-
formed reasonably well (ΔAICc = 0.77 and 2.10, respectively), although each was strongly
correlated with Tmax (r = -.96 and .92, respectively). Along with aboveground average cold
season daily minimum temperature (Tmin: ΔAICc = 8.8), these four were the only factors
related to pika status with ΔAICc < 10.
Warm exceedances within talus did not effectively explain pika status (R20+,
ΔAICc = 17.6; R24+, ΔAICc = 18.9). The direction of the effect of within-talus cold ex-
tremes (R(-2.5)-, ΔAICc = 11.3) on pika status was not as expected from a hypothesis of
cold stress—more cold extremes were positively associated with pika presence—but was
consistent with its correlation with air temperatures, including Tmax. No quadratic effect
was supported: pika presence was not maximized at intermediate values of cold exceedances
within the limits of our study. The direction of the R(-5)- effect also was not consistent with
a cold stress hypothesis within our data set, and a quadratic effect was not supported.
18
20
22
24
26
28
30
Present Marginal OldSign NoSign
Pika Status
Figure 4.—Mean daily-maximum air temperature June-September during 2001-2010, from BCM (Flint et al.
2013) for each site, versus pika status at that site. Cooler daily maximum warm season temperatures were the best
predictor of pika occurrence in our study (ordinal logistic regression, χ2 = 21.4, 1 df, P < 0.0001). Green diamonds
center on the mean and extend vertically to the upper and lower 95% condence limits of the mean; short internal
horizontal lines show signicant differences where not overlapping the internal lines of other diamonds. Dots
show individual sites.
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WITHIN-TALUS TEMPERATURES ARE NOT LIMITING FOR PIKAS
discussion
Soil acts to buffer deeper ground layers from variation in air temperature, and to
an extent this has been proven true of talus as well (Millar et al. 2014b). The deeper one
goes underground, the more variations in air temperature are buffered—rst daily variations
disappear, and ultimately annual variation is also smoothed to a constant value: the mean
annual temperature of the site, barring geothermal effects (Hillel 1982). Pikas presumably
are capable of exploring talus deposits to their very bottoms in many places, a freedom
human researchers lack. Yet even at very modest depths within talus, typically less than 1
meter, we found substantial buffering of temperatures. We may presume pikas are capable
of reaching depths within talus that are even more equable than we have documented.
Remarkably, Smith et al. (2016) found pikas occurring at a site with summer within-talus
temperatures (1 m depth) up to 30°C, albeit with some of the lowest feeding, haying, and
other activity rates known for American pikas. The authors mention a possibility of sub-
surface permafrost at the site, which in combination with pikas’ presumed ability to move
deep within talus or lava formations, might explain toleration of such warm temperatures
at intermediate depths of their talus refuge.
Even at the modest depths where we were able to place recorders, temperatures
did not appear particularly challenging to pikas. Otto et al. (2015) estimated 28°C as the
lower limit of the thermal neutral zone for resting pikas from Colorado and Wyoming, while
MacArthur and Wang (1973) observed a mortality at this temperature. Moyer-Horner et al.
(2015) suggested American pika activity would likely be restricted at temperatures above
20°C, due to the extra heat production of activity above resting metabolism, solar insolation,
and the limited capability of pikas to shed that heat. All but two of our sites never reached
28°C within talus even at only 0.5-1 m depth, and nearly one-third of 45 sites sampled dur-
ing 2011-2012 never reached 20°C within talus. Further, many of our sites are at elevations
lower than/experience air temperatures higher than sites where pikas currently occur, yet
had within-talus temperatures that were suitable for pikas. We did not observe evidence
these equable within-talus temperatures were related to the rock-ice features discussed by
Millar et al. (2014b) as contributing to pika resiliency. We believe rock-ice features, which
retain year-round ice beneath the talus surface, are rare or absent in the “hot zone” of the
lower elevational limit of pika distribution within our study area. Instead, equable within-
talus temperatures in or below the hot zone were likely the result of the buffering effects of
depth.
Amount of talus in the vicinity may be a factor that affects pika status at sampled
taluses (Stewart and Wright 2012, Stewart et al. 2015), but seems unlikely to be the whole
story. We elected to survey in and around Yosemite Valley and Lovers Leap/Horsetail Falls
because there are large areas of talus present, more than is typically found at elevations below
2000 m. The abundance of talus there did not support pikas, even though temperatures within
many of those taluses sampled were suitable; the lowest current pika occupancy we found in
Yosemite was at 2352 m, and near Horsetail Falls was at 2029 m (marginal). We also know
from old sign that, in the past, pika have reached and probably lived at many taluses we
sampled. These taluses overwhelmingly still have moderate within-talus temperatures, yet
are not currently occupied, suggesting that within-talus temperatures are not what excludes
pikas from these locations. These ndings echo those of Stewart et al. (2017), who found
a large, apparently recently extirpated area of former pika occupation north of Lake Tahoe,
CALIFORNIA FISH AND GAME Vol. 104, No. 4
190
California, where air temperatures have become increasingly challenging for pikas.
The tendency of researchers to study microclimates—such as subsurface tempera-
tures within talus—only where pikas occur may be responsible for unjustied optimism
about pika resilience to climate change (Millar and Westfall 2010, Smith et al. 2016). Varner
and Dearing (2014), based on more moderate within-talus temperatures at lower elevations,
suggested that lower elevation taluses might even provide better climate change refugia than
higher elevations. We found, however, by examining elevations below where pikas currently
occur, that pikas have been present in the past at low elevation taluses but appear extirpated
there now, despite generally suitable within-talus temperatures. These lower taluses have
warmer aboveground climates. Lower and warmer yet, we found no evidence of pika oc-
currence at all, despite comparable below-talus temperatures, even in taluses that appeared
to be within pika dispersal range (e.g., Yosemite Valley, Lovers Leap).
As opposed to within-talus temperatures, aboveground summer temperatures
appear more relevant to pika occupancy in our region. Elsewhere we argued that, among
available climatic variables, mean summer temperature had mechanistic appeal as an index
of chronic summer heat stress (Stewart et al. 2015), and it was a strong explanatory factor
in this study as well. Several authors have shown negative correlations of pika activity with
ambient aboveground temperature (MacArthur and Wang 1974, Smith 1974, Moyer-Horner
et al. 2015, Otto et al. 2015, Sta and O’Connor 2015). Warm aboveground temperatures
may impair pikas’ ability to forage or to disperse (Smith 1974, Wilkening et al. 2011), both
essential activities contributing to reproductive rate and metapopulation persistence. Heat
restricts foraging activity due to pikas’ heat sensitivity and limited ability to shed body
heat (MacArthur and Wang 1973, Otto et al. 2015). Moyer-Horner et al. (2015), based on
a biomechanical heat ux model, estimated that American pika activity generally would
be limited by ambient temperatures above about 20°C. Above this temperature, brief bouts
of activity (as opposed to resting metabolism) might still be possible, but would generate
body heat load that would have to be dissipated quickly to avoid stress and perhaps death.
Mathewson et al (2017), exploring further the consequences of the same heat ux model,
calculated under a “moderate” climate change scenario (mean western USA temperatures,
summer +2.6°C, winter daily minimums +0.8°C) that by the year 2070 the American pika
would be extirpated at 53 of 616 sites. A purely associative temperature model (i.e., not
incorporating pika morphology, physiology or behavior) under the same conditions projected
extirpation at more sites (69 of 616), the difference implying pika resilience—including use
of the thermal refuge of talus—would reduce but not eliminate pika range retraction in the
face of climate change.
Finding and establishing new territories is a potentially heat-sensitive period in the
life of juvenile pikas. Svendsen (1979) reported that subadult American pikas near Gothic,
Colorado, before they dispersed to nd and establish their own territories, attempted to use
“ephemeral home ranges” within their natal talus that sometimes overlapped adult territories.
Territorial adults harassed them at times when the adults were active. Svendsen observed that
subadults took advantage of times of day when adults were less active—prior to 0700 hours,
during midday, and beyond dusk—to have greater use of these areas without harassment.
Under conditions of high daytime air temperatures, subadult pikas might be less able to be
active aboveground during the warmest midday hours, which could affect their ability to
forage and avoid aggression by adults, and therefore to survive through the dispersal phase
of their life cycle. In addition, subadult dispersal to new territories sometimes involves
191
Fall 2018 191
WITHIN-TALUS TEMPERATURES ARE NOT LIMITING FOR PIKAS
movement through areas lacking thermal refugia such as deep talus, and if temperatures
are too warm, dispersing individuals may be subjected to stress or mortality during this
essential life stage (Smith 1974, Smith et al. 2016).
Within-talus winter cold stress has emerged repeatedly as another possible climate-
related factor leading to pika extirpations (Beever et al. 2010, 2011, Erb et al. 2011, Ray
et al. 2012, Yandow et al. 2015), but was not strongly supported as an explanation of pika
occupancy in our study. Instead, we found colder within-talus temperatures correlated to
higher pika occupancy within our scope of study. This correlation must not be taken too
literally because pikas can no doubt access more equable places within talus than those where
our loggers were placed—warmer during winters and cooler during summers. Nevertheless,
low temperatures within talus—below our focal thresholds—were weakly less frequent
with increasing mean air temperature and with decreasing elevation, which ran counter to
a cold-stress hypothesis to explain the broad absence of pikas at our lowest, warmest sites.
Overall, our results indicate, in agreement with Mathewson et al. (2017), that the
thermal refuge provided by talus is likely to be necessary and benecial to American pikas,
but sufcient only to partially offset the ongoing impacts of warming ambient temperatures
on waning pika distribution.
acknowledgMents
The US Fish and Wildlife Service State Wildlife Grant program contributed fund-
ing. We thank J. Thorne and L. and A. Flint for access to their BCM downscaled climate
dataset. E. Beever and C. Millar introduced us to temperature loggers. C. Nguyen guided
and provided input on the work, M. Armstrong-Russ collated much of the data. We thank
numerous eld staff and volunteers, notably M. Armstrong-Russ, Z. Bess, S. Birdsong, J.
Boulat, J. N. Henderson, K. Hooks, J. Kastner, P. Keating, K. Klingler, G. Mathews, T.
Nguyen, R. Pauloo, B. Raleigh, T. Rosenthal, S. Stewart, R. Strabel, and K. Wiggins.
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Approved 15 May 2018
Associate Editor was S. Osborn
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WITHIN-TALUS TEMPERATURES ARE NOT LIMITING FOR PIKAS
Site Latitude
(0)
Longitude
(0)Years Elevation
(m)
Pika Status
Deadman M 39.6254 -120.5495 2010-2012 1923 Present
Sardine L. Uppr. 39.6086 -120.6310 2011-2012 1861 NoSign
Sierra Buttes39.5997 -120.6402 2011-2012 2225 NoSign
Sierra City NE 39.5760 -120.6220 2011-2012 1489 NoSign
Hwy.80 nr.border 39.4368 -120.0288 2011-2012 1662 NoSign
Carpenter Ridge 39.4170 -120.3183 2011-2012 2595 Present
Donner W_14 39.3219 -120.4100 2010-2013 2008 OldSign
Donner E 39.3140 -120.3233 2010-2012 2187 Marginal
Cisco Butte NW 39.3101 -120.5632 2011-2012 1911 NoSign
Hwy.89 Bridge 6 39.2279 -120.2005 2011-2012 1880 OldSign
Mt. Watson 2 39.1875 -120.1877 2011-2012 2267 OldSign
Mt. Watson 1 39.1844 -120.1855 2011-2012 2248 OldSign
Big Bear 39.1619 -120.1825 2011-2012 2022 OldSign
Eagle Fls. 4 38.9537 -120.1134 2010-2013 1905 OldSign
Eagle Fls. 20 38.9473 -120.1172 2011-2012 2150 OldSign
Eagle Fls. 15 38.9470 -120.1197 2011-2012 2108 Present
EagleFls_100 38.9467 -120.1248 2010-2012 2157 Marginal
Eagle Lk. 38.9414 -120.1245 2010-2012 2160 Present
Velma Lk. Lower 38.9409 -120.1459 2010-2012 2382 OldSign
Mt. Tallac 38.9048 -120.0984 2010-2013 2933 Present
Heather Lk. Blw, S 38.8755 -120.1294 2010-2012 2363 Present
Heather Lk. 38.8745 -120.1365 2010-2012 2454 Present
Heather Lk. Below 38.8727 -120.1244 2010-2013 2311 Present
Echo Lk. S 38.8331 -120.0561 2010-2012 2336 Present
Pyramid Ck. E 38.8285 -120.1223 2011-2012 2126 Marginal
Twin Bridges NNE 38.8282 -120.1166 2011-2012 2364 Marginal
Pyramid Ck. SE2 38.8267 -120.1212 2011-2012 2060 Marginal
Hogback LL3 38.8050 -120.1375 2011-2012 1781 OldSign
American R., S fork 38.8028 -120.1392 2011-2012 1778 OldSign
Lovers Leap 2 38.8007 -120.1358 2011-2012 1838 OldSign
Hwy.89 WM 38.7926 -119.9671 2011-2012 2352 OldSign
Carson R. W 38.7633 -119.8519 2011-2012 1870 Marginal
Round Top 38.6435 -119.9891 2010-2012 2348 Present
Mosquito Lk. NW 38.5189 -119.9203 2011-2012 2530 Marginal
Pacic Gr. Summ. 10 38.5147 -119.9074 2010-2012 2452 OldSign
Calif. Falls N 37.9176 -119.4379 2011-2012 2396 Present
McGee Lk. 37.9014 -119.4311 2011-2012 2471 Present
HalfDome,CloudsR 37.7589 -119.5284 2011-2013 1394 OldSign
Bunnell Pt. 37.7454 -119.4652 2011-2013 2092 OldSign
Bunnell Crossing 37.7428 -119.4607 2011-2013 2020 OldSign
Moraine Dome 37.7412 -119.4763 2011-2013 1956 OldSign
Merced Lk. Camp 37.7410 -119.4098 2011-2013 2227 Marginal
Nevada Fall, Tr.Blw. 37.7269 -119.5354 2011-2013 1641 OldSign
Vernal Fall Tr. 37.7265 -119.5539 2011-2013 1360 OldSign
El Capitan SW 37.7233 -119.6543 2011-2013 1278 NoSign
Bridalveil E 37.7206 -119.6448 2011-2013 1208 OldSign
appendix i.—Study sites.
... Yet Wright and Stewart (2018) argued that subsurface temperatures may not predict pika persistence well, reporting extirpations at sites with suitably moderate subsurface temperatures in the northern Sierra Nevada. Wright and Stewart (2018) instead suggested that abovetalus temperatures might be more impor tant through negative effects on foraging and dispersal. ...
... Yet Wright and Stewart (2018) argued that subsurface temperatures may not predict pika persistence well, reporting extirpations at sites with suitably moderate subsurface temperatures in the northern Sierra Nevada. Wright and Stewart (2018) instead suggested that abovetalus temperatures might be more impor tant through negative effects on foraging and dispersal. Pikas experiencing warm surface temperatures have been shown to spend less time foraging (Hall et al. 2016), which could limit a pika's ability to cache food for winter and have negative effects on pika persistence (Dearing 1997, Morrison et al. 2009, Bhattacharya and Ray 2015. ...
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... Recent modeling has indicated that variability in the talus microclimate helps determine climate change response in pikas [69]. Although one study suggests that free-air temperature may be more important than subsurface temperature as a determinant of pika occupation and persistence in some regions [70], our study suggests that a warming trend in free air might be associated with disproportionate subsurface warming, an additional consideration that might not be evident from a study of shortterm temperature records. The amount of subsurface warming required to stress pikas should be related to the amount of heat they need to shed, which should be related to surface temperature and the amount of required surface activity-variables that would vary in space and time. ...
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... For example, a study of pikas in Washington found that corticosterone levels detected in summer coat hair were influenced by lower mean maximum daily temperatures, providing preliminary evidence that cold temperatures experienced in June to mid-July can cause thermal stress (Waterhouse et al., 2017). Finally, temperature (subsurface and ambient) is a predictor of pika occupancy and abundance in many systems (Hafner 1994;Beever et al., 2010;Wilkening et al., 2011Wilkening et al., , 2015Yandow et al., 2015;Schwalm et al., 2016;Wright and Stewart 2018), further highlighting the value of future studies that explicitly investigate the effects of temperature on seasonal patterns of stress. ...
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Temporal variation in stress might signify changes in an animal’s internal or external environment, while spatial variation in stress might signify variation in the quality of the habitats that individual animals experience. Habitat-induced variations in stress might be easiest to detect in highly territorial animals, and especially in species that do not take advantage of common strategies for modulating habitat-induced stress, such as migration (escape in space) or hibernation (escape in time). Spatial and temporal variation in response to potential stressors has received little study in wild animals, especially at scales appropriate for relating stress to specific habitat characteristics. Here, we use the American pika (Ochotona princeps), a territorial small mammal, to investigate stress response within and among territories. For individually territorial animals such as pikas, differences in habitat quality should lead to differences in stress exhibited by territory owners. We indexed stress using stress-associated hormone metabolites in feces collected non-invasively from pika territories every 2 weeks from June to September 2018. We hypothesized that differences in territory quality would lead to spatial differences in mean stress and that seasonal variation in physiology or the physical environment would lead to synchronous variation across territories through time. We used linear mixed-effects models to explore spatiotemporal variation in stress using fixed effects of day-of-year and broad habitat characteristics (elevation, aspect, site), as well as local variation in habitat characteristics hypothesized to affect territory quality for this saxicolous species (talus depth, clast size, available forage types). We found that temporal variation within territories was greater than spatial variation among territories, suggesting that shared seasonal stressors are more influential than differences in individual habitat quality. This approach could be used in other wildlife studies to refine our understanding of habitat quality and its effect on individual stress levels as a driver of population decline.
... p. schisticeps; Fig. 2b) is also the lineage that occurs in both eastern California and the Great Basin [22,23]. These two regions, which have produced most of the evidence for climate change-induced pika population extirpations [17,33,123,124], are also predicted to lose more of the currently suitable pika habitat during this century [16,22,125]. ...
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Background Distributional responses by alpine taxa to repeated, glacial-interglacial cycles throughout the last two million years have significantly influenced the spatial genetic structure of populations. These effects have been exacerbated for the American pika ( Ochotona princeps ), a small alpine lagomorph constrained by thermal sensitivity and a limited dispersal capacity. As a species of conservation concern, long-term lack of gene flow has important consequences for landscape genetic structure and levels of diversity within populations. Here, we use reduced representation sequencing (ddRADseq) to provide a genome-wide perspective on patterns of genetic variation across pika populations representing distinct subspecies. To investigate how landscape and environmental features shape genetic variation, we collected genetic samples from distinct geographic regions as well as across finer spatial scales in two geographically proximate mountain ranges of eastern Nevada. Results Our genome-wide analyses corroborate range-wide, mitochondrial subspecific designations and reveal pronounced fine-scale population structure between the Ruby Mountains and East Humboldt Range of eastern Nevada. Populations in Nevada were characterized by low genetic diversity (π = 0.0006–0.0009; θ W = 0.0005–0.0007) relative to populations in California (π = 0.0014–0.0019; θ W = 0.0011–0.0017) and the Rocky Mountains (π = 0.0025–0.0027; θ W = 0.0021–0.0024), indicating substantial genetic drift in these isolated populations. Tajima’s D was positive for all sites ( D = 0.240–0.811), consistent with recent contraction in population sizes range-wide. Conclusions Substantial influences of geography, elevation and climate variables on genetic differentiation were also detected and may interact with the regional effects of anthropogenic climate change to force the loss of unique genetic lineages through continued population extirpations in the Great Basin and Sierra Nevada.
... p. schisticeps; Fig. 2B) is also the lineage that occurs in both eastern California and the Great Basin [22,23]. These two regions, which have produced most of the evidence for climate change-induced pika population extirpations [17,33,123,124], are also predicted to lose more of the currently suitable pika habitat during this century [16,22,125]. ...
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Background: Distributional responses by alpine taxa to repeated, glacial-interglacial cycles throughout the last two million years have significantly influenced the spatial genetic structure of populations. These effects have been exacerbated for the American pika (Ochotona princeps), a small alpine lagomorph constrained by thermal sensitivity and a limited dispersal capacity. As a species of conservation concern, long-term lack of gene flow has important consequences for landscape genetic structure and levels of diversity within populations. Here, we use reduced representation sequencing (ddRADseq) to provide a genome-wide perspective on patterns of genetic variation across pika populations representing distinct subspecies. To investigate how landscape and environmental features shape genetic variation, we collected genetic samples from distinct geographic regions as well as across finer spatial scales in two geographically proximate mountain ranges of eastern Nevada. Results: Our genome-wide analyses corroborate range-wide, mitochondrial subspecific designations and reveal pronounced fine-scale population structure between the Ruby Mountains and East Humboldt Range of eastern Nevada. Populations in Nevada were characterized by low genetic diversity (𝜋=0.0006–0.0009; 𝜃W=0.0005–0.0007) relative to populations in California (𝜋=0.0014–0.0019; 𝜃W=0.0011–0.0017) and the Rocky Mountains (𝜋=0.0025–0.0027; 𝜃W=0.0021–0.0024), indicating substantial genetic drift in these isolated populations. Tajima’s D was positive for all sites (D=0.240-0.811), consistent with recent contraction in population sizes range-wide. Conclusions: Substantial influences of geography, elevation and climate variables on genetic differentiation were also detected and may interact with the regional effects of anthropogenic climate change to force the loss of unique genetic lineages through continued population extirpations in the Great Basin and Sierra Nevada.
... p. schisticeps; Fig. 2B) is also the lineage that occurs in both eastern California and the Great Basin [22,23]. These two regions, which have produced most of the evidence for climate change-induced pika population extirpations [17,33,123,124], are also predicted to lose more of the currently suitable pika habitat during this century [16,22,125]. ...
Preprint
Full-text available
Background: Distributional responses by alpine taxa to repeated, glacial-interglacial cycles throughout the last two million years have significantly influenced the spatial genetic structure of populations. These effects have been exacerbated for the American pika (Ochotona princeps), a small alpine lagomorph constrained by thermal sensitivity and a limited dispersal capacity. As a species of conservation concern, long-term lack of gene flow has important consequences for landscape genetic structure and levels of diversity within populations. Here, we use reduced representation sequencing (ddRADseq) to provide a genome-wide perspective on patterns of genetic variation across pika populations representing distinct subspecies. To investigate how landscape and environmental features shape genetic variation, we collected genetic samples from distinct geographic regions as well as across finer spatial scales in two geographically proximate mountain ranges of eastern Nevada. Results: Our genome-wide analyses corroborate range-wide, mitochondrial subspecific designations and reveal pronounced fine-scale population structure between the Ruby Mountains and East Humboldt Range of eastern Nevada. Populations in Nevada were characterized by low genetic diversity (𝜋=0.0006–0.0009; 𝜃W=0.0005–0.0007) relative to populations in California (𝜋=0.0014–0.0019; 𝜃W=0.0011–0.0017) and the Rocky Mountains (𝜋=0.0025–0.0027; 𝜃W=0.0021–0.0024), indicating substantial genetic drift in these isolated populations. Tajima’s D was positive for all sites (D=0.240-0.811), consistent with recent contraction in population sizes range-wide. Conclusions: Substantial influences of geography, elevation and climate variables on genetic differentiation were also detected and may interact with the regional effects of anthropogenic climate change to force the loss of unique genetic lineages through continued population extirpations in the Great Basin and Sierra Nevada.
... p. schisticeps; Fig. 2B) is also the lineage that occurs in both eastern California and the Great Basin [22,23]. These two regions, which have produced most of the evidence for climate change-induced pika population extirpations [17,33,111,112], are also predicted to lose more of the currently suitable pika habitat during this century [16,22,113]. ...
Preprint
Full-text available
Background: Distributional responses by alpine taxa to repeated, glacial-interglacial cycles throughout the last two million years have significantly influenced the spatial genetic structure of populations. These effects have been exacerbated for the American pika (Ochotona princeps), a small alpine lagomorph constrained by thermal sensitivity and a limited dispersal capacity. As a species of conservation concern, long-term lack of gene flow has important consequences for landscape genetic structure and levels of diversity within populations. Here, we use reduced representation sequencing (ddRADseq) to provide a genome-wide perspective on patterns of genetic variation across pika populations representing distinct subspecies. To investigate how landscape and environmental features shape genetic variation, we collected genetic samples from distinct geographic regions as well as across finer spatial scales in two geographically proximate mountain ranges of eastern Nevada. Results: Our genome-wide analyses corroborate range-wide, mitochondrial subspecific designations and reveal pronounced fine-scale population structure between the Ruby Mountains and East Humboldt Range of eastern Nevada. Populations in Nevada were characterized by low genetic diversity (𝜋=0.0006–0.0009; 𝜃W=0.0005–0.0007) relative to populations in California (𝜋=0.0014–0.0019; 𝜃W=0.0011–0.0017) and the Rocky Mountains (𝜋=0.0025–0.0027; 𝜃W=0.0021–0.0024), indicating substantial genetic drift in these isolated populations. Tajima’s D was positive for all sites (D=0.240-0.811), consistent with recent contraction in population sizes range-wide. Conclusions: Substantial influences of geography, elevation and climate variables on genetic differentiation were also detected and may interact with the regional effects of anthropogenic climate change to force the loss of unique genetic lineages through continued population extirpations in the Great Basin and Sierra Nevada.
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Microrefuges provide microclimates decoupled from inhospitable regional climate regimes that enable range-peripheral populations to persist and are important to cold-adapted species in an era of accelerated climate change. However, identifying and describing the thermal characteristics of microrefuge habitats is challenging, particularly for mobile organisms in cryptic, patchy habitats. We examined variation in subsurface thermal conditions of microrefuge habitats among different rock substrate types used by the American pika (Ochotona princeps), a climate-sensitive, rock-dwelling Lagomorph. We compared subsurface temperatures in talus and lava substrates in pika survey sites in two US national park units; one park study area on the range periphery and the other in the range core. We deployed paired sensors to examine within-site temperature variation. We hypothesized that subsurface temperatures within occupied sites and structurally complex substrates would be cooler in summer and warmer in winter than unoccupied and less complex sites. Although within-site variability was high, with correlations between paired sensors as low as 47%, we found compelling evidence that pikas occupy microrefuge habitats where subsurface conditions provide more thermal stability than in unoccupied microhabitats. The percentage of days in which microhabitat temperatures were between −2.5 and 25.5°C was significantly higher in occupied sites. Interestingly, thermal conditions were substantially more stable (p < .05) in the lava substrate type identified to be preferentially used by pikas (pahoehoe vs. a'a) in a previous study. Our study and others suggest that thermal stability appears to be the defining characteristic of subsurface microrefuges used by American pikas and is a likely explanation for enigmatic population persistence at the range periphery. Our study exemplifies an integrated approach for studying complex microhabitat conditions, paired with site use surveys and contextualized with information about gene flow provided by complementary studies.
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Alpine mammals are predicted to be among the species most threatened by climate change, due to the projected loss and further fragmentation of alpine habitats. As temperature or precipitation regimes change, alpine mammals may also be faced with insurmountable barriers to dispersal. The slow rate or inability to adjust to rapidly shifting environmental conditions may cause isolated alpine species to become locally extirpated, resulting in reduced biodiversity. One proposed method for mitigating the impacts of alpine species loss is assisted migration. This method, which involves translocating a species to an area with more favourable climate and habitat characteristics, has become the subject of debate and controversy in the conservation community. The uncertainty associated with climate change projections, coupled with the thermal sensitivity of many alpine mammals, makes it difficult to a priori assess the efficacy of this technique as a conservation management tool. Here we present the American pika (Ochotona princeps) as a case study. American pikas inhabit rocky areas throughout the western US, and populations in some mountainous areas have become locally extirpated in recent years. We review known climatic and habitat requirements for this species, and also propose protocols designed to reliably identify favourable relocation areas. We present data related to the physiological constraints of this species and outline specific requirements which must be addressed for translocation of viable populations, including wildlife disease and genetic considerations. Finally, we discuss potential impacts on other alpine species and alpine communities, and overall implications for conserving alpine biodiversity in a changing climate.
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When possible, many species will shift in elevation or latitude in response to rising temperatures. However, before such shifts occur, individuals will first tolerate environmental change and then modify their behavior to maintain heat balance. Behavioral thermoregulation allows animals a range of climatic tolerances and makes predicting geographic responses under future warming scenarios challenging. Because behavioral modification may reduce an individual's fecundity by, for example, limiting foraging time and thus caloric intake, we must consider the range of behavioral options available for thermoregulation to accurately predict climate change impacts on individual species. To date, few studies have identified mechanistic links between an organism's daily activities and the need to thermoregulate. We used a biophysical model, Niche Mapper, to mechanistically model microclimate conditions and thermoregulatory behavior for a temperature-sensitive mammal, the American pika (Ochotona princeps). Niche Mapper accurately simulated microclimate conditions, as well as empirical metabolic chamber data for a range of fur properties, animal sizes, and environmental parameters. Niche Mapper predicted pikas would be behaviorally constrained because of the need to thermoregulate during the hottest times of the day. We also showed that pikas at low elevations could receive energetic benefits by being smaller in size and maintaining summer pelage during longer stretches of the active season under a future warming scenario. We observed pika behavior for 288 h in Glacier National Park, Montana, and thermally characterized their rocky, montane environment. We found that pikas were most active when temperatures were cooler, and at sites characterized by high elevations and north-facing slopes. Pikas became significantly less active across a suite of behaviors in the field when temperatures surpassed 20°C, which supported a metabolic threshold predicted by Niche Mapper. In general, mechanistic predictions and empirical observations were congruent. This research is unique in providing both an empirical and mechanistic description of the effects of temperature on a mammalian sentinel of climate change, the American pika. Our results suggest that previously underinvestigated characteristics, specifically fur properties and body size, may play critical roles in pika populations' response to climate change. We also demonstrate the potential importance of considering behavioral thermoregulation and microclimate variability when predicting animal responses to climate change.
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The behavioral ecology of the American pika (Ochotona princeps) was investigated at a relatively hot south-facing, low-elevation site in the Mono Craters, California, a habitat quite different from the upper montane regions more typically inhabited by this species and where most prior investigations have been conducted. Mono Craters pikas exhibited a behavioral profile that contrasted significantly with that of pikas found in upper montane regions. Mono Craters pikas were less surface active than pikas in studies at high-elevation sites, although their rate of short-call vocalizations was similar. Mono Craters pikas did not exhibit typical foraging behavior: they fed and collected hay at significantly reduced rates, and did not construct large central-place hay piles. Social behaviors (conspecific aggression, social tolerance, avoidance) were infrequent compared with data from prior studies in upper montane environments. The Mono Craters site appears to be one of the warmest localities in which pikas have been observed. Recorded talus surface temperatures consistently exceeded 30 °C, and temperatures >40 °C were commonly recorded. In contrast, temperatures measured in the matrix of the talus were consistently cooler, and the apparent insulating effect of talus, as measured by the difference between surface and matrix temperatures, was typically most pronounced on the hottest days. Although pika activity was most frequent in early morning, late afternoon, and at night, pikas were also active during the hottest part of the day, presumably because of their ability to behaviorally thermoregulate by retreating into the cooler talus matrix. Data on populations of pikas which inhabit marginal sites can help us understand how pikas and other montane animals might respond in a world of climate change so that we may more effectively plan for their conservation.
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Background Contemporary climate change is affecting nearly all biomes, causing shifts in animal distributions, phenology, and persistence. Favorable microclimates may buffer organisms against rapid changes in climate, thereby allowing time for populations to adapt. The degree to which microclimates facilitate the local persistence of climate-sensitive species, however, is largely an open question. We addressed the importance of microrefuges in mammalian thermal specialists, using the American pika (Ochotona princeps) as a model organism. Pikas are sensitive to ambient temperatures, and are active year-round in the alpine where conditions are highly variable. We tested four hypotheses about the relationship between microrefuges and pika occurrence: 1) Local-habitat Hypothesis (local-habitat conditions are paramount, regardless of microrefuge); 2) Surface-temperature Hypothesis (surrounding temperatures, unmoderated by microrefuge, best predict occurrence); 3) Interstitial-temperature Hypothesis (temperatures within microrefuges best predict occurrence), and 4) Microrefuge Hypothesis (the degree to which microrefuges moderate the surrounding temperature facilitates occurrence, regardless of other habitat characteristics). We examined pika occurrence at 146 sites across an elevational gradient. We quantified pika presence, physiographic habitat characteristics and forage availability at each site, and deployed paired temperature loggers at a subset of sites to measure surface and subterranean temperatures. ResultsWe found strong support for the Microrefuge Hypothesis. Pikas were more likely to occur at sites where the subsurface environment substantially moderated surface temperatures, especially during the warm season. Microrefugium was the strongest predictor of pika occurrence, independent of other critical habitat characteristics, such as forage availability. Conclusions By modulating surface temperatures, microrefuges may strongly influence where temperature-limited animals persist in rapidly warming environments. As climate change continues to manifest, efforts to understand the changing dynamics of animal-habitat relationships will be enhanced by considering the quality of microrefuges.
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How climate constrains species’ distributions through time and space is an important question in the context of conservation planning for climate change. Despite increasing awareness of the need to incorporate mechanism into species distribution models (SDMs), mechanistic modelling of endotherm distributions remains limited in the current literature. Using the American pika (Ochotona princeps) as an example, we present a framework whereby mechanism can be incorporated into endotherm SDMs. Pika distribution has repeatedly been found to be constrained by warm temperatures, so we used Niche Mapper, a mechanistic heat-balance model, to convert macroclimate data to pika-specific surface-activity time in summer across the western United States. We then explored the difference between using a macroclimate predictor (summer temperature) and using a mechanistic predictor (predicted surface-activity time) in SDMs. Both approaches accurately predicted pika presences in current and past climate regimes. However, the activity models predicted 8-19% less habitat loss in response to annual temperature increases of ~3-5°C predicted in the region by 2070, suggesting that pikas may be able to buffer some climate-change effects through behavioral thermoregulation that can be captured by mechanistic modeling. Incorporating mechanism added value to the modeling by providing increased confidence in areas where different modeling approaches agreed and providing a range of outcomes in areas of disagreement. It also provided a more proximate variable relating animal distribution to climate, allowing investigations into how unique habitat characteristics and intraspecific phenotypic variation may allow pikas to exist in areas outside those predicted by generic SDMs. Only a small number of easily obtainable data are required to parameterize this mechanistic model for any endotherm, and its use can improve SDM predictions by explicitly modeling a widely applicable direct physiological effect: climate-imposed restrictions on activity. This more complete understanding is necessary to inform climate-adaptation actions, management strategies, and conservation plans. This article is protected by copyright. All rights reserved.
Article
American pikas (Ochotona princeps) are small alpine lagomorphs and talus obligates with a narrow range of temperature tolerance, along with physiological and ecological characteristics that make them especially vulnerable to local extirpation in the face of climate change. Since their initial colonization of the Great Basin during the Pleistocene geological epoch, the distribution of pikas in this region has become more restricted, with population losses occurring especially in lower-elevation sites characterized by relatively low precipitation and high temperatures. Even where pikas have persisted, many populations are now restricted to higher elevations. We surveyed several sites in the Bodie Hills of eastern California known to have been recently occupied by pikas. Here we report the recent extirpations of 2 of these sites: one small cluster of anthropogenic patches in the historic Masonic Mining District and one natural patch on Masonic Mountain. These extirpations are consistent with those reported in California and across the Great Basin and may indicate the impending loss of pikas from this region due to impacts from global climate change.
Article
American pikas (Ochotona princeps) are of concern with respect to warming montane temperatures; however, little information exists regarding their physiological ability to adapt to warming temperatures. Previous studies have shown that pikas have high metabolism and low thermal conductance, which allow survival during cold winters. It has been hypothesized that these characteristics may be detrimental, given the recent warming trends observed in montane ecosystems. We examined resting metabolic rate, surface activity, and den and ambient temperatures (T-a) of pikas in late summer (August 2011 and 2012) at 2 locations in the Rocky Mountains. Resting metabolic rate was calculated to be 2.02 mL O-2 . g(-1)h(-1), with a lower critical temperature (LCT) of 28.1 +/- 0.2 degrees C. No upper critical temperature (UCT) could be determined from our data; therefore, the estimated thermoneutral zone (TNZ) was 28.1 degrees C to at least 35.0 degrees C (upper experimental temperature). Pikas in this study showed the same bimodal above-talus activity patterns reported in previous studies. Den temperatures in Colorado were correlated with, but consistently lower than, current ambient temperatures. Wyoming den temperatures showed a weak correlation with T-a 20 min prior to the current den temperature. This study is one of few to present data on the physiological response pikas may have to current warming conditions, and the first to perform metabolic measurements in situ. Our data support conclusions of previous studies, specifically MacArthur and Wang (1973, 1974) and Smith (1974), which indicated American pikas may not have the physiological ability to cope with high T-a. Our results also highlight the importance of shaded regions below the talus rocks for behavioral thermoregulation by pikas.