Geoscience Reference
In-Depth Information
Subsequent.studies.of.this.system.relect.a.more.mechanistic.approach..To.explain.
patterns.of.loss,.Beever.et.al..(2010).posited.alternative.mechanisms.of.thermal.stress.
that.might.be.experienced.by.pikas.in.this.region,.including.chronic.heat-stress.(e.g.,.
high.average.summer.temperature),.acute.heat-stress.(e.g.,.a.daily.maximum.above.
28°C),. and. acute. cold-stress. (e.g.,. a. daily. minimum. below. -5°C).. To. estimate. the.
thermal.stress.experienced.by.pikas.in.the.25.locations.discussed.previously,.Beever.
et.al..used.thermal.sensors.to.record.temperatures.within.the.crevices.of.the.talus..
Sensors. recorded. temperatures. every. two. to. four. hours. during. 2005-2006.. These.
data.were.used.not.only.to.characterize.recent.temperatures,.but.also.to.model.tem-
peratures.within.each.site.over.the.past.six.decades..These.hindcasts.were.developed.
by. regressing. data. from. the. in-situ. sensors. on. data. from. nearby. weather. stations.
within. the. Historical. Climate. Network.. Hindcasts. for. each. site. were. divided. into.
two. periods,. 0-30ybp. and. 30-60ybp,. in. order. to. characterize. both. the. prevailing.
climate.(e.g.,.summer.temperature.averaged.over.both.periods).and.climate.change.
(e.g.,.the.difference.between.periods.in.average.summer.temperature)..In.all,.Beever.
et. al.. derived. 12. potential. predictors. of. extinction:. four. metrics. of. thermal. stress,.
each.calculated.from.data.on.recent.climate,.prevailing.climate,.and.climate.change..
Of.these.12.variables,.the.best.predictor.was.chronic.heat-stress.(the.average.of.daily.
maximum.and.minimum.temperatures.during.the.summer.months)..But.the.potential.
for.effects.of.cold.stress.were.also.apparent.in.this.analysis:.the.number.of.daily.min-
ima.below.-5°C.was.a.better.predictor.of.extinction.than.the.number.of.daily.minima.
below.0°C,.suggesting.increasingly.detrimental.effects.of.extreme.cold..In.a.post-hoc.
analysis.to.investigate.this.trend,.Beever.et.al..found.that.the.number.of.daily.minima.
below.-10°C.was.by.far.the.best.predictor.of.local.extinction.(Figure 5.3).
Ironically,.this.cold-adapted.species.may.be.stressed.most.by.cold.temperatures,.
at.least.within.the.Great.Basin..The.rationale.for.testing.a.cold-stress.hypothesis.was.
twofold:.winter.snowpack.has.been.in.decline.throughout.the.western.United.States.
(Mote.et.al..2005),.and.there.is.some.evidence.that.pika.mortality.is.higher.where.
winters. are. colder. and. snowpacks. are. thinner. (Smith. 1978;. for. similar. evidence.
related.to.other.ochotonids,.see.Smith.et.al..2004;.Morrison.and.Hik.2007)..Beever.
et.al..hypothesized.that.pikas.might.die.of.exposure.to.extremely.low.temperatures.
without. the. thermal. insulation. provided. by. suficient. snow. cover,. especially. if.
snowpacks.are.in.decline.due.to.global.warming..Although.they.did.not.ind.strong.
evidence. for. effects. of. climate. change,. their. evidence. for. effects. of. both. chronic.
heat-stress. and. acute. cold-stress. suggest. that. future. climate. change. will. impact.
pikas.if.summer.temperatures.continue.to.rise.and.snowpacks.continue.to.thin..It.
is. also. important. to. keep. in. mind. that. any. apparent. effects. of. temperature. may.
be. indirect. (e.g.,. allowing. the. emergence. of. a. disease). rather. than. directly. caus-
ing.thermal.stress.(Beever.et.al..2010)..A.better.understanding.of.the.relationship.
between. climate. and. the. distribution. of. the. American. pika. awaits. a. synthesis. of.
related.research.across.the.species'.range.(Millar.and.Westfall.2010).
As. with. broadly. “alpine”. species,. tracking. distributional. change. for. sensitive.
“arctic”. species. is. also. complex,. involving. unique. species. traits. and. habitat. fea-
tures.in.that.region..In.the.Arctic,.range.limits.are.largely.dictated.by.bioenergetics.
(Humphries.et.al..2004)..Relatively.few.species.can.survive.year-round.in.extreme.
northern.climates,.and.those.that.do.so.have.adaptations.allowing.for.energy.storage.
