Geoscience Reference
In-Depth Information
may. be. constrained. by. upper. range. limits. of. host. plants. (e.g.,. Merrill. et. al.. 2008;.
Chen.et.al..2009).
MODELING EFFECTS OF UPHILL RANGE SHIFTS ON BIODIVERSITY
Climate.envelope.models.have.been.widely.used.to.predict.changes.to.species.dis-
tributions.based.on.different.climate.change.and.dispersal.scenarios,.and.to.model.
the. consequences. for. extinction. rates. (e.g.,. Williams. et. al.. 2003;. Thomas. et. al..
2004).. These. models. have. usually. used. coarse-scale. data. on. climate. and. species.
distributions. to. produce. coarse-scale. predictions.. Fine-scale. topographic. variation.
in. mountainous. regions. means. that. many. high-elevation. species. are. restricted. to.
much. narrower. elevation. ranges. (or. ranges. of. climate. conditions). than. is. evident.
from.coarse-scale.grid.cells;.as.a.result,.many.bioclimate.models.may.have.under-
estimated.the.threat.posed.by.climate.change.to.the.persistence.of.montane.species.
(Trivedi.et.al..2008)..Conversely,.coarse-scale.climate.surfaces.might.overlook.local.
thermal. refugia. produced. by. processes. such. as. cold-air. drainage. or. shading. from.
local.topography.(see.Luoto.and.Heikkinen.2008;.Randin.et.al..2009).
The.equivalent.of.climate.envelope.models.for.mountain.regions.have.used.the.
current. elevation. ranges. of. species. as. measures. of. their. thermal. associations,. and.
have.predicted.future.regional.range.sizes.assuming.that.species.maintain.these.ther-
mal. associations. as. the. climate. warms. (e.g.,. Wilson. et. al.. 2005;. Raxworthy. et. al..
2008)..Inevitably,.because.of.the.roughly.conical.shape.of.mountains,.these.models.
predict.reductions.in.regional.distribution.sizes.as.species.are.forced.to.higher.eleva-
tions.. There. may. be. few. opportunities. for. such. species. to. colonize. other. climati-
cally.suitable.mountain.ranges,.because.such.areas.are.often.separated.by.lowland.
expanses.that.are.unsuitable.both.in.terms.of.habitat.and.climate.(Ohlemüller.et.al..
2006,.2008)..A.model.incorporating.the.elevation.ranges.of.landbird.species.(87%.
of.all.avian.species).predicted.that.400-550.landbird.species.would.face.extinction.
from.2.8°C.warming,.based.on.reductions.in.species.distribution.areas.(Sekercioglu.
et.al..2008)..However,.it.remains.necessary.to.identify.the.causal.inluence.of.climate.
over.species'.ranges.to.be.sure.whether.estimates.of.extinction.risk.from.bioclimate.
models. are. likely. to. be. accurate;. the. models. could. either. over-. or. underestimate.
threat.because.of.local.adaptation.(Harte.et.al..2004),.biotic.interactions.(Davis.et.
al..1998),.or.the.effects.of.spatial.scale.(Trivedi.et.al..2008;.Luoto.and.Heikkinen.
2008;.Randin.et.al..2009),.among.other.factors.(see.Pearson.and.Dawson.2003.for.a.
general.critique).
Climate.change.is.likely.to.have.a.major.impact.on.tropical.biodiversity.through.
its. effects. on. species'. elevational. ranges.. In. the. Tropics,. weak. latitudinal. thermal.
gradients. mean. that. species. are. much. less. likely. to. shift. to. higher. latitudes. than.
higher.elevations..As.a.result,.three.key.processes.might.threaten.biodiversity.in.the.
Tropics.(Colwell.et.al..2008):.
lowland biotic attrition
,.where.there.is.a.net.loss.in.
species. at. low. elevations. because. no. species. associated. with. hotter. conditions. are.
available. to. colonize. when. others. contract. to. higher. elevations;.
range-shift gaps
,.
where.future.elevational.ranges.of.species.do.not.overlap.with.their.current.ranges;.
and.
mountaintop extinctions
,. because. no. climatically. suitable. elevations. remain.
after.climate.change..Assuming.that.species'.thermal.associations.remain.the.same,.
