Geoscience Reference
In-Depth Information
Exposure. to. UV-B. radiation. may. be. especially. signiicant. for. those. amphibian.
species. for. which. selection. pressures. have. resulted. in. behaviors. that. expose. them.
to.relatively.large.doses.of.solar.radiation..For.example,.as.stated.previously,.many.
amphibians. lay. eggs. in. open,. shallow. areas. where. they. receive. maximum. expo-
sure. to. sunlight.. This. exposure. can. heat. egg. masses,. which. induces. fast. hatching.
and.developmental.rates.(Stebbins.and.Cohen.1995)..Similarly,.the.larvae.of.many.
amphibian.species.seek.shallow,.open.regions.of.lakes.and.ponds,.where.it.is.warm-
est.and.where.they.can.develop.quickly.(Hokit.and.Blaustein.1997;.Hoff.et.al..1999)..
Rate.of.development.is.especially.critical.for.amphibians.living.in.ephemeral.habi-
tats.. Species. living. in. temporary. habitats. must. metamorphose. before. their. habitat.
dries.or.freezes.(e.g.,.Blaustein.et.al..2001a)..Thus,.amphibians.are.often.faced.with.
conlicting. selection. pressures.. Some. species. must. develop. quickly. enough. before.
their. habitat. disappears.. Therefore,. they. seek. sunlight. where. exposure. to. warmer.
temperatures.enhances.development..Yet.evidence.from.a.number.of.recent.studies.
illustrates.that.many.amphibian.species,.even.those.that.seek.sunlight,.are.sensitive.
to.solar.radiation.(e.g.,.Fite.et.al..1998;.Belden.et.al..2000)..If.exposed,.amphibians.
may.die.or.accrue.sublethal.damage.that.can.severely.affect.them.(discussed.in.the.
following).
EVOLUTIONARY CONSIDERATIONS
Amphibians.have.evolved.behaviors,.morphologies,.and.overall.life.styles.that.have.
allowed.them.to.persist.for.millions.of.years..Such.attributes.were.obviously.a.result.
of.numerous.selection.pressures.over.evolutionary.time..Certain.behaviors.and.life-
history.attributes.have.been.instrumental.in.helping.amphibians.survive.for.millions.
of.years..However,.under.today's.environmental.conditions,.these.same.behaviors.and.
life-history.characteristics.appear.to.be.placing.amphibians.in.harm's.way.(Blaustein.
and.Bancroft.2007;.Bancroft.et.al..2008b)..This.seems.especially.true.in.light.of.cur-
rent. conditions. regarding. the. climate. and. stratospheric. ozone. with. accompanying.
increases. in. UV-B. radiation.. Moreover,. climate. models. suggest. that. under. certain.
scenarios. climate. change. will. affect. amphibian. distribution. and. abundance. more.
than.those.of.mammals.and.birds.
The.causes.for.amphibian.population.declines.are.complex.and.amphibians.may.
be.facing.unprecedented.environmental.changes,.some.of.which.are.intense.and.rela-
tively.new.(e.g.,.increasing.UV-B.radiation.due.to.ozone.depletion;.chemical.contam-
ination;.habitat.alteration)..These.dramatic.changes.in.the.environment.may.interact.
with.more.gradual.changes.in.precipitation.and.temperature.in.unpredictable.ways..
Therefore,.it.is.not.easy.to.assess.the.direction.a.population.will.take.in.the.face.of.
both.natural.and.an.increasing.array.of.human-induced.selection.pressures.
Amphibians. are. limited. by. historical. constraints.. Thus,. adaptations. in. the. face.
of.environmental.change.may.not.occur.before.populations.are.impacted.(Blaustein.
and.Bancroft.2007)..Limited.by.historical.constraints,.many.amphibians.(especially.
frogs. and. toads). must. lay. their. eggs. in. water.. Laying. their. eggs. in. water. exposes.
eggs.not.only.to.aquatic.predators.and.competitors.(Wells.2007).but.to.a.variety.of.
abiotic.parameters,.both.natural.and.human.induced,.that.may.be.especially.harm-
ful.to.them.under.today's.conditions..For.example,.extant.amphibians.face.chemical.
