Geoscience Reference
In-Depth Information
To. date,. many. sandy-beach. ecologists. have. addressed. structural. questions. via.
descriptive-correlative. studies. at. various. spatial. scales.. At. small,. within-beach.
scales,. an. important. distributional. paradigm. is. that. of. across-beach. zonation. (see.
McLachlan. and. Jaramillo. 1995. for. a. review).. Zonation. schemes. are. based. either.
on.physical.factors.(e.g.,.degree.of.saturation.of.the.sediments).or.biological.distri-
butions.. However,. the. term. “zonation”. may. be. inappropriate. because. the. across-
shore.distributional.boundaries.of.sets.of.species.may.not.coincide.(Hacking.1996)..
Furthermore,.many.species.migrate.tidally.across.the.shore.(Brown.and.McLachlan.
1990). causing. any. biologically. deined. “zones”. to. vary. with. tide. height.. There. is,.
however,.a.separation.of.high-shore.air.breathers.from.lower-shore.water.breathers.
with.the.low.shore.being.richer.in.species.than.the.high.shore.(Hacking.1996;.James.
and.Fairweather.1996).
At. larger,. among-beach. scales,. there. are. ecological. differences. associated. with.
beach. geomorphology.. Dissipative. beaches. have. more. species,. greater. abundance.
and.biomass,.and.more.soft-bodied.species.(e.g.,.polychaete.worms).than.relective.
beaches.(McLachlan.et.al..1993)..In.extreme.cases,.relective.beaches.may.support.
no.macrofaunal.species.at.all.(McLachlan.et.al..1993)..Patterns.of.species.richness.
are. correlated. with. composite. indices. such. as. the. Beach. Index. of. McLachlan. and.
Dorvlo.(2005),.which.is.based.on.tide.range,.beach.slope,.and.particle.size.
Because.most.patterns.are.correlated.with.physical.factors.rather.than.biological.
factors,.it.is.likely.that.physical.factors.are.most.important.in.structuring.the.eco-
logical.assemblages,.i.e.,.they.are.“physically.controlled”.rather.than.“biologically.
accommodated”. (Brown. and. McLachlan. 1990;. Jaramillo. and. McLachlan. 1993)..
Indeed,.Brown.et.al..(2000).state.that.wave.action.is.the.super-parameter.controlling.
South.African.sandy-beach.ecology,.suggesting.the.applicability.of.the.“autecologi-
cal. hypothesis”. (Noy-Meir. 1979;. McLachlan. 1990). in. which. species. in. physically.
controlled.habitats.respond.individualistically.to.physicochemical.factors.
Similarly,. the. ecological. differences. between. relective. and. dissipative. beaches.
were.attributed.to.the.hydrodynamic.force.in.the.swash.zone,.i.e.,.the.“swash.exclusion.
hypothesis”.(McLachlan.et.al..1993)..Recently,.this.has.been.extended.into.the.“mul-
ticausal.severity.hypothesis”.(Brazeiro.2001).and.the.“habitat.harshness.hypothesis”.
(Defeo.et.al..2003)..The.former.incorporates.the.additional.physical.factors.of.grain.
size,.organic.matter,.and.erosion-accretion.dynamics,.while.the.latter.combined.the.
autecological.hypothesis.with.the.swash.exclusion.hypothesis..It.predicts.a.“decrease.
in.the.performance.of.sandy.beach.macrofauna.descriptors.at.the.community.(lower.
species.richness,.diversity,.abundance).and.population.(lower.abundance,.growth.in.
weight.and.length,.fecundity,.reproductive.output,.and.higher.mortality.rates).levels.
from.dissipative.to.relective.conditions”.(Defeo.et.al..2003,.p..353)..A.consequence.
of.physical.control.is.that.the.loss.of.any.particular.species.would.have.little.effect.on.
others,.unlike.biologically.accommodated.assemblages.where.cascading.effects.may.
follow.the.loss.of.key.species.(e.g.,.Schiel.et.al.
2004)..If.true,.this.may.moderate.the.
ecological.effects.of.climate.change.on.beaches.
Although.the.claim.of.physical.control.is.plausible,.it.is.also.questionable.because.
the.explanatory.models.have.not.been.tested.via.controlled.experimentation,.largely.
because.of.feasibility.problems.in.an.energetic.and.unstable.environment..Moreover,.
recent. studies. have. indicated. the. inluence. of. biological. factors,. especially. on.
