Journalof
Ecology1996,
84, 31-42
salinity,
plantgrowth,
A positivefeedback:herbivory,
ofan Arcticsalt-marsh
and thedesertification
DIANE
S. SRIVASTAVA*
and R. L. JEFFERIES
of Toronto,25 WillcocksStreet,Toronto,Ontario,Canada M5S 3B2
ofBotany,University
Department
Summary
1 A 2-yearstudyis describedwhichsuggeststhata positivefeedbackprocessresults
in the destructionof salt-marshswardsand the exposureof bare sedimentsat La
PerouseBay, Manitoba,Canada. Lessersnowgeeseinitiatetheprocessby grubbing
and Carex
graminoids(Puccinelliaphryganodes
forrootsand rhizomesof salt-marsh
subspathacea)in spring.The increasedratesof evaporationfromsedimentsbeneath
disturbedor destroyedswardsin summerresultin highsoil salinitiesthatadversely
affectthegrowthof theremaininggrazedplants.
betweensitesin the salt marsh.
2 Above-groundbiomass and soil salinitydiffered
Soil salinitywas inverselyrelatedto above-groundbiomass and shoot densityof
Increasedbiomassled to reducedsoil salinityat siteswhere
Puccinelliaphryganodes.
exclosureswereerected.
3 Plantgrowth,measuredas therateofleafbirthson Puccinelliashoots,was reduced
by highsoil salinitiesat siteswhereexclosureswereerected.
in
4 Leaf demographyof transplantedexperimentalplants of Puccinelliadiffered
amountsof
into siteswithdifferent
1992,but not 1991,betweenplantstransplanted
above-groundbiomass.Leaf birthsand deathswerehighestforplantsgrownin sites
intobare
whereabove-groundbiomasswas highand lowestforplantstransplanted
increased
in 1991and onlymarginally
on leafdemography
sites.Grazinghad no effect
therateof leafdeathsin 1992.
highestat
individualsof Carex subspathaceawas similarly
5 Growthof transplanted
siteswherethe standingcrop of Puccinelliaand Carex was highand was lowestin
bare sites.
6 Algal crusts,whichformedon bare or poorlyvegetatedsites,also reducedthe
growthof Puccinelliaplants.
7 The effectsof thisdeleteriouspositivefeedbackon plant growthare discussedin
relationto changesoccurringin thelessersnowgoose coloniesat La PerouseBay and
elsewhere.
Keywords:geese, graminoids,grazing,halophytes,Hudson Bay, shoot and leaf
standingcrop
demography,
JournalofEcology(1996) 84, 31-42
Introduction
speciesmay
ofa keystone
Changesintheabundance
resultin abruptand rapidchangesin plantpopuprocesses
andecosystem
lations,speciesassemblages
1984;Power1990;
(Paine1980;Mann1982;Bertness
Kerbes et al. 1990; Strong1992). Positivefeedback
bringabout these changes
processesfreauentlv
? 1996 British
Ecological Society
* Presentaddress:CentreforPopulationBiology,Departmentof Pure and Applied Biology,ImperialCollege, Silwood Park,Ascot SL5 7PY, UK.
Correspondence:R. L. Jefferies.
(DeAngeliset al. 1986;Power 1992;Wilson& Agnew
1992). This studyexaminesthe effectsof intensive
foragingby a keystoneherbivore,the lesser snow
goose (Anser caerulescenscaerulescensL.), which
resultsin the progressivedestructionof salt-marsh
vegetationvia a positivefeedbackthatproducessoil
conditionsinimicalforplantgrowth.
Lessersnow geese,whichuse theNorthAmerican
and breedin theeastCentraland Mississippiflyways
ernCanadian Arctic,have increasedfrom1.2 million
to almosttwo millionbirdsbetween1973 and 1989
(Boyd et al. 1982; Cooch et al. 1989). At La Perouse
32
Arcticsalt-marsh
positivefeedback
?
1996British
Ecological Society,
JournalofEcology.
84, 31-42
Bay, Manitoba, a colony of lesser snow geese has
grownfrom2000 breedingpairsin 1968(Cooke et al.
1982) to 23 000 breedingpairs in 1992 (R.H. Kerbes
unpublishedaerialphotographic
censusdata) and has
expandedits geographicalrangeon the Cape Churchillpeninsula(Cooch et al. 1993). Othersnowgoose
colonieson thewestcoast of Hudson Bay (Kerbes et
al. 1990) and at Cape HenriettaMaria in Ontario
(K.R. Abraham, unpublisheddata) have shown a
comparable increase in numbersand in the geographicalextentof thecolonies.The rapidexpansion
in numbersof the greatersnow goose (AnsercaerulescensatlanticaL.) also has considerablyincreased
grazingpressureon arcticvegetation(Gauthieret al.
1995).
A consequenceof increasednumbersof birds in
the regionis thatforagingon salt-marshvegetation
has intensified
overthelast decade (Jefferies
1988a,b;
lacobelli & Jefferies
1991). Faecal densitieson saltmarshswardsat La PerouseBay, whichare an index
of foraging,have increasedbetween
of the intensity
1982 and 1990 from< 1 m-2 week-' to about 10 m-2
week-' duringthe snow-freeseason (R. L. Jefferies,
unpublisheddata). The foragingincludesboth grubbing and grazingby the geese. Grubbingfor roots
and rhizomesof graminoidspecies by both staging
birdsand the local populationof breedinggeese in
of salt marshswards
springresultsin thedestruction
(Jefferies
1988b).
Both thearea of salt marshcoveredby intactgraminoidswardsand theabove-groundbiomassofthese
swards have decreased over the last decade at La
Perouse Bay. Between1985 and 1989,the area covered by intactswardsdecreasedby 50% along permanenttransectstotalling800m in length(R. L. Jefferies, unpublisheddata). Between 1979 and 1991,
above-groundbiomass of intact swards decreased
fromabout 50 g m-2 to about 25 g m2 (Williamset
al. 1993).Similardecreasesin vegetationalcoverhave
been documentedfor othersalt-marshesgrazed by
lessersnow geese on the westcoast of Hudson Bay
(Kerbes et al. 1990).
Inland fromthe salt marsh at La Perouse Bay,
extensivegrubbingby geese around Salix bushes in
springhas ledto exposureofdarkpeatysoil,increased
surfacesoil temperatures
and increasedsoil salinity
(lacobelli & Jefferies
1991). The high soil salinities
have thencaused thedeathof theSalix bushes(lacobelli & Jefferies
1991). High goose numbersnot only
reducegraminoidbiomass of the salt-marshswards
by intensiveforaging,but also maytriggerlong-term
edaphic processesresponsibleforthedeclinein saltmarshgraminoidvegetationvia a positivefeedback
process.
A positivefeedbackoccurs'whenthe responseof
a systemto an initialdeviationof a systemacts to
reinforcethe directionof change' (DeAngelis et al.
1986). Intense grazing and extensivegrubbingby
geese appear to triggersalt-marshdestruction.The
decreasedabove-groundbiomassresultsin increased
soil evaporationrates, and the depositionof salts
in the upper layersof sediment.The resultanthigh
salinitiescan limitthegrowthand survivalof plants,
therebyfurther
reducingabove-groundbiomass and
exposing more soil surface which lead to further
increasesin evaporationand soil salinity.Otherprocesses,such as theformationof thickalgal crustson
the surfaceof sedimentsand soil erosion may also
reduceplantgrowthand survival,and contributeto
thispositivefeedbackcycle.
Validation of thispositivefeedbackbetweensoil
salinityand the salt-marshgraminoidvegetationat
La Nrouse Bay requiresthat:(1) a reductionof the
graminoidsward should resultin an increasein soil
salinityand (2) increasedsoil salinityleads to reduced
graminoidgrowth.Together,these lead to a third
proposal, that graminoidgrowthper shoot system
willbe lowerin areas of reducedbiomass.These predictionsweretested,by experimental
manipulations
in thefieldat La PerouseBay duringthesummersof
1991and 1992.
Methods
STUDY
SITE
AND
SPECIES
The easternsalt marshat La PerouseBay (58?04'N,
94?03'W),Manitoba,is dominatedby swardsof two
short(c. 2 cm tall) non seed-setting
graminoids,Puccinelliaphryganodes
(Trin.)Scribn.& Merrand Carex
subspathaceaWormsk.Puccinelliais both the more
abundant and the more salt tolerant(Srivastava&
Jefferies
1995a,b) of the two species. Both species
undergo extensiveclonal growth,with Puccinellia
producingstolons fromaxillaryshoots and Carex
rhizomes.
tillersfromunderground
DEMOGRAPHY
OF
PuccinelliaPLANTS
OF HIGH AND LOW BIOMASS
IN AREAS
IN 1991
Fieldmethods
Growthof Puccinelliaphryganodes
was examinedat
sitesdiffering
in theamountofabove-groundbiomass
and in soil salinityin a c. 300-mx 400-marea of saltmarsh on the easternshore of the bay. This area
was inundatedbytidesonlyin lateAugustaftertheexwascomplete.Sitesweredividedintotwocateperiment
goriesof biomasssubjectively
(Srivastava& Jefferies
1995b):low biomass(~ 25 g drymatterm2 in midsummer)and highbiomass(~ 50g drymattermr2in
and into threecategoriesof salinity
mid-summer),
whichformeda continuumof increasingsalinity.
In early June 1991 aftersnow melt, 1-mx 1-m
plots were established in high biomass and low
biomassareas and thesalinityof thewaterin thetop
1cm of soil in each area was measuredon 15 August
1991 (cf. Srivastava& Jefferies
1995b)usingan automated flame emission spectrophotometer
(Perkin
33
D.S. Srivastava&
R.L. Jefferies
? 1996British
Ecological Society,
JournalofEcology,
84, 31-42
Elmer Model 3110). Four categoriesof plots were
recognizeda posterioribased on the sodium concentrationin extractedsoil water:highbiomasswith
0-2 g LU Na+, low biomasswith0-2 g L` Na+, low
biomass wherethe sodium concentrationwas 2-4 g
L' Na+, and low biomasswith4-8 g L` Na+. Seven
plotsofeach typeweremonitored.In addition,shoot
biomass(g m-2)
density
(shootscm-2), above-ground
watercontent(gwate,
of Puccinellia,and gravimetric
selected
gdrysoil-') werealso measuredon one randomly
grazedturffromeach category(Srivastava& Jefferies
1995b).
ofsoil salinityand biomasson
Althoughtheeffects
plant growthwas the primaryfocus of this study,
resultsfromprevious investigationsindicatedthat
1989) and plant origin
grazing(Bazely & Jefferies
of Puc(Sadul 1987) were importantdeterminants
designwas used to
cinelliagrowth.A multifactorial
examinetherelativeimportanceofall ofthesefactors,
froma
Two plantsoriginating
and theirinteractions.
highbiomassand twofroma low biomassswardwere
transplantedinto each of the 28 plots to determine
whetherplants froma certain biomass type were
of the site
adapted to local conditionscharacteristic
wherethatbiomass typewas present.At each plot,
one plant of each biomass typewas exclosed from
goose grazing by poultrynetting(50cm x 50cm;
2.5 cm mesh). Each of the 28 plots (four cathad fourplants
egoriesx sevenreplicates)therefore
one ofeach possiblecombinationof orirepresenting
gin (high biomass, low biomass) and grazinglevel
(grazed,exclosed).All plants(c. 2.5 cmin length)were
by
transplantedon 6 June 1991 and were identified
colour-codedtoothpicksinsertedin the soil adjacent
to thetransplant.
Plant growthwas assessed usinga nondestructive
demographicmethod. Leaves (c. 8mm in length),
by a code of one to threesmallIndian
each identified
ink dots, were individuallymonitoredduring the
growingseason. At each census,thecondition(alive,
senescingor dead), size (emergentor fullsized), and
grazinghistory(intactor grazed) of existingleaves
and thebirthofnewleavesand shootswererecorded.
Leaves withnoticeableyellowingof 30-70% of the
surfacearea wereclassed as senescing,and thoseless
than 2 mm long as emergent.The demographyof
plantswas scoredon thefollowingfivecensusdates:
11-13 June,25-28 June,9-10 July,24-25 July,and
7-9 August.Plantsinplotswereexaminedin thesame
randomizedorderon all censusdates.
Leaf birthrates(numberofleavesproducedduring
the studyperiod dividedby days of the study)and
deathrateswerecalculatedforthemainshootofeach
plant, the combined axillaryshoots of each plant,
and the total shoots of each plant (main + axillary
shoots). Senescentleaves presentat the end of the
studyperiodwerescoredas equivalentto halfa leaf
leavesas equivalentto halfa leaf
death,and emergent
birth.These values weresummedas appropriate.
The calculateddemographicparametersincluded
data fromplantsthatdied,otherwiseestimateswould
be biased towardshealthyplants. Mortalityvalues
are presentedforcomparison.
Statisticalanalysis
The demographic(Kirk 1982;Day & Quinn 1989)exANOVA. The
perimentwas analysedwitha three-way
was designedwithsevenplotsnestedwithexperiment
levels,and witheach plot crossed
in fourtreatment
withtwo grazinglevelsand two plant origins.Each
possiblegrazingand origincombinationwithina plot
by four plants; demographicdata
were represented
were averagedover theseplantsto avoid the possiThe dependentvariables
bilityof pseudoreplication.
fortheANOVA analysisare thesix demographicvariables: main shootleafbirths,mainshootleafdeaths,
axillaryshoot leaf births,axillaryshoot leaf deaths,
and total (main + axillaryshoots) leaf birthsand
deaths(notethatcountdata wereused forthisanalysis because of expectationsof a Poisson errorstructure,althoughin the figureswe presentrate data to
enable comparisonwiththe 1992 results).The main
shoot leaf birthand death data were normallydiswithhomogeneousvariances(Shapiro-Wilks
tributed
Bartlett's
test,P > 0.05),so ANOVA analyseswere
test,
The axillaryshoot,and hencetotal
straightforward.
plant demographydata, contained numerouszero
values, and so we used log-linearmodels to relate
the values of the dependentvariablesto theirlinear
predictorsin an ANOVA withPoissonerrorsand a log
link(Crawley1993).Plannedcomparisonsused theF
statisticfordata withnormalerrors,and the X2 statisticfordata withPoissonerrors.The above analyses
were performedusing GLIM software(version4.0,
NAG Ltd, Oxford).
The effecton soil salinityof exclosurefromgeese
was examinedforeach biomass categoryseparately
using a two-way (grazing treatmentx time since
exclosure)ANOVA. Salinitydata (g Na+ L-') werelogto reduce correlationsbetweenmeans
transformed
and variances.
DEMOGRAPHY
BIOMASS,
OF
PuccinelliaPLANTS
LOW BIOMASS
IN HIGH
AND BARE AREAS IN 1992
Fieldmethods
Analysisof the 1991 resultsled us to change some
proceduresfor 1992. Plant growthwas examinedin
siteswhichincludedunvegetated'bare' sites,as well
as highbiomass and low biomass sites.In addition,
biomass,soil salinity,and othersoil variablesat sites
were examined throughoutthe season ratherthan
onlyat theend of theseason.
Each of threebiomass categories(high biomass,
by four
low biomass,bare sediment)was represented
siteswidelyplaced withinthesame studyarea as used
34
Arcticsalt-marsh
positivefeedback
in 1991. Each site contained 22 Puccinelliaplants
transplantedin 50-cmx 50-cmareas in each of two
adjacent plots; one plot of which was exclosed by
poultrynetting(1 m x 1m) to exclude geese. The
centresoftheadjacentareaswereat most90 cmapart,
so all plantswithina siteexperiencedsimilaredaphic
conditions.Unlikethedesignin 1991whichhad a few
plantsat manysites,thisdesignhad manyplantsat a
fewsites.Sinceplantoriginwas foundto havea minor
contributionto variancein 1991, all plantshad the
same origin,a 1-iM2 area of a high biomass Puccinellia-Carexsward.
The plotsforeach biomasscategorywereselected
to representtheaveragesoil salinityof thatbiomass
fromindependent
sites,see Sricategory(determined
1995b).Sincesoil-watersalinityis
vastava& Jefferies
uniformbetweensitesin theearlyspring,plantswere
intosixplotsperbiomasscategory.Four
transplanted
plots per biomasscategorywereselectedfromthese
between
sixplotson 12 July,whensalinitydifferences
biomass categorieswereevident.The salinityof the
soilwaterin theupper1cm ofa 10-cm x 10-cmblock
of soil was measured with a salinity/conductivity
meter(Yellow SpringsInstrumentCo., OH, model
33).
on 20-22 June1992.The
Plantsweretransplanted
very late spring precluded earlier planting. The
ofthetransplanted
plantswas examined,
demography
usingmethodsdescribedearlier,on threecensusdates
about 17 days apart:30 June-2July,16-19 July,and
3-4 August. Most plants did not produce axillary
shoots.Therefore,leaf birthand death ratesof axillaryshootswerenot calculated.Total leaf birthand
death rateswerecalculated,and are similarin value
values forthemain shoot. Uneven
to corresponding
samplesize,caused by theremovalof a fewplantsby
geese,was correctedby randomlyselecting18 plants
perplot foranalysis.
A block of soil (c. 8 cm x 8 cm x 4cm deep) was
removedfromeach of theexclosedand grazedplots
at all siteson 26 June,12 July,21 Julyand 30 July.
Care was takennot to disturbor affecttransplanted
plantsby soil removal.From each soil block,aboveof
groundbiomass(gdrywtm 2), sodiumconcentration
watercontent
extractedsoil-water(g L-'), gravimetric
bulk density(gdrysoilcm-3),and redox
(gwatergdrysoil-'),
as described
potential(Eh values) weredetermined,
elsewhere(Srivastava& Jefferies
1995b). Plots were
exclosedfrom26 June,thefirstdate of soil sampling,
conditions.
so thisdate represents
pre-exclosure
Statisticalanalysis
? 1996British
Ecological Society,
JournalofEcology,
84, 31-42
The experimentwas designedto be analysed by a
three-wayANOVA, withfourplots nestedin each of
threebiomass categories,all crossedby two grazing
treatments(grazed, exclosed). The seasonal demographicdata of totalleafbirthsand deaths,however,
had unequal variancesand were not normallydis-
This
tributed,even afternumeroustransformations.
in low biomass
was largelydue to extensivemortality
and bare sites.On manyplants no new leaves were
producedand all the original(2-3) leaves died,both
of whichled to low variancesin thedata. Data were
Friedman's
analysedby thenonparametric
therefore
test(versionforreplicateddesigns,chi-squaredstatistic;Gibbons 1985). Since Friedman'stestis appropriate only for two-waydesigns,sites of all three
biomass categorieswereexaminedtogetherin a site
(k = 12) by grazingtreatmentdesign. Once a significantplot effectwas established,the effectof
biomass categorycould be examinedby a planned
comparisonbetweenplotsof each biomasscategory.
DEMOGRAPHY
OF
Carex PLANTS
IN HIGH
BIOMASS AND BARE AREAS IN 1992
Given that Carex is more sensitiveto salinitythan
Puccinellia (Srivastava & Jefferies1995a), it was
expectedthatitsgrowthwould be at leastas reduced
betweenhighbiomass and bare sitesas thatof Pucfroma -iM2area of
cinellia.Carex plants,originating
intothegrazedarea ofeach
sward,weretransplanted
of the highbiomass and bare sites(n = 25 per site):
they were interspersed among the previously
describedPuccinelliaplants. Growthwas measured
usingthesame methodsas forPuccinellia.
DEMOGRAPHY OF PuccinelliaIN BARE AREAS
WITH AND WITHOUT ALGAL CRUSTS IN 1992
Three 1-mx 2-m siteswereselectedwitha uniform,
and dryalgal cruston the sediment
thick,blistering
surface.Siteswere l 00m apart and wereexclosed
fromgrazingwithpoultrynetting.At each site,the
removedwitha scalpelfrom
algal crustwas carefully
10
28 circlesof cm diameter.The distancebetweenthe
centresof adjacentcircleswas at least22 cm.
A total of 150 individualPuccinelliaphryganodes
plantsofsimilarsizewereisolatedfroma 15-cmx 20cm turf,in order to minimizegenetic variability
among Puccinellia phryganodesplants (cf. Sadul
1987).On 6 July1992,one Puccinelliaplantwas transplantedintothecentreof each circlefromwhichthe
crustwas removed,save threecirclesper site which
werereservedforsoil sampling(describedbelow). In
addition,a totalof 25 plantsper sitewereplantedin
an intactalgal crustbetweenthe circlesand at least
15cm fromthe centreof the nearestcircle(n = 25
per site).
plantsper treatment
Puccinelliaplantswas
Growthof thetransplanted
measured using demographictechniquesdescribed
earlier.Census dates were 13 days apart, on 7 July,
20 July,and 2 August1992. Sincevirtuallyno plants
producedaxillaryshoots,onlythedemographicparametersof total leaf birthand total leaf death rate
were calculated.Non-parametricstatisticalanalyses
wereused fordata of leafbirthand deathrates,since
35
D.S. Srivastava&
R.L. Jefferies
these data were both non-normallydistributed
(Shapiro-Wilks test, P < 0.01) and had heterogeneousvariances(Bartlett'stest,P < 0.05).
On 27 July,7-cmx 7-cmx 4-cm-deepblocks of
soil werecollectedfromeach of the threeremaining
circlesper site and under the adjacent intactalgal
crusts.Redox potentials(Eh values),fromjust below
the subsurfaceto a depthof 2 cm, were determined
bytheuse ofa platinumelectrodeforeach soilsample.
The top 1cm of soil of each sample was used for
soil salinity,soil bulk densityand gravimetric
water
contentmeasurements
as describedearlier.
Results
SOIL
SALINITY
IN SITES
OF DIFFERENT
BIOMASS
The soil salinityof siteswas inverselyrelatedto the
amountofgraminoidvegetation(measuredas abovegroundbiomass or shoot density)in both 1991 and
1992(Fig. 1). Highbiomassplotswerethuslesssaline
thanlow biomassplots,whichin turnwerelesssaline
8
a
6
4
_
o
O
thanbare plots (Table 1; a moredetailedanalysisis
in Srivastava& Jefferies
1995b). These differences
in
soil salinitybetweenbiomass categoriesare maintainedovermostofthegrowingseason (Srivastava&
Jefferies
1995b).
Such a correlation between soil salinity and
biomassdoes not necessitatea causal link.To testif
biomass reducessoil salinity,we used goose grazing
to manipulate above-groundbiomass: geese were
allowed to graze one half of high biomass, low
biomass,and bare plots(n = 4 plotsperbiomasscategory)whiletheotherhalfof each plot was exclosed
fromgeese.Beforeexclosure,thegrazedand exclosed
areas had similarvalues of biomass and soil salinity
(Fig. 2). One monthaftertheexclosureswereerected,
theexclosedareas of highbiomass plotshad 40 g
m 2 more biomass than grazed areas (Fig. 2d). This
was associatedwitha substantialreduction(33% on
average)inthesoilsalinityofexclosedareasrelativeto
grazedareas (Fig. 2a; ANOVA on postexclosuredates,
P < 0.02). Low biomassand bareareas wheregrazing
had no effect
on biomass(Fig. 2d) serveas a control,
and no effecton soil salinitywas detected(Fig. 2;
ANOVAS withsalinity
data, P > 0.05). In all plots,soil
salinityincreasedduringthe growingseason (date
inabove ANOVAS, P < 0.01),withno interaction
effect
withtreatment
(ANOVAS, P > 0.05). It is difficult
to
imaginewhat other aspects of grazingmighthave
caused this outcome and it thereforeappears that
biomassremovalincreasessoil salinity.
2
PuccinelliaGROWTH
0
0
SALINITY
2
0
4
6
8
Shootdensity
(shootscm)
25
_
b
C
20
0
15
10
a10
0
o
00
20
40
60
80
100
Biomass (g mr)
? 1996British
Ecological Society,
JournalofEcology,
84, 31-42
Fig. 1 (a) Sodium concentration
in soil wateras a function
of shoot densityof sites on 15 August 1991. Sites were
divided into four treatmentcategoriesfor analysis: The
regressionline(r = -0.61) was calculatedfora logarithmic
formofsodiumconcentration.
Low biomasssiteshad a soilwatersalinityof either0-2 (A), 2-4 (0) or 4-8 ([]) g L-'
Na+, and highbiomasssites(*) had a soil-watersalinityof
0-2 g L-' Na+ . (b) Sodium concentration
(0) in soil water
as a functionof sitebiomassforsitesin 1992(averagedata
for season used). The regressionline (r =-0.85)
was calculatedfora logarithmic
formof sodiumconcentration.
AND
IN SITES
DIFFERING
IN
BIOMASS
The positivefeedbackhypothesisrequiresthatplant
growthis reducedby increasingsalinityand consequentlyis lower in areas of reduced biomass. We
tested these requirementsby examining the leaf
demographyof transplantedplantsin sitesdiffering
in biomassand salinity.
In 1991,sitesweredividedintofourtreatment
categories based on site biomass and soil salinityas
describedearlier. For each Puccinelliaplant, leaf
birthsand deaths were counted for the main shoot
and axillaryshootsseparately,and thensummedfor
thewholeplant,resultingin six relateddemographic
measures(see Methods).The treatment
effect
was significantforall the demographicvariables(ANOVAS,
P < 0.05) except main shoot deaths (ANOVA,
P = 0.065). All othereffects,
includingtheeffectsof
grazingand plantoriginand all possibleinteractions,
werenot significant
(ANOVA, P > 0.05).
This treatmenteffectdoes not reflectdifferences
betweenhighbiomass and low biomass plotsper se.
If plantperformance
in all low biomass plots,irrespectiveof soil salinity,is comparedto that in high
biomassplots,thereis no significant
difference
in any
demographicvariable(plannedcomparison,usingan
36
Arcticsalt-marsh
positivefeedback
in sitesin (a) 1991 and (b) 1992. Data on
of sites,and Puccinelliaperformance
Table 1 Vegetationand soil characteristics
in soil waterand soil watercontent,werecollectedon (a) 15 August
shootdensity,vegetationbiomass,sodiumconcentration
1991(n = 7 sites)and (b) 30 July1992(n = 4 sites).Data forthesefourvariablesare presentedforgrazedareas only,as these
axillaryshoot production,and total leafbirth
variableswereaffectedby exclosure(see text).Data on Puccinelliamortality,
ratewerecollectedoverthegrowingseason and are forgrazedand exclosedareas combined,as exclosuredid not affectthese
variables(see text).For thesethreevariables,n = 56 in 1991(a) and n = 144 in 1992(b). Errorbars are + 1 SE
1991
High biomass
Low biomass
0-2 g Na+ L-'
2-4gNa+ L-'
4-8 g Na+ L-'
all sites
1992
High biomass
Low biomass
Bare
Shoot
Biomass
density
(shootscm-2) (gm-2)
Sodiumin
soil water
(g L-')
Soil water
content
4.64 + 0.49
1.11 + 0.15
1.18 + 0.08
0.95 +
0.82 +
0.71 +
0.83 +
1.46 +
2.93 +
5.90 +
3.43 +
1.45 +
1.11 +
1.22 +
0.26 +
0.15
0.15
0.11
0.08
60.4 + 2.1
21.9 + 4.5
0.2 + 0.1
(gwatergsoi-')
0.23
0.28
0.43
0.44
4.4 + 0.5
20.3 + 1.7
21.7 + 1.7
1234
1234
une 26
1234
a
I
4%
41%
0.135 + 0.008
0.11
0.09
0.12
0.07
7%
7%
14%
9%
73%
61%
39%
58%
0.189 +
0.184 +
0.108 +
0.160 +
0.65 + 0.02
0.55 + 0.02
0.49 + 0.04
3%
86%
94%
10%
1%
0%
0.135 + 0.008
0.189 + 0.016
0.184 + 0.017
1234
ocetato
I
0
30
~25-II
-
5~~~~~~~~~~~~~I
June26
1234
21
July12
July
1992
C
July30
1992~~~~
1234
12 -
0
June26
21
July12
July
1992
~~~~~
~~~~12341 ~~~~~ ~~~~~
123 4 199
120
heboms o br ste
andclosedsybosasabv.
Open
~
90 -
~
I
60050 -
I
*
.0~~~~~~~~~40
d
? 1996 British
Ecological Society,
JournalofEcology,
84, 31-42
June26
21
July12
July
1992
30
July
123 4
30
July
1 23 4
?
ad oisno
asnelgilhon70-
nlo
(c1irces
ihbims
bnc.d bv-rudboaso
24frbten()
0
1234
~30-
nolaeaubeestswihhihboms()owb020
~ ~ ~ ~ ~ ~ ~ ~~~~~~~~~~:9~~~~~~~~0
Cu
1 2 34
1 234
1234
40
I?~~~~~~~~
Fig. 2 oiu
0.016
0.017
0.010
0.009
grownin less saline sites (0-2 and 2-4 g L` Na+;
planned comparison,using an empiricalscale parameterwhere appropriate,P < 0.05; Fig. 3). As a
consequence,more plants died and fewerproduced
axillaryshoots in the more saline sites(Table 1). In
Puccinelliaplantscharacteristically
pot experiments,
reducethe productionof axillaryshoot leaves more
than those on main shoots when salinityincreases
empiricalscale parameterwhere appropriate,P >
0.05), contraryto thepositivefeedbackhypothesis.
By contrast,thereis evidencefora negativeeffect
of
of salinityon plant growth,anotherrequirement
biomass
Among
low
hypothesis.
thepositivefeedback
plots,plantsin themostsalinesites(4-8 g L-' Na+ in
extractedsoil water) had substantiallylower values
forall demographicvariablesthan those forplants
6
Puccinellia
plantswhich Total leaf
birthsper
Puccinellia produced
Puccinellia
plantswhich axillary
died (%)
shoots(%) plantperday
0
June
26
Jillge)u
1111as
July12
July21
1992
30
July
in soil waterat numberedsiteswithhighbiomass (a), low biomass (b), or whichare bare (c),
Fig. 2 Sodium concentration
with(open symbols)or without(closed symbols)grazing.Plots wereexclosedon 26 June1992. Note thatthe verticalaxes
differ
between(a), (b) and (c). (d) Above-groundbiomassof highbiomass(circles)and low biomass(triangles)numberedsites.
Open and closed symbolsas above. The biomassof bare siteswas negligibleand so is not shown.
37
D.S. Srivastava&
R.L. Jefferies
Mainshootleaves
shootleaves
Axillasy
0.13
0.16
0.12
0.14
r
0.12
Irr
80.11
0.1
0.1
0.09
0.08
1
:1
3
;,0~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
0.06
0.09
010
0.08
0.02
0.14
i
Low
Lowh Lowln
(0to2)
(21o4) (4to8)
Site (ima
High Low
Low
(214)
(01to2)
ln
(4 08)
ugh orlow)andsodium
concentration
ofsoilwater(g 1 )
Fig. 3 Seasonal changes sn leaf demography for Puccinellia
plantsofhighbiomassorigin
(C],*) andlowbiomassorigin
(0, *),
grazed (C], 0) and exclosed (U, *) areas of sites
in 1991. Dotted lines separate sites in differenttreatment
categories
(based on sitebiomassand the sodiumconcentration
ofsoilwater).Demographic
parameters
werecalculated for the period 11 June-9 August 1992. Error bars
are ? 1SE (untransformed
data).Notethatthevertical
axes
differ
between
thegraphsofmainshootandaxillary
shoot
demography.
(Srivastava & Jefferies
l995a). The same patternis
evidentamong the low biomass plots: the ratio of
axillaryshoot to main shoot leaf birthsdecreased
from1.21 in sitesof low salinity(0-2 g L-l Na+) to
0.69 in sitesof highsalinity(4-8 g Ll Na+).
In summary,in 1991leafbirthratesdid not differ
betweenhigh and low biomass plots, despite low
biomassplotsbeingmoresalineand salinityreducing
plant growth(Fig. 3). This apparent contradiction
may be resolvedby consideringthe effectsof intraspecificcompetition.If highand low biomassplotsof
the same salinity(0-2 g L' Na+) are compared,the
low biomass plots have more leaf births(in total;
Table 1), presumablybecause thelowerplantdensity
of such plots leads to the reducedintraspecific
competition.In 1992,leaf demographywas examinedin
plants fromthe four representative
sites for each
biomass category.Since axillaryshoot production
was verylow,onlytotal(mainshoot + axillaryshoot)
leafdemographywas examined.Total leafbirthand
death rates both varied considerablybetweensites
(Friedman test,sites not differentiated
by biomass
category,P < 0.001). However,bothrateswerestrikinglyhigherin highbiomassplotsthaninlow biomass
and bare plots,and the leaf birthrate was slightly,
but significantly,
higherin low biomassplotsthanin
bareplots(Wilcoxonranksum,Z-statistic,P < 0.05;
Fig.4). Leaf death rates were similarbetweenlow
biomassand bare plots (Wilcoxonranksum,Z-statistic, P > 0.05). These resultssupport the positive
feedbackhypothesis:plantgrowthis greaterin plots
withmore vegetation.This may reflectthe low soil
salinityof the morevegetatedplots. Indeed,the salinityof plots was a much betterpredictorof plant
mortality(logarithmicformof Na+; r2= 0.89) and
total leaf birth rate (logarithmicform of Na+;
r2= 0.94) thanplot biomass,or forthatmattersoil
water content, redox potential, or bulk density
(Fig. 5). Specifically,
in stepwiseregressions,
no linear
or logarithmicformof thesesite variableswas significant
afterlog Na+ had beenentered.
Grazing did not influencethe leaf birthrate of
plants(Friedmantest,P > 0.05). Bycontrast,theleaf
death rate of plants was slightlybut significantly
higherin exclosed areas (Friedman test, sites not
differentiated
by biomasscategory,P < 0.01).
0.120.12-a
.
0.12
ab
.
b
0.1
0.08
0102
0.06-00
CU
0.04
4
002 i
I
'
0 -v----0
high
? 1996British
Ecological Society,
JournalofEcology,
84, 31-42
0.06
0.0
x
:0.02
r-i-
low
bare
high
low
bare
Sitebiomass
Sitebiomass
Fig.4 Leaf birthrates(a) and death rates(b) of Puccinelliaplantsin 1992 as a functionof sitebiomass (highbiomass,low
biomass,bare) and grazingpressure(solid symbolsreferto grazedplants;open symbolsreferto exclosedplants).Errorbars
are + 1 SE (untransformed
data).
38
Arcticsalt-marsh
positivefeedback
120
100
80
60
~40
20
0v
0
5
10
15
0.12
20
b
~0.1
a0.08
0.06
0.04
0.02
0
5
10
15
20
Meansodium
concentration
ofsoilwater(g 1
Fig.5 Relationshipsbetweenthe sodium concentrationin
soil waterin 1992 (mean of data of 26 Juneand of 30 July)
and (a) deathsof Puccinelliaplantsas a percentageof total
plants (r2 = 0.89; n = 24) and (b) total leaf birth rate
(r2 = 0.93; n = 24). Each symbolrepresentsa percentage
value (a) or a mean value (b) for 18 plantsin eithergrazed
or exclosed areas of sites. Regressionlines are calculated
formof sodiumconcentration.
usinga logarithmic
THE GROWTH
SUBSPATHACEA
OF PLANTS
THE
EFFECT
OF
OF ALGAL
CRUSTS
PuccinelliaIN
BARE
Therewas no overalleffectof eitherthe presence
of algal crusts,or a siteby algal crustinteractionfor
the above soil variables (MANOVA, Wilks Lambda,
SITES
All plantsof Carex died in bare sites,comparedwith
17-37% deathsin the highbiomass sites.Like Puccinelliain 1992, Carex growthwas less in areas of
reducedbiomass.
Althoughall Carex plantsin bare siteshad died by
the end of the firstsamplinginterval,Puccinellia
plantswerestillalive in threeof the fourbare sites.
Similarly,in grazedplots at highbiomasssites,Puccinelliadeaths at the end of the studyperiod were
lowerthanthoseof Carex (0% vs. 17%,
consistently
5% vs. 29%, 0% vs. 24% and 6% vs. 37% forhighbiomasssites1, 2, 3 and 4, respectively).
GROWTH
P > 0.05).
P > 0.05).
OF Carex
AT DIFFERENT
deaths(Friedmantests,P < 0.001; Table 2). Almost
all leaf birthsoccurredon main shoots (main and
axillaryshootleaves are therefore
not examinedseparately).By the end of the study,leaf deaths had
exceededleafbirthsto suchan extenton someplants
that no living leaves remained;these plants were
declareddead. More plantsdied in areas withintact
algal crusts(64-92% of plants died, dependingon
site) than in areas withoutalgal crusts(24-64% of
plantsdied; G-test,G3 = 21.2, P < 0.005; Table 2).
Plantsgrownat different
sitesdiffered
significantly
in the rate of leaf births(Friedmantest,P < 0.001)
butnotdeaths(Friedmantest,P > 0.10). Differences
in plant growthbetweensites are linkedto spatial
variationin salinity.Site C, withthe lowestrate of
leaf birthsand axillaryshoot production,was more
saline(mean = 19.1g Na+ L' soil water)thansiteA
(mean= 11.8 g Na+ L-' soil water) or site B
(mean = 16.4g Na+ L` soil water).The sitesdiffered
in soil salinity(ANOVA, P < 0.05, folsignificantly
lowing overall effectof site on soil variables in
MANOVA,WilksLambda, P < 0.05) butnotsoil water
content,redox potentialor bulk density(ANOVAS,
ON THE
SITES
Plantsgrowninalgal crusts,comparedto thosegrown
withoutcrusts,had farfewerleafbirthsand moreleaf
Discussion
The threepredictions
arisingfroma positivefeedback
betweenpoor plant growthand high soil salinity
(Fig. 6a) were:(1) a decreaseinabove-groundbiomass
is associatedwithan increasein soil salinity;(2) high
soil salinitiesreduceplantgrowthand as a result(3)
plant growthis less at sites with reduced aboveground biomass. The evidence summarizedbelow
givesconsiderablesupportforthefirst
twopredictions
and some supportforthelast prediction.
Soil salinityappears to be substantially
influenced
bytheamountofabove-groundbiomass.Reductions
in biomass, such as that caused by heavy grazing,
result in increased soil salinity(Fig. 2). Localized
areas ofbaresediment,
usuallycaused bygoose foraginmid-summer,
whilenearby
ing,are thushypersaline
patchesof intactvegetationare associated withless
salinesoils (Fig. 1; Srivastava& Jefferies
1995b).The
Table 2 Growthof transplantedPuccinelliaphryganodes
plants in threesites,withand withoutalgal crusts(mean + SE,
n = 25)
SiteA
no crust
?
1996British
Ecological Society,
JournalofEcology,
84, 31-42
TLB (leavesplant-'day-')
1.18 + 0.20
ASLB (leavesplant-'day-') 0.12 + 0.09
TLD (leaves plant-'day-') 2.36 + 0.14
Plantdeaths(%)
0.28
Site B
SiteC
crust
no crust
crust
no crust
0.32 + 0.09
0.00
2.6 + 0.13
0.64
1.24 + 0.23
0.12 + 0.09
2.1 + 0.11
0.24
0.48 + 0.12
0.00
2.82 + 0.14
0.64
0.36 + 0.10 0.20 + 0.07
0.00
0.00
2.24 + 0.14 2.58 + 0.11
0.64
0.92
crust
39
D.S. Srivastava&
R.L. Jefferies
a. Positive feedbackin salt-marshdesertification
intense
spring
grubbing
andsurnrner
grazing
bysnowgeese
. .
reduced
graminoid
biomass
\<
reduced
growth
graminoid
soilsalinity
algalcrustformation, increased
erosion
rates
highevaporation
b. Competitionvs. salinityeffects
shootdensity
+
intraspecific
competition
soil
salinity
+
grarminoid
growth
c. Positive and negativefeedbacksof the salt marsh
_
soil
salinity
_
plant
biomass
andcover
grazing
by
geese
~~~~~~+
plant
growth
(NAPP)
inintact
sward
-
Fig.6 Diagramsoftheinterrelationships
between
feedback
andplantcompetition
processes,
forthegrazedsalt-marsh
swardsat La PerouseBay.(a) A positive
feedback
between
soil salinity
and graminoid
growth,
triggered
by intense
andresulting
inlossofsalt-marsh
gooseforaging
vegetation.
feedback
between
soilsalinity
and gram(b) Thispositive
inoidgrowth
is opposedby a negative
feedback
between
andgraminoid
intraspecific
Theposicompetition
growth.
tiveand negativesignsrepresent
thetypeof correlation
between
thefactors
linkedbyarrows,
and thedirection
of
thearrowsindicates
in both(b) and (c). (c) The
causality
feedback
soilsalinity
positive
between
andgraminoid
cover
in thesaltmarshis linkedto a positive
feedback
between
andprimary
inpatchesofintact
goosegrazing
productivity
swards(described
intext).Arrows
twovariables
connecting
in bothdirections
withthesamesignindicatea positive
A negative
feedback.
feedback
occurswherethesignsabove
thetwoarrowsare different.
Notethattheeffect
ofplant
biomasson thesizeofthegoosecolonywillonlybe evident
overmorethanoneseason(dottedarrows),
as opposedto
theother
interactions
so a timelagisexpected
(solidarrows),
inthispotentially
feedback.
regulatory
negative
?) 1996British
Ecological Society,
JournalofEcology,
84, 31-42
salinityof the La Perouse Bay soils is derivednot
directlyfromtidal sources,but ratherfromburied
marinesediments
whichunderliemuchoftheHudson
Bay Lowlands (Price & Woo 1988). Vegetation
reducestheevaporativewaterloss fromsurfacesediwe reportelsewhere
ments,as shownby experiments
(lacobelli & Jefferies
1991; Srivastava & Jefferies
1995b),thusslowingthe upwardmovementof salts
throughthe soil. SimilarresultsfromNew England
salt marsheshave been reportedby Bertnesset al.
(1992). Exposure of marshsoil to directsolar radiationresultedin elevatedsoil salinityin bare patches.
At bothlocations,theintactswardsmaintainedconditions conducive for plant growthby restricting
evaporationand thebuildup of salinity.
At La PerouseBay thegrowthof graminoidswas
depressedbyhighsoil salinities.This is evidentin the
leaf demographyof plants in low biomass sites in
1991 (Fig. 3), and in theplantmortality
data of 1992
(Fig. 5). Similar decreases in growthof these graminoidscan be generatedby wateringpottedplants,
grownin thefield,withsalinesolutions(Srivastava&
Jefferies
1995a).
In both years thereforethe firsttwo predictions
of the positivefeedbackhypothesiswere supported:
biomass reducedsalinity,and salinityreducedplant
growth.In 1992,the finalpredictionheld: leaf birth
rates of plants were greatestat high biomass sites,
intermediate
at low biomasssites,and lowestin bare
sites. In 1991, by contrast,leaf birthrates did not
differ
betweenhighand low biomasssites,despitelow
biomasssitesbeingmoresalineand salinityreducing
plant growth.It is possible that,in termsof plant
growth,the less intenseintraspecific
competitionof
low biomasssitescompensatedforthegreatersalinity
of such sitesin 1991 but not 1992 (Fig. 6b). Similar
interactionsbetweenintraspecific
competitionand
salinitytoleranceare evidentin thedemographyof a
Hordeumjubatum L. population (Badger & Ungar
1991).
This difference
in resultsbetweenyearscould be
due to differences
in experimental
designor meteorologicalfactors.Giventhatplantsgrownseparatelyat
La Perouse Bay, as part of otherexperiments,
also
showedmuchgreatersalttoleranceand axillaryshoot
productionin 1991 than in 1992 (Srivastava& Jefferies 1995a), it seemslikelythatweatherdifferences
betweenyearsare responsible.Indeed,theweatherin
1991and 1992differed
dramatically:1991was one of
the warmestand wettestyears on record,whereas
1992 was one of the coldest and driest(Churchill
WeatherOfficeMonthlyReports,Environment
Canada). On most days betweenMay and August,the
differencein temperaturebetween the two years
exceeded5 'C. Soil salinitywas similarbetweenyears
(Srivastava & Jefferies
1995b), but it may be that
salinitystress was exacerbated by the cold temperaturesand drywindsthatcharacterized1992.
The presenceofalgal crustsreducedshootsurvival
and leaf productionin bare sites. Similardry and
toughalgal crustsare also foundin low biomasssites,
butnotin highbiomasssites(thesoil surfacebeneath
the vegetationmat is moist).It is unlikely,however,
thatthepresenceofalgal crustsis entirely
responsible
forpoor plantgrowthin low and bare sitescompared
to that in high biomass sites. If this was the case,
differences
in demographyof plants between low
biomassand bare sites(1992) and withinlow biomass
sites(1991) would be minor(sinceall siteshave algal
crust)and would notbe explainedbyvariationin soil
salinity(soil salinitywas not correlatedwith algal
crustpresencein 1992).
Algal crusts are common in many salt-marshes
(Golubic 1973; Adam 1990), and thick dried
40
Arcticsalt-marsh
positivefeedback
algal crustshave been noted in nontidalsaline areas
(Ehrlich& Dor 1985). Blisteringalgal crusts,similar
to those at La Perouse Bay, are formedon welldrainedsedimentswithprolongedexposureto drying
conditions(Golubic,1973). Under theseconditions,
highratesof anaerobicdecompositionoccurand the
consequentaccumulationofCO2 underthealgal crust
causes blistering
(Golubi6 1973). The anaerobicconditionscoupledwiththemechanicallifting
ofthedrying algal crust,which may have exposed roots of
Puccinellia, could have led to the death of plants.
Althoughthepresenceof algal crustsdid not appear
to affecteithersoil watercontentor soil salinitysigin thisstudy,sucheffects
have been shown
nificantly
in other studies (Golubic 1973; Price et al. 1989;
Graetz 1991).
SALT-MARSH
GOOSE
? 1996British
Ecological Society,
JournalofEcology,
84, 31-42
VEGETATION
AND
THE
SNOW
COLONY
Over a periodof years,a netdecreasein theamount
of salt-marshvegetationis anticipatedas a resultof
the positivefeedback,with the rate being strongly
influencedby yearlyweatherpatterns.Since Puccinellia and Carex are thepreferred
forageforlesser
snow geese, and goslingsshow poor growthwhen
feedingon othertypesofforage(Gadallah & Jefferies
1995), this decrease in graminoidvegetationrepresentsa decreasein theavailabilityof qualityforage
forthegoose populationat La PerouseBay. Previous
reductionsof available forageoverthelast decade at
La PerouseBay are thoughtto have caused a marked
declinein goslinggrowthand survival(Cooch et al.
1991;Franciset al. 1992;Williamset al. 1993)and an
increasein posthatchdispersal(Cooch et al. 1991;
Cooch et al. 1993). Furtherloss of vegetationmeans
thatthesetrendscan onlybe expectedto continue.
Althoughposthatchdispersalis increasing,many
breeding pairs show strong breeding-sitefidelity
(snow geeseoftenliveup to 8 years)and have notleft
the La Perouse Bay salt marshes,despitelow brood
survival(Williamset al. 1993). The rateof reduction
in vegetation may thereforeexceed the rate of
reduction in grazing pressure,and the predicted
increasein desertification
(see below) willexaggerate
thisasynchrony.Since goose grazingis concentrated
in swards with high above-groundbiomass, any
reductionin grazing pressurewill not necessarily
affect
low biomassand baresitesinanycase (as shown
cf. Fig. 2).
by theexclosureexperiment,
At La Perouse Bay, between1979 and 1983,grazing by geese was reportedto increasesubstantially
both the nitrogencontentand the netabove-ground
primaryproduction(NAPP) of graminoidswards
(Cargill & Jefferies
1984; Bazely & Jefferies
1985,
1989). The increasein foragequantityand quality
caused by grazingallowed forfurther
goose grazing,
creatinga positivefeedbackbetweengoose grazing
and NAPP (Jefferies
et at. 1986; Fig.6c). This stimu-
lationof NAPP by grazingwas reflected
in a higher
rateof totalleafbirthsin graminoidplants(Kotanen
& Jefferies
1987; Bazely & Jefferies
1989). In this
study,however,totalleafbirthsof Puccinelliain high
biomass swardswerenot increasedby grazing.This
suggeststhat the positivefeedbackbetweenNAPP
and goose grazingis no longerdominatingvegetation
dynamics.
Experiments
withcaptivegoslingshaveshownthat
the enhancementof NAPP by goose grazingat La
levelsof grazPerouseBay is greatestat intermediate
ing pressure(Hik & Jefferies
1990; Hik et al. 1991).
The grazing-NAPP positive feedback therefore
as thegrazingpressureincreases
becomeslesseffective
(NAPP increaseswithgrazingpressureat a decreasing
rate).The salinity-biomass
feedback,however,results
in salinityincreasingwithgrazingpressure,at an ever
increasingrate,and so it becomesprogressively
more
effectiveas the grazinglevel increases.There is a
spatial componentto thesechanges,as soil salinity
increaseswiththe size of a bare patch (Srivastava&
Jefferies1995b). A similar situation has been
describedfora New England salt-marsh,wherethe
area of a bare patchis positivelycorrelatedwithsoil
salinityand negativelycorrelatedwithplant growth
(Bertness1991;Bertnessetal. 1992;Shumway& Bertness 1994). We suggestthat the increase in goose
densities,and hence grazingpressureover the last
decade at La Perouse Bay has resultedin the vegfrombeingdominatedby
etationdynamicsswitching
the grazing-NAPPpositivefeedbackto beingdominated by the salinity-biomass
positivefeedback.The
alternativestates of salt-marshvegetation development)at La Perouse Bay have been discussedby
Hik etal. (1992). Althoughthisplant-herbivore
interactioncan maintainsalt-marsh
graminoidswardsvia
a grazing-NAPPpositivefeedback,we do not regard
thisas a 'switch'process,as describedby Wilson &
Agnew(1992). The latterdiscussprocesseswherethe
modifiestheenvironment
plantcommunity
makingit
moresuitableforthatcommunity.
At La PerouseBay
plantcommunity
developmentis stronglydependent
on ratesofisostaticupliftand patternsofforagingby
geese. Ultimatelyupliftand the build up of soil
organic material facilitatethe establishmentof a
willow-grassland
communityin place of these saltmarshgraminoidswards.
Systemsdominated by positive feedbacksoften
show threshold effects(DeAngelis 1992). Rapid
changes in the state of the system occur when
thresholdsare crossed.As a resultof increasesin the
offoragingat La PerouseBay a vegetational
intensity
thresholdhas been crossed leading to a decreasein
the survivalof goslingsin recentyears.In contrast,
the cause of the increasedadult lesser snow goose
population over the last two decades is strongly
linked to human activitieselsewhere(use of agriculturalcrops by geese, refugia,declinein hunting)
(Franciset at. 1992;Warren& Sutherland1992).The
41
D.S. Srivastava&
R.L. Jefferisv
impoverishmentof such terrestrialarctic coastal
systems,
whichis ultimately
a consequenceof human
activities,is leadingto 'desertification'
(Graetz 1991)
ofthesesystems.The processesresulting
in theloss of
vegetationon thesetoastal flatsare similarto those
thathave led to destruction
ofvegetationbylivestock
in the Sahel regionof Africa(Graetz 1991). In both
cases weatherpatternsexacerbatetherateof destruction. Recoveryof both systemsis long-term,
beyond
the life expectancyof the presentcohorts of the
different
herbivorespecies.
Acknowledgements
We gratefully
acknowledgethe fieldassistanceof J.
Championand J. Morrowand othermembersof the
La PerouseBay Tundra ResearchStation.We thank
thefollowingorganizationsforfinancialsupport:the
Natural Sciencesand EngineeringResearchCouncil
of Canada, the Canadian WildlifeService(Environment Canada) and the Departmentof Indian and
NorthernAffairs
oftheGovernment
ofCanada. Prof.
M. J. Crawleygave much helpfulstatisticaladvice.
Mrs C. Siu kindlytypedthemanuscript.
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? 1996British
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JournalofEcology,
84, 31-42
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Received9 January1995
Revisedversionaccepted26 June1995