Download Interspecific Competition and Species Co

Nordic Society Oikos
Interspecific Competition and Species Co-Occurrence Patterns on Islands: Null Models and the
Evaluation of Evidence
Author(s): Edward F. Connor and Daniel Simberloff
Reviewed work(s):
Source: Oikos, Vol. 41, No. 3, Island Ecology (Dec., 1983), pp. 455-465
Published by: Wiley on behalf of Nordic Society Oikos
Stable URL: http://www.jstor.org/stable/3544105 .
Accessed: 21/02/2013 12:18
Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .
http://www.jstor.org/page/info/about/policies/terms.jsp
.
JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of
content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms
of scholarship. For more information about JSTOR, please contact [email protected].
.
Wiley and Nordic Society Oikos are collaborating with JSTOR to digitize, preserve and extend access to Oikos.
http://www.jstor.org
This content downloaded on Thu, 21 Feb 2013 12:18:56 PM
All use subject to JSTOR Terms and Conditions
1983
OIKOS 41: 455-465. Copenhagen
and speciesco-occurrence
patterns
competition
Interspecific
on islands:nullmodelsand theevaluationof evidence
Edward F. Connor and Daniel Simberloff
andspeciesco-ocD. 1983.Interspecific
competition
Connor,E. F. andSimberloff,
- Oikos41:
ofevidence.
on islands:nullmodelsandtheevaluation
patterns
currence
455-465.
rangesas evidence
geographic
Severalrecentstudieshaveadducednon-overlapping
excludeoneanother.
We havepreviously
pointedout
thattwospeciescompetitively
exclusive
is unusually
ofspecies'co-occurrence
thattoshowthata particular
pattern
withthatexpected
theobservedpattern
reason),one mustcontrast
(forwhatever
ofoneanother,
allspecies,
andonemustconsider
werespeciesplacedindependently
exclusive
ranges.Furthermore,
notonlythosethatappeara priorito haveunusually
ofspecies'distribulargenumber
evenwhenone is able to showthatan unusually
non-distributional
in the absenceof independent
exclusive,
tionsare improbably
fortheseodd disotherreasonableexplanations
evidencecapableof eliminating
causeisresponsible
oranyotherspecific
onecannotinfer
thatcompetition
tributions,
fortheobservedpattern.
and
alternative
procedures,
Otherworkershave critizedour methods,
suggested
andillusdamnedtheuse ofnullmodelsinecology.We respondto thesecriticisms
Wealsoreaffirm
ourviewthat
procedures.
trateproblems
thatbesetthesealternative
nullhypotheses
in ecologyareuseful.
CharlottesE. F. Connor,DeptofEnvironmental
Sci.,ClarkHall,Univ.of Virginia,
Deptof BiologicalSci.,FloridaStateUniv.,
ville,VA22903, USA. D. Simberloff,
FL 32306,USA.
Tallahassee,
B HeKOIopbmOOBpeMeHHbDCHCcJeipBaHw
HCfQTIb3Yi1TC5 HenepeepiaeBK4iOro- f
Beta HcxJarpa4qeclce
apeanL KacKgpKa3aTe~jicTBo, xTO xa KowypuyaIe
iT apyr ypyra. Mu npeOrapmemao ycTaHoBHTm,rIO xSs Toro, qrio( noica3aaa,
BFOB
CoBaoerT
scL
pieqaeMvCm
cmcXTaeTcs
(no uotIMo B KOHKPeTH~bI cYqao
&mibnpHwnnaM),cnenyT oonocTaBm
Ha6naemwo cyTyawo C TaKog, me npanonaraeTCsl He3aBHCxHe pacrpmemHHe
BO=B xyI OT xwyra; rlB 3TcmCnIewrT
0pacllipmnaTm BCe BHil, a He TobKO Te, KoTop
aIpHOpHO HmiOTH&NHME
nloKa~aTh,'ITO HeOO.HO
6oco6neHH
apeam. Bonee Toro, xwak ecu Bo3mmmO
o6ooo6leHo, rIpo oT01TTBmH
6owre
acmu B~upmQx apeaJnoB HenpaBpQm6Ho
cYIOoO6HOrO HCHe CB5[3aHHOIO c pacrpeeneHHem,
He3asBHCHNrO
rpKa3aTemcTBa,
Kxhom xpyrue pe3oHIwe o61IcHeHSI BTHXIQgI03HEixapeaToB, HeJYbS3f3leJnaTb 3aY
H m i HH5a cneWImecKaq npwwHa mme-T OTHm HIe K
xmeme, 'mr KOHam
ScHInnbHa6.IQna~y cnyqawo.B e HcanefopaTeni4 xpHTm oBam Hail! MeT,
3ynamTTpHaTMHbile meTow H OTptIaR ripHHPMocTh 0-rHrlOTObIB 31CKOJIC. MU
3mTHBThTePHZHTHBHL1em
sTy KPHTHK H BOanPOH3BQPWM
BOawmw~aem
nu. mai
Tam
YTBeKgaeM
HalWlyTOWY 3PeH}Iq 0
1pKaeHWMVCThB 3KQTK3H
O-noroi.
Accepted3 March1983
? OIKOS
29*
45 5
OIKOS 41:3 (1983)
This content downloaded on Thu, 21 Feb 2013 12:18:56 PM
All use subject to JSTOR Terms and Conditions
We disagreewiththeseconclusions.
First,
byisolating
thesethreeexamplesfroma largegroupof possible
141speciesofbirds
evidenceexiststhatin- examples(Diamond(1975) reports
Althoughsoundexperimental
occursin nature(Wilbur1972, or9870 pairsofspeciesintheBismarck
Islands)he has
terspecific
competion
wideerrorrate"and
Neill1975,Coen et al. 1981,CrowellandPimm1976, failedto controlthe"experiment
theprobability
ofmaking
inflated
hasgrossly
Mungerand Brown1981, and others),the inference therefore
thatthese
thenullhypothesis
in structuringa TypeI error,rejecting
thatit is of overwhelming
importance
arenotunusualwheninfactitis true.This
on ambigu- distributions
is based primarily
ecologicalcommunities
thattheprobability
of obous indirect
evidence(e.g.,Diamond1975,1978). For is analogousto concluding
fiveheadsina rowfromthetossofa faircoinis
example,Terborgh(1971) and Terborghand Weske taining
(1975) estimate
thatbetween37% and71% oftheele- (0.5)5 = 0.0313,wheniftheeventis embeddedin a
of
vationallimitsof birdsalongtheVilcabambaof Peru longerseriesof cointosses,say 10, theprobability
are determined
by interspecific
competition.
Theyar- obtainingat least one run of fiveheads is actually
rive at thisestimatesolelyby inspectionof known 0.1875 (6 X (0.5)5). The pointis that,yes,thesearexcluvegetation
discontinuities
and eachspecies'elevational rangements
isolatedpostfactoare improbably
thatare sive,butonewouldexpectbychancealonetofinda few
distributions
frequency
distribution.
Abutting
to improbablepairwisearrangements
when inspecting
notcoincident
withvegetation
changesare inferred
Widerspecies 9870 pairsofspecies.Second,to statethata particular
exclusion.
be maintained
bycompetitive
exclusive
andtherefore
is unusually
rangeson another
nearbymountain
rangearealsoattri- geographic
pattern
theobserved
onemustcontrast
release.Evidenceof aggressive competitively
produced,
butedto competitive
withthatexpectedwerespeciesindepenencounters
betweenspeciesor rangeexpansionsafter distribution
that
Nor dently
removalofa putative
is notpresented.
competitor
placedon islandssubjectonlyto constraints
is it shownthatindividual
otherrelevantbiologically
and environspecies'habitatpreferencesincorporate
do notaccountfortheobserveddegreeof elevational mentally
structure.
determined
Lastly,even werewe
ofspecies'
exclusivity
(cf.DueserandHallett1980).Is thiskindof ableto showthatan unusually
largenumber
basisto inferthatspecies' geographic
indirect
evidencea sufficient
patternsare improbably
exclusive,in the
distributions
are determined
Have we absence of independentnon-geographical
evidence
bycompetition?
inanyinstance
eliminated
otherbiologically
reasonable capable of eliminating
otherreasonableexplanations
in- fortheseodd distributions,
hypotheses?
Couldnotabutting
distributions
reflect
one cannotinferthatcomevolvedhabitatpreferences,
interaction petition
causeis responsible
forthe
dependently
oranyotherspecific
witha predator
or parasite,or purechance?In earlier observedpattern(Pielouand Pielou 1968,Simberloff
papers(ConnorandSimberloff
1979,1983,SimberloffandConnor1981).
andConnor1979,1981),we haveattempted
to answer Previously
(Connorand Simberloff
1979), we outthesequestionsbyexamining
one classofindirect
evi- lined a procedureto assess the null probability
of
dencethatis oftenadducedas evidenceofinterspecificobserving
a particular
amountof exclusivity
in species
data on geographical
Our goals co-occurrence
competition:
patterns.
patterns.
Based upontheanalysesperif a formed
were first,to developa procedureto determine
withthisprocedure,
weconcluded
thatadducing
particular
geographical
pattern
wasindeedodd;second, a roleforcompetition
inshaping
species'co-occurrence
if theseunusualpatterns
to determine
could bestbe patterns,
over and above whateverrole competition
ofinterspecific
explainedbyan hypothesis
competition,mayplayindetermining
howmanyspeciesanislandhas
or whether
otherreasonableexplanations
wereequally or howmanyislandsa speciesoccupies,is verydifficult
plausible;andthird,
to illustrate
whatwe believeto be sincemanynon-overlapping
distributions
are expected
the pitfallsof makingcausal inferences
fromindirect fornon-competitive
reasons.Recently,
Diamondand
evidence.
pattern
Gilpin(1982, 1983), Gilpinand Diamond(1982) and
Commonwisdomholdsthatifinterspecific
competi- Wrightand Biehl (1982), have criticized
our procetion is an important
forceshapinggeographical
dis- dures.We wouldlike to reiterate
our pointsand our
thencompetitive
tribution,
exclusionshouldtendto analytical
in an attempt
procedure
to relievewhatwe
produce non-overlappingspecies' distributions.perceiveto be theconsiderable
confusion
thathas ariDiamond(1975) suggests
thatthisis in facttrue,and sen overour interpretation
of theseissues,and to rewellexemplified
particularly
bywhathe terms"chec- spondto ourcritics.
kerboard"distributions
of birdspeciesamongislands.
He presents
threeexamples(Figs20, 21, 22) involving
two species in each of three genera of birds
2. The Connorand Simberloff
procedure
(Pachycephala,Ptilinopus,and Macropygia)in the Bis1. Introduction
marckIslands.He contendsthatthegeographical
patternwithineach genusis improbably
exclusiveand
therefore
primafacieevidencethatcompetition
is responsiblefortheexclusivity.
Connorand Simberloff
(1979) represented
the distributon
ofspeciesamongislandsas binary
0-1 matrices
in whicheach columnrepresented
an islandand each
row a species.A species'presenceon an islandwas
456
OIKOS 41:3 (1983)
This content downloaded on Thu, 21 Feb 2013 12:18:56 PM
All use subject to JSTOR Terms and Conditions
nor and Simberloff
(1979) suggested
severalpossible
causes forthe excessiveexclusivity
observed,among
themcompetitive
exclusion,predation,geographical
and unsettled
speciation,
We concludedby
taxonomy.
thatadducinga roleforcompetition
stating
in shaping
quartets,etc. thatshare 0, 1, 2, 3, . . . N islandsis then species'co-occurrence
overand above whatpatterns
simplya counting
problem(albeitnota smallone for ever role competition
mayplay in determining
how
largespeciosearchipelagos).
Fromthesecountstheac- manyspeciesan islandhas or how manyislandsa
tualfrequency
distribution
oflevelsofexclusivity
canbe speciesoccupies,is verydifficult
sincemanyexclusive
generated
forpairsor largergroupsof speciesforany patterns
are expectedfornon-competitive
reasons.
archipelago.
At issue,however,is to whatexpectedamountof
shouldtheobservedamountbe compared? 3. Criticismsof the Connor and Simberloffapproach
exclusivity
One could generatean expected distribution
of
3.1. Owlsandhummingbirds
exclusivity
simplyassuming
independent
placement
of
species(a totallyunconstrained
matrix).Such a dis- BothDiamondandGilpin(1982,1983) andWright
and
tribution
wouldbe "trulynull"withrespectto species Biehl (1982) claimthatthe Connorand Simberloff
butwouldlack realism.The factthat (1979) procedure
co-occurrences,
tendsto dilutetheeffect
ofcompetismallislandshavefewerspeciesthando largeislandsis tive exclusion by ".... submerginginstancesof comwellestablished
(ConnorandMcCoy1979).Thatsome petitorswithexclusivedistributions
in an irrelevant
speciesarerestricted
tojusta fewislandsandsomeare massof data fromecologically
remotepairsof species
." (Diamondand Gilpin1983). Wright
widelydistributed
is alsowellknown(Darlington
and Biehl
1957,
Cain 1944). To generatean expecteddistribution
of (1982) feel thatone should ".... restrictthe analysisto
exclusivity
thatignoressuchubiquitous
patterns
leads specieswhicharepotential
competitors".
Whiletheasonlyto a further
ofthecausesof anyap- semblyrulesproposedbyDiamond(1975) towhichwe
confounding
parentodd geographic
pattern,
anda markedtendency wereresponding
werenotrestricted
to guildsor "poto findthatrangesof speciespairsoverlapmorethan tentialcompetitors",
we agreethatitwillbe difficult
to
expected.
discernthe effectsof competition
on biogeographic
one cangenerate
Alternatively,
an expecteddistribu- patterns
froma community-wide
analysis.In fact,we
tion of exclusivity
thataccountsforspecies-areare- said exactlythat". . . statisticaltestsof properlyposed
lationships
and/orspeciesoccurrence
distributions,
To nullhypotheses
willnoteasilydetectsuchcompetition,
generatean expecteddistribution
of exclusivity
thatis sinceit mustbe embeddedin a massof non-competinullwithrespectto speciesco-occurrences,
yetposses- tivelyproduceddistributional
data" (Connorand Simses otherrelevant
biologically
andenvironmentally
de- berloff
1979).
termined
structure
(such as species/area
relationships However,restricting
theanalysistoguildsor "potenandspeciesoccurrence
distributions),
ConnorandSim- tialcompetitors"
is nota simplematter,
sinceto do so
berloff
(1979) generated
a seriesofbinary
with requiresassigning
matrices
speciesto guildsor a determination
rowand columnsumsidenticalto thoseof theactual of whichspeciesare in fact"potentialcompetitors".
matrix.
However,to assurethattheexpectedmatrices Partitioning
all thespeciesina community
intoguildsis
were nullwithrespectto speciesco-occurrence
pat- no easytaskandrequiresdataon resource
use foreach
terns,presencesof species(l's) wereplacedindepen- species(Root 1967). Krebs(1978) contendsthat"no
dentlyand uniform
randomly
subjectonlyto rowand one has yetbeen able to analyseall the guildsin a
columnsumconstraints.
Thisprocedure
wasperformed community".
Thosestudiesthathavedelineated
guilds
repeatedlyto generatethe expectedfrequencyof have reliedon massiveamountsof fielddata (Root
speciespairsor triosand itsvarianceforeach levelof 1967,Feinsinger
1976,AlataloandAlatalo1979).For
exclusivity,
i.e.,howmanypairsortriosareexpectedto no singleguild,muchlesstheentireaviancommunity,
share 0, 1, 2, . . . islands.The actual and expecteddis- have Diamond and Gilpin(1982, 1983) eitherdetributions
ofexclusivity
werethencomparedusinga x2 lineatedtheguildor presented
theevidencenecessary
statistic.
tojustify
theguildboundaries.
Furthermore,
evenwere
Employing
sucha procedure,
ConnorandSimberloffone able to delineatea singleguildsatisfactorily,
its
(1979) examinedthreefaunas.Theyobservedforthe geographic
of speciesco-occurrence
pattern
couldtell
New Hebrides birds a close agreementbetween us littleaboutcompetition
in general.Not onlycould
observedand expecteddistributions
of exclusivity
and different
guildsbe structured
by different
forces,but
fortheWestIndiesbirdsandbatsan excessivenumber also one wouldneedto knowthenullprobability
that,
of exclusivearrangements.
However,in thelattertwo of the N guildsexamined,one or moreguildswould
examplesa verylargenumberof allopatricarrange- producea pattern
thatwouldhavebeenstatistically
odd
mentswasexpected,
eventhoughtheexpectedwassig- hadonlythatsingleguildbeenexamined.
Ifa fewguilds
nificantly
lowerthantheactuallevelofallopatry.
Con- are chosenrandomly,
and notselectedbecauseof the
indicatedby a 1 and an absenceby a 0. Exclusiveor
exclusivedistributions
could thenbe recogpartially
nizedas pairsorgroupsofspecies(rows)inwhichno,or
just a few,matchesof l's occuracrossthe columns.
Determining
theactualnumberof speciespairs,trios,
457
OIKOS 41:3 (1983)
This content downloaded on Thu, 21 Feb 2013 12:18:56 PM
All use subject to JSTOR Terms and Conditions
of
a competitive
likelihoodof detecting
effect,
stronger absencescan be explainedby thejointdistribution
themarginal
totals.However,justas incontingency
inferences
canbe made.
tathenumberof
the analysisto only "potentialcom- ble analysis,forcertainfixedmargins
Restricting
can be so smallthatstatistical
petitors"
is equallyproblematic,
sinceone mustdeter- possiblerearrangements
minea prioriwhichof thetotalpossiblesetof species significance
cannotbe achieved.We have already
of speciesco-occurWhilean accu- shownthatthe possiblepatterns
pairsinvolve"potentialcompetitors".
rateassessment
toof thisis aptto be as laboriousas de- rencesachievablesubjectto a fixedsetofmarginal
thatare statistically
odd.The
lineating
guilds,one couldexaminegroupsarbitrarilytalscan includeinstances
forthe West Indies
defined
orany observedpatternsof exclusivity
bytaxonomy,
bodysize,billmorphology,
In factwedidso for birdsandbats(ConnorandSimberloff
othereasilyobserved
characteristic.
1979) arestatisunusualarrangements
in all threeexampleswe examined, tically
are
surprising.
taxonomic
families
Statistically
and Vuilleumier
and Simberloff
a also achievablefortherowandcolumnsumsobserved
(1980) performed
similar
fortaxonomic,
andrandomly forthe New Hebridesbirds(Connorand Simberloff
analysis
ecological,
partitioned
paramobirdfaunas.In anyevent,one can- 1983), but the observedpatternof speciesco-occurnotfocussolelyonthegroupof"potential
competitors" rencesintheNewHebridesbirdsisnotoneoftheseodd
sinceto concludethata particular
levelof exclusivitypatterns.
among"potential
is odd,one mustshow
Wereone able to demonstrate
competitors"
thatcompetition
had
thatsuch a patternis odd relativeto the level of affected
themarginal
thenfixing
distributions,
themarexclusivity
observedforspeciesnot consideredto be ginalsto equal the observedtotalswouldbe at least
"potential
competitors".
somewhatcircular.As we mentioned
above,without
We agreethatan analysisbasedon guildsor "poten- fixing
someofthemarginal
totals(rowsums,forexamtial competitors"
wouldbe interesting.
In eitherin- ple), it is impossible
to generatea reasonablenullexstance,to determine
the correctnullprobability
of a pectation
ofexclusivity
foranyparticular
If
archipelago.
particular
patternof speciesco-occurrences
requires we mustconcludethatthemarginal
totalsare affected
one to examinethe entirefauna,not merelythose bycompetition,
andtherefore
cannotfixthetotals,then
speciesthought
mostlikelyto be competitors.
itbecomesimpossible
to testthenullhypothesis
thata
ofspeciesco-occurrence
particular
pattern
isnotstatisticallyodd. If thisis whatGrantand Abbott(1980)
3.2. Hidden structurein the marginalconstraints
thentheyarearguing
intend,
thatitis impossible
totest
Diamondand Gilpin(1982, 1983) claimthatbycon- an hypothesis
the causes of geographical
concerning
the expectedrearrangements
straining
of speciesdis- arrangements
solelyfromdata on geographical
artributions
to satisfy
theobservedmarginal
totals(row rangements.
Butthisis ourverypoint!One cannotfrom
and columnsums),one builds"hiddenstructure"
into geographical
dataaloneeasilydetermine
thecausesofa
thenullexpectation
ofexclusivity.
In otherwords,they geographical
pattern.
feelthatcompetition
hasalreadyaffected
thenumber
of
Wrightand Biehl (1982) feel thatfixingthe row
speciesfoundon an islandandthenumber
ofislandsa sums,the numberof islandson whicheach species
speciesoccupies.Although
DiamondandGilpin(1982, occurs,is a reasonableapproachto generating
a null
1983)present
noevidencethatcompetition
hasaffected expectation
ofspeciesco-occurrence
patterns
(see disthemarginal
totalsor thepattern
ofspeciesco-occurr- cussionof alternative
proceduresbelow). However,
ences,it is surelypossiblethatcompetition
does affect theypointoutthatbyalso fixing
thenumber
ofspecies
thespeciesrichness
ofan islandorthegeographic
range on an island,as we did,our procedure
failsto detect
of a species,as we clearlystated(Connorand Simber- significant
in speciesco-occurrence
aggregation
patloff1979: 1136). This need not necessarily
translate terns.Once again,but for a different
reason,it is
intoalteredco-occurrence
patterns,
however.
Themar- suggested
thatour procedureis not"trulynull".But,
ginaldistributions
of the row and columnsums,not theintent
ofouranalysiswasnotto determine
ifthere
singlemarginal
totals,settheconstraints
forrearrangingwas any evidencefornon-randomness
in speciesdisthepatterns
ofspeciesco-occurrences.
One mustshow tributions
amongislands.We firmly
believethereis! A
thattheshapeorlocationsofthemarginal
distributionsvisual inspectionof a presence-absence
matrixof
are affected
bycompetition
to concludethatfixing
the speciesinalmostanyarchipelago
willrevealthatspecies
to equal theobservedvaluesbuildsin "hidden areaggregated.
margins
Thisis largely
becausethephysical
constructure".
ditionsand habitatsnecessary
fortheestablishment
of
GrantandAbbott(1980) raisethespecterthatfixing speciesinparticular
taxaarealso aggregated.
Thisvery
themarginal
totalsfortheexpectedrearrangements
to aggregationis at least partiallyresponsiblefor
equal theobservedis alwayscircular.
Eitherthisis not species-area
relationships.
Ourprocedure
wasdesigned
true,ortheentirepremise
ofcontingency
tableanalysis to examinethepattern
of speciesco-occurrences
after
is wrong,since,in a manneranalogousto contingencysuchaggregation
had been factored
outbyaccounting
tableanalysis,we are askingif the co-occurrence
as- forspecies-area
relationships,
notto ask merelyifany
pectsoftheobservedpattern
ofspecies'presences
and non-randomness
is evidentin speciesco-occurrence
458
OIKOS 41:3 (1983)
This content downloaded on Thu, 21 Feb 2013 12:18:56 PM
All use subject to JSTOR Terms and Conditions
each
It is interesting
to notethatthe procedures all pairsof speciesequallyratherthanweighting
patterns.
thata randomarrangement
of
advocatedbyDiamondandGilpin(1982, 1983) andby pairby theprobability
as excluoutthisaggrega- thatpairwouldhaveproduceda distribution
Wright
andBiehl(1982) failto factor
theycontendthata
tion,and thattheytherefore
concludethatspeciesare sive as thatobserved.Essentially
distributed
non-randomly
allopatric
speciesis lesslikely
among islands. But, the pairofwidelydistributed
thanis a pair
observednon-randomness
is a tendency
towardsag- to havearisenbyindependent
placement
distributed
gregation,
notexclusion.Wherein thisresultis there of narrowly
allopatricspecies.Again,we
wereresponding
to Diamond's(1975) assembly
evidenceofcompetition?
rules,
certainspecies
whichstatenothingabout weighting
groups,onlythatexclusivedistributions
occur,and do
3.3. Checkerboards
as Diamond
so forcompetitive
reasons.One canweight
Diamondand Gilpin(1982, 1983) questionthepower and Gilpin(1982, 1983) suggestusingourprocedure,
thebookkeeping
choresassociatedwith
ofourtestbyclaiming
thatitfailsto recognize
as non- butperforming
random". . . the simplest,clearest,and most non-ran- counting
thenumberofexclusive
pairsor triosslightly
dom pattern
causedby interspecific
. . .", differently.
For each rearrangement
of speciesprecompetition
checkerboard.
namelya perfect
On thecontrary,
per- sencesandabsencesgenerated
subjecttothefixedmarfectcheckerboards
are notnecessarily
causedbycom- ginal totals,one can countnot only the expected
petition,nor are the samplesof checkerboards
pre- numberof pairs,trios,etc. with0, 1, 2, 3, . . . N co-ocsentedby Diamondand Gilpin(1982, 1983) unusual. currences,
butalsorecordsimilar
information
as a funcTheycomputetheprobability
ofobserving
a particular tion of the combineddistributional
breadthof the
checkerboard
distribution
subjectonlyto fixedrow speciesin a group.For example,one can generatethe
sums.This,however,
allowsdegenerate
rearrangementsexpectednumberof species pairs whose combined
ofspeciespresences
andabsencesthatinvolvesiteswith occurrencetotals (row sums) are 2, 3, 4, .. . 2N that
no species(Connorand Simberloff
1983). Whyshould displayeachlevelofco-occurrence.
As an examplewe
thenullexpectation
of speciesco-occurrence
suchan analysisfortheWestIndies
patterns have performed
fora specific
groupofislandscountrearrangements
that bats (Fig. 1). While once again by x2 analysisthe
includeonlya subsetoftheactualsites?Whenexpecta- observedand expecteddistributions
are significantly
tionsof exclusivity
are computedsubjectto fixedrow different
(x2 = 105.16, df = 57, p < 0.0005, cells
and columnsums,no degenerate
rearrangements
are lumpedso thatno expectedvaluesare lessthan1 and
allowed,andtheexamplesofcheckerboards
presented less than20% are less than5) one can also see that
byDiamondandGilpin(1982, 1983) areshownnotto certain
categories
ofcombined
occurrence
breadth
have
differ
fromexpectation
(ConnorandSimberloff
1983). an excessivenumberof exclusivepairs.Determining
whichofthesedemandsomecausalexplanation
andthe
of
the
nature
cause
requires
evidence
other
than
thaton
3.4. Technical
flaws
geographical
distribution.
Theremaining
criticisms
lodgedbyDiamondandGilpin
(1982, 1983) againstourprocedure
concern
thetechnical detailsof our analysisratherthantheapproachto
the null expectationof exclusivity.
generating
procedures
They 4. Alternative
chastizeus forusingMonteCarlo procedures
rather
4.1. The Diamondand Gilpinprocedure
thanemploying
an explicit
solutionto generate
thenull
distribution
of exclusivity.
Whilewe agree thatthis To overcomewhattheybelieveto be theshortcomings
wouldbe desirable,it is certainly
notnecessary.
Fur- of our procedureGilpinand Diamond (1982) and
no explicitsolutionexistsevento countthe Diamondand Gilpin(1983) havesuggested
thermore,
theirown
number
ofpossiblematrix
rearrangements
forfixedrow 4-stepprocedure:
andcolumnsums,muchlessforthenulldistribution
of
fromthispopulation
exclusivity
ofrearrangements.
1) Computethe probability
that the i'th species
Theycontendthatourtestcannotdetectthedirection occurson thej'thislandas pij= Ri Cj/TwhereRi is
ofanyobservednon-randomness.
Butthiscancertainly thei'thcolumnsum,Cj is thej'throwsum,andT is
be accomplished
byan inspection
ofthedeviations
bethegrandsum(numberof l's in thematrix).
tweenobservedand expected,and byan inspection
of
2) Computetheexpectedoverlapbetweeneachpair
theindividual
cell contributions
to thex2statistic.
ofspeciesand itsstandard
In
deviation
fromtheabove
thoseinstances
wherewe concludedthattheobserved probabilities.
The expectedoverlapbetweenspeciesi
and expecteddistributions
and k isEik = Ij pijPki and itsstandard
differed,
we indicatedthe
deviation
is
directionof the difference
(Connorand Simberloff SDik = V PHPkj(1-pj Pkj)1979). Theirtestcan do no better.
3) Standardize
theobservedvaluesby substracting
The lastpointraisedbyDiamondand Gilpin(1982,
theexpectedand dividing
bythestandard
deviation
1983) in criticism
of ourprocedure
is thatit considers oftheexpected(dik = (Pik- Eik/SDik)), wheredik is
459
OIKOS 41:3 (1983)
This content downloaded on Thu, 21 Feb 2013 12:18:56 PM
All use subject to JSTOR Terms and Conditions
4~~~~~~~4
16~~~~
Fig.1. Deviationofobserved
fromexpectednumber
ofislandssharedforpairsofbatspeciesintheWestIndiesas a function
of
thecombined
occurrence
frequency
ofeachspeciespair.The plottedvaluesaretheindividual
cellcontributions
([0 - E]2/E)to
theoverallx2statistic
reported
inthetext.Data arefromBakerandGenoways
(1978).
theobservedoverlapexpressedas standard
deviates
oftheexpected,
and Oik is theobservedoverlap.
4) Comparethe frequency
distribution
of standardized observations
to a (0,1) standardnormaldistribution
via x2,and a graphical
representation.
degenerate.
Thispracticelowersthenullprobability
of
observing
anyspecified
levelof exclusivity
and biases
theirtestin favorof rejecting
the nullhypothesis
of
independent
speciesplacement
(ConnorandSimberloff
1983). Furthermore,
thosearrangements
thatare not
degeneratemay possessmarginaldistributions
quite
Ifthenullhypothesis
ofindependent
speciesplacement different
fromtheobservedmarginal
so
distributions,
is true,thehistogram
ofstandard
deviatesshouldfollow thattheexpectedpatternof speciesco-occurrences
is
a (0,1) normaldistribution.
An excessivenumberof confounded
withthisvariation
inthemarginal
distribuspeciespairsin theuppertailofthedistribution
would tions.
indicatesignificant
aggregation,
and an excessin the
GilpinandDiamond's(1982) motivation
forthisaslowertailsignificant
exclusivity.
pectoftheirprocedure
is to removethe"hiddenstrucwe applaudthespiritofthismethod,
Although
inas- ture"theyfeelisbuiltinbyfixedmarginal
distributions.
much as it representsan attemptto test the null However,thisapproachfailsto removesuch"hidden
ofindependent
hypothesis
speciesplacement
amongis- structure";
ratherit merelyobscuresanyrelationship
lands,severaltechnical
problems
besetthistechnique. betweenthe marginaldistributions
and the observed
patternof species'co-occurrences
sinceit obliterates
themarginal
distributions.
Our procedure
admitsthat
"hiddenstructure"
could
affect
the
marginal
distribu4.2. Expectedor fixedmarginal
distribution
tions,so we proceedbytesting
thenullhypothesis
that
The computational
procedureused by Gilpin and above andbeyondwhatever
"hiddenstructure"
is imDiamond(1982) and Diamondand Gilpin(1983) to partedby the marginaltotals,the patternof species
generatetheexpectedoverlapbetweenspecieseffec- co-occurrences
can be explainedto resultfromindetivelysetstherowand columnsumsfortheexpected pendentplacement.Were the Gilpinand Diamond
speciesco-occurrence
to be on averageequalto (1982) procedurecapable of removing
pattern
the so-called
theobserved
rowandcolumnsums,rather
thanequalto "hiddenstructure",
thenwe suggesttheterm"hidden
theobservedsums,as we did. In doingso, theGilpin structure"
to be a misnomer.
Rather,"hiddenstrucand Diamond (1982) procedurecountsdegenerate ture",ifit exists,willremainhidden.Whatever
strucrearrangements
withspeciesoccurring
nowhereand/or tureis impartedto the patternof speciesco-occurrsiteswithno speciesas possiblenullrearrangements.ences,hiddenor otherwise,
by themarginal
distribuThis,in essence,allowsmoredegreesof freedomin tionsshouldbe clearlyattributable
to themarginal
disrearranging
the matrixof speciespresenceand ab- tributions.
This can onlybe achievedif the marginal
sences,butmanyoftheseadditional
rearrangements
are distributions
are fixedand therefore
known.
460
OIKOS 41:3 (1983)
This content downloaded on Thu, 21 Feb 2013 12:18:56 PM
All use subject to JSTOR Terms and Conditions
the patternof
erfulregarding
questionsconcerning
species'co-occurrences
(Connorand Simberloff
1978,
The procedure
forcomputing
theprobability
thatthe Simberloff
1978),theywerenotintended
to
primarily
i'thspeciesoccurson thej'th island(step1) does not testsuchhypotheses.
Theseprocedures
weredesigned
computeprobabilities.
It computesexpectedfrequen- to testhypotheses
thesimilarity
of sitesin
concerning
cies. As mentionedby Gilpinand Diamond(1982) theirspeciescomposition,
notthesimilarity
of species
thesevaluescan exceed1, and oftendo (Connorand distributions
havebeen
amongsites.Similarprocedures
Simberloff
1983). Gilpinand Diamond(1982) adopt developedforand appliedto paleobiogeographic
and
severalad hocmethods
to dealwiththisproblem.
applications(Raup and Crick 1979,
They biostratigraphic
further
claimthattheirprocedure
confollows
thetraditional Harper1978). Once againthetestedhypotheses
log-linearmodelof contingency
tableanalysis.How- cernsimilarity
betweensitesor within
sitesovertime.
ever,suchan analysis
is appropriate
forfrequency
Theprocedure
andBiehl(1982)
suggested
byWright
data,
notbinary
presence-absence
data.Itisnotpossibletofit to supplantours (Connorand Simberloff
1979) for
a log-linear
or a logistic
modelto these"probabilities" examining
how manysitesa groupof speciesshares,
(Bishopet al. 1975). Of theseveralwayssuggested
theprobability
a patofobtaining
by startsbycomputing
Gilpinand Diamond (1982) to estimatecell prob- ternof speciesco-occurrence
as exclusiveas or more
abilitiesonlytheMonteCarloprocedure
is correct.
We exclusivethanthatobserved.The computation
is perhavealreadypointedoutDiamondandGilpin's(1982) formed for each species pair and follows the
aversionto MonteCarlomethods.
hypergeometric
distribution
suggested
by Connorand
Simberloff
ofthenumber
(1979: 1133).An expectation
of
sites
shared
between
can
also
be computed
species
4.4. Asymptotic
normality
andBiehl1982).
(Wright
The laststepin theprocedure
outlinedby Gilpinand
Using this procedure,Wrightand Biehl (1982)
Diamond(1982) is to comparetheobservedfrequency examined
fourpairsofspecies,thethreeavianexamples
ofstandard
deviates(step4) to a (0,1) normaldistribu- adducedbyDiamond(1975) as evidenceofcompetitive
tion.Thiscomparison
is predicated
on theassumption exclusion,
andLacerta lizardsintheAdriatic.
Theyalso
that,givenno speciesassociation,the observedfre- examinedtheNewHebridesbirdsandtheWestIndies
of standarddeviateswould be bats.
quencydistribution
asymptotically
(0,1) normal.We ask,asymptotic
with
The Wright
and Biehl (1982) procedure
essentially
respectto what?We haveno analogofsamplesize in fixesthe row totalsof the expectedspecies' presthisproblem,
so onecannotobservethebehavior
ofthe ence-absence
matrix
to equaltheobserved,
butletsthe
of standarddeviatesforlargersamplesto columntotalsvaryfreely.
distribution
As we statedabove,thisfails
ifit is, in fact,asymptotically
determine
normal.Why to constrain
theexpectedpatternof speciesco-occurassume normality?
Could the null distribution
be rencesto accountfortheomnipresent
species-area
reskewedor leptokurtic?
theoutcomeof the lationship
Obviously,
andtherefore
tendsto bias theirtesttoward
X2testis verysensitiveto theshapeassumedforthe rejecting
thenullhypothesis.
Again,thequestionofinexpecteddistribution
of standarddeviates.If thetrue terestis whether
or nottheobservedpattern
ofspecies
expecteddistribution
is skewed,thenthe conclusions co-occurrence
is consistent
witha hypothesis
of indereachedby Gilpinand Diamond(1982) fortheNew pendentplacement,
giventhe observedspecies-area
Hebridesand Bismarcksavifaunasmay be wrong. relationship
andspeciesoccurrence
distribution.
Thisis
However,without
a meansofassessingtheasymptotic thehypothesis
ourprocedure
wasdesigned
to test.Had
behaviorof theexpecteddistribution
of standardde- we been interested
in askingsimplywhether
thereis
viatesit is impossibleto tell. This rendersrigorous evidenceofnon-randomness
inspecies'distributions,
as
hypothesis
testingbased on the Gilpinand Diamond Wright
andBiehl(1982) appearto be, we wouldhave
(1982) procedure
rathertenuous.
fixedneither
therownorthecolumnsums.However,
sucha testwouldnotbe particularly
interesting
sincewe
know
to
species
be
non-randomly
distributed
withre4.5. TheWright
andBiehlprocedure
spect to the physicalenvironment,
particularly
at a
andBiehl(1982) feelthatourprocedure
Wright
(Con- geographic
scale.
norand Simberloff
1979) and otherswe haveoutlined In fact,we used the veryproceduresuggestedby
(Connorand Simberloff
1978, Simberloff
1978) are Wright
andBiehl(1982) tocompute
theprobabilities
of
"notappropriate
to determine
whether
islandcoloniza- obtainingby independentplacementthe levels of
tionis random".Theysuggestan alternative
approach exclusivityobservedforMacropygiaand Pachycephala
thattheyfeelis "betterabletodetecttheeffect
ofcom- as presented
by Diamond(1975), (Connorand Simpetitiveexclusionon insulardistributions".
Whilewe berloff1979: 1133). For Macropygiawe calculated
agreethattheprocedures
we havedevelopedto com- identical
probabilities,
butWright
andBiehl(1982) apputean expectation
of the numberof speciesshared pearforPachycephala
to haveerredintwoways.First,
betweensitesarenotparticularly
informative
norpow- usingtheirspeciesfrequencies,
we findtheirpublished
4.3. Cellprobabilities
461
OIKOS 41:3 (1983)
This content downloaded on Thu, 21 Feb 2013 12:18:56 PM
All use subject to JSTOR Terms and Conditions
Tab. 1. Hypergeometric
tailprobabilities
thatindependent
speciesplacement
wouldhaveresulted
inexclusivity
as extreme
as that
observedforfourselectedspeciespairs.
Numberofislandswith
Neither Species Species Both
species 1 only 2 only species
Ptilinopus
Pachycephala
Macropygia
Lacerta
3
21
17
21
17
13
30
11
18
18
14
18
15
14
14
28
9
9
12
18
11
6
6
18
2
2
0
0
0
0
0
0
Data source
Hypergeometric
probability
0.0003
Diamond(1975), Fig.22
0.0903
Diamond(1975),Figs22,8,15 andtext,p.357
0.0034
andBiehl(1982),Tab. 1
Wright
0.0035
Diamond(1975),p. 390
0.0037
Diamond(1975),Fig.21
0.0245
Diamond(1975), Fig.20
0.1226
Diamond(1975),Figs20,6,25 andtext,p. 357
0.0000001 Nevoet al. (1972)
value(theirTab. 1) too lowbya factorof 10. Second, exclusivity
appearsextraordinary.
We cannotpresenta
Diamond(1975) statesin histextthat11 islandssup- generaltreatment
of the bias thatthisnon-indepenportP. pectoralis,
18 support
P. melanura
dahliaand21 dence will likelyproduce,but the following
simple
support
neither.
His figure
21 has,foranalogoustotals, exampleshoulddemonstrate
theproblem.Wright
and
11, 15, and 17, respectively.
Wrightand Biehl have Biehl (1982) chooseto call a biogeographic
arrangeused 12, 14, and 17, respectively.
Tab. 1 depictsthe ment not predominantly
interactiveif the null
correcthypergeometric
forall threecon- hypergeometric
probabilities
ofindependent
probability
is
placement
figurations,
as wellas fortheotherexamplesgivenin lessthan0.05 foratleast5% ofthepairs.Supposethere
Tab. 1 ofWright
and Biehl(1982). A secondpointof were 12 speciesin a speciespool, arrangedover 10
interest
arisesfortheothertwoexamples,
Macropygia islands.Supposefurther
thatspecies1,2, 3, and4 were
andPtilinopus,
in theirTab. 1 fromDiamond(1975). each foundon 5 islands,and thatstrictcompetitive
FromDiamond'sFigs6, 8, 15,and25,andhistextonp. exclusion
existedbetweenspecies1 and2, 2 and3, and
357,itis apparent
thathisFigs20 and22 greatly
under- 3 and4. Finallysupposethattherewereno interactions
estimatethenumberof islandswithneither
speciesof betweenanyotherspecies.Thatis,exclusive
pairscomMacropygia
andPtilinopus,
respectively.
Whenthedata prise3/(1') = 4.5% of all pairs.Now,let A, B, C, D,
fromthefull50 surveyed
islandsare included,
thenull andE be theislandsoccupiedbyspecies1. ThenF, G,
probabilities
(ourTab. 1) are insignificant.
H, I, andJmustbe theislandsoccupiedbyspecies2, so
In anyevent,as we notedearlier,thesefourpairwise thatspecies3 mustoccupyA, B, C, D, and E. Then
examplesofWright
andBiehl'sTab. 1 areculledfroma species4 mustoccupyislandsF, G, H, I, andJ,and 1
massofbiogeographic
data precisely
becausetheyap- and4 arethusexclusively
eventhough
arranged
thereis
peared,a priori,to constitute
unusualexclusivity.
Un- no interaction
betweenthem.The nullhypergeometric
lessall availabledataare analyzedsimultaneously,
one probability
foreachofthe4 exclusive
pairsis0.004,and
cannotassessthe statistical
significance
of some par- therearethusatleast4/(12)= 6.1% ofthespeciespairs
ticularsubset.For thethreeavianexamplesfromthe exclusiveat thislevel.Theremaybe others,produced
BismarckArchipelagoneitherthe matrixsimulation by chancealonein theuniform
randomplacement
of
methodnorthepairwisehypergeometric
method(but theremaining
8 species,butjust these4 pairswould
expanded to all pairs) can be applied since the causethesystem
tobe classified
as interactive
byWright
biogeographic
data describedby Diamond(1975) as and Biehl (1982), even thoughtheactualnumberof
"givenbyMayrand Diamond1975" haveneverbeen interactions
is too lowbytheirstatedcriterion.
published.
one cannottellwhether
Consequently
three
One can conceiveofothersimplesituations
producsuchexclusively
arrangedpairscould have arisenby ingspuriousassociations,
eitherpositiveor negative.
If
independent
placement.
ForLacerta,Nevoetal. (1972) species1 and species2 wereobligatemutualists,
for
haveperformed
introduction
experiments
thatbuttress example,and species 1 and species3 competitively
theirconclusionof competitive
exclusionbetweenL. excludedone another,species2 and 3 (givenan apsiculaandL. melisellensis.
propriate
number
ofislandsandoccurrences)
wouldbe
Even whenthedata forall speciesin a pool are av- improbably
exclusiveby the pairwisehypergeometric
- assessing test,eventhoughtheydo notinteract.
ailable,theWright
andBiehl(1982) method
One can sayin
the null hypergeometric
probabilities
forall species generalthatlookingat all pairs,as Wright
and Biehl
pairsinthespeciespool- is defective.
Thespeciespairs (1982) suggest,
willlikelylead to overestimation
ofthe
arenotindependent
ofone another
andthismayeasily degreeofbiological
structure.
The GilpinandDiamond
inflatethe observednumbersof speciespairswhose (1982) procedurewhichalso examinesall pairs,al462
OIKOS 41:3 (1983)
This content downloaded on Thu, 21 Feb 2013 12:18:56 PM
All use subject to JSTOR Terms and Conditions
to
thoughnotusinghypergeometric
suffers 6) All thisis to saythatnotonlywillit be difficult
probabilities,
simulation
method concludethat a patternof speciesco-occurrence
fromthissameproblem.
Ourmatrix
is
An exactlyanalogous statistically
odd,butalsothatdataon geographical
does not producethisartifact.
patbasisto determine
the
problemwas recently
notedby Meagherand Burdick ternsalone are an insufficient
(1980) withrespectto nearestneighboranalysis.To causesofthosepatterns.
ifindividuals
determine
oftwospeciesor sexesare arinspace,sincepairsofindividuals Althoughwe feelthatGilpinand Diamond's(1982)
rangedindependently
andBiehl's(1982) procedures
are notindependent
ofone another(ifone knowsthat andWright
areincorrect
ofA's andinappropriate,
A is B's nearestneighbor,
theirintent
is to testnullhypotheses
one'spriorassessment
nearestneighboris modified),
thetraditional
Thiswe applaud.It is certainly
x2con- againstalternatives.
a
is invalid;theteststatistic
is notdistri- refreshing
changefromthecommonpracticeofinvoktingency
analysis
butedas x2.Meagherand Burdick(1980) use a re- ing,post hoc, competition
as the cause of exclusive
to generatethedistribution.
peatedsimulation
speciesranges(Diamond1975, 1978,Terborgh1971,
Terborgh
and Weske1975). However,in theviewof
Diamondand Gilpin(1982, 1983) and Wrightand
Biehl (1982), competitiveexclusionstill holds a
5. Discussion
totheotherpossiblecauses
paramount
relative
position
Our messageis simpleand short,so we reiterate.
ofexclusive
Availableevidencedoesnot
arrangements.
seemto us to support
thisview.
ofspeciesco-ocIn thisrespect,
1) In orderto inferthatsomepattern
we pointoutthatinspiteofthestrikcurrenceis odd, one mustknowwhatis expected. ingconcordance
in resultsbetweenourprocedure
and
some hypothesis
Therefore,
observedand thoseof Gilpinand Diamond(1982) and Wright
contrasting
and
expectedpatterns
ofco-occurrence
mustbe statedand Biehl(1982), theseauthorspersistin theirviewthata
tested.Thishypothesis
caninvolveeithera comparison case forcompetitive
exclusioncan be builton biogeooftheobservedpattern
ofco-occurrence
withthatex- graphicdataalone.FortheNewHebridesbirdsGilpin
pectedgivenindependent
ofspeciessubject and Diamond(1982) found,as did we, no excessof
placement
to fixedmarginal
or a comparison
ofthe exclusivespeciesrangesoverthatexpectedfromindedistributions,
co-occurrence
forspecieswithin
pattern
de- pendentplacement.Wrightand Biehl (1982) found
arbitrarily
finedgroupsof"potential
competitors"
withthatco-oc- onlya slightexcessof aggregative
over
arrangements
currencepatternexpectedof non-competitors.
Other thatexpectedforboththeNewHebridesbirdsandthe
are possible,buttheremustbe somenull West Indiesbats,whilewe foundsignificantly
comparisons
more
hypothesis.
exclusivepairsof speciesthanexpectedin the West
of species'co-occurrence
2) The expectation
should Indiesbats.For theBismarckIslandavifauna,Gilpin
accountfor species-arearelationships
and species' and Diamond(1982) also foundan excessofexclusive
occurrence
distributions.
distributions
overexpected,
buttheexcessisvanishingly
3) An excessivenumberof exclusivelydistributedsmall(Gilpinand Diamond1982: Fig. 4). Since the
speciesis consistent
withan hypothesis
ofcompetition,Bismarck
dataareunpublished,
we cannotapplyeither
butcanalsobe explainedas resulting
from
severalother ourmatrix
simulation
approachor thehypergeometric
causes,i.a., geographical
speciation,
multiple
coloniza- calculations.
The patternstrikingly
evidentfromboth
tionroutes,predation,
and independentlyGilpinand Diamond's(1982) and Wright
parasitism,
and Biehl's
evolvedhabitatpreferences.
is one ofa significant
(1982) analysis
tendency
towards
4) Since a uniquecause cannotbe associatedwitha aggregation
in speciesco-occurrences.
Wherein this
classofco-occurrence
particular
patterns,
itis impossi- resultis thereevidenceof an overwhelming
pattern
ble basedsolelyon biogeographical
evidenceto infer evenconsistent
withcompetitive
exclusion,
muchless
thatcompetition
oranyotherspecific
causeis responsi- causedbycompetition?
ble fora particular
geographic
pattern.
Evidenceofactivereplacement
orcolonizations
thatfailedbecauseof
activeexclusionis necessary
to inferthatcompetition
6. Postscript
on nullhypotheses
causesand maintains
an exclusivegeographic
pattern.
Therefore,
evidenceon geographical
patterns,
perse, is Diamondand Gilpin(1982, 1983), Schoener(1982),
no basisforinferring
a roleforcompetition.
and Harveyet al. (1983), laborto discredit
theuse of
5) Failureto rejectthenullhypothesis
ofindependent nullhypotheses
inecology.Theyclaimthatnotonlyare
speciesplacementdoes not constitute
its acceptance, thosewe havediscussedaboveillconceived,
weak,cirnordoes it implythatcompetition
or anyothercausal cular,difficult
to test,and of courseunsuccessful,
but
processesdo notoccur.It does implythatthereis no thatothernullhypotheses
posedandtestedbyvarious
evidencein thepatternof speciesco-occurrences,
per authors(Connorand Simberloff
1978, Connorand
se, thatcompelsone to positsomeeffect
ofone species McCoy 1979, Stronget al. 1979, Simberloff
and
on anotherspecies'geography.
Boecklen1981) are equallyreprehensible.
By implica463
OIKOS 41:3 (1983)
This content downloaded on Thu, 21 Feb 2013 12:18:56 PM
All use subject to JSTOR Terms and Conditions
thevalueofthisvariablefor
tionone couldadd to thislista growing
bodyofsimilar ranges.We can determine
of
by an inspection
of thisdisdain(Williams theobservedspecies'geographies
literature
equallydeserving
thevalueofthis
1964, Poole and Rathcke1979, Ricklefsand Travis thedata.But,howcan we determine
1980,Rabinowitz
et al. 1981,Dillon1981,Colemanet variableforthe"control"instancewherecompetition
al. 1982, Malanson1982, DeVita et al. 1982, and has playedno role?Whatrangeof valuescouldthis
of no importance?
others).We, of course,believethatthesenullhypo- variableassume,werecompetition
theseshavebeenuseful,startlingly
informative,
and,in This questionmustbe answeredin orderto testan
indetertheroleofcompetition
concerning
specific
instances,
vindicated
byfurther
analysis
(Strong hypothesis
biogeography.
and Simberloff
1981,Connoret al. 1983,Connorand mining
We suggestthata nullhypothesis
and a nullmodel,
Simberloff
1983,Simberloff
1983).
to generatetheexpecteddistributions
ofthe
We believeitwouldbe informative
heretosetoutthe contrived
underthenullhypothesis,
can userolewe envision
fornullhypotheses
andourmotivation variableofinterest
theroleofthe"control"in orderto
forusingthemin ecologyand biogeography.
Strong fullyapproximate
involving
evidence.
non-experimental
discussesthe virtuesof usingnull testanhypothesis
(1980) similarly
However,fromsuchan analysisone canconcludeonly
in thesetwodisciplines.
hypotheses
In a strictstatistical
biologiessufficeto
sense a null hypothesis
is an eitherthatspecies' independent
It is merelya conjecture
hypothesis
of no effect.
that explainthedataat hand,orthatsomeadditional
causal
in thedataat handwouldlead one to positan explanation
mustbe invoked.
Without
further
nothing
evidence,
otherthanchancefortheobservedresult. probablyof an experimental
explanation
nature,one can neither
While a null hypothesisis not the only kind of eliminateany particular
causal mechanism,
nor conmechanism
has operated.
hypothesis
thatcan serveas the"testedhypothesis",
it cludethata particular
is certainly
themostcommoninconventional
out thatwe have consisstatistical We concludeby pointing
tently
usednullmodelsas a meansto challengeinferusage.
In experimental
research,the resultsderivedfrom encesdrawnfromnon-experimental
evidence.We feel
ormanipulations
treatments
specific
appliedto subjects thatthisistheproperandusefulrolefornullmodels.As
are comparedwithuntreated
or controlsubjects.The longas thepractice
ofinferring
causefromnon-experitestedhypothesis
is usuallythe nullhypothesis
toflourish
of no mentalevidencecontinues
amongecologists,
difference
betweentreatments
and controls.Unfortu- thenwe expectnullmodelsalso to flourish.
natelymuchecologicalresearchis non-experimental.
Largesetsofobservations
themorphology,
concerning
- We would like to thankGeorge Hornand resourceuse of Acknowledgements
distribution,
abundance,
behavior,
forhiscomments
on thismanuscript
andBetsyBlizard
plantsand animalsare collectedprimarily
fordescrip- berger
forpreparing
the figure.This researchwas supported
by a
tivepurposes,
butare also examinedin reference
to a grantfromtheAcademicComputing
Centerof theUniv.of
widearrayof ecologicaland evolutionary
theories.If Virginia.
ofthesedataarefoundtobe consisparticular
portions
tentwitha particular
theory,
thentheyare adducedas
support
forthattheory.
onedoesnotaskif
Traditionally
somealternative
theory
might
explaintheavailableevidenceequallywellor evenifthedata are statisticallyReferences
withthetheory
consistent
theyare saidto support.
Alatalo,R. V. andAlatalo,R. H. 1979.Resourcepartitioning
Whyis thisso? Unlike in experimental
research
amonga flycatcher
guildin Finland.- Oikos33: 46-54.
wherean explicit"testedhypothesis"
is evaluatedby Baker,R. J. and Genoways,H. H. 1978. Zoogeography
of
bats.- In: Gill,F. B. (ed.),Zoogeography
inthe
use of a variablewithknowndistributional
properties, Antillean
Caribbean:
The1975LeidyMedalSymposium.
Acad.Nat.
in non-experimental
research,
thedistributional
propSci. Philadelphia,
pp. 53-97.
ertiesofthevariableofinterest
areoftenunknown.
For Bishop,Y. M. M., Feinberg,
S. E. and Holland,P. W. 1975.
an examplewereturn
againto dataon thegeographical Discretemultivariate
- MIT
analysis:
theory
andpractice.
Press,Cambridge,
MA USA.
distribution
ofspecies.Froman analysis
ofthedistribuS. A. 1944.Foundations
- Harper
ofPlantGeography.
tionofbirdsin theBismarck
Islands,Diamond(1975) Cain,
& Brothers,
NewYork.
developedhis "assemblyrules".These rulesstate,in Coen,L. D., Heck,K. L. andAbele,L. G. 1981.Experiments
sum,thatcompetition
is a majorforcedetermining
the
on competition
andpredation
amongshrimps
ofseagrass.
- Ecology62: 1484-1493.
of speciesamongislands.The evidence
arrangement
B. D., Mares,M. A., Willig,
M. R. andHsieh,Y. H.
adducedin support
ofthisidea is theobservedpattern Coleman,
- Ecology
1982.Randomness,
area,andspeciesrichness.
ofwhichbirdspeciesareon whichislands.However,if
63: 1121-1133.
is the"experimental
competition
effect"
forwhichwe Connor,E. F. and McCoy,E. D. 1979. The statistics
and
- Am.Nat. 113:
biologyofthespecies-area
relationship.
must test,to what "control"do we comparethe
791-833.
observedspecies'geographies?
How do we assesswhat - and
D. 1978. Speciesnumberand composiSimberloff,
effects
competition
has?
tionalsimilarity
of the Galapagosfloraand avifauna.The variableof interest
is theexclusivity
of species'
Ecol. Monogr.48: 219-248.
464
OIKOS 41:3 (1983)
This content downloaded on Thu, 21 Feb 2013 12:18:56 PM
All use subject to JSTOR Terms and Conditions
D. 1979. The assembly
of speciescom- Nevo,E. G., Gorman,
- and Simberloff,
M., Soule',M.,Yang,S. Y., Clover,R.
and Jovanovic,
V. 1972. Competitive
exclusionbetween
munities:chance or competition?- Ecology 60:
insularLacertaspecies(Sauria,Lacertidae):noteson ex1132-1140.
- Oecologia (Berl.) 10:
- andSimberloff,
D. 1983.Neutralmodelsofspecies'co-ocperimentalintroductions.
D. R., Simberloff,
D. and
- In: Strong,
123-190.
currence
patterns.
conceptual Pielou,D. P. and Pielou,E. C. 1968. Associationamong
Abele,L. G. (eds.), Ecologicalcommunities:
Univ.Press:Princeton,
speciesof infrequent
occurrence:
the insectand spider
issuesandtheevidence.Princeton
faunaofPolyporus
betulinis
NJ,USA. (in press).
(Bouliard)Fries.- J.Theor.
-, McCoy,E. D. andCosby,B. J.1983.ModeldiscriminaBiol. 21: 202-216.
studies.tionand expectedslopevaluesin species-area
Poole,R. W. and Rathcke,B. J. 1979. Regularity,
random- Science
Am.Nat. 122 (in press).
inflowering
ness,andaggregation
phenologies.
andniche
203: 470-471.
Crowell,K. L. andPimm,S. L. 1976.Competition
ontosmallislands.- Oikos27: Rabinowitz,
D., Rapp, J. K., Sork,V. L., Rathcke,B. J.,
shifts
ofmiceintroduced
251-258.
Reese,G. A. andWeaver,J.C. 1981.Phenological
propdisthegeographical
P. S. 1957. Zoogeography:
erties of wind-and-insect-pollinated
Darlington,
prairieplants.ofanimals.- Wiley,NewYork.
Ecology62: 49-56.
tribution
encounter Raup,D. M. and Crick,R. E. 1979. Measurement
DeVita,J.,Kelly,D. andPayne,S. 1982.Arthropod
offaunal
- J.Paleontol.
inpaleontology.
rate:a nullmodelbasedon randommotion.- Am.Nat.
similarity
53: 1213-1227.
119: 499-510.
Ricklefs,
R. E. andTravis,J.1980.A morphological
approach
- In:
ofspeciescommunities.
- Auk 97:
to thestudyofaviancommunity
Diamond,J.M. 1975.Assembly
organization.
321-338.
Cody,M. L. and Diamond,J. M. (eds.), Ecologyand
HarvardUniv. Press,Cam- Root,R. 1967.Thenicheexploitation
evolutionof communities.
oftheblue-gray
pattern
- Ecol. Monogr.37: 317-350.
gnatcatcher.
MA, USA, pp. 342-444.
bridge,
of interspecificSchoener,T. W. 1982. The controversy
- 1978. Niche shiftsand the rediscovery
over interspecific
- Am.Sci. 66: 322-331.
- Am.Sci. 70: 586-595.
competiton.
competition.
M. E. 1982.Examination
ofthe"Null"model Simberloff,
- andGilpin,
D. 1978.Usingislandbiogeographic
distributions
on
forspeciesco-occurrences
- Am.Nat.112:
of Connorand Simberloff
to determine
ifcolonization
is stochastic.
islands.- Oecologia(Berl.)52: 64-74.
713-726.
on
- 1983. Properties
- and Gilpin,M. E. 1983. Are speciesco-occurrences
of coexisting
birdspeciesin two arusefulin
and are null hypotheses
islandsnon-random,
chipelagoes.- In: Strong,D. R., Simberloff,
D., and
D.
community
ecology.- In: Strong,D. R., Simberloff,
Abele,L. G., (eds.),Ecologicalcommunities:
conceptual
concepandAbele,L. G. (eds.),Ecologicalcommunities:
issuesandtheevidence.Princeton
Univ.Press,Princeton,
tual issues and the evidence.PrincetonUniv. Press,
NJ,USA, (in press).
- andBoecklen,W. 1981.SantaRosaliareconsidered:
Princeton,
NJ,USA (in press).
size
- Evolution
in themorphology
anddistribuDillon,R. T. 1981.Patterns
ratiosandcompetition.
35: 1206-1228.
in OneidaLake, New York,detected - andConnor,
tionof gastropods
E. F. 1979.Q-ModeandR-Modeanalyses
of
- Am. Nat.
nullhypotheses.
usingcomputer-generated
biogeographic
distributions:
Null hypotheses
based on
- In: Patil,G. P. andRosenzweig,
118: 83-101.
random
colonization.
M.
and
Dueser,R. D. and Hallett,J. G. 1980. Competition
L. (eds.),Contemporary
quantitative
ecologyand related
in a forestfloorsmallmammalfauna.habitatselection
ecometrics.
Int.Co-operative
Publ.House,Burtonsville,
Oikos35: 293-297.
MD, USA, pp. 123-138.
of a tropicalguildof nec- - andConnor,E. F. 1981. Missingspeciescombinations.
P. 1976. Organization
Feinsinger,
tivorous
birds.- Ecol. Monogr.46: 257-291.
Am.Nat. 118: 215-239.
- Synthese
Gilpin,M. E. andDiamond,J.M. 1982.FactorscontributingStrong,
D. R. 1980.Nullhypotheses
inecology.
43:
inspeciesco-occurrences
on islands.to non-randomness
271-285.
Oecologia(Berl.)52: 75-84.
-, Szyska,L. A. and Simberloff,
D. 1979. Testsof comcompetition,
Grant,P. R. and Abbott,I. 1980. Interspecific
munity-widecharacter displacementagainst null
34:
islandbiogeography,
andnullhypotheses.Evolution
- Evolution33: 897-913.
hypotheses.
332-341.
- and Simberloff,
D. S. 1981. Straining
at gnatsand swal- Evolution35:
Harper,C. W. 1978. Groupingsby localityin community
lowingratios:Characterdisplacement.
- Lethaia
testsofsignificance.
ecologyandpaleoecology:
810-812.
11: 251-257.
J. 1971. Distribution
Terborgh,
on environmental
gradients:
J.G. andMay,R.
Harvey,P. H., Colwell,R. W.,Silvertown,
Theoryand a preliminary
interpretation
of distributional
M. 1983.Nullmodelsinecology. Ann.Rev.Ecol. Syst.
in the avifaunaof the Cordillera,
patterns
Vilcabamba,
(in press).
Peru.- Ecology52: 23-40.
analysis
ofdis- - and Weske,J. S. 1975. The role of competition
Krebs,C. J.1978.Ecology:The experimental
in the
- HarperandRow,NewYork.
tribution
andabundance.
distribution
ofAndeanbirds.- Ecology56: 562-575.
gardens:ef- Vuilleumier,
ofhanging
Malanson,G. P. 1982.The assembly
F. andSimberloff,
D. 1980.Ecologyversushis- Am.Nat.119:145-150.
fectsofage,area,andlocation.
toryas determinants
ofpatchy
andinsulardistributions
in
D. S. 1980.The use of nearest
Meagher,T. T. and Burdick,
highAndeanbirds. In: Hecht,M. K., Steere,W. C. and
analysesin studiesof association.
frequency
neighbor
Wallace,B. (eds.),Evolutionary
vol. 12. Plenum
biology,
Ecology61: 1253-1255.
Press,NewYork,pp. 235-379.
in desert Williams,
Munger,
J.C. and Brown,J.H. 1981.Competition
C. B. 1964. Patterns
in the balanceof nature.rodents:an experiment
exclosures.
withsemipermeable
AcademicPress,London.
Science211: 510-512.
H. M. 1972.Competition,
Wilbur,
predation,
andthestructure
- Ranasylvatica
- Ecology
studiesof microcrustacean oftheAmbystoma
Neill,W. E. 1975. Experimental
community.
of
and efficiency
community
composition
competition,
53: 3-21.
- Ecology56: 809-826.
resourceutilization.
S. J.andBiehl,C. C. 1982.Islandbiogeographic
Wright,
distributions:
testingforrandomregular,and aggregated
- Am.Nat.119: 345-357.
ofspeciesoccurrence.
patterns
465
OIKOS 41:3 (1983)
This content downloaded on Thu, 21 Feb 2013 12:18:56 PM
All use subject to JSTOR Terms and Conditions