Journalof Ecology (1991), 79, 1013-1030
AND RESPONSE:
COMPETITIVE
EFFECT
AND CORRELATED
HIERARCHIES
TRAITS
THE EARLY
STAGES
OF COMPETITION
IN
DEBORAH
E. GOLDBERG
AND
KEITH
LANDA*
Departmentof Biology, University
of Michigan,Ann Arbor, MI 48109, U.S.A.
SUMMARY
(1) Competitiveabilitycan be compared between species in two ways: effectof
differentneighbourspecies on performanceof a single targetspecies or response
of differenttarget species to a single neighbourspecies. In a 5-week glasshouse
experiment,an additive design was used for all combinationsof seven species as
both targetand neighbourspecies to determineif therewere consistenthierarchies
in competitiveeffectand/orresponse,what traitsof individualsdeterminedposition
in these hierarchies,and whetheror not effectand responsecompetitiveabilitywere
related duringthe early stages of competition.
(2) Five weeks aftersowing,significant
non-linearregressionsof targetbiomass
on neighbourdensitywere found for 59% of the forty-nine
species combinations
and significantlinear regressionson neighbourbiomass were found for 51% of
the species combinations.The slopes of these regressionsrepresentper-plantand
per-gramcompetitioncoefficients,
respectively.
in competitiveeffectper plant.Differences
(3) Neighbourspecies differedstrongly
in effectper gram,response per plant, and response per gram were much weaker.
Nevertheless,consistentcompetitivehierarchieswere found for both effectand
response on both a per-plantand per-grambasis.
(4) Differenttraitsdeterminedposition in the effectand response hierarchies.
Neighbour species with larger seed mass and larger maximumpotentialmass had
strongerper-plantcompetitiveeffects,whilstneighbourspecies withhighermaximum
relativegrowthrates had strongerper-gramcompetitiveeffects.The reverseof this
latterpatternwas seen forcompetitiveresponse: targetspecies withlower maximum
relativegrowthrates were betterresponse competitors.Mean effectand response
competitiveabilityof the seven species were uncorrelatedwitheach other.
(5) These differencesin traitsassociated with strongeffectand strongresponse
competitiveability emphasize the importanceof distinguishing
between them in
experimentalstudies,at least duringthe early stages of competition.
INTRODUCTION
Individual competitiveability can be compared between species in two distinct
ways: in theircompetitiveeffector abilityto suppressotherindividualsand in their
competitiveresponseor abilityto avoid being suppressed(Jacquard 1968; Goldberg
* Presentaddress: Departmentof Biology, Indiana University,Bloomington,IN 47405, U.S.A.
1013
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1014
Competitiveeffectand response
& Werner 1983; Goldberg & Fleetwood 1987). Operationally,these correspondto
positionsin rankingsof the reductionin some componentof individualfitnessof a
single target species grown with differentneighbourspecies (effectcomparisons
between neighbours)and positions in rankingsof reductionin fitnessof different
targetspecies in the presence of the same neighbourspecies (response comparisons
betweentargets).Positionin both effectand responsehierarchiesmay influencethe
long-termpopulation dynamicoutcome of competition(Goldberg 1990). However,
because it is verydifficult
to quantifypopulation-levelcompetitiveabilityfor longlived organismssuch as most terrestrialplants,most experimentalstudiesfocus on
individual-level
competitiveabilityand thesenecessarilyentailan explicitcomparison
of different
neighboursand/ordifferent
targets.
Despite the recognitionof the distinctionbetween comparisonsof competitive
effectand response (Aarssen 1983), discussionsof the natureof competitiveability
and traitsthatdeterminecompetitiveabilityrarelytake thisdistinctioninto account
(Goldberg 1990). Similarly,many of the studies that compare 'competitiveability'
amongspeciesexplicitycompareonlyeffect(e.g. Turkington& Harper 1979; Fonteyn
& Mahall 1981; Eissenstat& Caldwell 1988; Gaudet & Keddy 1988) or onlyresponse
(e.g. Gross 1984; Keddy 1989), even when the designwas such thatboth could have
been compared (e.g. Turkington& Harper 1979; Fonteyn& Mahall 1981; but see
Wilson & Keddy 1986; Miller & Werner 1987; Peart 1989; Gurevitchet al. 1990).
Consequently,a numberof theimportantcontroversies
concerningcompetitiveability
in plants (Thompson 1987; Tilman 1988; Thompson & Grime 1988) may simplybe
semantic- resultingfromconfusionof thetwotypesof competitiveability(Goldberg
1990). In thispaper, we compare competitiveeffectand response between a group
of species to address several importantquestions concerningcompetitiveability:
(i) How differentare species in competitiveeffectand in competitiveresponse?
Goldberg& Werner(1983) hypothesizedthatper-unitsize competitiveeffectsshould
be equivalent, at least withingrowthforms,because of the overall similarityin
resources required by plants, but that competitiveresponse would differbetween
in theirabilityto toleratelow resourceavailabilitydue
species because of differences
to the presence of neighbours. If true, this would greatlysimplifyanalyses of
competitiveinteractionsin plant communitiesbecause only response to competition
fromall neighbourswould need to be compared between species.
(ii) If effectsor responsesare not completelyequivalenton eithera per-individual
or per-grambasis, how consistentare hierarchiesof effector response competitive
abilitybetween species? If consistenthierarchiesoccur, similartraitsshould determine eithereffector response competitiveabilityregardlessof the particularpair
of competingspecies.
(iii) If consistenthierarchiesoccur,whattraitsdeterminepositionin the hierarchy?
A number of traits have been hypothesizedor demonstratedto be related to
competitiveability(Grime 1988; Tilman 1988; Keddy 1989). However, few attempts
have been made to separate expectedor actual relationshipsbetweenthe effectand
the response componentsof competitiveability(see Goldberg 1990).
(iv) Are the two componentsof competitiveabilitypositivelyor negativelycorrelated or uncorrelated?That is, do similartraitsdetermineeffectand responseor, if
different
betweentraitsthatconfercompetitiveeffectand
traits,are theretrade-offs
competitiveresponsesuch thatthereis a negativecorrelationbetweenthe two types
of competitiveability?If negativecorrelationsoccur, it becomes importantto consider how the two interactto determinedominanceand persistencein naturalplant
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D. E.
GOLDBERG AND
K.
1015
LANDA
communitiesand if thereare conditionsunderwhicheffector responsecompetitive
abilityis more important(Goldberg 1990).
The answersto all these questionsprobablydepend on time scale. For example,
several studieshave shownthatcompetitivehierarchieschange over timewithinthe
same environment(Connolly,Wayne & Murray1990; Menchaca & Connolly1990),
and thereforewhichtraitsdeterminecompetitiveabilitymustdepend on factorssuch
as relative sizes or stages of the life cycle of the competingplants. Because our
focus is on individualcompetitiveabilityand not the eventual population-dynamic
(5-week) experimentbut use
outcome of competition,we chose to use a short-term
a large numberof species combinations.Therefore,the resultsthroughoutapply
only to the initialstages of competition.
competitive
An additiveexperimentaldesignwas used to measure the short-term
species,
effectsand responsesbetween all pairwisecombinationsof seven different
combinations.In an additivedesign,a 'target'species is held
includingintraspecific
at a constantdensity,whilstdensityof a 'neighbour' species is increased. If the
competition,
targetspecies is at a densitylow enough that there is no intraspecific
the slope of a regressionof mean individualperformanceof the targetspecies on
This coefficient
can be
densityor biomass of neighboursis a competitioncoefficient.
interpretedas the per-individualor per-grameffectof the neighbouron the target
species or as the response of the target species to one individualor 1 g of the
neighbourspecies (Goldberg & Werner 1983). Comparison of differentneighbour
species on the same targetspecies gives comparisonsof effectcompetitiveability,
targetspecies grownwiththe same neighbourspecies
whilstcomparisonof different
gives comparisonsof responsecompetitiveability(Fig. 1). Bettereffectcompetitors
have steeper negative slopes, whilst better response competitorshave shallower
negativeslopes (Fig. 1). Facilitationis indicatedby positiveslopes.
METHODS
Species and traits
The experimentalspecies were all herbaceousplantscommonlyfoundin old fields
or pasturesof various ages (Table 1). Seed mass for each species was obtained by
(a )
0
0
E
(b)
~~N3>N2 >N,
T >T
>T3
3
F-
Neighbourdensity
FIG. 1. Examples of comparisonof (a) competitiveeffectamong neighbourspecies and (b)
competitiveresponseamongtargetspecies. For comparisonsof competitiveeffect,neighbour
species (N) with steeper slopes are bettercompetitors(larger per-individualor per-gram
effect).For comparisonsof competitveresponse, targetspecies (T) with shallowerslopes
are better competitors(smaller change in performancefor a given change in neighbour
densityor biomass).
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Competitiveeffectand response
1016
1. Traitsof species used in the experiment.Life formsare perennial(P) or annual (A).
Growthformsare erect grasses (EG), erect dicots (ED) or low-growingdicots (LD). Seed
mass is the mean froma minimumof five seed lots of 20-50 seeds each. Median days to
seeds per species. Maximumfinalmass and
emergencewas calculatedfroma minimumof fifty
percentageallocationof total biomass to shoots are means (n = 7) fromplantsgrownwithno
neighboursand harvestedat 5 weeks aftersowing.RGRmax. was calculatedas log (maximum
finalmass/seedmass)/time,where time= days since sowingor days since median emergence
date.
TABLE
Species
Lolium
perenneL.
Trifolium
pratenseL.
T. repensL.
Rumex
crispusL.
Chenopodium
album L.
Amaranthus
retrofiexusL.
Phleum
pratenseL.
Life
form
Seed
Growth mass
form (mg)
RGRmax.
(mgmg-' day-')
Max.
finalmass
(mg)
sowing
emergence
Median
days to
emerge
Allocation
to shoot
(%)
P
EG
2-90
130
0-123
0-105
5.4
56-3
P
P
LD
LD
1-74
1-68
36
39
0-099
0-097
0-084
0-085
5.5
4-4
68-8
74.5
P
LD
1-28
24
0-137
0-077
16-2
77.5
A
ED
0-64
21
0-155
0 100
12-2
90-2
A
ED
0 56
30
0 148
0.114
7.8
91 9
P
EG
0-35
27
0 165
0 125
81
66.7
seeds each. Emergencetime
weighinga minimumof fiveseed lots of twentyto fifty
seeds of each
was calculated as the median days to emergenceof a minimumof fifty
maximum
rate
potential
relative
Maximum
growth
potential
species.
(RGRmax.),
totalmass (root + shoot), and percentageallocationto shootswere determinedfrom
the targetplants grown with no neighbours(see below). RGRmax.was calculated
as log (final mass/seed mass)/days. Both days between sowing and harvest and
betweenmedianemergencetimeand harvestwere used in the denominator.SowingRGRmax.givesa measureof relativegrowthrate over the entireexperimentalperiod
allows separationof the influenceof time to emergence
while emergence-RGRmax.
and of post-emergencegrowthrate on competitiveability.
Densitygradientsand harvesting
The experimentswere carried out in a glasshouse at the Matthaei Botanical
Gardens of the Universityof Michigan.Containers(flats),25.4 cm x 25 4 cm in area
soil mix of equal parts peat, sand,
x 63 cm deep, were filledwith a nutrient-rich
compostedsoil and perlite.Each flatinitiallyhad nine targetindividualsand 0, 8, 16,
32, 64, 128 or 356 neighbourindividuals.This resultedin seven flatsforeach of the
species combinations,fora totalof 343 flats.The nine targetindividualsin
forty-nine
array. Neighbour individuals
each flat were arranged in a regular three-by-three
were also planted in regulararraysin the low-densityflatsbut were broadcastover
the soil surfacefordensities> 16 flat-'. For both targetand neighbourspecies, the
actual numberof seeds planted was based on expected percentagegermination.If
more thanone seed of the targetspecies germinatedat a givenlocationin each flat,
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D. E.
GOLDBERG AND
K.
LANDA
1017
the seedlings were thinnedto one, leaving the earliest germinatingseedling. No
attemptwas made to countor thinthe numberof germinating
neighbourindividuals.
Locations of targetseeds were marked with toothpicksso that targetindividuals
could be distinguishedfrom neighbourindividuals,especially in the intraspecific
competitionflats.Flats were wateredonce or twicedaily as needed and no fertilizer
was added at any time.
The flatswere harvestedafter5 weeks for above-groundbiomass of neighbours,
and above- and below-groundbiomass of targetindividuals.Neighbourindividuals
were countedas theywere harvested.All plantswere dried at 65 ?C forat least 48 h
and weighed.
Analysis
To compare targetspecies withdifferentmaximumsizes, mean mass per target
plant in each flatwas divided by the mean maximummass per plant of that target
species (mass in the absence of neighbours;n = 7, see Table 1). In addition,because
the values at 0 neighbourdensityforall seven neighbourspecies for a given target
species were actuallyreplicatesof the same treatment,these were averaged and the
mean used in all the regressionsfor that target species. These two procedures
from1
resultedin regressioninterceptsthat were only rarelysignificantly
different
for
(3/49
neighbourdensity,6/49 for neighbourbiomass) and thereforeallowed
comparisonsof slopes (competitioncoefficients)
thatwere independentof differences
in intercepts(performancein the absence of competition).
The proportionof maximumtargetmass was regressedagainstneighbourdensity
and againstneighbourabove-groundmass, yieldingper-plantand per-gramcompetitioncoefficients,
respectively.It is importantto note thattargetplantswere never
also includedas neighbours,so thatthereis no potentialforstatisticalconfounding
caused by using the same plants as both targetsand neighbours(cf. Mitchell-Olds
1987). Both untransformed
regressionsand regressionsusing the reciprocalof proportion of maximumtarget mass were calculated to determinethe form of the
relationshipbetween targetsand neighbours.In the transformed
regressions,positive slopes indicatenegative(competitive)effects.The reciprocaltransformation
is
based on a simple formof the reciprocal-yieldequation for intraspecificcompetition(Shinozaki & Kira 1956) and has been used extensivelyin this or modified
formto quantifyboth intraspecificand interspecificdensitydependence (Firbank
& Watkinson1990).
Slopes of the regressionsamong the seven neighbourspecies for each target
species (competitiveeffect)and among the seven targetspecies for each neighbour
species (competitiveresponse) were comparedwithanalysesof covariance. Pairwise
analyses of covariance were then used to test whethereach pair of neighbouror
targetspecies differedin competitiveeffector response,respectively.Because there
were twenty-one
possible pairwisecomparisonsforeach targetor neighbourspecies,
an alpha of 0-05/21= 0-0024 was used as a minimumsignificancelevel in the
pairwisecomparisonof species. Consistencyof hierarchiesof competitiveeffectsand
responses were tested by Kendall's coefficientof concordance (W). Relationships
among species traitsand between species traitsand mean effectsor responseswere
tested with Pearson correlationcoefficients.The BMDP (Dixon 1983) and SYSTAT
(Wilkinson1989) statisticalpackages were used for all analyses.
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1018
Competitiveeffectand response
RESULTS
Species traits
Of the quantitativetraitsmeasured for each species (Table 1), only seed mass
and maximumplant mass were significantly
correlatedwith each other (rs = 0-84,
P < 0.05). Not surprisingly,
species withlargerseeds had largermaximummass at
harvest. However, this significantrelationshipdisappeared if Lolium, with much
largerseeds and maximumplant mass than any of the otherspecies (Table 1), was
excluded (r4= 0 68, P > 0 10). Maximumplantmass was thensignificantly
negatively
correlatedwith time to emergence (r4= -0-86, P < 0.05), indicatingthat species
that took longer to emerge had smaller maximummass at harvest.Furthermore,
withoutLolium, seed mass was negativelycorrelatedwithRGRmaX. (fromsowing:
r4= -0-96, P< 001; fromemergence:r4= -0 87, P< 0 05). The observationthat
species with larger seeds have lower relative growthrates in the absence of any
competitionhas been foundin several other studies (e.g. Gross 1984).
Percentageallocation to shoot was significantly
higherin annuals than perennials
(F1,5= 13-13, P<0-05; see Table 1) and RGRmaX. was higherfor erect plants than
forlow-growingplants (fromsowing: F1,5 = 5 85, P = 0 06; fromemergence:F1,5 =
18 02, P<0.01). When Lolium was excluded fromthe analysis,erect species still
had significantly
higherRGRmax (fromsowing:F1,4= 10 47, P < 0 05; fromemergence: F1,4= 16 38, P<0.05) and also had much smaller seeds than did the lowgrowingplants (F1,4=38-93, P<0-01). Thus, not only does Lolium have much
larger seeds and maximummass than any of the other species, it seems to have
particularlylarge seeds for the erect species in this experiment.Lolium also has a
veryhighRGRmax. forits seed mass.
Descriptionof target-neighbourrelationships
The proportionof variance in targetmass explained by neighbourdensitywas
significantly
higherfor the transformedregressionsthan for the linear regressions
(Wilcoxon signed rank test, P < 0-001). Fifty-nineper cent of the transformed
regressionson neighbourdensitywere significantat the 0-05 level (Fig. 2). On
average, neighbourdensityexplained 61% of the variance in targetperformance
using the transformed
regressionsand 70% of the regressionsexplained more than
50% of the variancein targetperformance(Fig. 2). All regressionslopes except two
usingreciprocallytransformed
targetperformancewere positive(Fig. 2, Table 2a),
indicatingcompetitiveratherthan facilitativeinteractions.The two exceptionswere
both non-significant
and had R2 < 10%.
In contrastto the results for neighbourdensity,the proportionof variance in
targetmass explained by neighbourbiomass was not significantly
different
between
the linear and transformedregressions(Wilcoxon signed rank test, P = 0 77) and
thereforethe slopes of the linear regressionswere used in subsequent analyses.
The slopes of the forty-nine
target-neighbourcombinationswere highlycorrelated
between the transformedand untransformedregressions(r47= 0-61, P < 0.001),
suggestingresults would be similar regardless of which was used. On average,
neighbourbiomass explained 55% of the variancein targetperformance;thisvalue
was marginallyless than thatexplained by neighbourdensity(Wilcoxon signedrank
test, P = 0-06). Fifty-oneper cent of the linear regressionson neighbourbiomass
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1019
D. E. GOLDBERGAND K. LANDA
0.29
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0*86*
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S~~~~~~~~
1 +
S
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Amaranthus
T pratense T repens
Lolium
w~
_
0.90** Tprctense
tns
?81I*
0.90**
0*77**
0-43
po
~~~-
Trepens
Rumex
Chenopodlum
04
20
10
c;L;
L
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0
20 0*97*
0*21
0.66*
0652
Amaranxthus
0843*
0.19
0.90**
Ph/eAmarntu
0~~~~~~
L_
0
300 0
300 0
300 0
300 0
300 0
300 0
300
600
Neighbour density
FiG. 2. Reciprocal proportionof maximumtargetperformanceas a functionof neighbour
combinationsof seven species as neighbour(columns) and target
densityforall forty-nine
(rows). The values on each graph are the R2 and significancelevel (n = 7) fromlinear
regressionson transformeddata. Positive slopes indicate competitiverelationships.Note
that all graphsare on the same scale so that slopes can be visuallycompared. * P<0-05,
** P<O.O1, *** P<O.OO1.
at the 0O05level and 69% explainedmore than 50% of the variance
were significant
in targetperformance(Fig. 3). All but one of the regressionslopes using untransformedtargetmass were negative(Fig. 3, Table 2b), again indicatingthat targetneighbourrelationshipswere largelycompetitive.
To facilitatecomparisonsof competitiveeffector response on a per-plantand
per-grambasis, slopes on both neighbourdensityand biomass are presented as
positive in the rest of this paper. Larger positive slopes indicated strongereffect
competitors,but weaker response competitors.
Comparisonsof effectbetweenneighbourspecies
in their per-planteffectfor all target
Neighbour species differedsignificantly
species except T. repens (Table 2a), but relativelyfew of the possible pairwise
using Bonferonni-adjusted
comparisonsbetween neighbourspecies were significant
probabilities(none to six of twenty-onepossible for each targetspecies). Despite
therankingofper-plantcompetitiveeffectsbetweenneighbours
thesefewdifferences,
was significantly
concordantamong targets(W = 0 47, P <0001). Almost all of the
significantpairwise comparisons involved Lolium, which had a much stronger
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1020
Competitiveeffectand response
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+
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0
~~~~Rumex
2.5 0
2-5
50
2-5 0
2-5 0
2.5 0
2.5 0
2-5
Neighbour biomass (g )
FIG. 3. Proportionof maximumtargetperformanceas a functionof neighbourbiomass for
all forty-nine
combinationsof seven species as neighbour(columns) and target(rows). The
values on each graph are the R2 and significancelevel (n = 7) fromlinear regressionson
data. Note thatall graphsare on the same scale so thatslopes can be visually
untransformed
compared. Negative slopes indicate competitive relationships. * P < 0-05, ** P < 0.01,
*** P< 0.001.
effectthan any of the other neighbourspecies on all target species (Table 2a).
When Lolium was excluded fromthe comparisonsof competitiveeffect,neighbour
species differedoveall in per-planteffectsforonly threeof the seven targetspecies
(Chenopodium, T. repens and Rumex) and the rankingof per-plantcompetitive
concordantamong
effectsbetween neighbourspecies was no longer significantly
targetspecies (W = 0 15, P > 0 30). Based on mean slopes (averaged over all target
species), the hierarchyof per-plantcompetitiveeffectswas Lolium > T. pratense>
Amaranthus> Chenopodium> T. repens = Phleum > Rumex.
Mean per-planteffectswere significantly
strongerfor neighbour species with
largerseeds and largermaximumplant mass (Fig. 4a) but these trendsdisappeared
when the much larger Lolium was excluded (P> 0-35). With or withoutLolium,
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1022
Competitiveeffectand response
(a)
50 *
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0.4
%
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0.05
0*10
0.15
0.05
0.10
Emerge RGRmmax(mg mg-' day-')
5.0
-0.41
2*5
0*8
0
4
0
0
--0
12
20
4
12
Median time to emergence(days)
0-24
20
0*8
-0*66
2.5
0
0-4
0
0 00
50
0.15
0
0.4.*
5-0 -
0
0*18
0-8
0&
0
3.0
*
0
00
0.
75
50
75
100
Percentage allocation to shoots
0.01
100
FIG. 4. Mean effect(a) per-plant(x 10-2) and (b) per-gramas a functionof species traits:
(0) erect species; (0) low-growingspecies (see Table 1). Pearson correlationcoefficients
and significancelevels (n = 7) are shown each graph. + P < 0-10, * P < 0-05, ** P < 001,
***P<O.OO1.
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D. E. GOLDBERG AND K. LANDA
1023
com(slopes) fordifferent
TABLE 3. Means (? 1 S.D.) of competitioncoefficients
binations of growthform of target and neighbourspecies. F values are from
two-wayanalysesof variance of growthformof neighbourspecies (N) x growth
formof targetspecies (T). Resultsare qualitativelyidenticalforanalysesexcluding
Lolium.
Neighbour
Target
Densityslope
Erect
Low
Erect
0-021? 0028
0 007 ? 0008
Low
0 010? 0013
0 007? 0007
-0-516? 0151
-0 252? 0279
Biomassslope
Erect
Low
(n= 16)
(n=12)
(n= 16)
-0-470? 0224
(n =12)
(n= 12)
FN
2-64
FT
FNXT
1-85
0.99
0 52
0-01
(n=9)
(n= 12)
-0-214? 0108
19-47***
.(n=9)
*** P<0-001.
types (unpublished data) or
per-planteffectsdid not differbetween life-history
growthforms(Table 3).
different
for three of the
Per-grameffectsof neighbourswere only significantly
target species (Table 2b), and the F values for comparisonsbetween neighbour
lower than for per-planteffects(cf. Table 2(a) and 2(b);
species were significantly
Wicoxon signed rank test, P<0.05). Almost none of the possible pairwise com(none to one of twenty-onepossible for
parisonsof effectsper-gramwas significant
each target species). Nevertheless,rankingsof competitiveeffectper-gramwere
concordantbetween targetspecies (W= 0 57, P< 0001). Mean
again significantly
slopes foreach neighbourspecies indicatethe hierarchy:Phleum > Amaranthus>
Lolium > Chenopodium > Rumex > T. repens > T. pratense. This hierarchyis
unrelatedto thatforper-planteffects(r5= 0 16, P > 0 50). Note especiallythe lower
rankingof Lolium and T. pratensein the per-grameffecthierarchyand the higher
rankingof Phleum.
The significantpositive correlationsbetween competitiveeffectand maximum
plant mass and seed mass disappeared when neighbourbiomass instead of density
was used as the independentvariablein the regressions(Fig. 4b). Instead, neighbour
species withhigherRGRmax.(calculated fromtime of sowingor of emergence)had
significantly
strongerper-grameffects.In addition, erect species had significantly
greaterper-grameffectsthan did low-growingneighbourspecies (Table 3) and this
withoutLolium (F1,38= 19 47, P < 0.001). Because
relationshipremainedsignificant
of the confoundingof growthform and RGRmax.(Table 1), it is impossible to
separate whichof these is the primarycorrelateof per-grameffects.
All of the correlationswithquantitativespecies traitsthathave been presentedare
based on mean per-plantor per-grameffectfor each neighbourspecies, averaged
over all targetspecies. Correlationsfor each targetspecies using the slopes from
regressionsforeach neighbourspecies as the individualdata pointsshow verysimilar
(unpublisheddata).
patterns,althoughnot all the correlationswere significant
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1024
Competitiveeffectand response
(a)
3-0
0'6
c'-047
.
1'5
(b)
00
-0.58
@0
0
0.09
0-18
0
0-09
Maximummass (g plant-')
0
3.0 -
0.6
l's
.
.5 1-5
-0074+
0
-0.74+
0
0
0
~~0.3
0
00
0l
0
0.18
0
3.0
0
Seed mass (mg)
1'5
3 0 -
1.5
3.0
&6
@0
o0*
14-5
O
00
0.08
*
0.13
O
0.82*
0.18
0.08
OJ8
0#13
Sow R'GRma,. (mg mg-' day-')
o
c
0*3
0-80*
0.~~~~~~~~~
3.Q
~~~*
E
o
I'S
*
0.6
0.0
0o
0.3
.
0
0,05
00.39
023
0.10
0.15
0.05
0.10
0'15
0.05
Emerge I?GRGmax
(mg mg-' day-')
3-0
0-6
1.5
0
0.62
DO
0
4
0
0
50
0.3
0.85*
0
12
20
4
12
Median time to emergence(days)
3.0
1.5
0
*
*
0
20
0
0.6~
0
03
0.76*
0
*
0o
75
100
50
75
Percentage allocation to shoot
0'45
100
FiG. 5. Mean response to (a) neighbourdensity(x1O-2) and (b) neighbourbiomass as a
functionof species traits: (e) erect species; and (0) low-growingspecies (see Table 1).
Pearson correlationcoefficientsand significancelevels (n = 7) are shown each graph.
+ P<0 10, * P<o005, ** P<o 01, *** P<o 001.
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D. E.
GOLDBERG AND K. LANDA
1025
Comparisonsof targetspecies: competitiveresponse
Because neighbourdensityand biomass were significantly
positivelycorrelatedfor
all forty-nine
species combinations(P < 0-10 for all forty-nine
pairs, P < 0-05 for
forty-twoof forty-ninepairs), mean target responses to neighbour density and
biomass are highlycorrelated(r5= 0 78, P < 0 05) and are discussed together.The
targetspecies respondedsignificantly
to neighbourdensityforfourof the
differently
seven neighbourspecies (Table 2a) and to neighbourbiomass for only one of the
seven neighbourspecies (Table 2b). Few of the pairwise comparisonsof target
species to the same neighbourwere significant
of a possible
(none to fivesignificant
twenty-onecomparisonsfor each neighbourspecies). Nevertheless,the rankingof
response among targetspecies was highlysignificantly
concordantamong neighbour
species for density(W= 064, P < 0-001), although only barely significantly
concordantforbiomass (W= 0 37, P < 0 05). The hierarchyforbest to worstresponse
competitorforresponseto densitywas T. pratense> T. repens> Lolium > Phleum
> Rumex > Chenopodium> Amaranthus.The hierarchyfor response to biomass
was similarbut had several reversalsof nearbyspecies: T. repens> T. pratense>
Lolium > Amaranthus> Chenopodium> Rumex > Phleum.
Response competitiveabilityin terms of both neighbourdensityand biomass
increasedwithloweroverallRGRmax. althoughtherewas no relationship
significantly
withpost-emergenceRGRmax. (Fig. 5). There were also marginaltrends(P < 0 10)
for better response competitorsto have larger seeds (Fig. 5). In addition, good
response competitorsin termsof neighbourdensityhad lower allocation to shoots
and good response competitorsin terms of neighbour biomass had significantly
earlieremergence(Fig. 5a,b). Life-history
type(unpublisheddata) and growthform
(Table 3) had no influenceon response competitiveability.
Effectvs. response
Mean effectas a neighbourspecies and mean response to densityas a target
correlatedforeitherdensity(r5= -0 42, P > 0 10) or
species were not significantly
biomass (r5= 0-57, P>0-10). Note that a negative correlationmeans that species
thatare good-effect
competitorsas neighbours(steeperslopes) are also good-response
competitorsas targets(shallower slopes), whilsta positive correlationindicates a
trade-offbetween the two typesof competitiveability.
DISCUSSION
Target-neighbourrelationships
The average proportionof variancein targetperformanceexplainedby neighbour
densityand biomass was 55% and 61%, respectively,which is much higherthan
thatfoundin most neighbourhoodstudies,even underglasshouseconditions(Mack
& Harper 1977; Fowler 1984; Silander & Pacala 1985; Firbank& Watkinson1987).
This is especiallysurprising
giventheveryshortdurationof the experiment(5 weeks)
and indicates strongcompetitiveinteractions.The most likelyexplanationfor the
highvariationexplainedby competitionis thatthe targetperformancedata reported
in thisstudyrepresentthe mean performanceof 9 targetplantsflat-' ratherthan of
a single individual.This is likelyto have reduced variance among individualsthat
was independentof densityand so increasedthe proportionexplainedby neighbour
densityor biomass.
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1026
Competitiveeffectand response
The relationshipsbetween targetperformanceand neighbourdensitywere nondensitylinear, as has been foundin almost all of the many studiesof intraspecific
dependence (e.g. Weiner 1982, 1984; Silander & Pacala 1985; see Harper 1977 for
density-dependence
review of earlier work) and the fewer studies of interspecific
(Mack & Harper 1977; Goldberg 1987; Pacala & Silander 1987). However, the
relationshipswithbiomass were linear, suggestingthat the cause of the decline in
per-planteffectswith increasingdensityis simplythat individualneighbourplants
are smallerdue to greaterintraspecific
competition.In contrastto thisconstantpergram effect,the relativelyfew examples in the literatureall found decliningpergrameffectswithincreasingbiomass (Goldberg 1987; Goldberg & Fleetwood 1987;
Miller& Werner1987; Peart 1989). Because all of these studiesreportedon targetneighbourrelationshipsat the end of a growingseason or at the end of the plants'
life cycle, it may be that the constantper-grameffectfound in this studyis due
to our focus on the early stages of competition.For example, many plants shift
allocation fromleaves and roots to support,storage or reproductivetissues over
a season or with age (Harper 1977), which would result in decliningper-gram
resourceuptake rates and hence decliningper-gramcompetitiveeffects.If thisshift
is less pronounced at lower total neighbourbiomass (lower competitionbetween
neighbours),it would explain the contrastin resultsbetween this studyand other
studies in the literature.
Competitiveeffect:hierarchies,equivalenceand associated traits
Hierarchiesof competitiveeffectwere highlyconcordantamong neighbourspecies
on both a per-plantand per-grambasis, suggestingthat rankingsof competitive
effectsare independent of the target species. It is possible that extendingthe
interactionsover a longer time would have resultedin less-consistenthierarchies.
studiescomparingpairwisecompetitive
However, in a reviewof several longer-term
interactionsbetween a numberof plant species, Keddy & Shipley(1988) also found
that hierachiesof competitiveeffectare generallyconsistentamong targetspecies
(see also Miller& Werner1987; Peart 1989; Gurevitchet al. 1990). This generallack
of species-specific
pairwiseinteractionsin plantssuggeststhatthereis relativelylittle
betweenplantspecies so thatall species competeforcompletely
resourcepartitioning
shared resources.
Given consistenthierarchiesof competitiveeffects,what traitsdetermineposition
in hierarchieson eithera per-plantor per-grambasis? Competitiveeffectsshould be
relatedto abilityto deplete resourcesand make themunavailable to otherplants greater depletion rates of limitingresources should lead to greater competitive
effects(Goldberg 1990). Because a large componentof depletion abilityis simply
total biomass or surfacearea of resource-acquiring
organs,per-planteffectsshould
be stronglyrelated to plant size and species should be much more similar in
basis (Goldberg &
competitiveeffecton a per-unitsize basis thanon a per-individual
Werner 1983). The results of this study are consistentwith this hypothesis:the
proportionof variance explained by neighbourspecies was much smaller for the
comparisonsof per-grameffectsthan for per-planteffectsand the hierarchyof
per-planteffectswas highlycorrelatedwithboth initialplant size (seed mass) and
maximumpotentialplant size. However, some differencesbetween species in competitiveeffectwere found even on a per-grambasis, suggestingthat plant size (as
reflectedby totalbiomass) is not the onlydeterminant
of abilityto deplete resources.
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D. E.
GOLDBERG AND K. LANDA
1027
The hierarchyof per-gramcompetitiveeffectswas at least partiallydeterminedby
maximumpotentialrelativegrowthrate, consistentwiththe idea that,all else being
equal, species that grow fasterdeplete resources faster. However, the effectsof
RGRmax. on competitiveeffectwere confoundedwiththose of growthform.Because
the species in this studywith higherRGRmax. also had a more-erectgrowthform
that would be expected to deplete light to a greater extent, it is impossible to
determinefromthese resultsif both of these traitsinfluencecompetitiveeffector
only one of them.
Our resultsare consistentwith several recent studies that have also compared
per-plantand per-unitbiomass effectsor both, using somewhatlonger-term
experdifferentbetween neighbour
iments. Per-plant effectsare generallysignificantly
species (Goldberg 1987; Goldberg & Fleetwood 1987; Gordon et al. 1989; Gurevitch
et al. 1990). For per-grameffects,experimentsconductedin the fieldhave all found
statisticalequivalence (Goldberg 1987; Miller & Werner 1987; Peart 1989), whilst
those conducted under more-controlledconditionshave all found at least some
differences
betweenspecies (Goldberg & Fleetwood 1987; Gordon et al. 1989), as in
this study. Thus, the greater variance in other factorsaffectingplant growthwe
expect under field conditionsmay mask real differencesbetween species in their
competitiveeffects,at least fromthe viewpointof an ecologistwithan arbitrary
level
of significanceand power to detect differences.
Competitiveresponse:hierarchies,equivalenceand associated traits
As with competitiveeffect,hierarchiesin competitiveresponse among target
species were similarregardlessof neighbourspecies. This concordance was much
weaker, however, for responses to neighbourbiomass than to neighbourdensity.
This is probablybecause of the much weaker differencesamong targetspecies in
response to neighbourbiomass. Therefore,the greaterinconsistencyin rankingof
competitiveresponse to neighbourbiomass among targetspecies is unlikelyto be
importantbiologicallybecause the targetspecies were largelyequivalentin response
to neighbourbiomass. This does, however,leave open the question of why target
species were more similarin responseto biomass thanto densityforeach neighbour
species. Because neighbourdensityand biomasswere alwayshighlycorrelatedforall
species combinationsin this study,we can see no obvious reason for thispattern.
Position in competitiveresponse hierarchiesamong species should depend on
eitherrelativeabilities to tolerate depleted resource levels due to the presence of
neighboursor relativeabilitiesto avoid experiencingdepletedresourcelevels because
of pre-emptionof resources fromneighbours.Which of these is more important
should be related to relativesizes of targetsand neighbours(Goldberg 1990). When
targetsare small relativeto theirneighbours,theyare unlikelyto cause significant
depletionof resourcesrelativeto thedepletioncaused by theirneighbours.Therefore,
rankingof competitiveresponseshould be determinedby abilityto toleratedepleted
resourcelevels. Because species more tolerantto shade and low nutrientlevels often
have lower RGRmax, this translatesinto the predictionthat betterresponse competitorsshould have lower RGRmax. (see reviewsby Boardman 1977; Bazazz 1979;
Chapin 1980, 1988; Grime 1988). In contrastto this predictionfor stronglysizeasymmetrical
competition,iftargetplantsare similarin size to theirneighbours,preemptionof resourcesthroughearlier and/ormore rapid growthcould play a much
largerrole in determiningcompetitiveresponse (Goldberg 1990).
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1028
Competitiveeffectand response
Because seeds of targetand neighbourplantswere sown at the same time in this
study, the size-symmetrical
case seems more applicable, and early emergence, a
large initial size advantage, and/or high RGR max. should be positivelycorrelated
with competitiveresponse. The applicabilityof the size-symmetrical
case to our
experimentis reinforcedbecause of the shortdurationof our experiment.As a plant
stand develops, differencesin size between individualsor between species often
bcome accentuated(Weiner & Thomas 1986), whichcould lead to a switchover time
fromtraitsrelated to resource pre-emptionto traitsrelated to resource tolerance
being correlated with competitiveresponse. However, most of the results were
actuallymore consistentwith the size-asymmetrical
case. Earlier emergingspecies
were betterresponse competitorsas predictedforsize-symmetrical
interactions,but
only in response to neighbourbiomass, and no relationshipswith post-emergence
RGRmax., initial plant size or maximum plant size were found for response to
neighbourdensityor biomass. Furthermore,when RGRmax. was calculated from
date of sowingratherthan date of emergence,betterresponse competitorsto both
neighbourdensityand biomass actually had significantly
slower growthrates, as
predictedfor the size-asymmetrical
situation.
One hypothesisto explain why response hierarchiesseem to be associated with
low RGRmax. rather than with high RGRmax.as expected with this experimental
design is related to the variationin growthformamong species in this study. If
competitionwas largelyforlight,the low-growingspecies would not be able to preempt lightregardlessof theirrelativegrowthrate or size. Therefore,the alternative
mechanismof being a superior response competitor,tolerance of low resources
and its typicalcorrelate, low RGRmaX, would be expected to predominate.This
hypothesiswould predictthat position in the response hierarchyamong the erect
species only should be positivelycorrelatedwith RGRmax.. Unfortunately,
sample
size withingrowthformsis too small to test thisprediction.
Hierarchiesof effectvs. responsecompetitiveabilities
The hypothesesdescribed above for the traitsdeterminingeffectand response
competitiveabilitysuggestthe additionalhypothesisthatthe two typesof competitive
abilityshould be positivelycorrelatedfor size-symmetrical
competition(resource
pre-emptionpossible) and uncorrelatedor negativelycorrelatedforsize-asymmetrical
competitionwhen toleranceof low resourceavailabilitybecomes an importantcomponentof responsecompetitiveability.If the hypothesisabove about the importance
of growthformover initialconditions(seed-seed vs. seed-adult) in our studyis
correct,the resultsof this studyand others in the literaturecomparingeffectand
responsehierarchiesare generallyconsistentwiththishypothesis.In the short-term
glasshouse studyreportedin this paper, the two types of competitiveabilitywere
uncorrelatedwitheach otherbased on eitherneighbourdensityor neighbourbiomass
(althoughrelationshipswithRGRmaX. fromtimeof sowingwere opposite forthe two
types). In a much longer-termfield study of seedling-adult interactions,Peart
(1989) also found no correlationbetween effectand response competitiveability,
as would be predictedfor these size-asymmetrical
interactions.Three longer-term
studies startingall plants fromseed all found positive correlationsbetween effect
and response competitiveability, as predicted for size-symmetricalinteractions
(Goldberg & Fleetwood 1987; Miller & Werner 1987; Gurevitchet al. 1990).
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D. E.
GOLDBERG AND K. LANDA
1029
Conclusions
The observationthateffectand response are not always positivelycorrelatedand
can be related to differentplant traitsemphasizes the importanceof measuring
both of them if we are to develop generalizationsabout the traitsthat determine
competitiveability.Althoughour resultsapply directlyonly to the early stages of
competition,many of our results are consistentwith longer-termstudies in the
literature.Nevertheless,it is criticalthat the effectsof plant size and age on effect
and response competitiveability be evaluated directly.It also becomes critical
to establish the relative importanceof effectand response competitiveabilityof
individualsin determiningthe long-termpopulation-leveloutcome of competition.
Goldberg (1990) suggestedthat good-effectcompetitorsshould dominate early in
successionor withingaps whereresourcepre-emption
is possible,whilstgood-response
competitorsshould dominatelaterin successionand in undisturbedvegetationwhere
seedlings establish under adults. Testing this hypothesisabout effectvs. response
competitiveabilityor anyotherabout therelationshipbetweenindividualcompetitive
ability and population abundance will require a good understandingof the lifehistorystages that regulate populations; an understandingthat we have now for
veryfew species.
ACKNOWLEDGMENTS
We are verygratefulto the studentsin the General Ecology class at the University
of Michigan,Fall Semester 1986 for theirassistancein plantingand harvestingthe
Tom Miller,Earl Wernerand two anonymous
experimentsand to BetsyKirkpatrick,
refereesfor theircommentson earlierdrafts.This studywas partlysupportedby a
Rackham Faculty Grant to DEG fromthe Universityof Michigan.
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