Download THE INDIVIDUAL IN THE POPULATION

149
THE INDIVIDUAL IN THE POPULATION
BY JOHN L. HARPER
Department of AgriculturalBotany, University College of North Wales, Bangor
INTRODUCTION
One of the most striking and original ecological studies made by A. G. Tansley was an
attempt to determine experimentally how far the distribution of a species was explicable
in terms of its direct reaction to soil type and how far interference from its neighbours
modified this reaction. He grew Galium hercynicum (G. saxatile) and G. pumilum (G.
sylvestre) in pure and mixed stands on calcareous soil and acid peat, and from the results
he concluded 'Both species can establish and maintain themselves-at least for some
years-on either soil', but 'the calcicole species is handicapped as a result of growing on
acid peat and is therefore reduced to subordinate position in competition with its calcifuge rival, which is less handicapped' and '... the calcifuge species (Galium saxatile) is
heavily handicapped, especially in the seedling stage, as a direct effect of growing on
calcareous soil, and is thus unable to compete effectively with its calcicole congener,.
Galiumsylvestre' (Tansley 1917).
A major implication of Tansley's experiment is that the biology of a species seen in
isolation may not account for its ecology-yet this implication seems to be widely
ignored. It is the aim of the present paper to bring together some further examples of
plant interactions which result in changes in the behaviour of individuals as they become
influenced by the proximity of their neighbours.
Two major concepts of plant ecology, succession and climax, derive from observations
that plant species in an area modify each other's environment in such a way that they
progressively replace one another. Eventually, species are sifted by such a process until a
condition of apparent relative stability is reached. In this process each species changes
from being an invader and aggressor to being suppressed and eventually extinguished.
The ecology of a species in succession is therefore critically defined by its reaction to the
presence of others-those it ousts and those which in turn oust it. Similarly, the ecology
of a species which persists in a 'stable' climax is critically defined by those of its properties
which enable it to hold its own in the presence of associates. It may be argued, therefore,
that the essential qualities which determine the ecology of a species may only be detected
by studying the reaction of its individuals to their neighbours and that the behaviour of
individuals of the species in isolation may be largely irrelevant to understanding their
behaviour in the community.
Many aspects of the reaction of organisms to neighbours may be studied in model
populations.
A REVIEW OF EXPERIMENTS
1. Simple models of populationgrowth infree floating aquatics
A simple model of plant populations may be made from floating aquatics which can be
provided with a highly uniform controlled environment in glass beakers of culture
solutions at constant temperature and under constant light intensity (Clatworthy &
The individualin thepopulation
150
Harper 1962). Populations which are started with an inoculum of a few fronds follow a
growth curve similar to that described for Chlorella by Priestley & Pearsall (1922) (see
Fig. 1). In the growth curve, there is an initial period of exponential growth (Phase I) in
which the rate of increase of the population is a function of the plant capital available
(for example, Lemnaminor0 35 g/g/day). As fronds spread over the surface of the culture,
individual fronds overlap and a mat begins to form. The growth-rate ceases to be
exponential and eventually becomes linear-no longer a direct function of 'capital'but now a function of the 'size' (in this example, the surface area) of the habitat (Phase II,
L. minor 14-7 g/beaker/day). As the mass of fronds becomes thicker the lower fronds,
receiving negligible light, lose weight and die. The population then approaches a constant
3*0
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WEEKS
1. The growthin dryweightof Lemnaminorin self-crowdingcultures.Data areplotted
on a logarithmicscaleso that the varianceishomogeneous.The arrowindicatesgrowthafter
12 weeks. Phase I growth-ratewas determinedin an independentexperimentin which
crowding was prevented. Phase I, 0 35 g/g/day; Phase II, 14-7 mg/culture/day.(From
Clatworthy& Harper 1962.)
FIG.
size as the rate of loss equals the rate at which new fronds are produced (Phase III). The
qualities of species in such model populations may be defined by various parameters,
(i) the exponential growth-rate of Phase I, (ii) the linear growth-rate of Phase II, or (iii)
the population stock of Phase III. Four species, L. minor, L. gibba, L. polyrrhiza and
Salvinia natans were grown in single-species cultures. The species differed from each
other in all three parameters of growth.
Cultures of various mixtures of two species were also grown for 12 weeks and the
struggle for existence which developed was followed. An example of the behaviour of a
mixture is contrasted with the behaviour of pure cultures in Fig. 2. The following conclusions from the whole experiment are relevant to this paper.
(i) No single parameterof growth of two species in pure cultures was a reliable indicator
of their fate in mixtures. The exponential (Phase I) growth-rate of S. natans was lower
than that of Lemna minor, yet the proportion of Salvinia natans progressively increased
in mixed cultures of these two species. The Phase II growth-rate of Lemnapolyrrhizawas
JOHN L. HARPER
151
greater than that of L. gibba, but L. gibba succeeded at the expense of L. polyrrhiza in
mixtures. Pure cultures of L. polyrrhiza achieved higher yields in Phase III than L. gibba,
but L. gibba was the more successful in mixtures.
(ii) In a closely balanced struggle for existence, such as that which developed between
L. minorand L. polyrrhiza of which the outcome was still in doubt after 12 weeks, the role
of chance played a large part in determining the balance between species in replicate
cultures.
(iii) The development of a population of fronds within a highly uniform habitat rapidly
created heterogeneity within the habitat. In these experiments the heterogeneity was an
600 -
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.
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WEEKS
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FIG. 2. The growthin dry weightof Lemnapolyrrhizaand L. gibbain pure and mixedselfao,
*, L. polyrrhizaalone; 0
crowdingcultures.N.B. Scale not transformed.@
L. gibbaalone; *- - -@, L. polyrrhizain presenceofL.gibba; o- - -o,L.gibbain presence
of L. polyrrhiza.(From Harper1961.)
obvious gradient of light intensity through the depth of the frond mat and probably an
associated gradient of respiratorygases.
Many of the conclusions from this experiment confirm for populations of higher plants
(albeit rather peculiar plants) the conclusions of Thomas Park (1955) from experiments
with cultures of flour beetles and because of a joint concern with the dynamics of populations, provide a rare opportunity for contact between plant and animal ecology.
2. Reversal of habitatpreferences in thepresence of a second species
The essence of the experimental design of Tansley's study of the Galium species was
that the reaction of two species grown under different environmental conditions in pure
stand was contrasted with their growth in mixed stands. A comparable experiment was
made by Harper & Chancellor (1959) who sowed Rumex species with and without
The individualin thepopulation
152
Lolium perenne in a clay soil. The environment was varied by controlling the water
table at 10 cm from the soil surface or by allowing free drainage. The establishment of the
Rumex species, measured as the number of plants present after 12 months, is shown in
Fig. 3. In the absence of the grass the establishment of both species of Rumex, R. crispus
and R. obtusifolius, was more successful when the water table was maintained. In the
presence of grass, establishment of both species of Rumex was reduced but the most
successful establishment of R. obtusifoliusnow occurred under freely drained conditions.
The following conclusions are important to the arguments of this paper:
(i) The habitat 'preferences' of a species may become less marked (e.g. R. crispus) or
reversed (e.g. R. obtusifolius)in the presence of a further species.
4T FIDUCIAL
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-GRASS
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FIG. 3. The establishmentfrom seed of Rumes crispus (above) and R. obtusifolius (below)
under two water regimes (oi, freely drained; *, maintainedwater tablesee text) in the
presenceand absence of Lolium perenne. The right-handgraph is of data transformedto
squareroots to give homogeneousvarianceand permitfiduciallimits to be shown.
(ii) This experiment illustrates interferencebetween cohabiting species of very different
systematic position, Rumex and Lolium. In view of the stress commonly laid on the
special problem of the cohabitation of closely relatedspecies (e.g. Tansley 1917) it must be
emphasized that mutual interference is not limited to congeners.
3. An associaltionbetweensoil reacltionand the vigourof interferenceof two species
De Wit (1960) has introduced a subtle and sensitive experimental design (the replacement series) with which to study the mutual influences of species in mixtures. The design
consists essentially in maintaining constant the overall density of a sown or planted
mixture of two species, A and B, while at the same time varying the proportions of A to B.
The results of such an experiment involving mixtures of oats and barley are illustrated in
Fig. 4. This experiment was made in various parts of the Netherlands on sandy soils in
JOHN
L.
153
HARPER
which the reaction of the soil was one significant variable (Fig. 4 a and b) or in which the
reaction was deliberately varied (Fig. 4 c-f). Each experiment contained pure stands of
both species which are represented at opposite ends of the horizontal scale in the graphs
and an equi-proportioned mixture occupies the intermediate position on the horizontal scale. The vigour (or aggressiveness) of a species in a mixture is shown by the
relative convexity or concavity of the yield curves. Various parameters of the population
may be plotted in such graphs-in Fig. 4 the number of grains produced is shown in
relation to the number of grains sown. The following features of the results are
particularly significant.
!
(b)
(a)
(c)
120
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Barley
50
100
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~ d 7 e -,()
0 Barley
100
50
50 Oats
.i 16..).....
100
0
OF OATSAND BARLEYSOWN (0h)
PROPORTIONS
FiG. 4. Seed productionby barley and oats when sown at normal agriculturalseed rates,
in pure and mixed stands. @, oats; x, barley. (a) Averagefrom a range of fields of pH-
KCIl>4-6. (b) Averagefrom a range of fieldsof pH-KCI< 4 -6. (c) to (f) from a subsidiary
experimentin which soil reaction was altered by fertilizertreatment;pH-KCI: (c) 4-0,
(d) 3-7, (e) 3-2, (f) 3-1. (From de Wit 1960.)
(i) The relative reproductivecapacity of two species in pure stands does not necessarily
indicate their relative performance in mixtures-on soils of high pH barley was the more
successful component in mixtures with oats, although it was the lower yielder of the two
species in pure stands (Fig. 4a).
(ii) The yields of pure stands of both oats and barley showed great constancy over a
range of soil reactions (Fig. 4 a-c), but in mixtures the oat became relatively more successful with increasing acidity of the soil.
(iii) In soils of very low pH values the yield of barley was lowered even in pure stands
and at the lowest pH (Fig. 4f) barley ceased to yield at all; but the yield of barley in
mixtures was reduced even at pH values which did not influence yield in pure stands.
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The individualin thepopulation
(iv) De Wit showedthat if environmentalconditionswereexactlyrepeated,successive
sowingsof the progenyof a mixtureof oats andbarleyon soils of highpH wouldlead to
the progressivedominanceof barley-despite its lowerreproductivecapacity.
Variousother examplesare known in which,of a pairof speciesor varieties,the one
which yields best in pure stand does not survivewhen repeatedlysown in association
with the other speciesor variety.Gustafsson(1951)lists examplesof this 'Montgomery
effect'whichhe namedafterMontgomery(1912)who madeone of the firstreportsof this
phenomenon.
4. Interferencebetweenplants involvingexploitation of the canopy
It has now been demonstratedfor a wide range of naturaland artificialplant communitiesthat duringtheheightof the growingseasonalmostallincidentlightis trappedor
reflectedby the vegetationand that leavesplacedlow in the vegetationexist below the
compensationpoint (Monsi & Saeki 1953). It seems probable that, in the ultimate
analysis,interferencewith suppliesof light is the most potentway in which one species
the roles of water
may succeedat the expenseof another.This is not to under-emphasize
andnutrientsin a strugglefor existence,but to suggestthat theirrolelies oftenin modifying the timingand extentof an ultimatestrugglefor light.
Amongstthe simplestmodelpopulationsin whichthe role of light in the strugglefor
studiedby Black.In comexistencehas beenstudiedare those of Trifoliumsubterraneum
parisonsof populationsdevelopedfromlargeand smallseeds,Black(1957, 1958)found
that pure stands derivedfrom large seeds were eventuallyequalledin yield by stands
from an equal numberof small seeds. The populationsfrom large seeds more rapidly
reachedceilingyield but wereultimatelyconstrainedwith limitsset by the environment,
and the differencein the seed 'capital'investedthen became relativelyunimportant.
However,when large and small seeds were sown in mixture,the situationwas very
different.The plants derivedfrom large seedshad largercotyledonsand maintainedan
increasingsuperiorityin the mixtureuntil after 11-12 weeks 97% of incidentlight was
beinginterceptedby those plantsderivedfromlarge seedsand less than 3% by the now
suppressedplants from small seeds. Essentiallysimilarbehaviourwas found by Black
whichdiffered
(1960)whenhe grewpureandmixedstandsof varietiesof T. subterraneum-i
in petiole length. When pairs of varietieswere grownin mixtures,there was alwaysa
rapidassumptionof superiorityby the longerpetioledformwhichquicklycame to trap
the greaterpartof the incidentlight.
In these two examples,the successfulpartnerin a mixturewas that one whichcontributed most to closing the canopy and monopolizingincidentlight. In both of these
examplesthe mixtureswereintraspecific(differentseed sizes from a seed sampleof one
varietyor differencesin petiolelengthbetweenvarietiesof the samespecies).
Stern& Donald (1962;see also Donald 1961)havefoundmuchthe samephenomenon
andLoliumrigidumin whichdifferencesin
in mixturesof two speciesof T. subterraneum
nitrogensupplymay bias the outcomeof interferencein favourof one or the othercomponentand showedthat the dominatinginfluenceof nitrogenwas upon the relativerate
and heightat whichthe componentsformeda closedcanopy.
The followingconclusionsmay be drawn:
(i) The differentialgrowth-rateof two speciesin purestandstendsto becomereduced
or maskedwhentheirpopulationsarelimitedby the supplyingpowerof the environment
(see also the studieson Bromusspp. reportedby Harper1961).The supplyingpowerof
JOHN
L.
HARPER
155
the environment then becomes the main determinant of production. This exactly parallels
the development of Lemna and Salvinia cultures as they changed from Phase I to Phase II
of population growth. Most plant populations (unless severely restricted by water or
nutrient shortage or short growing season) tend to develop until most of the incident
light is intercepted.
(ii) In mixed stands the simple differential growth of two species, involving cotyledon
size, hypocotyl length, petiole or stem length, may determine the way in which light
resources are shared. Once one form is in the ascendancy its domination is likely to lead
to monopolistic trapping of the light.
5. More complexpatterns in the behaviourof mixtures
The foregoing examples illustrated the progressively monopolistic utilization of an
environment by one of a pair of contrasted plant forms, varieties or species. This may
happen when two components of a population differ in a single character which gives one
a cumulative advantage over the other. The ability to put a canopy higher than that of
the neighbour may be a common way in which plant succession is brought about. However, not all of the models of interference between plant species are so simple and it may
be that more complex behaviour may be necessary for the formation of communities in
which relatively stable cohabitation of mixtures of species occurs.
Examples of interferencebetween species which may lead to their continued persistence
together have been discussed by Harper, Clatworthy, McNaughton & Sagar (1961). Two
may be cited here by way of illustration. Two species of clover, Trifoliumrepens and T.
fragiferum, commonly cohabit in alluvial grassland and on sand dunes. When sown
together in mixtures these species show a fascinating alternation of advantages one over
the other through the first season of growth (Harper & Clatworthy 1963). T. fragiferum
has the larger seeds and starts growth with greater embryonic capital, but it possesses a
greater proportion of hard seed than T. repens which may give it an advantage or place
it at a disadvantage, depending on the hazards of establishment. T. fragiferum bears
larger cotyledons than T. repens, but has a lower relative growth-rate. T. repensproduces
new leaves faster than T. fragiferum and its hypocotyl elongates more sensitively in
response to shading (e.g. by neighbours). When individuals are crowded in mixture, the
hypocotyls of T. repens elongate to carry the small cotyledons of this species to the top
of the developing canopy. After this advantage to T. repens, T. fragiferum remains for a
period of several weeks in a position underneath the main canopy and most of the
incident light is intercepted by T. repens. T. fragiferum has still, however, two remaining
strings to its bow. It is capable of some vertical stem growth in contrast to T. repens
which is wholly stoloniferous, and it is capable of greater petiole extension than T.
repensand eventually overtops T. repensin the canopy. Such an alternation of advantages
in the first year of growth ensures each species a period of dominance in the canopy and
the light supply of the season is partitioned in time between the two species. At no stage
(in the first year) is a cumulative advantage gained by one species.
A second complex interaction between individuals has been reported for populations
of poppies (Harper & McNaughton 1962). Mixtures of two or more species of Papaver
are very common in Britain, in fact, P. rhoeas is the only species which is normally found
living in areas without its congeners. The ability of these species to persist together without one ousting, or succeeding at the expense of, another seems to be accounted for in the
way that individuals respond to interferencefrom neighbours.
In populations of each species of Papaverthe mortalityrisk of individuals increaseswith
156
The individualin the population
increasing density. This has the effect of placing an upper limit on population size, irrespective of the number of seeds sown. In mixtures of species, individuals of the most
abundant species suffer the highest mortality risk and individuals of the minority
species are relatively favoured. This effect penalizes any species which gains numerical
predominance in mixture and gives mixtures more stability than pure stands.
Differencesbetween species which are crucial in determiningtheir success or failurewhen
grown together may only be exposed and demonstrated when the species are grown
together. For example, the plasticity of the hypocotyl and petiole of clovers largely determines which leaves in a mixed stand form the canopy and trap the most incident light.
The plasticity of these organs is not obvious in isolated individuals and its significance is
not apparent in pure stands. Only when the species are grown together is the critical
nature of this plasticity evident. Likewise, in the example from the poppies, self-thinning
or density-induced mortality is a phenomenon of individuals in populations, not manifest in the behaviour of isolated plants. The behaviour of mixed populations is, moreover,
not predictable from the behaviour of pure stands.
The experiments described above represent only a small part of a rapidly growing
section of experimentalecology concerned with the mutual influences of plants in populations. Other examples of this type of experimentation are representedin New Zealand by
the work of Brougham (1962), by Kasanaga & Monsi (1954), Monsi & Saeki (e.g. 1953)
and by Kira (e.g. 1953) and his colleagues in Japan, and in Germany in the school of
Knapp (e.g. 1954).
THE PHYSIOLOGY OF POPULATIONS
The results of experiments on interference between plants pose significant challenges in
most fields of ecological study. The form, tolerances and persistence of species may be
profoundly modified by the proximity of neighbours of the same or other species. It
follows that the characteristicsof individual species shown by isolated individuals or pure
populations may offer no significant guidance to their behaviour in the presence of others.
Conversely, the ecology and distribution of a species in the presence of others may offer
no significant guide to the behaviour of isolated individuals. These conclusions are, of
course, readily accepted by gardeners who make use of the astonishingly wide ecologic
tolerance and geographic range of species grown in isolation.
Individuals free from the influence of neighbours are anomalies in nature. That plants
in nature are normally under stress from their neighbours can usually be shown by the
removal of the neighbours (thinning of woodland, thinning of garden crops, selective
killing in grassland and road verges with herbicides, selective defoliation by predators)
which leads very regularly to enhanced growth.
A part of the influence of neighbouring plants upon each other derives from the forced
sharing of environmental resources and the resultant modification of individual physiology. As the behaviour of individuals is modified by their neighbours, so the population
acquires its own distinctive physiology-different from that of isolated plants. This may
be illustrated in two examples:
(1) Reactions of plants to light intensity
The influence of light intensity on assimilation by isolated leaves is illustrated
in Fig. 5(a) (Alexander & McCloud 1962). However, as whole plants grow, their
leaves overlap and shade each other or are shaded by the leaves of neighbours. A light
157
JOHN L. HARPER
intensity which is supra-optimalfor the top-most leaves in a canopy may then be far from
optimal for the leaves beneath. Further increases in light intensity may then increase the
photosynthetic rate of the population as the shaded leaves receive more optimal light
intensities. The response of a population of plants to light intensity then takes a different
form from that of isolated leaves, e.g. contrast Figs. 5 (a) and (b).
(2) Transpiration
Water loss by an isolated plant or leaf may be relatively easily measured and related
to factors of its surrounding atmosphere. The physiology of transpiration built on such
models has little relevance in populations because the presence of neighbours may
(a)
-
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E
N100
sI
15
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()
1
E
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.
2
3
4
ILLUMINATION
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(b)
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31
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-
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3
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ILLUMINATIONft candies
FIG.
5. (a) The relationshipbetweenCO
2
5
6
7
x1000)
uptakeper unit leaf area and light intensityfor
isolated leaves of Cynodon dactylon (L.) Pers. (b) Similarly for swards of C. dactylon cut
daily to 1, 2 and 8 in. (2 5, 5 and 20 cm) height. (FromAlexander& McCloud 1962.)
introduce between leaves on a plant, and between individuals in the population, differences as profound as those between contrasting habitats. Penman's approach (1948) to
the water relations of plant communities recognizes that the population can often usefully be regarded as a physiologic unit and that water loss from such populations becomes, within limits, a function of the energy supply to an area, independent of the
number of plants, species or the composition of the community.
CONCLUSIONS
In descriptive ecology there is a widening gulf between the description of communities
in terms of species composition and description in terms of production. In experimental
ecology there is a comparable gulf between experiments on individuals and experiments
on populations. This paper is intended to stress the need for more than lip service to be
L J.E.
The individualin thepopulation
158
paid to the ways in which plants may interfere with each other and the consequences of
this interference. It is intended to focus attention,on the reaction of a plant to its neighbours as a critical, often the most critical, part of the autecology of a species and to
suggest that this type of study has a cementing and unifying function in the science of
plant ecology. It is appropriate at the Jubilee Meeting to remember that it was the first
president of the British Ecological Society who made the classical study in this field.
REFERENCES
Alexander,C. W. & McCloud,D. E. (1962). C02 uptake (net photosynthesis)as influencedby light
intensity of isolated Bermuda grass leaves contrasted to that of swards under various clipping
regimes. CropSci. 2, 132-5.
L.)
Black, J. N. (1957). Seed size as a factor in the growthof subterraneanclover(Trifoliumsubterraneum
under spaced and sward conditions. Aust. J. Agric. Res. 8, 335-51.
Black, J. N. (1958). Competitionbetweenplants of differentinitial seed sizes in swardsof subterranean
L.) with particularreferenceto leaf area and the light micro-climate. Aust.
clover (T. subterraneum
J. Agric.Res. 9, 299-318.
Black, J. N. (1960). The significanceof petiole length, leaf area, and light interceptionin competition
L.) grownin swards. Aust.J. Agric.
betweenstrainsof subterraneanclover(Trifoliumsubterraneum
Res. 11, 277-91.
Brougham,R. W. (1962). The leaf growth of Trifoliumrepensas influencedby seasonal changesin the
light environment. J. Ecol. 50, 449-59.
Clatworthy,J. N. & Harper,J. L. (1962). The comparativebiology of closely relatedspeciesliving in the
withinculturesof Lemnaspp. and Salvinianatans.
same area. V. Inter-and intra-specificinterfereence
J. Exp. Bot. 13, 307-24.
De Wit, C. T. (1960). On competition. Versl.Landbouwk.Onderz.66, 1-82.
Donald,C. M. (1961).Competitionfor light in cropsand pastures. Symp.Soc. Exp. Biol. 15, 282-313.
Gustafsson,A. (1951). Mutations,environmentand evolution. Cold Spr. Harb. Symp. Quant.Biol. 16,
263-80.
Harper,J. L. (1961).Approachesto the studyof plant competition. Symp.Soc. Exp. Biol. 15, 1-39.
Harper,J. L. & Chancellor,A. P. (1959). The comparativebiology of closely relatedspeciesliving in the
same area. IV. Rumex: Interferencebetween individualsin populations of one and two species.
J. Ecol. 47, 679-95.
Harper,J. L. & Clatworthy,J. N. (1963).The comparativebiology of closely relatedspeciesliving in the
same area. VI. Analysis of the growth of Trifoliumrepensand T. fragiferumin pure and mixed
populations. J. Exp. Bot. 14, 172-90.
Harper,J. L., Clatworthy,J. N., McNaughton,I. H. & Sagar, G. R. (1961).The evolution and ecology of
closely relatedspeciesliving in the same area. Evolution,15, 209-27.
Harper,J. L. & McNaughton,I. H. (1962).The comparativebiology of closelyrelatedspecieslivingin the
same area. VII. Interferencebetweenindividualsin pure and mixed populationsof Papaverspecies.
New Phytol. 61, 175-88.
Kasanaga,H. & Monsi,M. (1954).On the light-transmissionof leaves,and its meaningfor the production
of matterin plant communities. Jap.J. Bot. 14, 304-24.
Kira,T., Ogama,H. & Sakayaki,N. (1953).Intraspecificcompetitionamong higherplants. J. Polytech.
OsakaCity Univ.4, 1-16.
Knapp, R. (1954). Experimentelle Soziologie der Hdheren Pflanzen.
Stuttgart.
und Bedeutungfur die
Monsi,M. & Saeki, T. (1953).Ueberden Lichtfaktorin den Pflanzengesellschaften
Stoffproduktion. Jap.J. Bot. 14, 22-52
Montgomery,E. G. (1912).Competitionin cereals. Bull.Nebr.Agric.Exp. Sta. 24.
Park, T. (1955).Experimentalcompetitionin beetles, with some generalimplications. In TheNumbers
of Man and Animals(Ed. by J. B. Cragg and N. W. Pirie). Edinburgh.
Penman, H. L. (1948). Natural evaporationfrom open water, bare soil and grass. Proc. Roy. Soc.
A 193, 120-46.
Priestley,J. H. & Pearsall,W. H. (1922).An interpretationof some growthcurves. Ann.Bot., Lond.36,
239-49.
Stern,W. R. & Donald,C. M. (1962).The influenceof leaf area and radiationon the growthof clover in
swards.
Aust. J. Agric. Res. 13, 615-23.
Tansley, A. G. (1917). On competitionbetween Galiumsaxatile L. (G. hercynicumWeig.) and Galium
sylvestrePoll. (G. asperumSchreb.)on differenttypes of soil. J. Ecol. 5, 173-9.