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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 --A'ASE 0 III 1 *0 C-) o~~~~~~ Oi cD 2-0O 0 -S 1*5 0 2 4 6 8 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 - 400 E / LU 200 - L 0 . I 2 . 4 WEEKS , 6 8 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 10- -GRASS 0-05 3-~~~~~~~~~~~jP= +GRASS 2- -GRASS FIDUCIAL ?1o -T3 J - GRASS +GRASS +GRASS -GRASS LIMIT +GRASS 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 00 Barley 50 100 L40cr 0r 6 KC1>4 120O I- (f) X 80 120 (e) A fo(d) ~ -.(rmd ~ x~~~~~~~~~~~~~~~ ~ 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. 154 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) - E 20 E N100 sI 15 0 () 1 E 0 . 2 3 4 ILLUMINATION (ft. candles x 1000) 5 C (b) 5 r 6 8 daily 0 E_/ 31 1 daily - 2s o 0 1 2 3 4 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.) 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