THE INDIVIDUAL IN THE POPULATION

149

THE INDIVIDUAL IN THE POPULATION

BY JOHN L. HARPER

Department of Agricultural Botany, 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,. Galium sylvestre' (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 population growth in free 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 &

150 The individual in the population

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, Lemna minor 0 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 III

0

o~~~~~~ Oi

1 *0 C-)

cD 2-0O

0 -S 1*5

0 2 4 6 8 WEEKS

FIG. 1. The growth in dry weight of Lemna minor in self-crowding cultures. Data are plotted on a logarithmic scale so that the varianceis homogeneous. The arrow indicates growth after 12 weeks. Phase I growth-rate was determined in an independent experiment in which crowding was prevented. Phase I, 0 35 g/g/day; Phase II, 14-7 mg/culture/day. (From

Clatworthy & Harper 1962.)

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 parameter of 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 Lemna polyrrhiza was

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. minor and 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 I . , .

0 2 4 6 8 WEEKS

FIG. 2. The growth in dry weight of Lemna polyrrhiza and L. gibba in pure and mixed self- crowding cultures. N.B. Scale not transformed.@ *, L. polyrrhiza alone; 0 ao, L. gibba alone; *- - -@, L. polyrrhiza in presenceofL.gibba; o- - -o,L.gibbain presence

of L. polyrrhiza. (From Harper 1961.)

obvious gradient of light intensity through the depth of the frond mat and probably an associated gradient of respiratory gases.

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 habitat preferences in the presence 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


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. obtusifolius now 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.

4-

T FIDUCIAL

- -GRASS +GRASS -GRASS +GRASS

?1o -T3 F IDUCIAL J LIMIT

10- 3- ~~~~~~~~~~~jP= 0-05

2-

- GRASS +GRASS -GRASS + GRASS

FIG. 3. The establishment from seed of Rumes crispus (above) and R. obtusifolius (below) under two water regimes (oi, freely drained; *, maintained water tablesee text) in the presence and absence of Lolium perenne. The right-hand graph is of data transformed to

square roots to give homogeneous variance and permit fiducial limits to be shown.

(ii) This experiment illustrates interference between 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 related species (e.g. Tansley 1917) it must be emphasized that mutual interference is not limited to congeners.

3. An associaltion between soil reacltion and the vigour of interference of two species De Wit (1960) has introduced a subtle and sensitive experimental design (the replace-

ment 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. HARPER 153

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.

(a) (b) ! (c)

120

00 Barley 50 100

L40-

cr 0r

KC1>4 6 A fo(d) (e) (f) 120O I-

120 ~ ~ ~ d 7 e -,() -.(rmd .i 16..).....

80 X

x~~~~~~~~~~~~~~~

0 Barley 50 100 100 50 Oats 0

PROPORTIONS OF OATS AND BARLEY SOWN (0h) FiG. 4. Seed production by barley and oats when sown at normal agricultural seed rates, in pure and mixed stands. @, oats; x, barley. (a) Average from a range of fields of pH- KCIl>4 -6. (b) Average from a range of fields of pH-KCI< 4 -6. (c) to (f) from a subsidiary experiment in which soil reaction was altered by fertilizer treatment; pH-KCI: (c) 4-0,

(d) 3-7, (e) 3-2, (f) 3-1. (From de Wit 1960.)

(i) The relative reproductive capacity 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.


(iv) De Wit showed that if environmental conditions were exactly repeated, successive sowings of the progeny of a mixture of oats and barley on soils of high pH would lead to the progressive dominance of barley-despite its lower reproductive capacity.

Various other examples are known in which, of a pair of species or varieties, the one which yields best in pure stand does not survive when repeatedly sown in association with the other species or variety. Gustafsson (1951) lists examples of this 'Montgomery effect' which he named after Montgomery (1912) who made one of the first reports of this phenomenon.

4. Interference between plants involving exploitation of the canopy It has now been demonstrated for a wide range of natural and artificial plant com-

munities that during the height of the growing season almost all incident light is trapped or reflected by the vegetation and that leaves placed low in the vegetation exist below the compensation point (Monsi & Saeki 1953). It seems probable that, in the ultimate analysis, interference with supplies of light is the most potent way in which one species may succeed at the expense of another. This is not to under-emphasize the roles of water and nutrients in a struggle for existence, but to suggest that their role lies often in modify- ing the timing and extent of an ultimate struggle for light.

Amongst the simplest model populations in which the role of light in the struggle for existence has been studied are those of Trifolium subterraneum studied by Black. In com- parisons of populations developed from large and small seeds, Black (1957, 1958) found that pure stands derived from large seeds were eventually equalled in yield by stands from an equal number of small seeds. The populations from large seeds more rapidly reached ceiling yield but were ultimately constrained with limits set by the environment, and the difference in the seed 'capital' invested then became relatively unimportant. However, when large and small seeds were sown in mixture, the situation was very different. The plants derived from large seeds had larger cotyledons and maintained an increasing superiority in the mixture until after 11-12 weeks 97% of incident light was being intercepted by those plants derived from large seeds and less than 3 % by the now suppressed plants from small seeds. Essentially similar behaviour was found by Black (1960) when he grew pure and mixed stands of varieties of T. subterraneum-i which differed in petiole length. When pairs of varieties were grown in mixtures, there was always a rapid assumption of superiority by the longer petioled form which quickly came to trap the greater part of the incident light.

In these two examples, the successful partner in a mixture was that one which contri- buted most to closing the canopy and monopolizing incident light. In both of these examples the mixtures were intraspecific (different seed sizes from a seed sample of one variety or differences in petiole length between varieties of the same species).

Stern & Donald (1962; see also Donald 1961) have found much the same phenomenon in mixtures of two species of T. subterraneum and Lolium rigidum in which differences in nitrogen supply may bias the outcome of interference in favour of one or the other component and showed that the dominating influence of nitrogen was upon the relative rate and height at which the components formed a closed canopy.

The following conclusions may be drawn:

(i) The differential growth-rate of two species in pure stands tends to become reduced or masked when their populations are limited by the supplying power of the environment (see also the studies on Bromus spp. reported by Harper 1961). The supplying power of

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 complex patterns in the behaviour of 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. How- ever, 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 interference between 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, Trifolium repens 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. repens produces 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. repens and eventually overtops T. repens in 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 interference from neighbours.

In populations of each species of Papaver the mortality risk of individuals increases with


increasing density. This has the effect of placing an upper limit on population size, irre- spective 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.

Differences between species which are crucial in determining their success or failure when 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 deter- mines 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 mani- fest 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 experimental ecology concerned with the mutual influences of plants in populations. Other examples of this type of experimentation are represented in 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 characteristics of 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

JOHN L. HARPER 157

intensity which is supra-optimal for 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

sI N . 100 0

15 ()

E

0

0 1 2 3 4 5 6 ILLUMINATION (ft. candles x 1000)

5 C r (b) 8 daily

E_/

3- 1 1 daily

2s -

o

0 1 2 3 4 5 6 7 ILLUMINATION ft candies x1000)

FIG. 5. (a) The relationship between CO 2 uptake per unit leaf area and light intensity for 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. (From Alexander & 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 use- fully 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.


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 influenced by light intensity of isolated Bermuda grass leaves contrasted to that of swards under various clipping regimes. Crop Sci. 2, 132-5.

Black, J. N. (1957). Seed size as a factor in the growth of subterranean clover (Trifolium subterraneum L.) under spaced and sward conditions. Aust. J. Agric. Res. 8, 335-51.

Black, J. N. (1958). Competition between plants of different initial seed sizes in swards of subterranean clover (T. subterraneum L.) with particular reference to leaf area and the light micro-climate. Aust. J. Agric. Res. 9, 299-318.

Black, J. N. (1960). The significance of petiole length, leaf area, and light interception in competition between strains of subterranean clover (Trifolium subterraneum L.) grown in swards. Aust. J. Agric. Res. 11, 277-91.

Brougham, R. W. (1962). The leaf growth of Trifolium repens as influenced by seasonal changes in the light environment. J. Ecol. 50, 449-59.

Clatworthy, J. N. & Harper, J. L. (1962). The comparative biology of closely related species living in the same area. V. Inter- and intra-specific interfereence within cultures of Lemna spp. and Salvinia natans. J. Exp. Bot. 13, 307-24.

De Wit, C. T. (1960). On competition. Versl. Landbouwk. Onderz. 66, 1-82. Donald, C. M. (1961). Competition for light in crops and pastures. Symp. Soc. Exp. Biol. 15, 282-313. Gustafsson, A. (1951). Mutations, environment and evolution. Cold Spr. Harb. Symp. Quant. Biol. 16,

263-80. Harper, J. L. (1961). Approaches to the study of plant competition. Symp. Soc. Exp. Biol. 15, 1-39. Harper, J. L. & Chancellor, A. P. (1959). The comparative biology of closely related species living in the

same area. IV. Rumex: Interference between individuals in populations of one and two species. J. Ecol. 47, 679-95.

Harper, J. L. & Clatworthy, J. N. (1963). The comparative biology of closely related species living in the same area. VI. Analysis of the growth of Trifolium repens and T. fragiferum in 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 related species living in the same area. Evolution, 15, 209-27.

Harper, J. L. & McNaughton, I. H. (1962). The comparative biology of closely related species living in the same area. VII. Interference between individuals in pure and mixed populations of Papaver species. New Phytol. 61, 175-88.

Kasanaga, H. & Monsi, M. (1954). On the light-transmission of leaves, and its meaning for the production of matter in plant communities. Jap. J. Bot. 14, 304-24.

Kira, T., Ogama, H. & Sakayaki, N. (1953). Intraspecific competition among higher plants. J. Polytech. Osaka City Univ. 4, 1-16.

Knapp, R. (1954). Experimentelle Soziologie der Hdheren Pflanzen. Stuttgart. Monsi, M. & Saeki, T. (1953). Ueber den Lichtfaktor in den Pflanzengesellschaften und Bedeutung fur die

Stoffproduktion. Jap. J. Bot. 14, 22-52 Montgomery, E. G. (1912). Competition in cereals. Bull. Nebr. Agric. Exp. Sta. 24. Park, T. (1955). Experimental competition in beetles, with some general implications. In The Numbers

of Man and Animals (Ed. by J. B. Cragg and N. W. Pirie). Edinburgh. Penman, H. L. (1948). Natural evaporation from open water, bare soil and grass. Proc. Roy. Soc.

A 193, 120-46. Priestley, J. H. & Pearsall, W. H. (1922). An interpretation of some growth curves. Ann. Bot., Lond. 36,

239-49. Stern, W. R. & Donald, C. M. (1962). The influence of leaf area and radiation on the growth of clover in

swards. Aust. J. Agric. Res. 13, 615-23. Tansley, A. G. (1917). On competition between Galium saxatile L. (G. hercynicum Weig.) and Galium

sylvestre Poll. (G. asperum Schreb.) on different types of soil. J. Ecol. 5, 173-9.

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THE INDIVIDUAL IN THE POPULATION