Proc. Nat. Acad. Sci. USA Vol. 70, No. 2, pp. 585-590, February 1973

Competition Among

F. W. WENT Laboratory of Desert Biology, Desert Research Institute, University of , Reno, Nev. 89507

Competition is a word of various meanings. In biology, it exception of 1969, almost as many ripe seeds were produced as originally was introduced to account for the low survival rate seeds germinated, which means that a rather precarious dy- of the potential offspring of all creatures. The number of namic equilibrium exists for the few annual plants growing in seeds formed by a pea may be a dozen; most annual the center of the valley (Geraea canescens, Chorizanthe rigida, plants produce hundreds or thousands of seeds; cottonwood and Chaenactis carphoclinia). Exceptional seed years like trees and orchids seeds run to the millions; and, in the case of 1935, 1947, and 1969 are needed to replenish the seed popula- fern and mushroom spores, there are billions formed by a tion in the driest parts of Death Valley, where normally no single individual. Since in a state of equilibrium each plant seed surpluses are produced to feed harvester ants (Veromes- can be replaced by only a single other one, processes were sor) and seed-eating rodents (Dipodomys). Consequently, considered that eliminated the excess offspring (such as the these seed eaters are mostly absent in the driest areas of the activity of predators). desert. With Darwin's evolution theory, competition took on addi- In the less dry parts of Death Valley 12 plots, each about tional meaning in relation to survival of the fittest. Competi- 0.2 i2, were surveyed from January through June. Table 3 tion was not anymore a struggle between equals, but a mech- shows the germination and survival data for these plots in the anism to award superiority. Competition became a contest, years 1968 and 1969. A total of 2893 seedlings were marked in and considerations of combat, struggle, territorial exclusion, these plots, of which 1178 (41%) survived, flowered, and and even war entered in the wake of Darwin's ideas. As Warm- set seed, and 33,951 mature seeds were formed. This is an ing (1) states, competition is "a consideration of the means by average of 29 seeds per plant, or 12 new seeds per germinated which plants oust each other from habitats." But, it is hard seed. In other years and other deserts a 10- to 20-fold increase to conceive of any mechanisms by which stationary plants in the number of seeds produced per seed germinated was also can combat each other to result in an ousting. found. The survival of 41% of all seedlings was slightly lower In an important experimental investigation, Clements, than in other deserts [Southern (3), over 50%; Weaver, and Hanson (2) studied competition. They con- Southern Nevada (4), 56%; Southern Nevada (Went 1972, cluded that "Competition is purely a physical process. With unpublished data), 53%]. Some seedlings disappeared because few exceptions, such as the crowding up of tuberous plants they were eaten by rodents or insect larvae (Oenothera clavae- when grown too closely, an actual struggle between competing formis by Altica torquata), but most died in the early stages of plants never occurs.. In the exact sense, two plants, no mat- germination when their roots did not penetrate properly in the ter how close, do not compete with each other as long as the soil. But, once established, the seedlings survived for practi- water-content, the nutrient material, the light and heat are inl cally 100% to flowering. Only in a few of the taller plants excess of the needs of both. When the immediate supply of a (Mfalacothrix californica, platyphylla, and Chaen- single necessary factor falls below the combined demands of actis carphoclinia) was reproduction poor, because their flower- the plants, competition begins." buds were grazed off. The general conclusion to be drawn from When growing sunflower, wheat, and other plants at differ- these observations, therefore, is that the selection of the sur- ent distances of each other, Clements et al. (2) found that the vivors in the population of annuals in the desert is not a result closer the plants were spaced to one another, the more they of competition among themselves. Since there is a 10- to 20- inhibited each other. But, it appeared from their data (see fold increase in seeds with each germination event, what is the Table 1) that: (i) all plants in a competition plot were equally selection process that keeps the desert seed population from reduced in growth, and (ii) with increasing density of the increasing exponentially? Anywhere from 90 to 95% of all planting, the production of the plants per unit area tended to seeds produced have to disappear. This disappearance is not reach a maximum value, which was not changed with further due to decomposition of seeds by microorganisms. In the first decreases in spacing. This is a common experience in all agri- place, we do not find partially digested seeds in these desert cultural spacing tests, a result that shows that this form of soils. Besides, we know that the seeds of annuals under dry competition does not provide a mechanism for selection, since desert conditions remain fully viable for at least 20 years (5). all individuals are equally inhibited. The same experience was Removal of seeds by seed-eating animals must be considered gained from observations in the field. seriously. Tevis (6) found in the desert that "the estimated In the center of Death Valley near the headquarters of the amount of seeds taken by the ants (Veromessor pergandei) National Monument, with an average yearly rainfall of about from an acre in 12 months compared with the estimated num- 40 mm, the vegetation is exceedingly poor. Only very few ber of seeds -produced in a poor year showed that the insects shrubs-such as Larrea, Atriplex hymenelytra, and Tidestroe- do not seriously affect the total seed supply." But Tevis' ants mia-grow in that area, and the number of seedlings of annual had gone through a long drought period, and were very much plants appearing after rain is small (see Table 2). With the reduced in numbers. An ant nest (also of Veromessor per- 585 Downloaded by guest on October 2, 2021 586 Went Proc. Nat. Acad. Sci. USA 70 (1973)

TABLE 1. Effects of spacing on Helianthus annuus 80 days TABLE 2. Germination of annual plants and seed after planting (2) production in a 1_M2 plot in the center of Death Valley Plants Height, Leaf area, cm2 Dry weight, grams Number of seeds Number of new per 10 cm per Year germinated seeds produced m2 plant per plant per m2 per plant per m2 1966 17 11 1 220 27000 2700 491.4 49 1967 30 31 4 235 9800 3900 279.2 70 1968 37 9 16 207 2600 4200 85.5 135 1969 32 274 64 150 580 3800 20.8 130 1970 0 0 250 115 64 1600 4.6 115 1971 19 35 1000 100 41 4100 2.1 210 1972 1 0

gandei) observed in Death Valley was at least 10 times as populous as those observed by Tevis, and its population col- to their nests. In years with abundant seed production, they lected over 10 times as many seeds. Therefore, it seems likely may bury their cheek pouch contents in spots all over the that the harvester ants adjust rapidly to the total amount of desert and, after an appropriate rain, these superficially buried seeds available, and, together with rodents [with a comparable seed caches will germinate. Then, anywhere from 100 to 200 biomass in the desert (7)], actually remove most of the over- seedlings will sprout in an area of less than 1 cm2; always these production of seeds, resulting in an "ever-normal granary." green tufts contain only a single species of seedlings, and al- Therefore, it is not competition among themselves, but preda- ways over 95%, often 100%, of all the seeds germinate. All tion of their seeds by ants and rodents, that keeps the popula- of the seedlings survive in these most extreme cases of com- tion of desert annuals on an even keel. petition, and most of them manage to flower and fruit. In one This lack of competition among seedlings and mature plants of those tufts of Plantago insularis, I counted 126 plants and was also found by Kooper (8) among weeds in fallow fields in three ungerminated seeds, of which two plants had produced Java, where the number of seedlings was counted soon after two flowers, and 74 had one flower each. Since per flower two plowing. This contrasts with an observation of Darwin, who seeds are produced, this patch of plants of 1 cm2 produced 152 mentions in The Origin of Species (9): ". on a piece of ground seeds per 129 seeds in the original cache. A well-developed 3 feet long and two wide, dug and cleared, and where there Plantago insularis, growing on 1 dM2 of ground, probably could be no choking from other plants, I marked all the seed- would have produced 20 ears with 30 flowers each, or 1200 lings of our native weeds as they came up, and out of 357 no seeds, or only 10% of what the closely spaced plants of the less than 295 were destroyed, chiefly by slugs and insects." seed cache managed to produce per unit space. Similar ob- In Darwin's case, only 18% of the weeds survived, although servations were made on seed caches of Pectocarya penicillata not by competition with each other. and other Boraginaceae; in each case at least as many seeds Whereas the previous observations show that close spacing were produced as had been present in the original cache. of seedlings does not result in differential survival and elimina- Summarizing all my desert observations, I found that (i) tion of-presumably-the weaker plants, the individuals re- competition under most extreme conditions results in a 1:1 main smaller the closer the spacing. The mechanism of this replacement of all germinated seeds, (ii) under the usual desert mutual inhibition has not been established, but even in the conditions there is a 10- to 20-fold increase in seed population most extreme case it does not result in elimination of plants. after each germination event, (iii) there is no differential In the desert, kangaroo rats (Dipodomys) and pocket mice survival during growth of these desert annuals, and (iv) the (Perognathus) collect seeds that they carry in cheek pouches number of seeds available for germination in successive sea- TABLE 3. Number of seedlings germinated after early winter rains in 0.1- to 1-M2 plots in Death Valley, the number that survived to the flowering and fruiting stage, and the number of ripe seeds produced per plot

1968 1969 Altitude in m Germinating Surviving Seed Germinating Surviving Seed Valley Bottom -80 37 5 9 32 19 274 Road to Beatty -40 303 42 270 63 50 897 Bennett's Camp -80 148 69 3093 28 1086 Bennett's Well -80 33 11 210 Jubilee Pass 100 149 74 140 - 48 2778 Titus Canyon 200 35 12 372 63 33 1553 Weir 600 186 51 375 80 72 150 Grapevine SW 800 222 47 612 170 62 2830 Grapevine NW 800 145 13 82 124 61 736 Grapevine SE 800 102 16 686 61 45 500 Grapevine NE 800 144 45 1005 104 33 600 Entrance 1000 194 168 6000 151 83 6258 Scotties 1100 272 80 3335 75 11 100 All plots - 1970 633 16,189 923+ 545 17,762 Downloaded by guest on October 2, 2021 Proc. Nat. Acad. Sci. USA 70 (1973) Competition Among Plants 587

sons is controlled by removal of the excess seeds by ants and correlations between epiphytic orchids, mosses and lichens, rodents, and not by differential production related to differ- and the host tree (e.g., ref. 16). In the desert, several plants ential survival of mother plants. Therefore, these annuals do (Rafinesquia neomexicana) grow only near shrubs [Franseria not exert any measurable selection pressure on each other; dumosa, (17)1. Many mushrooms are always growing in associ- their survival is conditioned by (a) the germination condi- ation with certain trees, and grasses and legumes, like clover, tions, (b) the physical conditions immediately after germina- are mutually stimulatory. This effect is so important that tion that determine whether the seedling roots will penetrate most of the pasture development in Australia and New Zea- properly into the soil, (c) the degree to which they are eaten land is based on the coexistence of grasses and subterranean by animals (a minor factor for these diminutive desert an- clover. nuals), and (d) the degree to which their seeds have not been Thus far, we have looked only into interplant competition. removed by ants, rodents, birds, and, to a small extent, liz- An entirely different aspect is intraplant competition. This ards. These findings perhaps account for the fact that except effect has, of course, no evolutionary significance, for it will for germination controls these desert annuals do not show any not change the species composition of the vegetation. obvious adaptations to desert conditions. Each plant should be considered not as an individual, but In the previously discussed cases much of the interplant as a colony, very much like in animals a coral, a bryozoon, or a inhibition was strong or very strong, when a 100- or even 1000- syphonophore is a colony of similar or of differentiated in- fold size difference occurred between solitary and closely- dividuals. There is a strict control of growth that keeps the spaced plants. But, there was no elimination of individuals; individual parts of a plant-stems, roots, buds, flowers-in when germination occurred at the same time, the mutual in- equilibrium, and that insures a harmonious development. fluence was evenly distributed and quantitative. Through correlative inhibition only a limited number of buds The situation is very different when we consider the es- develops into shoots, and this control is part of the mechanism tablishment of new plants in an existing vegetation. In a by which plants competing for space keep their proportions. closed vegetation essentially no seeds germinate. Exceptions The more space a wheat plant has, the more basal buds de- are seeds of parasites, such as Striga and Euphrasia, which velop into tillers and produce ears. Since the expansion growth need the secretions of their host plants for germination. Also, of new leaves is controlled by existing leaves, the leaf size of bulbils and other vegetative propagules (e.g., of Polygonum closely spaced sunflowers, already reduced by strong competi- vwiparum) can develop in a closed vegetation. But even in tion, will remain small (see Table 1). In this way, the inter- open spaces of an established vegetation, germination and plant competition causes a more severe intraplant competi- growth of plants is severely inhibited; these examples offer the tion, through hormonal control. In the case of the sunflowers of best cases of plant competition. Typical examples of such Table 1, the intraplant competition almost completely bal- apparent competition are the open unvegetated spaces around ances the interplant competition, a balance that results in a Salvia leucophylla and other chaparral shrubs, the lack of constant amount of plant mass per unit growing surface. undergrowth in walnut orchards and Eucalyptus forests, and This effect makes the planting density uncritical for yield per the absence of desert annuals under Encelia farinosa shrubs. acre for a farmer, at least beyond a minimal density. At higher Hundreds of other cases have been described in the literature. densities the farmer may waste seed in planting, and he may They are generally attributed to the effects of allelopathic have trouble in controlling weeds, but he does not significantly substances, produced by the dominant plant, which either change his total yield per acre. prevent germination or inhibit growth of other plants. An In a montane Sierran forest, intratree competition turned extensive literature has sprung up about Allelopathy, since out to be more severe than intertree competition. In Little Molisch (10) coined this term 35 years ago. Much informa- Valley, a valley located 40 km south of Reno between Lake tion can be found in Grummer (11), Evenari (12), and Whit- Tahoe and Washoe Lake at 1900 m altitude, the meadow in aker (13). the center of the valley is bordered by dense forests of Pinus Allelopathic substances may be volatile, for example, they murrayana, where the ground water stays within 10-100 cm may be terpenes (14) and operate via the air, or they may dif- of the surface the year around. In the densest stands the light fuse through the soil; they may be produced by living roots, intensity is only a few percent of that outside the forest, and or may be derived from decaying above-ground parts; they in the darkest areas there is no undergrowth, except for some may inhibit other plants indiscriminately or be most effective pine and fir seedlings and saprophytic phanerogams such as against younger plants of the same species; they may persist Sarcodes sanguinea, Pterospora andromedea, Corallorhiza for a long time in the soil or lose their effectiveness as soon as maculata, and Pyrola secunda. the inhibiting plant disappears; some are present in the seeds, In one of the darker parts of this forest, with a pure stand of but most are produced during the growth of the plant. A Pinus murrayana, the following observations were made. In long list of chemicals has been established with inhibitory Fig. 1 the percent of living trees in any one of five size clas- effects on other plants but, curiously, there are very few allel- sifications was entered. The age of the trees is fairly well cor- opathic substances known that have growth stimulating ef- related with the diameter of the stem, with the 24- to 40-cm fects. diameter trunks probably 90 years old, when Little Valley was Yet, there are many positive interactions between plants, logged to provide timbers for the mines of Virginia City, 15 where the presence of one plant enhances rather than inhibits km east of Little Valley. It is interesting that about half of growth of another, where cooperation rather than competition the trees were alive, even among the smallest ones. These reigns, where inclusion rather than exclusion is involved in small ones had grown up in the deep shade of the large trees, community development. This relationship was pointed out with 2-6% of the outside light reaching their needles, a light by Kropotkin (15) for animal communities, but the same intensity at which all branches of the older trees were dead. effect is important in plants as well. There are strong positive Fig. 2 shows the distribution of living branches with green Downloaded by guest on October 2, 2021 588 Went Proc. Nat. Acad. Sci. USA 70 (1973)

100 competition all branches below 4- to 5-m height were killed, the branches of young trees were alive. Therefore, intertree competition is weaker or less effective than intratree com- 50 petition. It was also interesting to note that all dead trees over 5 m in height had produced cones, still attached to their dead branches. So, even those trees that had succumbed at a fairly early age had still had a chance to reproduce. 0 3 6 12 24 40 In this case the number of seedlings that develops in the established forest is small, and only after good seed years a FIG. 1. Percentage of living Pinus murrayana trees in a mon- few Pinus murrayana seeds germinate. Their seedlings have tane forest (ordinate) as a function of their size (abscissa: diameter at base in centimeters). the ability to grow at a very low light intensity, perhaps be- cause they are partially fed by mycorrhiza, which are able to supply not only minerals but also organic food to their roots. needles along the height of three different-sized trees growing This feeding they accomplish in a manner similar to that by within 1 In of each other. In the largest and tallest tree, all which the mycorrhizal fungi feed phanerogamic saprophytes. side branches over 6.5 m from the ground were alive; there This food is derived from the decomposition of forest litter, was one live branch at 5.3 m, while all other branches below which is available in abundance in the Pinus murrayana 6.5 m, in deep shade, were dead. Of a 9.5-cm diameter tree, forest and which greatly stimulates pine seedling growth all branches below 4 m were dead, from 4 to 5 m 70% of the (18). A fair number of these seedlings grow into young trees, branches were alive, and in its upper 2 m all branches were which are able to develop even in very deep shade, as de- alive with green needles, even though they were strongly scribed earlier, and where the similar death rate of trees of all shaded most of the time by the surrounding taller trees. A sizes (Fig. 1) suggests that little intertree competition occurs. tree with a 5-cm diameter trunk was only 3.9 m tall, and en- In several tropical forests, where according to common tirely shaded by all surrounding trees. Yet, in its upper 0.4-m concensus the fiercest interplant competition occurs, the seed- top all branches were alive, with living branches to within 1 ling situation is similar to that in the Pinus murrayana forest. m from the ground. Trees growing outside the forest in the In rain forests, seedlings grow very slowly in the deep shade; meadow had only live branches, down to ground level. This after 5-10 years they may be still only a few decimeters tall was the typical behavior of all trees in this forest: the youngest with very few leaves, far too small to compete with each other. and smallest ones, of which almost half were alive, had living In a dark forest in the Central Amazon basin near Manaus, branches far below the level where the branches of older, with hardly any herbs as undergrowth (0.5/M2), there were taller trees were dead, not on account of age (because these 10.2 5- to 10-year-old tree seedlings per M2, equally distrib- lower branches were all alive in free-standing trees) but be- uted over classes with 1, 2, 3, 4, 5, 6, 7, and 8 leaves, with cause of intratree competition. And, where through intratree only 1.0 dead seedling per M2. A similar situation was found in a forest near the mouth of the Amazon near Belem (Table 14 4), with on the average only five seedlings of the 5- to 10-year- old class per M2. Similar numbers were found near Borba along the Madeira River and below Tingo Maria along the 12 Huallaga River, both tributaries to the Amazon. With my experience in a mountain rain forest in Java (16) and observations in forests in Trinidad and Costa Rica, I can 10 state positively that in tropical rain forests normally only a few tree seedlings germinate per year per M2, and that many of these seedlings survive for a long time, with hardly any 8 foliage, in deep shade, probably partially fed by mycorrhiza, for all of them have an extensive superficial root system in the forest litter. The only two exceptions to this rule I have seen 6 were (i) in a Javanese mountain forest, where thousands of

41 TABLE 4. Number of trees and tree seedlings in Pirelli Forest near Belem*

2 Light areas Dark areas Trees (4 plots) (2 plots) Seedlings with cotyledons only 8.7 3.0

100 Seedlings less than 20 cm 51.7 37.5 A M°° ° B C ° D Seedlings 20-80 cm 30.5 16.0 FIG. 2. Percentage of living primary lateral branches of Pinus Seedlings 100-200 cm 7.2 5.0 murrayana (abscissa) as a function of height of insertion on tree Trees over 200-cm tall 10.5 9.5 (ordinate, in meters). Trees A, B, and C were growing within 1 m Dead plants (% of live) 3.2 (3.0%) 6.0 (8.3%) of each other, tree D was growing in an open meadow. Stem diameters: 5.0 cm in A, 9.5 cm in B, 30 cm in C, and 40 cm in D. * Each plot is 10 M2. Downloaded by guest on October 2, 2021 Proc. Nat. Acad. Sci. USA 70 (1973) Competition Among Plants 589

Cestrum aurantiacum seedlings covered the forest floor (but excretes or exudes inhibitory substances that reduce or pre- Cestrum was the only introduced tree in that forest) and (ii) vent growth of other plants in their neighborhood-so-called a group of 12 closely-spaced unidentified seedlings of the same allelopathic substances-the term competition, in the sense of species in the Mocambo forest near Belem, of which three had strife against one another, ultimately resulting in the ouster died (through competition?). These were the only cases in of one, seems appropriate. It is remarkable, however, that which I found seedlings of the same species adjacent to each such inhibition can be quite specific, and that in several cases, other, and the only cases of extensive death. Otherwise, I specifically among desert plants such as Larrea, the strongest found in the above-mentioned Mocambo forest only one to inhibition is exerted on individuals of its own species. In those two dead plants per 100 for those less than 30 cm in diameter, cases it seems appropriate to consider the allelopathic sub- and six dead per 100 trees of over 50-cm diameter. When one stances not as means to cut down competition, but to prevent takes into account that big dead tree stumps last much longer excessive growth under temporarily favorable conditions, such than small ones, this finding would suggest a similar death as after a heavy rain. If desert shrubs immediately responded rate for old as for young trees. to such rain with fast growth, they might very well overgrow Table 4 suggests that in the Pirelli forest near Belem there their capacity to survive a subsequent drought period, for is a higher death rate of seedlings in the early years, but still a which they should be prepared with a low shoot to root ratio. majority survives to provide a pool from which a few grow up A second set of cases was discussed in which dense planting to tall trees when an old shading tree dies and provides a causes mutual inhibition of adjoining individuals. In this case chance for fast growth of the suppressed undergrowth. This we can use the term competition in the sense of a scramble for pattern was confirmed in the Tjibodas forest in Java, where the same commodity, such as light, nutrients, or water. If the many trees 10 cm or less in diameter had not grown at all in plants grow so close together that they shade each other, they 40 years, whereas the big trees of 50 cm and more in diameter compete for light and can only grow to the extent that light is either had died or had grown much in the 40 years since Koor- available. This type of competition has no evolutionary sig- ders had measured and numbered those trees. nificance, since all competing individuals are equally inhibited, Summarizing these observations in tropical rain forests, we while the total plant mass and seed produced per unit surface can say: tends to reach a maximum as the individuals grow closer (i) Germination of most tropical tree seeds is a rare phe- together. The mechanism of this type of competition between nomenon. Most of these seeds have a very short life span, and individuals of the same species is probably hormonal, with a the very rapid fungal decomposition of organic matter in limiting amount of, for example, leaf growth hormone pro- tropical soils prevents the build-up of seed reserves in the soil. duced per unit leaf or ground surface. It is likely that growth (ii) The few seedlings that come up (a few per m2 per year) factors produced in the root system play an important part in grow very slowly, even those with very large seeds. this growth control. (iii) In the deep shadow of the forest, where so little photo- A third set of cases involves the development of desert an- synthesis is possible that very few herbs will grow there, most nuals. Here, no competition is found at all in the early stages tree seedlings have only a few leaves, although measurements of germination and growth. Under extreme conditions, such as have shown that they photosynthesize slowly. Probably their in the center of Death Valley, later growth of the annuals principal food source comes from mycorrhiza. seems to be limited by the small amount of precipitation, yet (iv) Survival of these tree seedlings is remarkably high, with seed production offsets the attrition of the seedlings and young a half-life of 10-20 years. Their death is not attributable to plants due to drought, and a seed germination to seed produc- competition, but to attrition by predators, diseases, and me- tion ratio close to 1:1 occurs. Everywhere else there is a 10- chanical injury (dropping branches, e.g.). to 20-fold increase in seed production over the number of (v) Suppression by their physical environment, rather than seeds that germinates per germination event. The excess seed competition by the surrounding vegetation, is the term to be is removed by ants and rodents. Thus, the equilibrium that used in connection with the development of these tree seed- we find in the desert between the seeds produced and seeds lings. Thus, none has an advantage over its neighbors and no used in propagation is controlled by predation of animals, and differential survival exists. no competition in the sense of differential survival or survival (vi) The same situation continues during the later growth of of the fittest exists. The vegetation that develops after a rain the trees. They still are suppressed by the forest giants shad- is strictly the result of germination of the seeds left by rodents ing them, and all 5- to 10-m tall treelets have an even chance and ants. to take over the role of principal tree when a shading tree dies. In a fourth set of cases, the development of trees in a conifer (vii) The natural attrition, resulting in a few percent dead forest and in tropical rain forests was considered. It turns out trees of all ages, apparently does not favor or disfavor any that intraplant competition is an important factor in the de- species; this lack of competition explains the great abundance velopment of conifer trees, whereas interplant competition is of tree species in the rain forest. The main characteristic all of secondary importance. In tropical rainforests, there does these rain forest trees have in common is the ability to grow not seem to be any competition between plants, at least be- very slowly under suppressed conditions. This slow growth in tween seedlings of the forest trees. They are suppressed by youth makes most of them unsuitable for forestry practice. the lack of light due to the dense canopy, but there is no obvi- (viii) We find completely opposite behavior in trees of the ous struggle between seedlings or between young trees. secondary forest, such as Cecropia, Triplaris, and Ochroma, It would be possible to discuss dozens of other cases where which germinate in abundance in a clearing and which grow either there is or is not interaction within or between plants very rapidly, 5 to 10 m/year. that might come under the heading of competition. The gen- We have now considered several cases in which competition eral conclusion about plant competition would still remain occurs among plants. In one set of cases, where one plant the same. Downloaded by guest on October 2, 2021 590 Went Proc. Nat. Acad. Sci. USA 70 (1973)

There definitely are cases where competition in the sense germination of annuals in the Joshua Tree National Monu- of strife exists among plants, and where this competition is ment, California," Ecology 37, 318-330. 4. Beatley, J. C. (1967) "Survival of winter annuals in the a factor in survival, and where allelopathy is a factor in evolu- Northern Mojave desert," Ecology 48, 745-750. tion. We do get elimination of such forms that are less effec- 5. Went, F. W. (1969) "A long term test of seed longevity," tive in their struggle for existence. Yet, the successful survival Aliso 7, 1-12. of so many primitive plants such as algae or conifers makes us 6. Tevis, Jr., L. (1958) "Interrelations between the harvester wonder about the general effectiveness of competition in ant Veromessor pergandei (Mayr) and some desert ephemer- evolution. Besides, only competition between organisms that als," Ecology 39, 695-704. 7. Went, F. W., Wheeler, J. & Wheeler, G. C. (1972) "Feeding have not reached the 100% level of adaptedness could result and digestion in some ants," BioScience 22, 82-88. in differential survival, and it is remarkable in how few genera- 8. Kooper, W. J. C. (1927) "Sociological & ecological studies on tions the 100% adaptedness can be reached in breeding experi- the tropical weed vegetation of Pasuruan (the island of ments [examples: sugar level in beets and sugar cane, growth Java)," Rec. Trav. Bot. Neer. 24, 1-255. rates in tomato plants (19)]. If selection through competition 9. Darwin, C. (1859) The Origin of Species, from Reprint of were the all-important factor in evolution, we could expect 6th ed. 10. Molisch, H. (1937) Der Eingfluss einer Pflanze auf die andere- evolution to stop at some time in the future. Allelopathie (Fischer, Jena). But we have also seen that competition plays no, or only a 11. Grummer, G. (1955) Die gegenseitige Beeinflussung hoherer minor, role in environments that are still in full evolution, Pflanzen- Allelopathie (Fischer, Jena). such as in a tropical rain forest. Therefore, my general con- 12. Evenari, M. (1961) "Chemical influences of other plants clusion is that competition is an overrated factor in the plant (allelopathy)," Encycl. Plant Physiol. XVI, 691-736. 13. Whittaker, R. H. (1970) "The biochemical ecology of higher world. It exists, but does not have the overriding importance plants," in Sondheimer and Simeone eds., Chemical Ecology that is imputed to it in the animal kingdom, where active (Academic Press, New York), pp. 43-70. competition and elimination of competitors is possible. Even 14. Muller, C. H. (1966) "The role of chemical inhibition among zoologists the importance of competition in evolution (allelopathy) in vegetational composition," Bull. Torrey has been questioned (15). Bot. Club 93, 332-351. 15. Kropotkin, P. A. (1917) Mutual Aid; A Factor of Evolution, Much of the observational work reported was supported by (Knopf, New York). Grants GB 6681 and GB 17731X of the National Science Founda- 16. Went, F. W. (1940) "Soziologie der Epiphyten eines tropis- tion. chen Urwaldes," Ann. Jard. Bot. Buitenzorg 50, 1-98 1. Warming, E. (1896) Lehrbuch der okologischen Pflanzengeo- 17. Went, F. W. (1942) "The dependence of certain annual graphie (Fischer, Jena). plants on shrubs in southern California deserts," Bull. 2. Clements, F. E., Weaver, J. E. & Hanson, H. C. (1929) Torrey Bot. Club 69, 100-114. "Plant competition an analysis of community functions," 18. Went, F. W. (1973) Effect of soil litter and Mycorrhiza on Carnegie Inst. Wash. Publ. 398. growth of Pinus murrayana, in press. 3. Juhren, M., Went, F. W. & Phillips, E. (1956) "Ecology of 19. Went, F. W. (1957) "The experimental control of plant desert plants. IV. Combined field and laboratory work on growth," Chron. Bot. 17, XVII + 343. Downloaded by guest on October 2, 2021