Competition Among Plants
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Proc. Nat. Acad. Sci. USA Vol. 70, No. 2, pp. 585-590, February 1973 Competition Among Plants F. W. WENT Laboratory of Desert Biology, Desert Research Institute, University of Nevada, 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 plant 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 California (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, Atrichoseris 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.