Chapter 13 Competition.docx

1、Chapter 13 CompetitionChapter13 CompetitionAlong a coral reef off the north coast of Jamaica, threespot damselfish guard small territories of less than 1 m2 (fig. 13.1 ). These small territories are regularly dispersed across the reef and contain most of the resources upon which the damselfish depen

2、d: nooks and crannies for shelter against predators, a carefully tended patch of fast-growing algae for food, and in the territories of males, an area of coral rabble kept clean for spawning. The damselfish constantly patrol and survey the borders of their territories, vigorously attacking any intru

3、der that presents a threat to their eggs and developing larvae, or to their food supply. If you look carefully, however, you may find that not all members of the population have a territory. Damselfish without territories live in marginal areas around the territorial members, wandering from one part

4、 of the reef to another.FIGURE 13.1 Territorial reef fish, such as this threespot damselfish, Eupomacentrus planifrons, compete intensely for space.If you create a vacancy on the reef by removing one of the damselfish holding a territory, other damselfish appear within minutes to claim the vacant te

5、rritory. Some of the new arrivals are threespot damselfish like the original resident, and some are cocoa damselfish, which generally live a bit higher on the reef face. These new arrivals fight fiercely for the vacated territory. The damselfish chase each other, nip each others flanks, and slap eac

6、h other with their tails. The melee ends within minutes, and life among the damselfish settles back into a kind of tense tranquillity. The new resident, which may have driven off a half dozen rivals, is usually another threespot damselfish.This example demonstrates several things. First, individual

7、damselfish maintain possession of their territories through ongoing competition with other damselfish, and this competition takes the form of interference competition, which involves direct aggressive interaction between individuals. Second, though it may not appear so to the casual observer, there

8、is a limited supply of suitable space for damsel fish territories, a condition that ecologists call resource limitation. Third, the threespot damselfish are subject to intraspecific competition, competition with members of their own species, as well as nterspecific competition, competition between i

9、ndividuals of two species that reduces the fitness of both. The effects of competition on the two competitors may not be equal, however The individuals of one species may suffer greatly reduced fitness while those of the second are affected very little. The observation that threespots generally win

10、in aggressive encounters with cocoa damselfish suggests this sort of competitive asymmetry.Competition is not always as dramatic as fighting damselfish nor is it always resolved so quickly-. In a mature white pine forest in New Hampshire, tree roots grow throughout the soil taking up nutrients and w

11、ater as they provide support. In 1931, J. Toumey and R. Kienholz designed an experiment to determine whether the activities of these tree roots suppress the activities of other plants. The researchers cut a trench, 0.92 m deep, around a plot 2.74 m by 2.74 m in the middle of the forest. In so doing,

12、 they cut 825 roots, which removed potential competition by these roots for soil resources. They also established control plots on either side of the trenched plot and then watched as the results of their experiment unfolded. The experiment continued for 8 years, with retrenching every 2 years and o

13、ver 100 roots cut each time. By retrenching, the researchers maintained their experimental treatment, suppression of potential root competition.In the end, this 8-year experiment yielded results as dramatic as those with the damselfish. Vegetative cover on the section of forest floor that had been r

14、eleased from root competition was 10 times that present on the control plots. Apparently the roots of white pines exert interspecific competition for some combination of nutrients and water that is strong enough to suppress the growth of forest floor vegetation (fig. 13.2). In addition, the growth o

15、f young white pines was also much greater within the trenched plots than in the control plots. Therefore, considerable intraspecific competition also occurred on the forest floor.FIGURE 13.2 Competition in a forest can be as intense as competition on a coral reef However, much of the competition in

16、a forest takes place underground where the roots of plants compete for water and nutrients.Ecologists have long thought that both interspecific and intraspecific competition are pervasive in nature. For instance, Darwin thought that interspecific competition was an important source of natural select

17、ion. While ecologists have shown that interspecific competition substantially influences the distribution and abundance of many species, they have also questioned the assumption that competition is an all-important organizer of nature. Such questioning has stimulated more careful research and more r

18、igorous testing of the influence of competition on populations, and while this testing continues, sufficient evidence has accumulated to make some tentative generalizations.CONCEPTS Studies of intraspecific competition provide evidence for resource limitation. The niche reflects the environmental re

19、quirements of species. Mathematical and laboratory models provide a theoretical foundation for studying competitive interactions in nature. Competition can have significant ecological and evolutionary influences on the niches of species.CASE HISTORIES: resource competitionStudies of intraspecific co

20、mpetition provide evidence for resource limitation.In chapter 11, we saw that slowing population growth at high densities produces a sigmoidal, or S-shaped, pattern in which population size levels off at carrying capacity. Our assumption in that discussion was that intraspecific competition for limi

21、ted resources plays a key role in slowing population growth at higher densities. The effect of intraspecific competition is included in the model of logistic population growth. If competition is an important and common phenomenon in nature, then we should be able to observe it among individuals of t

22、he same species, individuals with identical or very similar resource requirements. Thus we begin our discussion of competition with intraspecific competition.Intraspecific Competition Among Herbaceous PlantsIn chapter 6, we reviewed experiments by David Tilman and M. Cowan (1989) that showed how pla

23、nts alter root: shoot ratios in response to availability of soil nitrogen. The plants in these experiments reduced their allocation to roots as soil nitrogen concentration increased. The experiments also included evidence for intraspecific competition. Tilman and Cowan grew the grass Sorghastrum nut

24、ans at low density (7 plants per pot) and high density (100 plants per pot). The results showed that the root: shoot ratios are higher when the plants are grown at high density, suggesting that competition for nutrients was more intense underthese conditions.The results of Tilman and Cowans experime

25、nts also show that soil nitrogen concentration and population density substantially influence growth rates and individual plant weight. For example, the weight of S. nutans increased with increased soil nitrogen (fig. 13.3). Therefore, we can conclude that both these responses were limited by nitrog

26、en availability at the lower concentrations in the experiment. Now compare the growth rates and plant weights shown by plants grown at low and high densities. How are they different? Both growth rate and plant weight are higher in the low-density populations, and we can conclude that competition for

27、 nutrients (resources) is more intense at the higher plant population density. Such competition for limited resources in natural populations usually leads to mortality among the competing plants.Self-Thinning in Plant PopulationsThe development of a stand of plants from the seedling stage to mature

28、individuals suggests competition for limited resources. Each spring as the seeds of annual plants geminate, their population density often numbers in the thousandsFIGURE 13.3 Population density, soil nitrogen, and the size attained by the grass Sorghastrum nutans (data from Tilman and Cowan 1989).pe

29、r square meter. However, as the season progresses and individual plants grow, population density declines. This same pattern occurs in the development of a stand of trees. As the stand of trees develops, more and more biomass is composed of fewer and fewer individuals. This process is called self. t

30、hinning.Self-thinning appears to result from intraspecific competition for limited resources. As a local population of plants develops, individual plants take up increasing quantities of nutrients, water, and space for which some individuals compete more successfully. The losers in this competition

31、for resources die, and population density decreases, or thins, as a consequence. Over time the population is composed of fewer and fewer large individuals.One way to represent the self-thinning process is to plot total plant biomass against population density. If we plot the logarithm of plant bioma

32、ss against the logarithm of plant density, the slope of the resulting line averages around 1/2. In other words there is an approximately one-unit increase in total plant biomass with each two-unit decrease in population density; plant population density more rapidly than biomass increases (fig. 13.4

33、).FIGURE 13.4 Self-thinning in plant populations (data from Westoby 1984).Another way to represent the self-thinning process is to plot the average weight of individual plants in a stand against density (fig. 13.5). The slope of the line in such plots averages around -3/2. Because self-thinning by many species of plants comes clo