Competition

Chapter 12 Competition CONCEPT 12.1 Competition occurs between individuals of two species that share the use of a resource that limits their growth, survival, or reproduction. CONCEPT 12.2 Competition, whether direct or indirect, can limit the distributions and abundances of competing species.

Chapter 12 Competition CONCEPT 12.3 Competing species are more likely to coexist when they use resources in different ways. CONCEPT 12.4 The outcome of competition can be altered by environmental conditions, species, interactions, disturbance, and evolution.

Figure 12.2 Competition Decreases Growth in a Carnivorous Plant Ecology3e-Fig-12-02-0R.jpg

Interspecific competition: Between members of different species Introduction Competition: An interaction between individuals in which each is harmed by their shared use of a limiting resource. Interspecific competition: Between members of different species Intraspecific competition: Between individuals of a single species

CONCEPT 12.1 Competition occurs between individuals of two species that share the use of a resource that limits their growth, survival, or reproduction.

Concept 12.1 Competition for Resources Resources: Features of the environment required for growth, survival, or reproduction, and that can be consumed to the point of depletion. 7

Concept 12.1 Competition for Resources Examples of resources: Food Water in terrestrial habitats Light for plants Space, especially for sessile organisms For mobile animals, space for refuge, nesting, etc.

Figure 12.3 Space Can Be a Limiting Resource Ecology3e-Fig-12-03-0.jpg

Concept 12.1 Competition for Resources Species are also influenced by features of the environment that are not consumed, such as temperature, pH, salinity. These factors are not consumed and are not considered to be resources.

Concept 12.1 Competition for Resources Competition reduces availability of resources. Experiments with 2 diatom species by Tilman et al. (1981) showed that when each species was grown alone, a stable population size was reached. When grown together, they competed for silica, and one species drove the other to extinction.

Figure 12.4 Competing Organisms Can Deplete Resources (Part 1) Ecology3e-Fig-12-04-1R.jpg

Figure 12.4 Competing Organisms Can Deplete Resources (Part 2) Ecology3e-Fig-12-04-2R.jpg

Figure 12.4 Competing Organisms Can Deplete Resources (Part 3) Ecology3e-Fig-12-04-3R.jpg

Concept 12.1 Competition for Resources Competition can intensify when resources are scarce. Competition among plants should increase in nutrient-poor soils. Wilson and Tilman (1993) studied grass plants that were transplanted into fertilized and unfertilized plots.

Concept 12.1 Competition for Resources Each plot type had 3 treatments: 1. Neighbors left intact (belowground and aboveground competition). 2. Neighbor roots left intact but neighbor shoots tied back to reduce shading (belowground competition). 3. Neighbor roots and shoots both removed (no competition).

Concept 12.1 Competition for Resources Belowground competition (treatment 2) was most intense in nitrogen-limited plots. Aboveground competition (treatment 1 minus treatment 2) for light increased when light levels were low.

Figure 12.5 Resource Availability Affects the Intensity of Competition Ecology3e-Fig-12-05-0R.jpg

CONCEPT 12.2 Competition, whether direct or indirect, can limit the distributions and abundances of competing species.

Concept 12.2 General Features of Competition Exploitation competition: Species compete indirectly; individuals reduce the availability of a resource as they use it. Examples: The pitcher plants (Brewer), and the 2 diatom species (Tilman et al.).

Concept 12.2 General Features of Competition Interference competition: Species compete directly for access to a resource. Individuals may perform antagonistic actions (e.g., when two predators fight over a prey item, or voles aggressively exclude other voles from preferred habitat).

Concept 12.2 General Features of Competition Interference competition in sessile species: The acorn barnacle crushes or smothers nearby individuals of another barnacle species as it grows and directly excludes the other species from portions of the rocky intertidal zone.

Concept 12.2 General Features of Competition Interference competition in plants: Individuals of one species grow on or shade other species, reducing their access to light, for example kudzu. Allelopathy: Plants of one species release toxins that harm other species.

Figure 12.6 Interference Competition in Plants Ecology3e-Fig-12-06-0.jpg

Concept 12.2 General Features of Competition For a resource in short supply, competition will reduce the amount available to each species. The effects of competition are often unequal, or asymmetrical, and one species is harmed more than the other. Example: When one species drives another to extinction.

Concept 12.2 General Features of Competition There is a continuum in how strongly each competitor affects the other. The ends of this continuum represent amensalism: –/0 interactions; individuals of one species are harmed while individuals of the other species are not affected at all. 26

Figure 12.7 A Continuum of Competitive Effects Ecology3e-Fig-12-07-0.jpg

Concept 12.2 General Features of Competition Competition can influence species distributions. Connell (1961) examined factors that influence the distribution, survival, and reproduction of two barnacle species on the coast of Scotland. 28

Concept 12.2 General Features of Competition Distribution of the drifting larvae of the 2 species overlapped. Adult distributions did not overlap: Chthamalus were found only near the top of the intertidal zone. Semibalanus were found throughout the rest of the intertidal zone.

Figure 12.9 Squeezed Out by Competition Ecology3e-Fig-12-09-0R.jpg

Concept 12.2 General Features of Competition Using removal experiments, Connell found that Semibalanus excluded Chthamalus from all but the top of the zone. Semibalanus smothered, removed, or crushed the other species. However, Semibalanus dried out and survived poorly at the top of the intertidal zone.

Concept 12.2 General Features of Competition A “natural experiment” is a situation in nature that is similar in effect to a controlled removal experiment. Patterson (1980, 1981) studied chipmunk species in mountain forests and found: When a species lived alone on a mountain, it occupied a wider range of habitats than when it lived with a competitor species.

Figure 12.10 A Natural Experiment on Competition between Chipmunk Species Ecology3e-Fig-12-10-0R.jpg

CONCEPT 12.3 Competing species are more likely to coexist when they use resources in different ways.

Concept 12.3 Competitive Exclusion If the ecological requirements of competing species—the ecological niches—are very similar, the superior competitor may drive the other species to extinction. 35

Concept 12.3 Competitive Exclusion In the 1930s, Gause did experiments on competition using 3 species of Paramecium. Populations of all 3 species reached a stable carrying capacity when grown alone. When paired, some species drove others to extinction.

Figure 12.11 Competition in Paramecium (Part 1) Ecology3e-Fig-12-11-1R.jpg

Figure 12.11 Competition in Paramecium (Part 2) Ecology3e-Fig-12-11-2R.jpg

Concept 12.3 Competitive Exclusion P. aurelia drove P. caudatum to extinction; both feed on yeast cells floating at the top of the growth medium. P. caudatum and P. bursaria were able to coexist, but the carrying capacity of both species was lowered. P. caudatum ate floating bacteria, while P. bursaria ate yeast cells that settled to the bottom.

Concept 12.3 Competitive Exclusion Experiments on these and many other species led to the competitive exclusion principle: Two species that use a limiting resource in the same way cannot coexist indefinitely.

Concept 12.3 Competitive Exclusion Resource partitioning: Species using a limited resource in different ways. Stomp et al. (2004) studied two cyanobacteria species in the Baltic Sea. BS1 absorbs green wavelengths most efficiently; BS2 absorbs red most efficiently.

Concept 12.3 Competitive Exclusion Each species could survive when grown alone in either wavelength. When grown together, one drove the other to extinction, depending on the wavelength used. Under white light (all wavelengths) they both persisted.

Figure 12.12 Do Cyanobacteria Partition Their Use of Light? Ecology3e-Fig-12-12-0.jpg

Concept 12.3 Competitive Exclusion The Lotka–Volterra competition model:

Concept 12.3 Competitive Exclusion N1 = population density of species 1 r1 = intrinsic rate of increase of species 1 K1 = carrying capacity of species 1 α and β = competition coefficients— constants that describe the effect of one species on the other

Concept 12.3 Competitive Exclusion Density of species 2 (N2) does not change in size when: These two equations describe straight lines; N2 is a function of N1 (zero population growth isoclines).

Concept 12.3 Competitive Exclusion Zero population growth isoclines: The population does not increase or decrease in size for any combination of N1 and N2 that lies on these lines. Zero growth isoclines can determine the conditions under which each species will increase or decrease.

Figure 12.13 Graphical Analyses of Competition Ecology3e-Fig-12-13-0R.jpg

Concept 12.3 Competitive Exclusion This graphical approach can be used to predict the end result of competition between species. The N1 and N2 isoclines are plotted together. There are four possible ways the isoclines can be arranged relative to each other.

Concept 12.3 Competitive Exclusion When the isoclines do not cross (A and B), competitive exclusion results. Depending on which isocline is above the other, either species 1 or species 2 always drives the other to extinction.

Figure 12.14 Outcome of Competition in the Lotka–Volterra Competition Model (Part 1) Ecology3e-Fig-12-14-1R.jpg

Concept 12.3 Competitive Exclusion In (C), competitive exclusion occurs, but which species wins depends on how densities change over time. In only one case (D), the two species coexist, although competition still has an effect—the equilibrium density of each species is lower than its carrying capacity.

Figure 12.14 Outcome of Competition in the Lotka–Volterra Competition Model (Part 2) Ecology3e-Fig-12-14-2R.jpg

CONCEPT 12.4 The outcome of competition can be altered by environmental conditions, species interactions, disturbance, and evolution.

Figure 12.15 Herbivores Can Alter the Outcome of Competition Ecology3e-Fig-12-15-0R.jpg

Concept 12.4 Altering the Outcome of Competition If herbivores prefer to feed on the superior competitor, it reduces the growth, survival, or reproduction of that species. The same is true of predators, pathogens, and mutualists: change in abundance of such species can change the outcome of competition among the species with which they interact.

Concept 12.4 Altering the Outcome of Competition Disturbances such as fires or storms can kill or damage some individuals, while creating opportunities for others. Some species can persist in an area only if such disturbances occur regularly. 57

Concept 12.4 Altering the Outcome of Competition Forest plants that need sunlight are found only where disturbance has opened the tree canopy. As trees recolonize and create shade, these plants cannot persist in the patch. Such species are called fugitive species because they must disperse from one place to another as conditions change.

Concept 12.4 Altering the Outcome of Competition The brown alga called sea palm coexists with mussels, a superior competitor, in the rocky intertidal zone. Large waves sometimes remove the mussels, creating temporary openings for the alga. In low disturbance areas, competition with mussels causes sea palm populations to decline over time.

Figure 12.16 Population Decline in an Inferior Competitor Ecology3e-Fig-12-16-0R.jpg

Concept 12.4 Altering the Outcome of Competition Competition can cause evolutionary change, and evolution can alter the outcome of competition. This interplay has been observed in many studies.

Concept 12.4 Altering the Outcome of Competition Natural selection can influence the morphology of competing species and result in character displacement. The phenotypes of competing species become more different over time.

Figure 12.18 Character Displacement Ecology3e-Fig-12-18-0R.jpg

Concept 12.4 Altering the Outcome of Competition In two species of Galápagos finches, beak sizes, and hence sizes of the seeds eaten, are different on islands that have both species. On islands with only one of the species, beak sizes are similar.

Figure 12.19 Competition Shapes Beak Size Ecology3e-Fig-12-19-0R.jpg