Competitive Exclusion: One Species May Drive Another to Extinction
Competition: Important concepts
Competitive Exclusion: one species may drive another to extinction Interspecific: among different species Intraspecific: within the same species • Intraspecific competition is most intense Exploitative competition Interference competition . Territoriality . Allelopathy Resource Partitioning: eliminates direct competition Competition
Competition: An interaction between individuals in which each is harmed by their shared use of a limiting resource.
Competition for Resources
Resources: Features of the environment required for growth, survival, or reproduction, and that can be consumed to the point of depletion.
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. Space Can Be a Limiting Resource Interspecific Competition
Competitive Exclusion
Competitive exclusion principle: Two species that use a limiting resource in the same way cannot coexist indefinitely. When paired, some species drove others to extinction.
Gause’s Experiment
Species grown together
Paramecium caudatum
Paramecium aurelia Forms of Competition
Exploitative competition • Species have equal access to resource; one is simply better at exploiting it
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. Competing Organisms Can Deplete Resources Competing Organisms Can Deplete Resources Competing Organisms Can Deplete Resources
Competition for Resources
Competition can intensify when resources are scarce. Forms of Competition
Interference competition • One species prevents another from using resource . usually through aggressive behavior Intraspecific Competition Intraspecific Competition Territoriality
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. Intraspecific Competition Allelopathy (sibling harming)
Eucalyptus oils harm young saplings of same species. Can also affect other species Interference Competition in Plants: Kudzu grows fast and can smother other plants
General Features of Competition Competition can influence species distributions.
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. Squeezed Out by Competition Interference Competition
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. A Natural Experiment on Competition between Chipmunk Species
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.
Competitive Exclusion
Resource partitioning: Species using a limited resource in different ways. Resource Partitioning
Apparent competitors may have slightly different niches May use resources in a different way or time Minimizes competition and allows coexistence
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.
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. Cyanobacteria Partition Their Use of Light
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. Connections in Nature: The Paradox of Diversity
Noncarnivorous plants are superior competitors for light and soil nutrients. But pitcher plants are tolerant of fire and grow mostly when competitors are absent or reduced (e.g., after a fire). This strategy may allow them to persist with their competitors. Figure 12.21 Coexistence in a Nutrient-Poor Environment 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. Population Decline in an Inferior Competitor Sea Palm
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.
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. Character Displacement
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.
Competition Shapes Beak Size