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Lecture 11

Lecture 11 - Competition

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Biology 2483A
Hugh Henry

LECTURE 11: COMPETITION  Competition – an interaction between individuals in which each is harmed by their shared use of a limiting resource  Competition occurs between species that share the use of a resource that limits the growth, survival or reproduction of each species  Interspecific competition: Interaction between two species in which each is harmed when they both use the same limiting resource.  Intraspecific competition: Between individuals of a single species. Competition for Resources  Resources – features of the environment required for growth, survival, or reproduction, and which can be consumed to the point of depletion  Examples of resources: o Food o Light for plants o Water in terrestrial habitats o Space, especially for sessile organisms o For mobile animals, space for refuge, nesting, etc.  Species are also influenced by physical factors (abiotic) that are not consumed, such as temperature, pH, salinity. o These factors are not considered to be resources  Competition reduces availability of resources. o Experiments with two diatom species by Tilman et al. (1981) showed that when each species was grown alone, a stable population size was reached. o When grown together, they competed for silica, and one species drove the other to extinction. o Minimum concentration of silica that both species can live at (shown by first two graphs) o Evidently, being able to tolerate the lowest drawdown of resources can be a big advantage for some species  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  Each plot had three treatments: 1. Neighbors left intact (belowground and aboveground competition) 2. Neighbor roots left intact but neighbor shoots tied back (belowground competition) 3. Neighbor roots and shoots both removed (no competition)  Belowground competition (treatment 2) was most intense in nitrogen-limited plots  Aboveground competition for light increased when light levels were low  How important is competition? o Connell (1983) found that competition was important for 50% of 215 species in 72 studies. o Gurevitch et al. (1992) analyzed the magnitude of competition in 93 species in 46 studies: Competition had significant effects on a wide range of organisms.  Potential biases: Researchers may not publish studies that show no significant effects, and a tendency for investigators to study species they suspect will show competition.  Still, they document that competition is common, though not ubiquitous.  Exploitation competition—Species compete indirectly: Individuals reduce the availability of a resource as they use it.  Interference competition: Species compete directly for access to a resource. o Individuals may perform antagonistic actions (e.g., when two predators fight over a prey item, or voles aggressively exclude other voles from preferred habitat) o Interference competition in sessile species (those that aren’t able to move around):  The acorn barnacle crushes or smothers nearby individuals of another barnacle species as it grows, and directly excludes the other species from portions of a rocky intertidal zone o Interference competition in plants:  Individuals of one species grow on or shade other species, reducing their access to light  Allelopathy – plant of one species release toxins that harm other species  “Chemical warfare” between plants  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.  Competition can occur between distantly related species.  In experiments with rodents and ants that eat the same seeds, Brown and Davidson (1977) set up plots with four treatments, as follows: 1. Rodents excluded: ant colonies increased by 71% 2. Ants excluded: rodents increased in number and biomass 3. Both rodents and ants excluded: seed density increased 450% - implies that seeds are a much-needed resource 4. Undisturbed control plots  Competition can influence species distributions: o Connell (1961) examined factors that influence the distribution, survival, and reproduction of two barnacle species on the coast of Scotland.  Distribution of larvae of the two species overlapped.  Adult distributions did not overlap: o Chthamalus were found only near the top of the intertidal zone o Semibalanus were found throughout the rest of the intertidal zone  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  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 that when a species lived alone on a mountain, it occupied a wider range of habitats than when it lived with a competitor species. Competitive Exclusion  Competing species are more likely to coexist when
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