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Ecology Notes

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Biology 2483A
Hugh Henry

Ecology Notes – Oct. 16/12 - Competition is 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 is interaction between two species in which each is harmed when they both use the same limiting resource. Intraspecific competition is between individuals of a single species. - Competition for resources: Resources are features of the environment required for growth, survival, or reproduction, and which can be consumed to the point of depletion. Examples of resources are food, light for plants, water in terrestrial habitats, space especially for sessile organisms, and space for refuge and nesting for mobile animals. - Species are also influenced by physical factors (abiotic) that are not consumed, such as temperature, pH, and salinity. These factors are not considered to be resources. - Competition reduces the availability of resources. Experiments with two diatom species by Tilman et al 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. - 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. Neighbours left intact (belowground and aboveground competition). 2. Neighbour roots left intact but neighbour shoots tied back (belowground competition). 3. Neighbour 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?: Connell found that competition was important for 50% of 215 species in 72 studies. Gurevitch analyzed the magnitude of competition in 93 species in 46 studies and found that competition had significant effects on a wide range of organisms. Potential biases exist where researchers may not publish studies that show no significant effects, and there is a tendency for investigators to study species they suspect will show competition. Still, they document that competition is common, though not ubiquitous. - General features of competition: Competition, whether direct or indirect, can limit the distributions and abundances of competing species. Exploitation competition is when species compete indirectly – Individuals reduce the availability of a resource as they use it. Interference competition is when species compete directly for access to a resource. Individuals may perform antagonistic actions ((ex. when two predators fight over a prey item, or voles aggressively exclude other voles from preferred habitat). 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 a rocky intertidal zone. Interference competition in plants – Allelopathy is when plants of one species release toxins that harm other species. Individuals of one plant species may also grow on or shade other species, reducing their access to light. - 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. An example is 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 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%. 4. Undisturbed control plants. These two distantly related groups compete for the same food source. - Competition can influence species distributions. Connell 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 – Chthamalus were found only near the top of the intertidal zone. 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 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 they use resources in different ways. If the ecological niches of competing species are very similar, the superior competitor may drive the other species to extinction. - In the 1930s, Gause did experiments on competition using three species of Paramecium. Populations of all three species reached a stable carrying capacity when grown alone. When paired, some species drove others to extinction. When P. aurelia and P. caudatum were grown together, P. caudatam went extinct. P. caudatum and P. bursaria were able to coexist despite competition, but the carrying capacity of both species was lowered. P. caudatum usually ate floating bacteria, while P. bursaria u
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