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Ecology 11.docx

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Biology 2483A
Mark Moscicki

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Ecology-Lecture 11 Competition  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: Interaction between two different species in which each is harmed when they both use the same limiting resource.  Intraspecific competition: Between individuals of a single species. As a result, the resources available to members of a high density population can be reduced to such an extent that growth, survival, or reproduction decreases or emigration increases 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:  Food  Light for plants. (overstory tall trees absorb all the light, thus consuming it. Understory trees are outcompeted)  Water in terrestrial habitats.  Space, especially for sessile organisms.  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. These factors are not considered to be resources. Even though these factors may be required for growth, reproduction and survival, they are not depleted. The same substance can be a physical factor for some and a resource for others (Ex: terrestrial mammals and insects consume oxygen but do not deplete it, thus a physical factor but in some aquatic or soil environments, organisms may consume oxygen more rapidly than it is replenished, thus resource)  Competition reduces availability of resources. (too much consumption that resource depletes and populations decline)  Experiments with two diatom species by Tilman et al. (1981) They studied competition for silica. Two diatom species were grown alone and in competition with one another. They measured how population densities and silica concentrations in water changed over time.  This study showed that when each species was grown alone, a stable population size was reached. (used up all of the silica) Synedra had a lower stable population size than Asterionella  When grown together, they competed for silica, and one species drove the other to extinction.  Combined graph tell us minimum concentration of silica that a species lives in. We see #2 has better survival! (A-blue)  Competition can intensify when resources are scarce. There is aboveground competition (light) and belowground competition (soil nutrients) Importance of two depends on which resource is more 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. they selected a series of plots of vegetation grown in sandy, nitrogen poor soils. For 3 years, they treated half with high nitrogen fertilizer. After the 3 yearsm they planted the grass in the plots. Once added to high and low nitrogen plots, they were grown under 3 treatments.  Neighbors left intact (belowground and aboveground competition).  Neighbor roots left intact but neighbor shoots tied back (tied back shoots did not shade the grass but the act of tying did not appear to affect neighbour roots, so that was belowground competition only).  Neighbor roots and shoots both removed (no competition).  Total competition did not differ between low and high nitrogen plots. However, belowground competition (treatment 2) was most intense in nitrogen-limited plots. Aboveground competition for light increased when light levels were low. THUS INTENSITY OF COMPETITION INCREASES IF A RESOURCE IS SCARCE  Competition is common  Connell (1983) found that competition was important for 50% of 215 species in 72 studies.  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.  The above surveys face potential sources of bias. 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. Features of Competition  Exploitation Competition: Species compete indirectly through mutual effects on availability of a shared resource: Individuals reduce the availability of a resource as they use it.  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). These are most familiar in mobile organisms. However, this can also occur 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. This grows half a meter every day crushing competitors  Interference competition also occurs in plants. Individuals of one species grow on or shade other species, reducing their access to light. This can also take the form ofallelopathy. Allelopathy: Plants of one species release toxins that harm other species.  For a resource in short supply, competition will reduce the amount available to each species. Each species obtains less of the resource than it could have obtained if the competitor were not present. The effects of competition are often unequal, or asymmetrical, and one species is harmed more than the other. Before it goes extinct, the inferior competitor uses some resources and hence reduces the resources available to the superior competitor  This is seen when one species drives another to extinction.  Competition can occur between closely and 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%. (exclusion via a fence. Rodents within fence were removed by trapping) 2. Ants excluded: Rodents increased in number and biomass. (insecticide) 3. Both rodents and ants excluded: Seed density increased 450%. 4. Undisturbed control plots.  Treatments 1,2,4 all resulted in similar densities of seeds  When either rodents or ants were removed, the remaining group ate more seeds than the two species in the control plots did combined  Competition can influence species distributions. It may restrict a species to certain portions of a particular habitat, or it may determine the broad geographic regions in which a species is found to a particular set of environmental conditions  Competition can restrict the local distribution of a species  Connell (1961) examined factors that influence the distribution, survival, and reproduction of two barnacle species on the coast of Scotland. Larvae drift through ocean waters, then settle on rocks or other surfaces where they metamorphose into adults forming a hard shell  Distribution of larvae of the two species overlapped. (upper/middle intertidal zones)  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. (chose some individuals in each zone and removed all nearby members. For other individuals, he left neighbours in place) Semibalanus smothered, removed, or crushed the other species. Semibalanus was not affected strongly by competition. Semibalanus dried out and survived poorly at the top of the intertidal zone. It appears to have been excluded by dessication, not competition  Competition can also prevent a wide range of species (mammals, birds, plants, marine invertebrates) from occupying geographic regions in which they would otherwise thrive  A “natural experiment” is a situation in nature that is similar in effect to a controlled removal experiment. This shows evidence of this scenario  Patterson (1980, 1981) studied chipmunk species in mountain forests (separated by desert flats) 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. Competition may have prevented the chipmunks from living in areas of otherwise suitable habitat  Chipmunks and Barnacles are cases in which a species may limit the distribution of a competitor without driving the competitor to extinction (unlike diatoms) 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. Ecological niche is the physical, biological conditions a species needs to grow, survive and reproduce  In the 1930s, Gause did experiments on competition using three species of Paramecium. He created mini aquatic ecosystems with bacteria and yeast as food supply.  Populations of all three species reached a stable carrying capacity when grown alone. (logistic) When paired, some species drove others to extinction  P. Aurelia drove P. Caudatum to extinction when grown together. (both fed on bacteria-overlap in food source)P. caudatum and P. bursaria were able to coexist despite competition, but it as clear that they competed for one or more resources because the carrying capacity of each was lowered by the presence of the other. They could coexist because P. caudatum usually ate floating bacteria, while P. bursaria usually fed on yeast cells that settled to the bottom.  Experiments like the one above and many others (with various species) led to the competitive exclusion principle.  Competitive exclusion principle: Two species that use a limiting resource in the same way cannot coexist.  Competitors may coexist when they use resources differently. Such differences are referred to as resource partitioning  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. Thus, it reflects red light and uses red light inefficiently. BS2 absorbs red most efficiently. Thus, it reflects green light and uses green light inefficiently  Each species could survive when grown alone in either wavelength.  When grown together, one drove the other to extinction, depending on the wavelength used. When grown under green light, red BS1 drove BS2 to extinction. When grown under red light, green BS2 drove red BS1 to extinction  When grown under white light (full light spectrum containing red and gr
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