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Lecture 9

Lecture 9-10 Notes.pdf

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Biological Sciences
Course Code
Marc Cadotte

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Lecture 9-10: Competition Introduction ▯ - A.G. Tansley did one of the first experiments on competition n 1917. He wanted to explain the distribution of ▯ two species of bedstraw: Galium hercynicum, which was restricted to acidic soils, and G. pumilum, restricted ▯ to calcareous soils ▯ - Tasley found that if grown alone, each species could survive on both acidic and calcareous soils ▯ - But when grown together, soil type determined which would survive ▯ - Tansley inferred that competition restricted the two species to particular soil types in nature ▯ - Competition can limit the distributions and abundances of competing species ▯ - As far back as Darwin, competition between species has been seen as an influence on evolution and species ▯ distributions ▯ - Interspecific competition: is an interaction between two species in which each is harmed when they both ▯ use the same limiting resource ▯ - Intraspecific competition: can occur between individuals of a single species Competition for Resources ▯ - Competition occurs between organisms that share the use of a resource that limits the growth, survival, or ▯ reproduction of each species ▯ - Examples of resources that can be consumed to depletion: ▯ ▯ - Food ▯ ▯ - Water in terrestrial habitats ▯ ▯ - Light for plants ▯ ▯ - Space, especially for sessile organisms ▯ ▯ - For mobile animals, space for refuge, nesting, etc ▯ ▯ - These resources are usually in low availability ▯ - Example: A coral reef. Thereʼs no open space, so space if a limited resource. Coral reefs are very crowded. ▯ Thereʼs a constant conflict for space, they try to digest each other, battling with each other. The more space ▯ they occupy, the more chance to get nutrients to reproduce, to grow ▯ - Experiments using two species of diatoms (single-celled algae that make cell walls of silica) were by by ▯ Tilman et al. ▯ ▯ - When each species was grown alone, they reached a carrying capacity and silica concentrations ▯ ▯ were reduced. They grew according to their regular growth ▯ ▯ - When grown together, the two species competed for silica, and one species drove the other to ▯ ▯ extinction. The population was a lot lower and silica concentration was lower ▯ - The population that is able to persist at lower resource concentrations will out compete other species ▯ - This is referred to as R* or R-star ▯ - How important is competition in ecological communities? ▯ - Results from many studies have been compiled and analyzed to answer this question ▯ - Schoener found that 390 species studied, 76% showed effects of competition under some conditions; 57% ▯ showed effects under all conditions tested ▯ - Connell found that competition was important for 50% of 215 species in 72 studies ▯ - Gureitch et al. analyzed the magnitude of competitive effects found for 93 studies in 46 studies. They showed ▯ that competition had significant effects on a wide range of organisms General Features of Competition ▯ - Exploitation competition: species compete indirectly through their mutual effects on the availability of a ▯ shared resource ▯ - Competition occurs simply because individuals reduce the availability of a resource as they use it ▯ - R-star ▯ - Interference competition: species compete directly for access to a resource ▯ - Individuals may perform antagonist actions (e.g. when two predators fight over a prey item, or voles ▯ aggressively exclude other voles from preferred habitat) ▯ - Interference competition can also occur in sessile species (organisms that do not move) ▯ - E.g. The acorn barnacle: as they grow, they push other barnacles off the substrate ▯ - Allelopathy: a form of interference competition in which individuals of one species release toxins that harm ▯ other species ▯ ▯ - Cattle do not eat the introduced spotted knapweed, giving it an edge over native plants that cattle do t a ▯ ▯ e ▯ ▯ - It also releases a toxin called catechin into surrounding soils, which has been shown to reduce ▯ ▯ germination and growth of native grasses ▯ - For a resource in short supply, competition will reduce the amount available to each species ▯ - In man cases, the effects of competition are unequal, or asymmetrical, and one species is harmed more than ▯ the other ▯ - E.g. When one species drives another to extinction ▯ - Usually, competition is strongest between very similar or closely-related species ▯ - Competition can also 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: ▯ ▯ ▯ 1. Wire mesh fence excluded seed-eating rodents ▯ ▯ ▯ 2. Seed-eating ants were excluded by applying insecticides ▯ ▯ ▯ 3. Both rodents and ants were excluded ▯ ▯ ▯ 4. Undisturbed control plots ▯ ▯ - When rodents were excluded, ant colonies increased by 71% ▯ ▯ - When ants were excluded, rodents increased in both number and biomass ▯ ▯ - When both were excluded, the number of seeds increased by 450% ▯ ▯ - When neither rodents or ants were removed, the group that remained ate roughly as many seeds as ▯ ▯ rodents and ants combined ate in the control plots ▯ ▯ - In natural conditions, each group would be expected to eat fewer seeds in the presence of the other ▯ ▯ group than it could eat when alone ▯ - Competition can also limit distribution and abundance of species ▯ - Connell examined factors that influenced the distribution, survival, and reproduction of two barnacle species, ▯ Chthamalus stellatus and Semibalanus balanoides, on the coast of Scotland ▯ ▯ - Distribution of larvae of the two species overlapped throughout the upper and middle intertidal zones ▯ ▯ - Adult distributions did not overlap: Chthamalus were found only near the top of the intertidal zone; ▯ ▯ adult Semibalanus were found throughout the rest of the intertidal zone ▯ - Competition can also affect geographic distribution ▯ - A natural experiment refers to a situation in nature that is similar in effect to a controlled removal experiment ▯ - Chipmunk species in the southwestern U.S. live in mountain forests ▯ ▯ - Patterson found that when a chipmunk species lived alone on a mountain range, it occupied a ▯ ▯ broader range of habitats and elevations than when it lived with a competitor species Competitive Exclusion ▯ - Competitive species are more likely to coexist when they use resources in different ways ▯ - In the 1930ʼs, G.F. Gause performed laboratory experiments on competition using three species of ▯ Paramecium
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