Chapter 11 eco case studies (2).docx

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Biological Sciences
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Marc Cadotte

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BIOB50: Ecology – Marc Cadotte Chapter 11: Competition - Case Studies Tilman = laboratory experiment: - Examined competition of silica (SiO ) in 2resh water diatoms = single-celled algae - Diatoms use silica to construct cell walls - Tilman grew 2 diatom species = Synedra ulna and Asterionella formosa - Grew S. ulna and A. Formosa both separately and then in completion w/ each other = measures how diatom pop densities and silica concentrations in water changed over time - When grown alone, each species reduced resource = silica to a low, constant concentration – each species reached stable pop size - S. ulna had a lower stable pop size than A. formosa and also reduced silica levels lower than A. formosa - When 2 species competed w/ each other, S. ulna drove A. Formosa to extinction b/c it reduced silica levels so low that A. formosa couldn’t survive Scott Wilson & David Tilman = transplant experiment: - Used Schizachyrium scoparium = perennial grass species native to Minnesota - 5x5 plats of natural vegetation growing in low-nitrogen, sandy soils - For 3 years, treated half of the plots w/ high-nitrogen fertilizer/year - After 3 years, the planted S. scoparium in the plots – some to high-nitrogen and some to low-nitrogen plots - Had 3 treatments: o 1) w/ neighbours left intact o 2) w/ neighbour roots left intact but neighbour’s shoots tied back o 3) w/ neighbour roots and shoots both removed - treatment 1 = had both below-ground and above-ground competition (nitrogen in soil and light from sun) - treatment 3 = no competition - treatment 2 = only below-ground competition - Wilson and Tilman found that while the total competition (sum of below-ground and above-ground competition) didn’t differ between low and high nitrogen plots, below-ground competition was more intense in low-nitrogen plots - *intensity of competition can increase when the resource being competed for becomes scarce* Schoener - Examined results of 164 published studies on competition and found that of 390 species studied, 76% showed effects of competition under some circumstances, and 57% showed effects of competition under all circumstances tested Connell - Examined results of 72 studies and found that competition was important for 50% of 215 species Gurevitch BIOB50: Ecology – Marc Cadotte - Analyzed magnitude of competitive effects found for 93 species in 46 studies published 1980-89 - Showed that competition had significant (and variable) effects on wide range of organisms (carnivores, herbivores, producers/plants) Acorn Barnacle = interference competition - As it grows, acorn barnacle = Semibalanus balanoides often crushes/smothers nearby individuals of another barnacle species = Chthamalus stellatus - Therefore, Semibalanus directly prevents Chthamalus from living in most portions of rocky intertidal zone Spotted knapweed = allelopathy - Aka Centaurea maculosa = native to Eurasia, accidently introduced to North America in 1800s - A contaminant in crop seeds – rapidly spread across US and Canada causing ~42 million $ annual losses - Has ability to spread so rapidly because: o Cattle avoid eating it, therefore more likely to survive than native grasses that cattle do eat o Spotted knapweed releases toxin = catechin into surrounding soils  Catechin reduces germination and growth of native grasses  Exposure to catechin activates 100s of genes in native grasses that compete w/ spotted knapweed – some genes cause root cells of grasses to ‘commit suicide’ which kills the root and allows spotter knapweed to grow = potential competitive advantage Brown & Davidson - Experiments on rodents and ants = test whether competition also occurs between groups of distantly- related species o Thought the 2 species might compete because they both eat the seeds of desert plants and the sizes of the seeds they prefer overlap a lot - In Tucson, Arizona – used 3 years and 4 treatments: o 1) Plots where ¼-inch wire mesh fence excluded seed-eating rodents and rodents w/in the fence were removed by trapping o 2) plots where seed-eating ants were excluded by insecticides o 3) plats where both rodents and ants were excluded by fencing, trapping, and insecticides o 4) plot where rodents and ants were left undisturbed = control group - *found that ants and rodents compete for food* - # of ant colonies increased by 71% in plots where rodents were excluded (plot 1) - # of rodents increased by 18% and 24% in biomass in plots where ants were excluded (plot 2) - Plot 3 = both ants and rodents excluded – seed availability increased by 450% compared to all other plots o Plots 1, 2, and 4 all had somewhat same seed densities - Under natural conditions, each group ex
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