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BIOC50H3 Study Guide - Schizachyrium Scoparium, Centaurea Maculosa, Semibalanus Balanoides

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 (SiO2) in fresh 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
- 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
- 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*
- 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
- Examined results of 72 studies and found that competition was important for 50% of 215 species
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
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
o Plots 1, 2, and 4 all had somewhat same seed densities
- Under natural conditions, each group expected to eat fewer seeds in presence of the other group than it
could eat when alone
BIOB50: Ecology Marc Cadotte
Joseph Connell
- Examined factors that influenced distribution, survival, and reproduction of 2 barnacle species =
Chthamalus stellatus and Semibalanus balanoides
- In Scotland, the distribution of these 2 species have overlapped a lot
- Larvae of both species were found on rocks located throughout upper/middle intertidal zones
o adult Chthamalus usually found only near top of intertidal zone
o adult Semibalanus usually found elsewhere, not on top of intertidal zone
- Connell looked at effects of competition and physical factors e.g. desiccation = drying out b/c of
exposure to air (greatest at upper intertidal zone)
- Found that competition w/ Semibalanus excluded Chthamalus from all but the top of the intertidal zone
o As Semibalanus grew, it smothered/grew on top of, removed/grew underneath of, and crushed
- Only 14% of Chthamalus survived 1st years when competing against Semibalanus (were still small and
had poor reproduction)
- *When Semibalanus was removed, 72% of Chthamalus survived = competition*
- Semibalanus wasn’t affected strongly by competition
o Whether Chthamalus was removed or not, the Semibalanus still dried out and survived poorly
near the top of intertidal zone
o *Semibalanus is excluded from top of intertidal zone because of its sensitivity to desiccation, not
because of competition*
G.F. Gause = laboratory experiment:
- Used 2 species of Paramecium = single-celled protest
- Grew paramecium in tubes filled w/ liquid medium that contained bacteria and yeast cells as food supply
- Found that pops of each of the 3 paramecium species reached stable carrying capacity when grown alone
- When pairs of paramecium species competed against each other, diff outcomes occurred:
o When P. aurelia grown in competition w/ P. caudatum, P. aurelia drove P. caudatum to extinction
b/c both species fed on bacteria = overlap in food requirements
o When P. caudatum grown w/ P. bursaria, neither species went extinct = coexisted but still
competed for resources = had lower carrying capacity when grown together compared to
Used diff resources 1 fed on bacteria, 1 fed on yeast
Schoener = lowland Anolis lizards:
- 4 lizard species = A. opalinus, A. graham, A. lineatopus, A. valencienni
o All species live together in trees/shrubs and eat similar foods
- Have differences in height/thickness of their perches and time they spent in sun/shade
- w/in species, individuals of diff sizes used diff portions of their habitat
o larger inds found on thicker perches
- but in a diff species, larger inds were actually found on thinner perches
- when diff species were found near one another on tree/shrub, they tended to be species that differed
considerably in size