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Lecture 11

Biology 2483A Lecture Notes - Lecture 11: Allelopathy

Course Code
BIOL 2483A
Lisa Henry

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Lecture 11: Competition
Competition: an interaction between individuals in which both are 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 species in which each is
harmed when they both use the same limiting resource
Intraspecific competition: between individuals of a single species
Competition for Resources
Resources: 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  one can shade another
- Water in terrestrial habitats
- Space  for sessile organisms, and for refuge/nesting for mobile ones
Species are also influenced by physical factors (abiotic) that are not consumed,
such as temperature, pH, salinity  they are not considered to be resources
Competition reduces availability of resources
- Experiments with two diatom species 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

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Competition can intensify when resources are scarce
Competition among plants should increase in nutrient-poor soils
- Wilson and Tilman studied grass plants that were transplanted into fertilized
and unfertilized plots
- Each plot has three treatment types:
1) Neighbours left intact (belowground and aboveground competition)
2) Neighbour roots left in tract but neighbor shoots tied back (belowground
3) Neighbor roots and shoots both removed (no competition)
- Belowground competition (treatment 2) was most intense in nitrogen-limited
- Aboveground competition for light increased when light levels were low
How important is competition?
Connell found that competition was important or 50% of 215 species in 72 studies
Gurevitch et al. analyzed the magnitude of competition in 93 species in 46 studies
competition had significant effects on a wide range of organisms
Potential biases: researchers may not publish studies that show no significant
effects, and a tendency for investigators to study species they suspect will show
- They document that competition is common, though not ubiquitous
Competition Types
Exploitations competition: species compete indirectly  individuals reduce the
availability of a resource as they use it
Interference competition: species compete directly for access to a resources
- Individuals may perform antagonistic actions (i.e. when two predators fight
over a prey item, or voles aggressively exclude other volvs from preferred
Interference competition in sessile species:
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