LECTURE 11: COMPETITION
Competition – an interaction between individuals in which each is harmed by their shared use of a
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:
o Light for plants
o Water in terrestrial habitats
o Space, especially for sessile organisms
o For mobile animals, space for refuge, nesting, etc.
Species are also influenced by physical factors (abiotic) that are not consumed, such as temperature,
o These factors are not considered to be
Competition reduces availability of resources.
o Experiments with two diatom species by
Tilman et al. (1981) showed that when each
species was grown alone, a stable
population size was reached.
o When grown together, they competed for
silica, and one species drove the other to
o Minimum concentration of silica that both
species can live at (shown by first two
o Evidently, being able to tolerate the lowest
drawdown of resources can be a big
advantage for some species
Competition can intensify when resources are
Competition among plants should increase in nutrient-poor soils
Wilson and Tilman (1993) studied grass plants that were transplanted into fertilized and unfertilized
Each plot had three treatments:
1. Neighbors left intact (belowground and
2. Neighbor roots left intact but neighbor
shoots tied back (belowground
3. Neighbor roots and shoots both removed
Belowground competition (treatment 2) was
most intense in nitrogen-limited plots
Aboveground competition for light increased when light levels were low
How important is competition?
o Connell (1983) found that competition was important for 50% of 215 species in 72 studies. o 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.
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.
Exploitation competition—Species compete indirectly: Individuals reduce the availability of a resource
as they use it.
Interference competition: Species compete directly for access to a resource.
o Individuals may perform antagonistic actions (e.g., when two predators fight over a prey item, or
voles aggressively exclude other voles from preferred habitat)
o Interference competition in sessile species (those that aren’t able to move around):
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
o Interference competition in plants:
Individuals of one species grow on or shade other species, reducing their access to light
Allelopathy – plant of one species release toxins that harm other species
“Chemical warfare” between plants
For a resource in short supply, competition will reduce the amount available to each species.
The effects of competition are often unequal, or asymmetrical, and one species is harmed more than the
Example: When one species drives another to extinction.
Competition can occur between 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
1. Rodents excluded: ant colonies increased by 71%
2. Ants excluded: rodents increased in number and biomass
3. Both rodents and ants excluded: seed density increased 450% -
implies that seeds are a much-needed resource
4. Undisturbed control plots
Competition can influence species distributions:
o Connell (1961) examined factors that influence the distribution, survival, and reproduction of
two barnacle species on the coast of Scotland.
Distribution of larvae of the two species overlapped.
Adult distributions did not overlap:
o Chthamalus were found only near the top of the
o 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
Semibalanus smothered, removed, or crushed the other
However, Semibalanus dried out and survived poorly at
the top of the intertidal zone
A “natural experiment” is a situation in nature that is
similar in effect to a controlled removal experiment.
Patterson (1980, 1981) studied chipmunk species in
mountain forests 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. Competitive Exclusion
Competing species are more likely to coexist when