Competition is an interaction between individuals in which each is 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 different species in which each is harmed
when they both use the same limiting resource.
Intraspecific competition: Between individuals of a single species. As a result, the resources
available to members of a high density population can be reduced to such an extent that
growth, survival, or reproduction decreases or emigration increases
Competition for Resources
Resources are features of the environment required for growth, survival, or reproduction, and
which can be consumed to the point of depletion.
Examples of resources:
Light for plants. (overstory tall trees absorb all the light, thus consuming it. Understory
trees are outcompeted)
Water in terrestrial habitats.
Space, especially for sessile organisms.
For mobile animals, space for refuge, nesting, etc.
Species are also influenced by physical factors (abiotic) that are not consumed, such as
temperature, pH, salinity. These factors are not considered to be resources. Even though these
factors may be required for growth, reproduction and survival, they are not depleted. The same
substance can be a physical factor for some and a resource for others (Ex: terrestrial mammals
and insects consume oxygen but do not deplete it, thus a physical factor but in some aquatic or
soil environments, organisms may consume oxygen more rapidly than it is replenished, thus
Competition reduces availability of resources. (too much consumption that resource depletes
and populations decline)
Experiments with two diatom species by Tilman et al. (1981) They studied competition
for silica. Two diatom species were grown alone and in competition with one another.
They measured how population densities and silica concentrations in water changed
over time. This study showed that when
each species was grown alone, a
stable population size was
reached. (used up all of the
silica) Synedra had a lower
stable population size than
When grown together, they
competed for silica, and one
species drove the other to
Combined graph tell us
minimum concentration of silica that a species lives in. We see #2 has better survival!
Competition can intensify when resources are scarce. There is aboveground competition (light)
and belowground competition (soil nutrients) Importance of two depends on which resource is
Competition among plants should increase in nutrient-poor soils.
Wilson and Tilman (1993) studied grass plants that were transplanted into fertilized and
unfertilized plots. they selected a series of plots of vegetation grown in sandy, nitrogen
poor soils. For 3 years, they treated half with high nitrogen fertilizer. After the 3 yearsm
they planted the grass in the plots. Once added to high and low nitrogen plots, they
were grown under 3 treatments.
Neighbors left intact (belowground and aboveground competition).
Neighbor roots left intact but neighbor shoots tied back (tied back shoots did
not shade the grass but the act of tying did not appear to affect neighbour
roots, so that was belowground competition only).
Neighbor roots and shoots both removed (no competition).
Total competition did not differ between low and high nitrogen plots. However,
belowground competition (treatment 2) was most intense in nitrogen-limited
plots. Aboveground competition for light increased when light levels were low.
THUS INTENSITY OF COMPETITION INCREASES IF A RESOURCE IS SCARCE
Competition is common Connell (1983) found that competition was important for 50% of 215 species in 72 studies.
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.
The above surveys face potential sources of bias. 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
Features of Competition
Exploitation Competition: Species compete indirectly through mutual effects on availability of a
shared resource: Individuals reduce the availability of a resource as they use it.
Interference competition: Species compete directly for access to a resource. Individuals may
perform antagonistic actions (e.g., when two predators fight over a prey item, or voles
aggressively exclude other voles from preferred habitat). These are most familiar in mobile
organisms. However, this can also occur in sessile species
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
zone. This grows half a meter every day crushing competitors
Interference competition also occurs in plants. Individuals of one species grow on or shade
other species, reducing their access to light. This can also take the form ofallelopathy.
Allelopathy: Plants of one species release toxins that harm other species.
For a resource in short supply, competition will reduce the amount available to each species.
Each species obtains less of the resource than it could have obtained if the competitor were not
present. The effects of competition are often unequal, or asymmetrical, and one species is
harmed more than the other. Before it goes extinct, the inferior competitor uses some
resources and hence reduces the resources available to the superior competitor
This is seen when one species drives another to extinction.
Competition can occur between closely and 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 follows:
1. Rodents excluded: Ant colonies increased by 71%. (exclusion via a fence. Rodents within fence
were removed by trapping)
2. Ants excluded: Rodents increased in number and biomass. (insecticide)
3. Both rodents and ants excluded: Seed density increased 450%.
4. Undisturbed control plots.
Treatments 1,2,4 all resulted in
similar densities of seeds
When either rodents or ants
were removed, the remaining
group ate more seeds than the two species in the control plots did combined
Competition can influence species distributions. It may restrict a species to certain portions of a
particular habitat, or it may determine the broad geographic regions in which a species is found
to a particular set of environmental conditions
Competition can restrict the local distribution of a species
Connell (1961) examined factors that
influence the distribution, survival, and
reproduction of two barnacle species on
the coast of Scotland. Larvae drift
through ocean waters, then settle on
rocks or other surfaces where they
metamorphose into adults forming a hard
Distribution of larvae of the two species
overlapped. (upper/middle intertidal
Adult distributions did not overlap. Chthamalus were found only near the top of the
intertidal zone. 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 the zone. (chose some individuals in each zone and removed all nearby
members. For other individuals, he left neighbours in place) Semibalanus smothered,
removed, or crushed the other species. Semibalanus was not affected strongly by
competition. Semibalanus dried out and survived poorly at the top of the intertidal
zone. It appears to have been excluded by dessication, not competition
Competition can also prevent a wide range of species (mammals, birds, plants, marine
invertebrates) from occupying geographic regions in which they would otherwise thrive
A “natural experiment” is a situation in nature that is similar in effect to a controlled
removal experiment. This shows evidence of this scenario
Patterson (1980, 1981) studied chipmunk species in mountain forests (separated by
desert flats) and found
that when a species
lived alone on a
mountain, it occupied a
wider range of habitats
than when it lived with a
Competition may have prevented the chipmunks from living in areas of otherwise
Chipmunks and Barnacles are cases in which a species may limit the distribution of a
competitor without driving the competitor to extinction (unlike diatoms) Competitive Exclusion
Competing species are more likely to coexist when
they use resources in different ways.
If the ecological niches of competing species are very
similar, the superior competitor may drive the other
species to extinction. Ecological niche is the physical,
biological conditions a species needs to grow, survive
In the 1930s, Gause did experiments on competition using three species of Paramecium.
He created mini aquatic ecosystems with bacteria
and yeast as food supply.
Populations of all three species reached a stable
carrying capacity when grown alone. (logistic)
When paired, some species drove others to
P. Aurelia drove P. Caudatum to extinction when
grown together. (both fed on bacteria-overlap in
food source)P. caudatum and P. bursaria were
able to coexist despite competition, but it as clear
that they competed for one or more resources
because the carrying capacity of each was
lowered by the presence of the other. They could
coexist because P. caudatum usually ate floating
bacteria, while P. bursaria usually fed on yeast
cells that settled to the bottom.
Experiments like the one above and many others (with various species) led to the competitive
Competitive exclusion principle: Two species that use a limiting resource in the same
way cannot coexist.
Competitors may coexist when they use resources differently. Such differences are referred to
as resource partitioning
Resource partitioning: Species using a limited resource in different ways.
Stomp et al. (2004) studied two cyanobacteria species in the Baltic Sea. BS1 absorbs green wavelengths most efficiently. Thus, it reflects red light and
uses red light inefficiently. BS2 absorbs red most efficiently. Thus, it reflects
green light and uses green light inefficiently
Each species could survive when grown alone in either wavelength.
When grown together, one drove the other to extinction, depending on the
wavelength used. When grown under green light, red BS1 drove BS2 to
extinction. When grown under red light, green BS2 drove red BS1 to extinction
When grown under white light (full light spectrum containing red and gr