Ecology Notes – Oct. 16/12
- 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 is interaction between
two species in which each is harmed when they both use the same limiting resource. Intraspecific
competition is between individuals of a single species.
- 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 are
food, light for plants, water in terrestrial habitats, space especially for sessile organisms, and space
for refuge and nesting for mobile animals.
- Species are also influenced by physical factors (abiotic) that are not consumed, such as temperature,
pH, and salinity. These factors are not considered to be resources.
- Competition reduces the availability of resources. Experiments with two diatom species by Tilman et
al 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.
- Competition can intensify when resources are scarce. Competition among plants should increase in
nutrient-poor soils. Wilson and Tilman (1993) studied grass plants that were transplanted into
fertilized and unfertilized plots. Each plot had three treatments – 1. Neighbours left intact
(belowground and aboveground competition). 2. Neighbour roots left intact but neighbour shoots
tied back (belowground competition). 3. Neighbour roots and shoots both removed (no competition).
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?: Connell found that competition was important for 50% of 215
species in 72 studies. Gurevitch analyzed the magnitude of competition in 93 species in 46 studies
and found that competition had significant effects on a wide range of organisms. Potential biases
exist where researchers may not publish studies that show no significant effects, and there is a
tendency for investigators to study species they suspect will show competition. Still, they document
that competition is common, though not ubiquitous.
- General features of competition: Competition, whether direct or indirect, can limit the distributions
and abundances of competing species. Exploitation competition is when species compete indirectly –
Individuals reduce the availability of a resource as they use it. Interference competition is when
species compete directly for access to a resource. Individuals may perform antagonistic actions ((ex.
when two predators fight over a prey item, or voles aggressively exclude other voles from preferred
habitat). Interference competition 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. Interference competition in plants – Allelopathy is when
plants of one species release toxins that harm other species. Individuals of one plant species may also
grow on or shade other species, reducing their access to light.
- 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
other. An example is 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 set up plots with four treatments as follows – 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%. 4. Undisturbed control
plants. These two distantly related groups compete for the same food source. - Competition can influence species distributions. Connell 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 – 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. Semibalanus smothered, removed, or crushed the other species. 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 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
- 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.
- In the 1930s, Gause did experiments on competition using three species of Paramecium. Populations
of all three species reached a stable carrying capacity when grown alone. When paired, some species
drove others to extinction. When P. aurelia and P. caudatum were grown together, P. caudatam went
extinct. P. caudatum and P. bursaria were able to coexist despite competition, but the carrying
capacity of both species was lowered. P. caudatum usually ate floating bacteria, while P. bursaria