Ecology Lecture No. 11: Competition
Tuesday October 16 , 2012
-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. There are two types of competition in species:
interspecific competition (an interaction between two species in which each is harmed when they both
use the same limiting resource) and intraspecific competition (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 include: food, light (for plants), water
(in terrestrial habitats), space (especially for sessile organisms), and space for refuge or nesting (in
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. Note that oxygen is
generally considered to be a more of a physical factor than a resource (although this depends on the
-Competition reduces the availability of resources. Experiments with two diatom species by Tilman et al.
(1981) showed that when each species was grown alone, a stable population size was reached. When
grown together, they competed for silica (common resource), and one species drove the other to
extinction (because it was the better competitor). In the graphs, Synedra is the better competitor
because it can tolerate the lowest concentrations of silica as a resource.
-Competition can intensify when resources are scarce. Competition among plants should increase in
nutrient-poor soils. This was noted by Wilson and Tilman (1993) when they studied grass plants that
were transplanted into fertilized and unfertilized plots. Each plot type had three treatments: one where
the neighbours were left intact (belowground and aboveground competition), another where the
neighbour roots were left intact but the neighbour shoots were tied back (belowground competition)
and finally where neighbour roots and shoots were both removed (no competition).
-They concluded that belowground competition (treatment 2) was most intense in nitrogen-limited plots
and aboveground competition (treatment 1) for light increased when light levels were low.
Relative Importance Of Competition:
-There are potential biases when studying competition among species as 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. General Features Of Competition:
-Competition, whether direct or indirect, can limit the distributions and abundances of competing
species. Exploitation competition is where species compete indirectly: Individuals reduce the availability
of a resource as they use it. Interference competition describes species that compete directly for access
to a resource. Examples of this may include individuals performing antagonistic actions (e.g. territorial
display, dominance in predators.
Interference Competition In Sessile Species & Plants:
-E.g. 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. Plants practise
interference competition in two ways: either by allelopathy (where plants of one species release toxins
that harm other species) or individuals of one species grow on or shade other species, reducing their
access to light. Of the two methods of interference competition, allelopathy is not as commonly
Effects Of Competition In Reducing Available Resources:
-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 often more harmed than
the other. An example of this is when one species drives another to extinction.
Competition 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 follows: treatment 1 with rodents excluded (ant colonies increased by 71%),
treatment 2 with ants excluded (rodents increased in number and biomass), treatment 3 with both
rodents and ants excluded (seed density increased 450%), and treatment 4 with undisturbed control
plots. In the graph, there is much overlap observed as the species are both feeding on the same seeds.
Effect Of Competition On Species Distributions:
-Connell (1961) examined factors that influence the distribution, survival, and reproduction of two
barnacle species on the coast of Scotland. Even though the distribution of larvae in the two species
overlapped, the adult distributions did not overlap. Chthamalus were found only near the top of the
intertidal zone and 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, but Semibalanus dried out and
survived poorly at the top of the intertidal zone.
Natural Experiments For Studying Competition:
-A “natural experiment” is a situation in nature that is similar in effect to a controlled removal
experiment. For example, 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 l