Tansley found that competition restricted two species to particular soil types in nature.
Competition can limit the distributions and abundances of competing species. It occurs between
organisms that share the use of a resource that limits the survival, growth and reproduction of other
IntERspecific Competition – occurs between two or more different species
IntRAspecific Competition – occurs between individuals of a single species.
The population that is able to persist at lower resource concentration will out compete other species. R-
Exploitation Competition – species compete indirectly, and is only competition because individuals
reduce the availability of a resource as they use it.
Interference Competition – species compete directly. E.g. fighting over a prey item. Sessile species
(those that are fixed to a surface, like coral, can also perform interference competition)
Allelopathy – form of interference competition in which individuals of one species release toxin that
harm other species. (example is the spotted knapweek that rereleases catechin)
Competing species are more likely to co-exist when they use resources in different ways.
Resource Partitioning – species use a limited resource in different ways
Competitive Exclusion Principle – two species that use a limiting resource in the same way cannot exist.
Ecological Niche – the overall ecological requirements of a species.
Joseph Grinnel said “no two species of birds or mammals will be found to occupy precisely the same
Natural selection can influence the morphology of competing species resulting in Character
Competitive Reversal – the species that was inferior in the habitat becomes the superior competitor in
another. Environmental conditions results in it.
In the houseflies-blowflies experiment, the houseflies started off superior but as time went on the
situation reversed and the houseflies went extinct.
The population does not increase or decrease for any combination of N1 and N2 on the Zero Population
Growth Isoclines. (Equilibrium) The determine under which conditions each species will increase or
decrease. When the isoclines do not cross, competitive exclusion results.
‘ Predators don’t cause prey extinction because:
- Habitat complexity
- Spatial refuges
- Prey switching
- Prey responding to predation
In experiments, Hairston et al. found that populations cycled, but not synchronously, i.e. asynchronous.
Predators peaked when prey reached their lowest and vice-versa.
Algal genotypes that were most resistant to predators were poor competitors (this is an example of a
trade off) when predator density is high, resistant genotypes increase then predator numbers decrease.
When predator density is low, the resistant genotype is outcompeted by other genotypes and they
increase in number and then the predators increase.
Hare and lynx populations act in synchrony, but we do not have a sufficient understanding as to why.
The Hymenoepimecis argyraphaga induces the spider to spin a special cocoon to protect it from being
swept away by torrential rains.
Symbionts – organisms that live in/on other organism. More than half of the species on earth are
Parasite (+/-) – consumes tissues or body fluids of the organism on which it lives. It’s host.
Pathogens are parasites that cause diseases.
Macroparasites are large,
Microparasites are microscopic. They also include herbivores such as aphids and nematodes as
Ectoparasites live on the outer body surface of the host. (e.g. Dodder, a plant parasite that