EEB 2244 Lecture 12: Competition
22 Dec 2018
School
Department
Course
Professor
Competition
Competition plays a huge role in shaping natural systems. It impacts the realized niche
of species.
It is also incredibly difficult to observe in progress.
Competition: interaction with negative effects between two individuals depending on
the same limiting resource.
Intraspecific competition: within species, relates to density dependent population
growth and more recently with sexual selection
Interspecific competition: among individuals of different species
Why Should We Be Interested in Competition?
Competition determines ranges and niches and therefore affects their occurrence within
and among communities.
Ultimately helps determine community composition – which species are found where
and with what other species.
Important consideration with regards to the maintenance of natural resources – such as
protected areas, harvested forests, harvested wildlife populations.
Many of the species we want or rely upon compete with each other.
Measuring Competition Among Closely Related Plants
Darwin suggested that competition is most intense between related species because
they have similar traits and consume similar resources.
For related species that compete strongly, natural selection should favor differences in
habitat use.
The Tansley experiments are an early example of measuring competition in plants.
Focused on closely related species showing differences in habitat use according to soil
type.
Heath bedstraw prefers to grow on acidic soils, whereas the related white bedstraw
prefers to grow on alkaline soil.
Hypothesis is that each species is adapted to particular soil conditions and will be
competitively superior in its native habitat.
Two Predictions:
1) They can grow in each soil type but will do best in its own because that is
where it is the superior competitor.
2) When grown together, outcome will depend on soil type.
find more resources at oneclass.com
find more resources at oneclass.com
• 2 bedstraw species can grow alone in different soil conditions
although each species grew best in the soil type of its natural
habitat.
• While both can grow in either soil type, competition has led to
different habitat use and restricted their distribution.
• Results suggest, it is competition causing spatial habitat use
pattern.
Competition and Niche Relationships
Degree of overlap influences the degree of competition between two species. More
overlap→ more competition.
Complete competitors have high levels of competition because they share the same
niche and are limited by the same one resource.
The competitive exclusion principle tells us that such species cannot coexist indefinitely.
When two species are limited by the same resource, one species is often a better
competitor and survives better when resources are scarce.
Which Resource Will Be Limiting?
Not all resources limit consumer populations. Liebig’s law of the minimum: population
increases until the supply of the most limiting resource prevents it from increasing further.
If availability of one resource increases, then another may become limiting.
Potentially limiting resources include:
• Food (living or dead, plants or animals) for animals, protists, and carnivorous
plants.
• Inorganic nutrients for plants (e.g. N, P, K) and sometimes for animals (e.g. salt
for ungulates, calcium for birds and many mammals).
• Water
• Solar Energy (visible photosynthetically active light) for plants.
• Open space for sessile organisms (plants, intertidal invertebrates) [but can be a
proxy for other resources].
• Hiding or nesting sites or territories for reproduction for animals (e.g. bluebird
nest holes, dens for snakes, breeding territories for antelope) and safe sites for
plants (e.g. for germination of seeds).
find more resources at oneclass.com
find more resources at oneclass.com
