Ricklefs Chapter 16 Notes

Chapter 16 – Competition

Competition is the use of defense of a resource by one individual that reduces the availability of that resource to other individuals.

Intraspecific Competition – competition between individuals of the same species
Interspecific Competition – competition between individuals of different species

The more crowded a populations, the stronger the effects of competitions between individuals.

When one population can continue to grow at a resource level that curtails the growth of a second population, the first will eventually replace the second.

Resource -
• Is consumed and its amount or availability is thereby reduced
• A consumer uses a resource for its own maintenance and growth
• When resource availability is reduced, biological processes are affected in such a way as to reduce consumer population growth

Not a resource – temperature, nonconsumable physical and biological factors

Nonrenewable resource – are not regenerated. Such as space.
Renewable resource – constantly regenerated or renewed.

Liebig’s law of the minimum – each population increases until the supply of some resource, the limiting resource, no longer satisfies the population’s need for it. This applies strictly only to resources having an independent influence on the consumer population.

Synergistic resources together enhance the growth of a consumer pop more than the sum of both individually

Competitive Exclusion Principle – two species cannot coexist indefinitely when the same resource limits both species. Leads to the question, how much difference in resource requirements is sufficient to allow coexistence?

If two species are to coexist, the populations of both must reach a stable equilibrium size greater than zero.

dN1/N1dt = 0 when N1=K1-a1,2N2

So the change in the population size over time = 0 when the population is equal to its carrying capacity minus (the degree to which each individual of the competing species uses the resources of an individual of this species times the population of the competing species.)

When interspecific competition is weak, the equilibrium populations of each is greater than zero and competition coefficients are less than 1. In other words, to coexist, species must limit themselves more than they limit each other.

Note: the outcome of competition does not depend on the exponential growth rates of the populations when they are small.

Interspecific competition reduces the equilibrium level of a population below the carrying capacity.

Asymmetric competition – when species coexist in nature because they are limited by different factors (e.g. the barnacles – one was stronger and pushed out the other, but the other could resist desiccation and environmental stress at higher altitude nearby that the evicting species could not.

Allelopathy – Chemical competition, occurs most frequently in terrestrial plants and may take on a variety of forms. I.e. black walnut produces juglone, an aromatic organic compound that inhibits certain enzymes in other plants, so few species are capable of germinating and becoming established under black walnut trees.
Consumers can influence the outcome of competition. (i.e. removing sea stars, that feed on several species of beings, decreased diversity of species present because some prey species, left uncontrolled by predators, populate faster and push out other species – thus the lower instance of diversity)

A negative association between two species can occur without direct competition for a shared resource. (i.e. algae and corals, the negative interaction is caused by a chain of events that is initiated by the algae emitting polysaccharides, which fed and led to the overabundance of microbes that typically live in coral at lower levels and both thrive. With the addition of the polysaccharides from the algae, the microbes increased to a point where they were consuming all of the surrounding oxygen and suffocating the coral.

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