Lecture 17: Competition

Reading:  None.

Competition

Intraspecific Competition: Competition within a species.

Interaction between individuals for a limited resource, leading to reduced survivorship, growth, and/or reproduction

Intraspecific competition is competition between individuals of one species.

Consequences:

	Decreased resource uptake/individual
	Decreased individual rates of growth, or development
	Decreased stored reserves
	Increased predation risk

Leading to: Decreased survivorship and/or decreased fecundity

Resulting in: Decreased reproductive output/individual

These outcomes are not necessarily distributed evenly among competing individuals.

Forms of Competition

Exploitation Competition
            Resource use by one individual results in less resource available for another individual when a resource is in limited supply
(can occur without direct interaction between individuals)

Interference Competition
            Resource use by one individual limits access to that resource for other individuals (direct interaction between individuals)

Intensity of Competition
            Increases with density of competitors
            Expect density dependent effects on deaths, births, growth

Density dependence of death rates in flour beetle, Tribolium confusum, in cultures of different initial egg numbers. Region 1: Mortality density independent. Region 2: Mortality density dependent but mortality rate does not increase as fast as density so increasing density yields increasing numbers of surviving adults. Region 3: Mortality density dependent and exceeds rate of density increases, mortality overcompensates density increases and number of adults surviving decreases. (after Begon, Harper and Townsend, 1996, p 218, Fig. 6.3)

Density dependence of mortality in soybean, Glycine soja (after Begon, Harper and Townsend, 1996, p 219, Fig. 6.5).

Density dependent fecundity in fingernail clams, Musculium securis (data from Begon, Harper and Townsend, 1996, p 220, Fig. 6.6).

Density dependent seed production in an annual dune plant, Vulpia fasciculata (after Begon, Harper and Townsend, 1996, p 220, Fig. 6.6).

If both birth rates (fecundity) and death rates (mortality) respond to density (A), then recruitment rates may respond to density (B). As density approaches carrying capacity, net recruitment decreases from its maximum level, and population growth will have a characteristic logistic (sigmoid) form (C) (after Begon, Harper and Townsend, 1996, p 225, Fig. 6.10).

Density dependent responses in both fecundity and mortality are seen in fruit flies, Drosophila melanogaster (Ricklefs, 1996, p 344, Fig. 15.13).

Density effects are also be seen in individual biomass (Ricklefs, 1996, p 348, Fig. 15.20).

 Intraspecific competition effects are not the same at all densities, consequences can be asymmetric. First established individuals (older) may force adjustments in later established new arrivals (younger). Size distributions can become skewed because older individuals are less affected by competition than are young individuals. Population distribution of flax plant, Linum, dry weights changes with the density of seeds sown (Ricklefs, 1996, p 347, Fig. 15.18).

Types of Interspecific (Between Species) Interactions

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