Competition Individual Interactions, part 1

Niche A concept that encompasses all of the individual environmental requirements of a species This is definitely an abstract concept, but it helps us to organize and explain ecological phenomena

What are some possible elements of this organism’s niche?

Niche A species’ niche is composed of both physical (e.g., average temperature) and biotic (e.g., food sources) components Resources comprise a critical subset of these niche elements

Resources Include such things as: Food or nutrients Shelter Space to grow Water Light (plants)

Resources Resources are usually limited Individuals compete for limited resources

Competition Competition can be separated into two broad categories of interactions: Interference Competition – direct antagonistic behavior towards other individuals (e.g., defending territory) Resource Competition – individuals compete indirectly through the exploitation of a shared resource

Competition Competition for resources between individuals of the same species = Intraspecific competition

Intraspecific Competition Leads to the process of self-thinning in plants

Intraspecific Competition Intraspecific competition regulates population growth in a density-dependent manner: as individuals deplete resources, population growth slows until the population size = K dN/dt = r max N(K-N/K) K

Intraspecific Competition When genetic factors influence the efficiency of resource use, evolution tends to increase the competitive ability within a population K1K1 K2K2 Resource availability Population density

Competition Competition for resources between individuals of different species = Interspecific competition

Interspecific Competition Played a prominent role in Darwin’s theory of natural selection “struggle for existence” based on competition for limited resources Competition should be most intense between closely related species

Modeling Interspecific Competition: Lotka-Volterra Model Population growth of species 1: dN 1 /dt = r max1 N 1 (K 1 -N 1 /K 1 ) Population growth of species 2: dN 2 /dt = r max2 N 2 (K 2 -N 2 /K 2 ) We can modify these logistic growth equations to account for interspecific competition by adding competition coefficients… -  12 N 2 -  21 N 1 and

Modeling Interspecific Competition: Lotka-Volterra Model Population growth of species 1: dN 1 /dt = r max1 N 1 (K 1 -N 1 -  12 N 2 /K 1 ) Population growth of species 2: dN 2 /dt = r max2 N 2 (K 2 -N 2 -  21 N 1 /K 2 )

Modeling Interspecific Competition: Lotka-Volterra Model dN 1 /dt = r max1 N 1 (K 1 -N 1 -  12 N 2 /K 1 ) = 0 dN 2 /dt = r max2 N 2 (K 2 -N 2 -  21 N 1 /K 2 ) = 0 When population growth has stopped… This can be rearranged algebraically…

Modeling Interspecific Competition: Lotka-Volterra Model N 1 = K 1 -  12 N 2 N 2 = K 2 -  21 N 1 To predict when population growth in each species will stop: and

Modeling Interspecific Competition: Lotka-Volterra Model

K 1 > K 2 /  21 Species can only coexist when: K 2 > K 1 /  12 and …that is, when intraspecific competition is greater than interspecific competition

Interspecific Competition G.F. Gause (1934) integrated the idea of the niche and interspecific competition: Competitive Exclusion Principle – Two species with identical niches cannot coexist indefinitely (one will out-compete the other for limited resources)

K of P. aurelia alone = 195 K of P. caudatum alone = 137

Competitive Exclusion Grown separately, P. aurelia had a higher K than P. caudatum Grown together, P. aurelia out-competed P. caudatum for resources (growth medium), and P. caudatum was eliminated

Interspecific competition & the niche Hutchinson: fundamental niche defines the environmental conditions in which a species might live, in the absence of interactions with other species; realized niche is the actual niche of a species, which is limited by biotic interactions (competition, predation, etc.)

Interspecific competition - Gallium G. saxatileG. pumilum A. Tansley (1917)

Interspecific competition

Interspecific competition - Gallium On limestone (basic) soils, G. pumilum overgrew and eliminated G. saxatile by the end of the first growing season On acidic soils, G. saxatile was completely dominant, but G. pumilum was not completely eliminated by the 6 th year. Growth of both species was much slower on the acidic soils. A. Tansley (1917)

Interspecific competition - Gallium The fundamental niches of both Gallium species include a wider variety of habitats (soil types) than those they actually inhabit in nature Interspecific competition restricts the realized niche of each species to a narrower range of soil types Asymmetric competition – each species is able to specialize in its realized niche because each is better at doing a different thing (e.g., exploiting resources vs. tolerating stress)

Interspecific competition - Gallium Asymmetric competition – each species is able to specialize in its realized niche because each is better at doing a different thing (e.g., exploiting resources vs. tolerating stress)

Interspecific competition - barnacles Joseph Connell (1961)

REVIEW QUESTION What type of selection pattern (stabilizing, directional, disruptive) would you expect to observe in a population undergoing intense intraspecific competition for resources?

REVIEW QUESTION How might the realized niches of two competing species evolve?

Disruptive selection