Competition

What is competition?

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

• Competition among individuals may be:– intraspecific (within species)– interspecific (between species)

Types of competition

Competition occurs when use of a needed resource (food, water, space, light, nutrients, etc.) by one individual reduces the availability of that resource to another - whether of the same or another species – Ricklefs

• Intraspecific competition most intense for common species• Interspecific competition most important for sparse species

What is a resource?

• Resources include foods that are eaten, but also include:– open space for sessile organisms– hiding places and other safe sites

• Conditions are not resources:– for example, temperature is not a resource:

it may affect growth and reproduction, but is not consumed by organisms

Competition?

Types of competition - 1

Exploitation (scramble, diffuse) Competition• Consumptive - use of renewable resource• Preemptive - based on occupation of open space Interference (contest, direct) Competition• Overgrowth - when one grown on or over another, depriving it of resources• Chemical - toxin that acts at a distance• Territorial - defense of space• Encounter - transient interactions over a resource

Types of competition - 2

• Symmetric competition: ability to capture resources is proportional to size.

• Asymmetric competition: the large organisms capture a disproportionate amount of the resource

Intraspecific competition

Relationship between density and yield in herbaceous plants.

The “Law of constant final yield”

Note that after a critical density is reached, yield is independent of density.

Plant weight as a function of plant density, and its change with plant growth

y=w.d1/w = d(1/y)1/w = Ad + B

Survivorship in 7 populations of Loblolly Pine established in 1925 in the Duke Forest.

G= { ΣiΣj |yi-yj| }/(2n**2)ybarall plants equal --> 0.0 ; all plant but one infinitely small --> 1.0

The -3/2 thinning rule

w= c.p-3/2

area proportional to L2

weight proportional to volume proportional to L3

Therefore, area proportional to w2/3

and area proportional to 1/density

Thus, weight = c.p-3/2

Natural thinning in loblolly pine populations of differing initial density.

Limiting Resources

The potential of a resource to limit a population depends on availability relative to demand:

This concept is embodied in Liebig’s law of the minimum:

• each population increases until the supply of some limiting resource becomes depleted

• this law applies strictly to resources that do not interact to determine population growth rate

The Competitive Exclusion Principle

Results from many studies were summarized by Gause and others in the competitive exclusion principle:

• two species cannot coexist indefinitely on the same limiting resource

• although similar species exist, careful study usually reveals that they differ in their habitat or diet requirements

Lotka - Volterra modelIncorporates interspecific

competition into the logistic equation as follows:

• Two models are needed to describe the joint effects of species upon one another.

• For two species to coexist, each population must reach a stable size greater than zero:

dN1/N1dt = 0 when:

N1 = K1 - αN2

dN2/N2dt = 0 when:

N2 = K2 - βN1

Specie

s 2

Species 1K1

Specie

s 2

Species 1K1

K1/a

Specie

s 2

Species 1

K2/β

K2

G.F. Gause conducted early experimental studies of competition: he grew Paramecium aurelia and P. caudatum alone and in mixture in nutritive media:

• each species grew well alone, but in mixture only P. aurelia persisted

• similar experiments conducted on a wide variety of species have tended to show the same thing - one species persists and the other dies out, usually within 30-70 generations

24% 29% 34%

70 F confusum71 - 29

castanum14 - 86

castanum0 - 100

30 F confusum100 – 0

confusum87 – 13

confusum90 - 10

1. Relative intrinsic growth rateL minor > S. natans > L. gibba > L. polyrhiza

2. Arithmetic growth rate when crowdedS. natans > L. polyrhiza > L. gibba > L. minor

3. Asymptotic yield / cultureL. polyrhiza > L. minor > > S. natans > L. gibba

4. Succession, mixedS natans & L gibba > L. polyrhiza >= L. minor

Resource Ratio Model

• Dave Tilman constructed a model of competition based on differential depletion of two resources.

• When there is an abundant supply of resources, the population of each species increases and the supply of resources per unit competitor declines until growth becomes zero (ZNG = Zero Net Growth)

Tansley’s Classic StudyBritish ecologist A.G. Tansley was the first to

experimentally demonstrate competition between closely related species:

• He selected two species of the plant species Galium:G. saxatile is normally found on acid, peaty soilsG. sylvestre is normally found on limestone hills and pastures

• These two species were grown alone and in mixture with the other species on both soil types in a common garden

Tansley’s Results• When grown alone in common garden experiments,

each species performed better on its preferred soil, although each could grow on the other soil type.

• When grown in mixture, each species overgrew and shaded the other on its preferred soil type.

• Tansley concluded that each species was at a disadvantage in competition when grown on the other soil type; this helped explain the observed distributions of the two species in nature.

Tansley’s Conclusions

Tansley’s conclusions have far-ranging implications for all competitive situations:– the presence or absence of species can be

determined by competition with other species– conditions of the environment affect the

outcome of competition– competition may be felt very broadly

throughout the community– the present segregation of species may have

resulted from past competition

Competition among barnacles

Experimental studies of competition among intertidal barnacles by Joe Connell enhanced our understanding of how competition can affect spatial distributions of species:

Barnacles are not food-limited, but compete intensely for limited space on rocks in the intertidal zone:

• The barnacle Chthamalus is more tolerant of desiccation and thrives in the upper intertidal zone

• The barnacle Balanus is less tolerant of desiccation but can displace Chthamalus in the lower intertidal zone

Competition may occur through exploitation or interference.

• Exploitation competition occurs when individuals compete indirectly through their mutual effects on shared resources:– a common mechanism, typical of examples

we’ve considered thus far

• Interference competition occurs when individuals defend resources through antagonistic behaviors:– less common, requires that resources can be

profitably defended