Competition and Models - Exam 2

Competition

A -/- interaction in which organisms negatively affect one another through shared use of limiting resources

Intraspecific competition

Competition between individuals of the same species

Interspecific competition

Competition between individuals of different species

Direct or interference competition

Organisms directly prevent others from accessing resources through aggression or physical interactions

Indirect or exploitative competition

Organisms reduce resource availability for competitors by consuming or using shared resources

Fundamental niche

The full range of environmental conditions and resources a species could use without competition

Realized niche

The portion of a species' fundamental niche it actually occupies when competition and other biological interactions are present

Competitive exclusion principle

Two species with complete niche overlap cannot coexist indefinitely when competing for the same limiting resources

Competitive coexistence

Two competing species persist together when their ecological differences prevent one from eliminating the other

Resource partitioning

Species reduce competition by using different resources or different portions of a shared resource

Character displacement

Evolutionary divergence in traits between competing species that reduces resource overlap

Gause competition experiments

Paramecium aurelia outcompeted P. caudatum when grown together because it more effectively exploited their shared bacterial food resource

Recognizing the superior competitor in Gause experiments

The superior competitor maintains population growth while the inferior competitor declines toward extinction in mixed culture

Resource utilization curves

Graphs showing the frequency with which species use resources along a resource gradient

Competition and resource utilization curves

Competition is greatest where the resource utilization curves overlap most

Resource partitioning and utilization curves

Less overlap between curves indicates reduced competition and greater potential for coexistence

Lotka-Volterra competition model

A modification of logistic growth that incorporates the effects of both intra- and interspecific competition on population growth

Lotka-Volterra equation for species 1

dN1/dt = r1N1[1-(N1+αN2)/K1]

Lotka-Volterra equation for species 2

dN2/dt = r2N2[1-(N2+βN1)/K2]

Components of Lotka-Volterra equations

N = population size • r = intrinsic growth rate • K = carrying capacity • α and β = interspecific competition coefficients

Alpha α

The competitive effect of one species 2 individual on species 1 expressed in equivalent species 1 individuals

Beta β

The competitive effect of one species 1 individual on species 2 expressed in equivalent species 2 individuals

Alpha greater than 1

An individual of species 2 has a stronger competitive effect on species 1 than another species 1 individual

Alpha less than 1

An individual of species 2 has a weaker competitive effect on species 1 than another species 1 individual

Beta greater than 1

An individual of species 1 has a stronger competitive effect on species 2 than another species 2 individual

Beta less than 1

An individual of species 1 has a weaker competitive effect on species 2 than another species 2 individual

Alpha or beta equal to 1

Individuals of either species have equivalent competitive effects on the focal species

Lotka-Volterra isocline

A line representing combinations of the two population sizes where one species has zero population growth

Lotka-Volterra graph axes

Species 1 population size N1 is on the x-axis and species 2 population size N2 is on the y-axis

Species 1 isocline intercepts

Connect K1 on the x-axis with K1/α on the y-axis

Species 2 isocline intercepts

Connect K2/β on the x-axis with K2 on the y-axis

Growth on an isocline

The species represented by that isocline has zero population growth

Growth below the species 1 isocline

Species 1 increases because its total competitive pressure is below its carrying capacity

Growth above the species 1 isocline

Species 1 decreases because its total competitive pressure exceeds its carrying capacity

Growth below the species 2 isocline

Species 2 increases because its total competitive pressure is below its carrying capacity

Growth above the species 2 isocline

Species 2 decreases because its total competitive pressure exceeds its carrying capacity

Lotka-Volterra population vectors

Species 1 growth produces horizontal arrows and species 2 growth produces vertical arrows

Interpreting combined population vectors

Combine horizontal and vertical arrows to determine whether populations move toward coexistence or competitive exclusion

Lotka-Volterra outcome 1 species 1 wins

Species 1 excludes species 2 when K1 > K2/β and K1/α > K2

Lotka-Volterra outcome 2 species 2 wins

Species 2 excludes species 1 when K1 < K2/β and K1/α < K2

Lotka-Volterra outcome 3 stable coexistence

Isoclines intersect with K1/α > K2 and K2/β > K1 → populations move toward a stable equilibrium

Lotka-Volterra outcome 4 unstable coexistence

Isoclines intersect with K1/α < K2 and K2/β < K1 → initial population sizes determine which species wins

Priority effects

The species that establishes first or begins at sufficiently high abundance can prevent its competitor from establishing

Stable versus unstable Lotka-Volterra equilibrium

Stable equilibrium attracts population trajectories while unstable equilibrium causes trajectories to move toward exclusion of one species

When should Lotka-Volterra be used

Use it to predict competition outcomes from population sizes • carrying capacities • competition coefficients and zero-growth isoclines

Limitation of Lotka-Volterra

The model summarizes competition without explicitly tracking individual resources or their different requirements

Tilman resource-ratio hypothesis

A model predicting competition outcomes based on species-specific requirements for two limiting resources and their availability

Tilman versus Lotka-Volterra graphs

Tilman graphs resource concentrations while Lotka-Volterra graphs population abundances

Zero net growth isocline ZNGI

A line representing resource combinations where population births and deaths balance and net growth equals zero

R* value

The minimum concentration of a limiting resource required for a species to maintain zero net population growth

Interpreting R* values

The species with the lower R* for a resource is the better competitor for that resource because it can persist at lower concentrations

Tilman population growth relative to ZNGI

Above and to the right of both resource thresholds growth is positive • on the ZNGI growth is zero • below either essential threshold growth is negative

Resource supply point S

The concentrations of the two resources available in the environment without consumers

Consumption vector

A directional arrow showing how a species reduces resource concentrations through consumption and the relative amounts of each resource it uses

Renewal vector

A directional arrow showing how environmental resource replenishment increases resource concentrations

Tilman outcome with nonintersecting ZNGIs

The species with lower R* requirements for both resources excludes the other when resource supply supports its persistence

Tilman outcome with intersecting ZNGIs

Each species is the better competitor for a different resource → outcomes depend on resource supply ratios and consumption vectors

Tilman outcome when resource X is limiting

The species with the lower R* for X wins when X is scarce relative to Y and resource consumption leads toward exclusion

Tilman outcome when resource Y is limiting

The species with the lower R* for Y wins when Y is scarce relative to X and resource consumption leads toward exclusion

Tilman stable coexistence

With intermediate resource supply ratios each species consumes more of the resource that limits itself → populations stabilize together

Tilman priority effects

When consumption patterns create an unstable equilibrium the initially established species can exclude the other

Tilman outcome when both resources are insufficient

Neither species can maintain positive population growth when essential resource concentrations remain below its requirements

Interpreting Tilman consumption vector slopes

A steeper consumption vector indicates greater Y consumption relative to X while a shallower vector indicates greater X consumption relative to Y

Predicting Tilman outcomes from a graph

Compare species R* values → locate the resource supply point → follow consumption vectors → determine whether exclusion or stable coexistence occurs

Three long-term outcomes of competition

Extinction • exclusion from part of a habitat or niche • evolutionary change that reduces competition

Extinction as a competition outcome

A competitively inferior species may disappear entirely when it cannot maintain a viable population

Exclusion as a competition outcome

A species may persist elsewhere but be excluded from habitats or resources occupied by a superior competitor

Evolution as a competition outcome

Competition can favor heritable changes in resource use that reduce overlap between species

Ghosts of competition past

Present-day species distributions and niche differences may reflect historical competition even when little competition is currently observabl