Competition

Lotka-Volterra

equations are variations of the logistic growth equation that can be used to estimate the equilibrium population sizes for two competing species

Phase planes

• isocline identify the location os stability within populations

• combining bother isocline together shows the phase plane

• If resources are unlimited, populations growth exponentially and competition does not occur

• When resources are limited, similar organism compete for those resources

• Resource limiting results in competition

• Sometimes competition is difficult to detect

• Interactions with another species restrict where a population can persist

Fundamental niche

• Set of environmental conditions under which it can survive and reproduce

Realized niche

Portion of a species’ fundamental niche in which it is actually found

Competitive exclusion principle

No two species can coexist in exactly the same ecological niche. One is expected to win the competition

Direct effect

• Produces when an individual or species directly impacts another without the interaction being mediated by or transmitted through a third party

  • Predation, fighting

Indirect effect

• When an interaction between two individuals or species is mediated by a third part or by another agent, such as a resource

  • Predation affecting herbivore abundance

Resource competition/exploitation competition

• Individuals compete indirectly by consuming a shared resource

• Indirect effect

Self-Thinning

• A decline in population density resulting from intraspecific competition

  • Shrubs

Interference competition

• When competing individuals directly interact in an attempt to reduce the other’s access to resources

  • Fighting over food, water, space, mates

Allelopathy

• Releasing toxins to affect competition

  • Black walnut

Territoriality

• The establishment of an area by one or more organisms from which others are at least partly excluded

• Aggressive, animals only

  • Pissing on stuff

Preemption

• When an individual prevents other individuals from occupying a location by occupying the space first

  • Barnacles, plants

• Direct effect

Intraspecific compeititon fast facts

• Strength of intra factors strongly into a population’s change in size

• As a population grows, intra reduces the BR and increase the DR

• Actually population growth rate drops below the maximum 

• Individuals grow slower in high density populations, they also die faster

• Individuals in lower density populations are more likely to grow and reproduce

Carrying capacity

• Limiting resource can be space, chemical nutrients, sunlight

• A smaller K means the density at which individuals begin to experience the effects of intra is reduced

• Logistic growth

  • Competition is limiting further growth at K

• Twice the maximum growth rate will result in the population growing more rapidly

Competition coefficient

• Represent the per capita effect on the growth of one species of another species.

• Quantify the relative competitive strengths of different verses same species competitors

• Effect of species one on species 2: a₁₂

• If a is 0, there is no displacement, no effect

Lotka-Volterra

• Used to predict which species should “win” a competitive interaction

• If the two isocline do not intersect, coexistence is not possible and the species whose isocline lies farthest from the origin will eliminate the other from the system

• If the winner of the competition between two species depends on their initial population sizes, interspecific competition is stronger than intraspecific competition

Phase plane

• Plots the population sizes of both species against each other to help visualize these predictions

• At eq, all feature points are plotted on top of each other

• Steady state point

  • No change in population size over time

  • Globally stable if it does not depend on initial population size

  • When intraspecific competition is stronger than interspecific competition

• Phase portrait

  • Stupid ass lines

How to experiment

Identify places where two species seem to compete, remove one of them, examine what happens to the other

Meta-analysis

• A statistical technique that can be used to combine data from multiple studies to identify trends

• Gurevitch

  • Evaluate how changing the density of a competitor impacts biomass

  • In most cases, competition had a large effect on the biomass of each species

Common garden experiments

• Explore whether an observed pattern is due to environmental effects or interaction between species

• Grow both species under same conditions within the same garden, it becomes clear how much of an observed pattern is due to the environment and how much must be due to something else

Hypothesis for competitive coexistence

• Environmental factors can shift the competitive interaction so it favors one species under certain conditions and another species under different conditions

• Environmental variation in space or time can lead to coexistence 

Spatial variation

• Describe differences that result from observations made in different locations, but at the same time

Temporal variation

Describe differences in observations that are made at the same location, but at different points in time

Trade-offs

• Two competing species may evolve in ways that reduce the strength of competition between them through opposing shifts along some trade-off

Character displacement

• When two species compete, natural selection may reduce the strength of competition by changing some characteristics of one for both species

• occur when niche overlap

Species can coexist if there was a trade-off in competitive ability vs. colonization ability

• Adaptations that increase how efficiently a species can reach and disperse in a newly opened area may decrease that species’ ability to compete

• As long as the system is periodically disturbed, competition vs. colonization can promote coexistence

Competition can be made weak due to predation or other ecological interactions

• Predation reduces population sizes for multiple competition species, then resources might never become limiting