Competition in Ecological Communities

Competition

Organisms in Ecological Communities

• Organisms live in ecological communities and interact with each other.

Competition

• Organisms compete for resources.

• Resources can be consumed or depleted.

• Resources are required for growth, survival, or reproduction.

Resources

• Components of the environment (e.g., food, water, light, space) required by species.

• Food: Essential for life; no food leads to population decline.

• Water: Important in terrestrial ecosystems, especially arid ones.

• Light: Plants compete for light, with some shading others.

• Space: Important for growth, especially for sessile animals; mobile animals need space for hunting, refuge, and mates.

Fundamental and Realized Niche

• Fundamental niche: The full set of resources plus other biotic and abiotic requirements of a species.

• Realized niche: The restricted set of resources a species is limited to due to species interactions.

Types of Competition

Interspecific Competition

• Between members of different species.

• Limiting resources drive this competition.

Intraspecific Competition

• Between individuals of a single species.

General Features of Competition

Exploitation Competition

• Species compete indirectly by reducing the supply of a resource as they use it.

  • Example: Herbivores competing for forage, birds competing for berries.

Interference Competition

• Species compete directly by interfering with a competitor's ability to use a limiting resource.

  • Examples: Carnivores fighting over prey, voles aggressively excluding other species from habitat.

  • Meadow voles dominant over red-backed voles.

Interference Competition in Plants

• Individuals of one species grow on or shade other species, reducing their access to light (e.g., kudzu).

Allelopathy

• Plants release toxins that harm other species.

  • Example: Garlic mustard releases sulfur-containing compounds, inhibiting seed germination and growth, and killing mycorrhizal symbionts; toxic to native butterfly larvae.

Competition Between Distantly Related Species

• Exclusion experiments with rodents and ants that eat the same seeds (Brown and Davidson 1977).

  • When either rodents or ants were removed, the remaining group ate as many seeds as both groups combined.

Continuum of Competitive Effects

• There is a continuum in how strongly each competitor affects the other.

• Amensalism: "–/0" interactions; one species is harmed, while the other is unaffected.

Interspecific Interaction: Competitive Exclusion & Competitive Coexistence

• Interspecific competition can lead to two possible outcomes:

Competitive Coexistence

• Ecologists consider interspecific competition important in communities.

• Darwin argued it influences both ecological and evolutionary processes.

Outcomes of Interspecific Competition

1. Competitive exclusion: A dominant species prevents another from using essential resources; the inferior species may become locally extinct.

2. In reality, most species show some sort of competitive coexistence—ability to coexist despite sharing limiting resources.

Gause's Experiments with Paramecium

• In the 1930s, Gause experimented on competition using three Paramecium species: P. caudatum, P. bursaria, P. aurelia.

  • Populations reached a stable carrying capacity when grown alone.

  • P. caudatum and P. bursaria were able to coexist by feeding on different food items; growth rates for both were slower.

  • P. aurelia drove P. caudatum to extinction; both feed on floating yeast cells.

Competition in Paramecium

• Two different species of Paramecium, when grown in identical but separate cultures, both survive.

• When grown together, one species outcompetes the other.

• One species dies out (P. aurelia is dominant, P. caudatum dies out).

• Both feed on floating yeast cells.

Coexistence of P. Bursaria and P. caudatum

• When grown together, P. caudatum and P. bursaria coexist, although at lower densities than when alone.

• Stable densities were much lower than when grown alone.

• They were in competition with one another.

• P. caudatum tended to live and feed on the bacteria suspended in the medium, while P. bursaria concentrated on the yeast cells.

Competitive Exclusion Principle

• Experiments on these and many other species led to the competitive exclusion principle:

  • Two species that use a limiting resource in the same way cannot coexist indefinitely.

Competition and Coexistence

• How do so many species coexist? Why do some species not outcompete others to extinction?

  • Competition, Predation, rapid evolution

Interspecific Competition and Coexistence: Lions and Leopards

• Lions and Leopards prioritize different landscape features that meet their individual ecological needs (Miller et al 2018).

• Leopards preferred areas with more bushy, “hideable” habitat, human (tourist) activity, and topographic ruggedness.

• Lion habitat preferences were related to large prey abundance.

Coexistence Mechanisms

• Resource partitioning: Species using a limited resource in different ways can coexist.

• Behavioral Change: Avoiding areas, restricting foraging time, seeking refuge during risky times

Resource Partitioning

• MacArthur (1958) studied resource partitioning in a community of warblers in New England forests.

  • He recorded feeding habits, nesting locations, and breeding territories.

  • He found that the birds were using different parts of the habitat in different ways.

  • Warblers although using the same habitat and food resources were able to coexist by partitioning those resources.

Temporal Partitioning: Coexisting Gerbils in the Negev Sand Dunes

• Gerbillus pyramidum (GP) and Gerbillus andersoni allenbyi (GA) coexist in the semi-stabilized sand dunes.

• G. pyramidum (GP) is the bigger and dominant species; it forages early at night, exploits richer patches, and excludes G. a. allenbyi (GA) by interference competition.

• The smaller G. a. allenbyi is a more efficient forager and benefits from patches already abandoned by G. pyramidum.

• $40g$ GP, $25g$ GA

Competitive Coexistence and Character Displacement

• Character displacement: When two species compete for resources, natural selection may favor phenotypes that allow them to partition their limiting resources, thus decreasing the intensity of competition.

Character Displacement in Finches

• Character displacement appears to have occurred in two species of finches on the Galápagos archipelago: Geospiza fuliginosa and G. fortis (medium ground finch and small ground finch).

Competition Shapes Beak Size

• Beak sizes, and hence sizes of the seeds eaten, are different on islands that have both species.

• On islands with only one of the species, beak sizes are similar.

Evolution Due to Competition

• Competition for resources can cause competing species to change over time.

• When they first come together, the two species catch prey of about the same size.

• After living together for some time, the two species often catch prey of different sizes.

The Lotka–Volterra Competition Model

• Evidence for resource partitioning has been used as an explanation for the patterns of species diversity found in communities.

• Mathematical models can predict whether the outcome of competition results in competitive exclusion or competitive coexistence.

Logistic Equation

• The Lotka–Volterra model is a modification of the logistic equation:

  • $\frac{dN}{dt} = rN \left( \frac{K - N}{K} \right)$

    • $N$ = population size

    • $r$ = intrinsic rate of increase

    • $K$ = carrying capacity

Lotka–Volterra Competition Model

• The Lotka–Volterra competition model:

  • $\frac{dN<em>1}{dt} = r</em>1N<em>1 \left( \frac{K</em>1 - N<em>1 - \alpha N</em>2}{K_1} \right)$

  • $\frac{dN<em>2}{dt} = r</em>2N<em>2 \left( \frac{K</em>2 - N<em>2 - \beta N</em>1}{K_2} \right)$

  • To incorporate the effects of competition, competition coefficients are subtracted to indicate how strong the competitive effect of one species is on another.

    • $\alpha$ and $\beta$ = competition coefficients (constant)

    • $\alpha$ = effect of species 2 on species 1

    • $\beta$ = affect of species 1 on species 2