Interspecific Competition Study Notes

Interspecific Competition

Definition and Overview

• Interspecific competition involves interactions between two or more species competing for the same resources. This concept is crucial in the study of ecology as it influences population dynamics, community structure, and evolutionary processes in ecosystems.

Types of Interspecific Competition

• Divergence and Specialization: Differentiation among species that allows them to utilize different resources, minimizing direct competition.

• Exploitation vs Interference Competition:
    - Exploitation: Occurs indirectly as species compete for a limited resource, leading to reduced availability for others.
    - Interference: Direct interactions where one species disrupts another's access to resources, often through aggressive behaviors.

• Alternate Classification:
    - Consumption: This is analogous to exploitation, where the act of consuming resources limits other species’ access.
    - Encounter: Nonterritorial conflicts among species, which are similar to interference scenarios; e.g., interactions between an eagle and a fox or an elephant and a warthog.
    - Territorial: Behavioral exclusion of certain species from areas, such as exhibited by species like the sarcastic fringehead.
    - Preemption: When one species occupies a space such that it prevents others from establishing.
    - Overgrowth: Typically involves sessile species where one organism physically overgrows another, relying on chemical interactions, often involving the release of toxins or growth inhibitors (allelopathy).

Lotka-Volterra Interspecific Competition Model

• The Lotka-Volterra model describes the dynamics of two competing species using differential equations:
    - Species 1:
      $\frac{dN_1}{dt} = r_1 N_1 \left(1 - \frac{N_1}{K_1}\right)$
    - Species 2:
      $\frac{dN_2}{dt} = r_2 N_2 \left(1 - \frac{N_2}{K_2}\right)$

• Competition Coefficients: These quantify the competitive impact between species:
    - The per capita effect of species 2 on species 1 is denoted as α.
    - The per capita effect of species 1 on species 2 is denoted as β.

• For species 1, the equation incorporates the competitive effect of species 2:
    - $\frac{dN_1}{dt} = r_1 N_1 \left(1 - \frac{N_1 + \alpha N_2}{K_1}\right)$

• For species 2, the equation incorporates the effect of species 1:
    - $\frac{dN_2}{dt} = r_2 N_2 \left(1 - \frac{N_2 + \beta N_1}{K_2}\right)$

• An increase in N2 or a larger α leads to a greater reduction in the growth rate of species 1.

Example Calculations of $\frac{dN_1}{dt}$

• Given the variables: r1 = 0.25, K = 1000, consider the scenarios:
    1. Scenario 1: N1 = 200, N2 = 200, α = 0.5
    2. Scenario 2: N1 = 200, N2 = 200, α = 1
    3. Scenario 3: N1 = 400, N2 = 0, α = 1
    4. Scenario 4: N1 = 200, N2 = 200, α = 2

Graphical Representation of Outcomes

• Phase Plane Diagrams: Outcomes of interspecific competition can be illustrated via isoclines, indicating population sizes where growth rates equal zero:
    - The zero-growth isocline for each species shows combined values of N1 and N2 that stabilize populations.

• Graphs display competitive dynamics, leading to potential extinction, coexistence, or turnover among species.

Experimental Support for Lotka-Volterra Model

• A key experiment involving Paramecium species, found:
    - P. aurelia (200 individuals) outcompetes P. caudatum (150 individuals) when grown in mixed populations, illustrating competitive exclusion.

• Gause's experiment (1934) demonstrated distinct population dynamics when both species were cultured separately vs. together.

Competitive Exclusion Principle

• In the Lotka-Volterra model, three of the four possible outcomes result in competitive exclusion:
    - The Competitive Exclusion Principle posits that