Introduction to Competition

Test Questions

• In which of the following situations will the reproductive ratio of infections ($R_0$) increase?

  • a) The rate of infection/transmission (eta) decreases

  • b) The number of susceptible individuals ($S$) decreases

  • c) The rate of recovery or death ($m$) decreases

  • d) The threshold density ($S_T$) increases

  • Formula: R_0 = S imes rac{eta}{m}

Threshold Density of Susceptible Individuals

• The formula for threshold density of susceptible individuals ($S_T$).

  • Demonstration with a new pathogen introduced to a population of 75 birds:

  • Infection Rate: 0.01

  • Recovery Rate: 0.5

  • Will the pathogen spread through the bird population?

    • Options:

    • a) Yes

    • b) No

    • c) There is not enough information to answer

    • Calculation:

    • S_T = rac{m}{eta} = rac{0.5}{0.01} = 55

    • Since S > S_T, the disease will spread.

Introduction to Interspecific Competition

Types of Competition

• Exploitative Competition: Indirect competition through shared resources where both species reduce the availability of resources.

• Interference Competition: Direct interference by one species affecting the abilities of others to utilize shared resources.

  • Examples include:

  • Carnivores competing for prey

  • Barnacle species overgrowing and crushing others

  • Plant species competing for light.

General Features of Competition

• Limiting Resources:

  • Include food, water, light, and space necessary for survival, growth, and reproduction.

• Impacts of Competition:

  • Can limit a species’ fundamental niche resulting in a narrower realized niche.

  • Decreases growth and reproduction.

  • Varies in intensity and mechanism utilized based on environmental conditions.

Competitive Exclusion Principle

• Competitive Exclusion Principle: Proposed by G.F. Gause in the 1930s.

  • Definition: Two species that use a limiting resource in the same way cannot coexist indefinitely, with the superior competitor excluding the inferior.

  • Coexistence requires differential resource use.

Experimental Analysis of Competition

A.G. Tansley's 1917 Experiment

• Tansley tested competition between two species of Galium bedstraw.

  • Species:

  • Galium saxatile (Heath bedstraw): Grows in peat soil (acidic).

  • Galium sylvestre (White bedstraw): Grows in calcareous soil (alkaline).

  • Experimental Design:

  • Grow species together and alone in different soil types.

  • Results:

  • Both species grow well alone in their respective soils.

  • When grown together, one species dominates based on soil type.

  • Conclusion: Must compare growth with and without competitors.

Indirect vs Direct Competition

• Indirect Competition (Exploitation):

  • Competitors use shared resources, affecting each other's ability to access the resource.

• Direct Competition (Interference):

  • One species actively hinders another's access to a shared resource.

Case Studies and Experiments

• David Tillman’s Experiment:

  • Focus: Two species of freshwater diatoms competing for silica.

  • Findings:

  • Each species reduced silica levels when grown alone, achieving stable population densities; however, significant difference in maximum densities was observed.

  • When grown together, the outcome favors the species that reduces silica slower, indicative of competitive dynamics.

Outcomes of Competition

• Outcomes can either result in:

  • Competitive Exclusion: Inferior competitors are driven locally extinct (e.g., Chthamalus and Balanus).

  • Competitive Coexistence: Both species can persist despite sharing limiting resources, often seen in natural community structures.

Implications of Competition

• Competition is vital in shaping ecosystems and influences species diversity and local extinctions.

  • Character Displacement: Occurs when competition causes the evolution of distinct physical or behavioral traits in species that coexist, leading to resource partitioning.