Session4-Langetal-2013

Species Interactions and Competition

Introduction

• Ecological Community: An assemblage of populations of different species that interact within a certain geographic area.

• Species interactions play a crucial role in ecosystem properties (e.g., nutrient cycling, food webs).

• Interaction dynamics depend on evolutionary and environmental contexts, making them hard to define and measure.

• Categories of interactions:

  • Intra-specific: Interactions among individuals of the same species.

  • Inter-specific: Interactions among different species.

• Classes of interactions discussed include competition, predation, herbivory, and symbiosis.

Competition

• Definition: Interaction among individuals for limited resources, leading to affected fitness.

• Generally considered negative for weaker competitors.

• Three major forms of competition:

  1. Interference Competition: Direct competition, e.g., physical aggression in animals.

  2. Exploitation Competition: Indirect competition, where individuals use shared resources reducing availability for others.

  3. Apparent Competition: Indirect interaction where two competitors impact each other through shared predation.

Interference Competition

• Direct alteration of resource-attaining behavior among individuals (e.g., a dominant male gorilla limiting mate access).

Exploitation Competition

• Indirect resource competition. One competitor’s resource usage impacts availability for others.

• Result: Over time, a superior competitor may outcompete an inferior one, leading to competitive exclusion.

Competitive Dynamics

• Lotka-Volterra model: Describes outcomes of competition between species:

  1. Species A excludes Species B.

  2. Species B excludes Species A.

  3. Coexistence possible, contingent on population dynamics.

• Competitive exclusion can be mitigated by:

  • Adopting alternative life history and dispersal strategies.

• Local disturbances (e.g., fire, tides) promote coexistence by allowing dispersal and nutrient acquisition.

Disturbance and Coexistence

• Intermediate disturbance hypothesis:

  • Disturbances help prevent exclusion by balancing competitive interactions, allowing inferior competitors to remain in habitats.

Apparent Competition

• Indirect competition where two species share a predator.

• Example: Increased squirrel population can indirectly harm mice populations due to common predation by hawks.

Predation

• Predator-prey interaction necessitating one individual kills another for nutrition.

• Commonly observed inter-species, can also occur within species (cannibalism).

• Cannibalism can provide population stability when food is limited but increases competition for scarce resources.

• Predators and prey engage in an evolutionary arms race with adaptations enhancing survival.

Herbivory

• Interaction where organisms consume parts of plants, potentially harming them.

• Herbivores categorized based on plant part consumed:

  • Granivores: Seeds

  • Grazers: Grasses and shrubs

  • Browsers: Leaves from trees

  • Frugivores: Fruits

• Plants evolve defenses against herbivory, including physical (thorns) and chemical defenses (toxins).

Symbiosis

• Definition: Interaction of two or more species living in direct contact, commonly subdivided into three types:

  1. Mutualism: Both species benefit.

     • Obligate: Species cannot survive without the interaction.

     • Facultative: Species can survive independently but benefit together.

  2. Commensalism: One species benefits, the other unaffected.

  3. Parasitism: One species benefits at the cost of another (host).

Summary

• Various species interactions complicate ecosystem dynamics, including evolutionary and environmental influences.

• Previously thought that competition was the primary community driving force, but interactions create a complex framework that shapes communities and ecosystems.

References and Recommended Reading

• A collection of scientific references is provided to further explore species interactions and competition.