Community Ecology

Learning Goals

  • Understand various types of interspecific interactions and their impact on population growth rates.
  • Analyze how interspecific interactions influence community formation and structure.
  • Grasp the basic model of density-dependent population growth, modified for interspecific competition.
  • Differentiate between bottom-up and top-down effects on community structure.
  • Comprehend ecological succession and how interspecific interactions and community effects drive this process.

Interspecific Interactions

  • Definition: Shedding light on the effects of interactions between different species.
  • Symbiosis: A general term encompassing all close, long-term interactions between species. Mutualism is a specific type of symbiosis where both species benefit.

Types of Interspecific Interactions

  1. Competition
    • Affects both species negatively.
  2. Predation
    • Affects predator positively and prey negatively.
  3. Parasitism
    • Hosts suffer while parasites benefit.
  4. Mutualism
    • Positive interaction for both species involved.
  5. Commensalism
    • One species benefits while the other is unaffected.

Competition

  • Mathematical representation:

    • Growth of species is influenced by competitive interactions:
      • Species 1: $N1' = r1 N1(1 - (N1 + a N2)/K1)$
      • Species 2: $N2' = r2 N2(1 - (N2 + b N1)/K2)$
    • where "a" and "b" represent competitive effects of one species on another, and
    • $K1$, $K2$ are carrying capacities.

  • Intraspecific vs. Interspecific Competition:

    • Strong intraspecific competition leads to smaller carrying capacity (K).
    • Strong interspecific competition significantly increases competitive effects between species.

  • Equilibrium Points:

    • Two equilibrium points may exist depending on the population sizes: when one species goes extinct or both coexist at stable equilibria.

Community Structure Influencers

Top-down Effects

  • Definition: The impact of predators on prey species, which can alter community structure and population sizes.
  • Example: Removal of a predator can lead to an increase in prey populations, potentially causing competitive dominance among prey species.

Bottom-up Effects

  • Definition: The influence of lower trophic levels (e.g., plants) on higher trophic levels (e.g., herbivores).
  • Example: Increased vegetation during certain climatic conditions can enhance energy availability and alter population dynamics.

Ecological Succession

  • Definition: The process through which ecosystems change and develop over time, often influenced by interspecific interactions.
  • Mechanisms for succession can be driven by disturbances that open niches, allowing new species to establish and alter community composition.

Adaptations and Defensive Mechanisms

  1. Crypsis: Camouflaging patterns and colors to avoid predation.
  2. Masquerading: Resembling objects in the environment to escape attention from predators.
  3. Aposematism: Bold, warning colors that signal toxicity or danger to potential predators.
  4. Mimicry: Imitating the appearance of a harmful or unpalatable species to deter predators; notable examples include mimicry in butterflies.

Mutualism Examples

  1. Pollinator-Plant Interactions: Pollinators gain food (nectar) while plants achieve reproduction through pollination.
  2. Nitrogen-Fixing Bacteria in Legumes: Legumes obtain altered nitrogen forms, while bacteria get sugars to nourish themselves.
  3. Mycorrhizal Fungi: Fungi extend root networks for better nutrient absorption while receiving carbohydrates from plants.