Community Ecology Notes

Community Ecology Introduction

• Community ecology explores biodiversity by examining species interactions and environmental factors.
• A community is composed of interacting species within a specific space and time.
• Community ecology studies the patterns of diversity, abundance, and composition of species.
• Community ecologists formulate theories and test them by comparing predictions to data.

Measuring Biodiversity

• Biodiversity refers to the variety of life at all levels of organization.
• Metrics for characterizing communities include:
  • Species richness (number of species)
  • Abundance (number of individuals)
  • Evenness (homogeneity of abundances)
  • Diversity indices (e.g., Shannon's index H = -\sum{i=1}^{S} pi \ln p_i)
  • Species composition (identity and relative abundance)
  • Turnover (changes in composition over space or time)

Fundamental Patterns of Biodiversity

• Species-area relationships (SARs) describe the relationship between habitat area and the number of species.
  • S = cA^z where:
    • S = number of species
    • A = area
    • z = fitted exponent (slope of the curve)
    • c = coefficient
  • More area generally correlates with more species.
• Species abundance distributions (SADs) describe the abundance of each species in a community.
  • They can be represented graphically or as histograms.
  • SADs capture information about richness, evenness, and community structure.

Mark Vallon's Framework for Community Ecology

• Organizes community ecology around four main processes:
  • Selection/niches: Species differences regarding resource use, environment responses, and interactions.
  • Dispersal: Movement of species through space.
  • Drift/demographic stochasticity: Randomness in birth and death events.
  • Speciation: Origination of new species.

Two Species Lotka-Volterra Competition Model

• Describes the interaction between two species competing for the same resource
• \frac{dN1}{dt} = r1N1\frac{K1 - N1 - \alpha{12}N2}{K1}
• \frac{dN2}{dt} = r2N2\frac{K2 - N2 - \alpha{21}N1}{K2}
• Where:
  • N_i = population density of species i
  • r_i = per capita population growth rate of species i
  • K_i = carrying capacity of species i
  • \alpha_{ij} = competition coefficient, effect of species j on species i
• Equilibria are found when \frac{dN_i}{dt} = 0
• Four possible equilibria:
  • (0,0) (both species extinct)
  • (K_1, 0) (species 1 dominant)
  • (0, K_2) (species 2 dominant)
  • (\hat{N1}, \hat{N2}) (coexistence):\hat{N1} = K1 - \alpha{12}N2 , \hat{N2} = K2 - \alpha{21}N1
• For coexistence equilibrium to be biologically sensible(positive abundances), either: \frac{K1}{K2} > \alpha{12} and \frac{K2}{K1} > \alpha{21}, OR \frac{K1}{K2} < \alpha{12} and \frac{K2}{K1} < \alpha{21}