Community Ecology Notes

Last Class Review
  • Topics Covered:
    • Top-down control and keystone species
    • Co-adapted vs. individualistic communities
Today's Class Agenda
  • Community dynamics
    • Disturbances such as fire, flood, and wind
  • Global patterns of communities
    • Example: Island biogeography
Top-Down Control
  • Definition: Predators that control the structure of ecological communities.
  • Key Concept: Top predators are often keystone species that have a disproportionate effect on their environment.
  • Example: The reintroduction of wolves in Yellowstone led to a trophic cascade impacting various species:
    • Increase in Species:
    • Rough-legged hawk, coyote, mouse, beaver, elk, mule deer, willows, grasses, aspen, cottonwood.
    • Decrease in Species: Herbivores like elk due to predation by wolves.
Views on Community Structure

  1. Community as Interactive Units (Clements)

    • Species evolved to function interconnectedly.
    • Concept of a climax community.

  2. Community as Random Groups (Gleason)

    • Changes in species composition are less significant.
    • Emphasizes redundancy within communities.
Patterns of Density
  • Density Variation:
    • Tree species density shown across latitudes (from equator to boreal regions).
    • Important in assessing community structure views.
Community Dynamics: Disturbances
  • Types:
    • Severe: Complete reset of the ecosystem (e.g., volcanic eruptions).
    • Less Severe: Pushes back succession but retains some elements (e.g., forest fires, hurricanes).
Succession Mechanisms
  1. Primary Succession:
    • Occurs after severe disturbances in bare rock environments.
    • Stages:
      • Pioneer Species: Lichens
      • Intermediate: Grasses and shrubs
      • Climax: Oak and hickory forests
  2. Secondary Succession:
    • Occurs after less severe disturbances (e.g., wildfire).
    • Stages:
      • Pioneer: Annual plants
      • Intermediate: Grasslands and young trees
      • Climax: Mature forests
Island Biogeography
  • Concept: Species richness depends on island size and distance from the mainland.
    • Larger islands have lower extinction rates and higher colonization rates.
    • Islands closer to the mainland have greater richness compared to remote islands.
    • Factors:
    1. Island Size: Larger islands support more species due to resources and habitat.
    2. Distance: Proximity to the mainland influences immigration rates.
Global Patterns of Species Richness
  • Proximity to Equator: More species are found near the equator due to:
    • More stable environments and time for speciation.
    • Higher energy and productivity leading to longer food chains.
    • Increased ecological interactions resulting in more niches available.
  • Latitudinal Gradient: Species richness of birds and mammals shows a strong latitudinal gradient, with richness generally higher near the equator.
Practice Graph Reading
  • Observation: Mammal species richness varies with latitude, promoting analysis of patterns.
  • Result: Determine whether mammal species richness is greater at 40°N or 40°S using data from biodiversity graphs.