(4/17) Community Ecology: Succession

Introduction

• The biotic elements of our environment are continually changing, building upon concepts from Evolution, Mendelian Genetics, Population Genetics, Population Ecology, and Exploitative and Mutualistic Interactions.
• The focus shifts from the individual to the population, then to interactions between species, and now to the community level.
• Community Ecology: How species interact within a community, concerned with species diversity, how it changes over time, and what causes those changes.
• Community ecology studies how a community functions and changes over time as a whole.
• Species Diversity: The number and relative abundance of species in an area.
• Species Richness: The number of different species in an area.
• Species richness is typically what people mean when they say "species diversity."
• Example:
  • Two forests with the same size and total number of trees (111 each) but different species compositions.
  • Forest #1: Eastern Hemlock (15), Sugar Maple (59), American Beech (37)
  • Forest #2: Eastern Hemlock (33), Sugar Maple (20), American Beech (58)
  • Both forests have the same species richness (3 species) but different species diversity because the relative numbers of each species differ.
• Maintaining species diversity actually means maintaining species richness.
• It is important to have precise language to understand ecological discussions and protect the planet and to avoid conceptual confusion.

Community Change

• Succession: The gradual change in an area over time, involving changes in species diversity and richness.
• Different species may appear, and the relative abundance of existing species may change.
• Successional study types:
  • Vegetative Succession
  • Animal Succession
  • Geologic Succession
  • Includes Stream Succession, Old Field Succession, and Aquatic Plant Succession.
  • Any change over time in an area can be a study in succession.
• Vegetative succession is commonly studied:
  • It's well-studied, allowing generalizations about succession.
  • It is more predictable than other forms of succession.
  • Changes in vegetation lead to predictable changes in other areas (e.g., animal diversity changes with vegetation).
• Types of Succession:
  • Primary Succession: Occurs on newly created substrates (e.g., volcanic ash, shifting sand dunes, glaciers).
    • No original soil remains; new soil must be created by pioneer plants and their decomposition.
  • Secondary Succession: Occurs when the community is diminished, but the soil remains intact (e.g., after wind, fire, overgrazing, logging, hurricanes).
    • Primary succession starts with bare rock, then lichens and mosses, then herbaceous species.
    • Secondary succession starts with exposed soil and moves into herbaceous species.
    • Example: Glacier Bay, Alaska
• Traditional vegetative succession pattern: bare soil → herbs → shrubs → pioneer trees → intermediate trees → climax forest.
• A local community is defined by the species composition of the late successional stage (climax).
  • Example: Binghamton University area is considered a sugar maple, hemlock forest.
• Important considerations:
  • Succession does not necessarily lead to a local climax forest.
  • Succession may not follow the idealized progression.
  • Repeated disturbances can interrupt succession.
  • Successional stages may be skipped or repeated.
• The pathway of succession is influenced by:
  • Randomness: Random events (e.g., fire) can destroy community members.
  • History of the area: Lack of source populations can prevent colonization.
• Mechanisms of Succession:
  • Facilitation: One species makes it easier for subsequent species to live in the same habitat.
    • Example: Legumes fix nitrogen, benefiting other species that colonize the area.
  • Tolerance: Some species have greater tolerance of extremes in abiotic factors (light, temperature, wind, dryness).
    • Later succession stages are more tolerant and better competitors.
    • Example: Shrubs are more tolerant of shade than herbs; pioneer trees more tolerant than shrubs.
  • Inhibition: Early succession plants are inhibited by later succession plants.
    • Late succession species are more tolerant of subdued light.
    • Some herbaceous species inhibit shrub growth by outcompeting young shrubs for resources.
  • All three mechanisms play a role in successional change in most habitats.

Stability in Communities

• Stability: The ability of a community to resist abrupt changes in species diversity and richness.
• A stable community is generally considered healthy.
• Abrupt changes: Addition of an invasive species or extinction of a species.
• Co-evolutionary relationships add stability because species buffer each other from adverse effects.
• Keystone Species: Species with a greater impact on diversity and richness than predicted by their biomass. For example, sea stars.
• Species Richness: Greater species richness leads to more checks and balances, making the community less likely to experience changes in species richness or composition.