15.3 Island Biogeography and Ecological Succession

Island biogeography explores how size and isolation affect species diversity on islands. It's all about balance – new species arriving versus existing ones going extinct. This concept helps us understand biodiversity patterns in isolated habitats beyond just islands.

Ecological succession shows how ecosystems change over time after disturbances. It starts with hardy pioneer species and ends with a stable climax community. This process shapes landscapes, from volcanic islands to forests recovering from fires.

Island Biogeography and Species Diversity

Principles and Factors Influencing Species Richness

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• Island biogeography studies the factors affecting species richness and biodiversity in isolated natural communities (islands)
• Theory proposes species number determined by balance between immigration and extinction rates
  • Influenced by island size and distance from mainland or source populations
• Larger islands have higher species diversity due to:
  • Increased habitat diversity
  • Reduced extinction risk
  • Higher immigration rates
• Islands closer to mainland or source populations have higher species diversity because of:
  • Higher immigration rates
  • Lower extinction rates

Species-Area Relationship and Equilibrium Theory

• Species-area relationship states number of species increases with island size following logarithmic curve
  • Fundamental principle of island biogeography
• Equilibrium theory suggests number of species reaches dynamic equilibrium
  • Rate of new species arriving balanced by rate of species going extinct
• Implications for species diversity:
  • Island size and isolation play critical roles in determining species richness
  • Understanding these principles helps predict and conserve biodiversity in isolated habitats (nature reserves, fragmented landscapes)

Immigration, Extinction, and Equilibrium

Processes of Immigration and Extinction

• Immigration process of new species arriving and colonizing an island from mainland or source populations
  • Immigration rate influenced by distance between island and source, species dispersal abilities
• Extinction process of species disappearing from an island due to various factors:
  • Habitat loss, competition, predation, stochastic events
  • Extinction rate influenced by island size (smaller islands have higher rates due to limited resources, smaller populations)

Equilibrium and Factors Affecting It

• Equilibrium number of species reached when immigration rate equals extinction rate
  • Number of species remains relatively stable, but composition may change over time
• Theory predicts islands closer to mainland will have:
  • Higher immigration rates, lower extinction rates
  • Resulting in higher equilibrium number of species compared to isolated islands
• Factors disrupting equilibrium include changes in:
  • Sea level, climate, habitat availability
  • Human activities (introducing invasive species, habitat destruction)

Ecological Succession and Ecosystem Shaping

Process and Drivers of Succession

• Ecological succession gradual process of change in structure and composition of ecological community over time
  • Follows disturbance or formation of new habitat
• Driven by changes in physical environment and species interactions during colonization, growth, habitat modification
• Influenced by factors such as climate, soil type, topography, resource availability (water, nutrients)
• Begins with pioneer species (hardy, fast-growing, high dispersal abilities) colonizing newly formed or disturbed habitat
• As succession progresses:
  • Pioneer species modify environment, making it suitable for other species to colonize
  • Leads to increased species diversity and complexity over time

Climax Community and Ecosystem Stability

• Final stage of succession is climax community
  • Relatively stable state where ecosystem reaches dynamic equilibrium with environment
• Climax community characterized by:
  • Diverse array of species well-adapted to local conditions
  • Complex interactions and nutrient cycling
• Ecosystem stability maintained through:
  • Resistance to disturbances
  • Resilience in recovering from disturbances
  • Examples: Old-growth forests, coral reefs, grasslands

Primary vs Secondary Succession

Primary Succession Characteristics

• Primary succession occurs when new habitat is formed
  • Examples: volcanic eruptions, glacial retreat, emergence of new islands
• Substrate initially devoid of soil and organic matter, no pre-existing community of organisms
• Slower process compared to secondary succession
  • May take hundreds or thousands of years for climax community to develop
• Pioneer species often algae, lichens, mosses that can grow on bare rock or inhospitable substrates
  • Examples: Lichens on volcanic rock, mosses on glacial moraines

Secondary Succession Characteristics

• Secondary succession occurs when existing community is disturbed or destroyed
  • Examples: fires, floods, human intervention (logging, agriculture)
• Substrate contains soil, organic matter, remnants of previous community (seeds, roots)
• Generally faster than primary succession due to:
  • Presence of nutrients and organic matter in substrate
  • Remnants of previous community facilitating recovery
• Pioneer species typically fast-growing herbaceous plants (grasses, weeds) that quickly colonize disturbed areas
  • Examples: Grasses in abandoned agricultural fields, wildflowers after forest fires
• Sequence of species replacement and final climax community may differ from primary succession based on environmental conditions and species involved