13_Habitat Fragmentation

Habitat Fragmentation/Metapopulations

  • Habitat fragmentation: process of breaking a large habitat into smaller isolated pieces.

  • Metapopulations: populations divided into subpopulations with potential migration between them.

Types of Extinction

  • Extirpation: extinction of a local subpopulation.

  • Regional extinction: all subpopulations of a species become extinct.

  • Example: Male Ivory-billed Woodpecker, last sighting 1937 by James T. Tanner.

Randomness Affecting Populations

  1. Catastrophes: Unpredictable events causing reproductive failure and death in populations.

  2. Variable physical conditions: Ongoing changes affecting population growth and environmental carrying capacity.

  3. Random sampling: More significant impact on smaller populations than larger ones.

  4. Complex extinctions: Extinctions often caused by multiple interacting factors.

Population Structure

  • Key demographic categories:

    • Calves and juveniles

    • Reproductive females

    • Births

    • Adult males

    • Post-reproductive females

  • Births and deaths represent overall health of the population.

Extinction Probability

  • As population size increases, the probability of extinction decreases.

  • Example: Bird censuses from the Channel Islands (1917, 1968) show lower extinction risk with larger populations:

    • 4-10 breeding pairs: 30% extinction

    • 60 breeding pairs: 2% extinction

Habitat Fragmentation Effects

  • Scrub fragments in San Diego show correlation between species presence/absence and:

    • Fragment size

    • Time since isolation

    • Population density

  • Habitat structure from fragmentation affects species differently; some succeed while others fail.

Characteristics of Fragments

  • Fragments less than 10 hectares lose scrub bird species.

  • Species area relationship determined through modeling shows significant differences in species presence between larger and smaller fragments.

  • Age of fragments influences extinction rate; older fragments have reduced native species diversity.

Klamath River Restoration

  • Four dams previously blocked natural flow; restoration has opened nearly 400 miles of habitat for salmon and other species.

Metapopulation Dynamics

  1. Within-patch dynamics: growth regulation within subpopulations.

  2. Among-patch dynamics: colonization of empty patches and extinction of established ones.

  3. Migration significance: High migration leads to metapopulation behaving like a single large population; no migration results in isolated subpopulations at higher risk of extinction.

Models of Metapopulation Dynamics

  • p = fraction of occupied sites

    • p = 0: all sites unoccupied

    • p = 1: all sites occupied

  • Dynamics rely on immigration (I) and extinction (E) rates, describing changes in patch occupancy over time.

  • Internal-colonization model expresses the probability of extinction and colonization.

Importance of Quality and Distance

  • Source-sink dynamics: differences in habitat quality influence population exchange.

  • Wildlife corridors increase movement between habitat patches, essential for conservation planning.