bio 2.21

Overview of Population Growth

• Logistic Growth Model: A density-dependent model where population growth changes as population size approaches carrying capacity.

  • Y-Axis: Represents the total number of individuals in the population (population size/density).

  • X-Axis: Represents time or density of the population.

Key Concepts in Population Growth

• Per Capita Growth Rate (r):

  • Maximum growth occurs at an initial high capacity; however, growth rate decreases as the population becomes denser due to limited resources.

  • At carrying capacity, growth stops, illustrating the balance between births and deaths.

• Birth and Death Rates:

  • Increased population density leads to reduced birth rates and higher death rates due to competition and resource scarcity (e.g., starvation, stress).

  • Increased severe predation risk may also be observed with higher densities.

Metapopulation Dynamics

• Definition of Metapopulation: A larger population comprising several somewhat isolated subpopulations.

  • Geographic isolation affects gene flow and population dynamics.

  • Different species (e.g., birds versus terrestrial slugs) have varying isolation levels based on mobility.

Research on Daphnia Populations

• Experiment Design:

  • Researchers established artificial rock pools with different water volumes (12L, 50L, 300L) to represent various subpopulations and tracked Daphnia population sizes over four years.

  • Measurements were taken for total counts and averages over the four years.

• Results:

  • The extinction rates for various pool sizes:

    • 12L pools: 18.5%

    • 50L pools: 16%

    • 300L pools: 9.1%

  • Average sizes of persisting versus extinct subpopulations varied significantly across pool sizes:

    • 12L Pools: Persistent = 1,500; Extinct = 65

    • 50L Pools: Persistent = 2,700; Extinct = 530

    • 300L Pools: Persistent = 10,000; Extinct = 1,800

Patterns and Predictions of Extinctions

• Smaller subpopulations tend to have higher extinction probabilities compared to larger ones.

• Example of Correlation:

  • Smaller persistent subpopulations were significantly larger than smaller extinct ones, indicating a trend of resilience among larger populations.

• Implications of Connectivity:

  • Immigration/Emigration could recover populations that are dwindling; interconnected metapopulations can enhance survival prospects.

Real-world Example: Aedes Checkerspot Butterfly

• The butterfly relies on specific host plants found in serpentine rock formations.

• Historical drought led to local butterfly population extinctions, but large subpopulations sustained themselves.

• As recovery happened in the 1980s, nearby subpopulations recolonized areas where butterflies had gone extinct, demonstrating the importance of metapopulation dynamics.

Conservation Considerations

• Ensuring habitat connectivity allows for inter-population gene flow and resilience against local extinctions.

• Effective Population Management:

  • Protecting a large population across several subpopulations can minimize extinction risk from localized disasters.

Human Population Context

• As of February 2020, human populations significantly rose due to agricultural transition and advancements (e.g., hygiene) since 1650, affecting overall death and birth rates worldwide.

• Population studies continue to debate the long-term impacts of these changes on global dynamics.