301_Chapter 13 Population Dynamics Space Time_F2024

Chapter 13: Population Dynamics over Space and Time

Key Concepts

  • Populations fluctuate naturally over time.

  • Density dependence with time delays can cause populations to be inherently cyclic.

  • Chance events can cause small populations to go extinct.

  • Metapopulations are composed of subpopulations that can experience independent population dynamics across space.

Population Fluctuations

  • Factors Influencing Fluctuations:

    • Resource availability

    • Predation

    • Competition

    • Disease

    • Parasites

    • Climate

  • Stability Examples:

    • Red deer population on the Isle of Rum, Scotland has remained stable for over 30 years.

  • Wide Fluctuations:

    • Algae in Lake Erie fluctuates from 0–7,000 cells per m³ within a year.

  • Reproductive Rates:

    • Smaller organisms like algae reproduce rapidly, responding quickly to environmental changes.

    • Larger organisms like red deer maintain homeostasis better due to lower surface-area-to-volume ratios.

Age Structure Fluctuations

  • Birth and Death Rates:

    • High or low numbers in age groups indicate past birth or death rate fluctuations.

    • Whitefish age analysis from 1946–1951 showed dominance of a cohort from 1947.

  • Long-term Age Structure Analysis:

    • Tree ring data shows forest composition changes over time, e.g., shifts from oak to white pine during the mid-1600s due to fire and drought.

Overshoots and Die-offs

  • Definitions:

    • Overshoot: Population growth beyond carrying capacity due to reduced resources.

    • Die-off: Significant decline in population density below carrying capacity, often following an overshoot.

Cyclic Population Fluctuations

  • Population Cycles:

    • Regular oscillations of population size over time, such as the gyrfalcon population cycles in Iceland recorded every 10 years.

  • Synchrony Across Species:

    • Regular cycles can be observed in different species across regions, e.g., capercaillie and grouse in Finland.

Delayed Density Dependence

  • Concept:

    • Populations may exhibit cycles due to delays in the response to environmental changes before reproduction occurs.

  • Modeling:

    • Using logistic growth models, influences of time delays (τ) increase the risk of populations overshooting or undershooting carrying capacity (K).

    • When rτ < 0.37, populations approach K without oscillations; between 0.37 and 1.57, damped oscillations occur; above 1.57, stable limit cycles emerge.

Extinctions in Small Populations

  • Vulnerability:

    • Small populations face higher extinction risks (e.g., bird studies on Channel Islands showed an inverse correlation between extinction probability and population size).

  • Growth Rate Contradictions:

    • Despite models suggesting small populations grow rapidly, stochastic variations lead them to higher extinction susceptibility.

  • Stochasticity Types:

    • Demographic Stochasticity: Variability in birth/death rates among individuals.

    • Environmental Stochasticity: Variability due to environmental changes.

Metapopulations

  • Definition:

    • A collection of local populations connected by dispersal.

  • Dynamics:

    • Each subpopulation has its own birth/death rates, and connectivity influences overall dynamics. Metapopulation dynamics are determined by colonization and extinction processes.

  • Patch Characteristics:

    • Habitat fragmentation can lead to isolated patches, impacting colonization and extinction rates.

Source-Sink Dynamics

  • Model Explanation:

    • Source patches produce excess individuals, while sink patches rely on immigration to prevent extinction.

  • Implications for Conservation:

    • Protecting habitat fragments and connecting areas can enhance metapopulation stability and species survival.

Conclusion

  • Metapopulation Theory Application:

    • Case study on black-footed ferrets demonstrates the successful implementation of metapopulation theory in managing population recovery and extinction resistance.