Risks of Population Extinction from Demographic and Environmental Stochasticity and Random Catastrophes.

Area of Science:

• Ecology
• Population Dynamics
• Conservation Biology

Background:

• Stochastic factors significantly influence population dynamics and extinction risk.
• Understanding the relative contributions of different stochastic processes is crucial for conservation.
• Previous research has not fully elucidated the interplay between population size and extinction risk under various stochastic scenarios.

Purpose of the Study:

• To analyze stochastic factors affecting single population demography.
• To determine the relative risks of extinction from demographic stochasticity, environmental stochasticity, and random catastrophes.
• To compare extinction dynamics under different stochastic models based on population growth rate.

Main Methods:

• Analysis of asymptotic scaling relationships for average time to extinction (T) versus carrying capacity (K).
• Modeling stochastic factors within a simple exponential growth model up to carrying capacity.
• Assessment of extinction dynamics based on the long-run growth rate (r̃) below carrying capacity.

Main Results:

• If r̃ is positive, extinction time (T) scales exponentially with K for demographic stochasticity and as a power of K for environmental stochasticity or catastrophes.
• If r̃ is negative, T increases logarithmically with K under demographic stochasticity.
• Demographic stochasticity poses a lower extinction risk than environmental stochasticity or catastrophes for large populations.

Conclusions:

• Demographic stochasticity is less critical for large populations compared to environmental stochasticity or catastrophes.
• The relative importance of environmental stochasticity and catastrophes depends on growth rate variance and catastrophe characteristics.
• Populations with a substantially positive r̃ can persist long-term, even with environmental stochasticity or catastrophes, challenging prior assumptions.