Estimating Allee dynamics before they can be observed: polar bears as a case study

Area of Science:

• Ecology and Evolutionary Biology
• Population Dynamics
• Conservation Biology

Background:

• Allee effects, where population growth rate declines at low densities, are critical for species viability but difficult to quantify, especially for large mammals.
• Estimating low-density growth rates is essential for population viability analyses and conservation management.
• Data on population growth at low densities are often unavailable, hindering accurate assessments of Allee effects.

Purpose of the Study:

• To present a novel mechanistic modeling framework for estimating expected low-density population growth rates under mate-finding Allee effects.
• To provide a method for parameterizing and validating models using data on Allee effect mechanisms, rather than direct population growth data.
• To apply this framework to polar bears (Ursus maritimus) to estimate their low-density growth and assess the impact of Allee effects on population recovery.

Main Methods:

• Developed a mechanistic modeling framework integrating a mating dynamics model with a matrix projection model.
• Parameterized and validated the model using data on the mechanisms driving the Allee effect, specifically mate-finding in polar bears.
• Analyzed the density- and frequency-dependence of the Allee threshold, considering age-structure, sex ratio, and life history parameters.

Main Results:

• The Allee threshold is influenced by age-structure, sex ratio, and life history parameters, demonstrating density- and frequency-dependence.
• Sensitivity analyses revealed that different life history parameter combinations can yield varying Allee thresholds, even with the same stable-stage growth rate.
• Mate-limitation can lead to prolonged transient population dynamics before reaching carrying capacity.
• Application to a low-density polar bear population suggests a mate-finding Allee effect contributes to slow recovery.

Conclusions:

• The developed mechanistic modeling framework offers a viable method for estimating low-density growth rates and Allee thresholds when direct data are scarce.
• This approach is generalizable across various mating systems and life cycles, aiding proactive conservation and management strategies.
• Provides a priori estimates for conservation targets for rare species and eradication targets for invasive species.