Aggregation and a strong Allee effect in a cooperative outbreak insect

Area of Science:

• Ecology
• Population Dynamics
• Entomology

Background:

• Many species aggregate to mitigate negative impacts of low population densities, a phenomenon known as the Allee effect.
• Aggregation can significantly alter the population-level expression of Allee effects.
• The mountain pine beetle (Dendroctonus ponderosae Hopkins) is an irruptive bark beetle known to aggregate to overcome tree defenses.

Purpose of the Study:

• To investigate the interaction between aggregation behavior and Allee effects in the mountain pine beetle.
• To model the population dynamics of mountain pine beetles considering aggregation and tree-level attack thresholds.

Main Methods:

• Development of spatiotemporal models based on the Nicholson-Bailey framework.
• Incorporation of beetle aggregation and a tree-level attack threshold into the models.
• Fitting the models to data from a mountain pine beetle outbreak.

Main Results:

• Aggregation enables mountain pine beetles to overcome tree defenses and reproduce by surpassing critical attack numbers.
• Models demonstrated that aggregation allows tree-level attack thresholds to be met even with a low ratio of beetles per susceptible tree.
• This finding highlights how aggregation can mitigate strong Allee effects at the population level.

Conclusions:

• Aggregation is a crucial factor in understanding the population dynamics of species exhibiting strong Allee effects.
• The study confirms the importance of incorporating aggregation into ecological models for organisms like the mountain pine beetle.
• Findings have implications for predicting and managing bark beetle outbreaks.