Disease-Induced Hydra Effect with Overcompensatory Recruitment

Area of Science:

• Ecology
• Epidemiology
• Mathematical Biology

Background:

• The hydra effect describes counterintuitive population increases driven by elevated mortality.
• This phenomenon has been observed in various species and modeled mathematically.
• Recent empirical studies suggest disease can also induce a similar effect.

Purpose of the Study:

• To investigate the conditions under which infectious diseases can induce a hydra effect.
• To determine when an endemic equilibrium population size exceeds the disease-free equilibrium population size.
• To analyze the role of intra-specific competition and reproductive inhibition in disease-induced hydra effects.

Main Methods:

• Development of theoretical models in continuous and discrete time.
• Conducting simulations to explore population dynamics under disease pressure.
• Analyzing conditions for overcompensatory recruitment and Ricker recruitment.

Main Results:

• A disease-induced hydra effect occurs when intra-specific competition is intense and infection severely curtails reproduction.
• A necessary and sufficient condition for this effect in continuous time models is overcompensatory recruitment.
• Discrete time models demonstrate that overcompensatory recruitment is essential for the disease-induced hydra effect.

Conclusions:

• Infectious diseases can lead to population increases via the hydra effect under specific ecological conditions.
• Overcompensatory recruitment is a critical factor for observing this phenomenon in both continuous and discrete population models.
• Simulations confirm the potential for disease-induced hydra effects with Ricker recruitment dynamics.