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Related Experiment Video

Updated: Oct 9, 2025

Generation and Long-term Maintenance of Nerve-free Hydra

Generation and Long-term Maintenance of Nerve-free Hydra

Published on: July 7, 2017

Disease-Induced Hydra Effect with Overcompensatory Recruitment.

J M Jaramillo¹, Junling Ma², P van den Driessche²

¹Department of Mathematics and Statistics, University of Victoria, Victoria, BC, V8W 2Y2, Canada. juanj@uvic.ca.

Bulletin of Mathematical Biology

|December 19, 2021

Summary

The hydra effect, an increase in population due to higher mortality, can be triggered by infectious diseases. This occurs when strong competition and reduced reproduction in infected individuals lead to a larger overall population at endemic equilibrium.

Keywords:

Hydra effect Infectious disease Overcompensation Ricker recruitment

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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.