What Is the Enemy Release Hypothesis?

The enemy release hypothesis (ERH) posits that when a species is introduced to a new environment, it leaves behind many of the natural enemies—such as predators, parasites, and pathogens—that kept its population in check in its native range. Freed from these ecological constraints, the introduced species experiences reduced mortality and increased productivity, allowing it to grow, reproduce, and spread more rapidly than native competitors124.

How Does Enemy Release Work?

1. Escape from Natural Predators and Pathogens

In their native habitats, species have co-evolved with a range of natural enemies. These enemies play a crucial role in regulating population size and competitive ability. When a species is transported to a new region—intentionally or accidentally—many of these enemies are left behind. The result is a dramatic reduction in biotic pressures that would otherwise limit the species’ success124.

2. Competitive Advantage Over Native Species

Native species continue to be regulated by their own natural enemies, while the invader enjoys a period of “enemy release.” This often gives the invasive species a significant competitive edge, allowing it to outgrow and outcompete native plants or animals for resources such as light, nutrients, or space25.

3. Rapid Population Growth and Spread

With fewer threats, invasive species can allocate more energy to growth and reproduction. This leads to rapid population increases and the ability to colonize large areas in a relatively short time, sometimes resulting in ecological dominance156.

Evidence Supporting the Enemy Release Hypothesis

The enemy release hypothesis is one of the most widely supported explanations for the dominance of exotic invasive species. Numerous studies have documented cases where invasive plants and animals experience less damage from herbivores, pathogens, or predators in their introduced ranges compared to their native ranges125. This pattern has been observed across a variety of taxa and ecosystems.

However, it is important to note that enemy release is not the only explanation for invasive success. Other factors, such as greater phenotypic plasticity, higher reproductive rates, and more efficient resource use, can also contribute to invasiveness. In some cases, the advantage of enemy release may diminish over time as local enemies adapt to the invader or new enemies are introduced578.

Related Concepts: Evolution of Increased Competitive Ability (EICA)

The enemy release hypothesis is closely related to the Evolution of Increased Competitive Ability (EICA) hypothesis. EICA suggests that, once freed from the need to invest heavily in defense against enemies, invasive species may evolve to allocate more resources to growth and reproduction, further enhancing their competitive edge6.

Why Other Hypotheses Don’t Fully Explain Invasive Success

• Ecological Niche Hypothesis: Focuses on how species fill available ecological roles but does not specifically address the absence of enemies.

• Intermediate Disturbance Hypothesis: Explains species diversity in relation to disturbance frequency, not specifically the success of invasives.

• Biotic Resistance Hypothesis: Suggests that diverse native communities can resist invasion, which is often true, but does not explain why some invasives thrive despite resistance.

Practical Implications

Understanding the enemy release hypothesis is critical for managing invasive species. It highlights the importance of monitoring for potential natural enemies of invaders and considering the introduction of biological control agents—carefully selected predators or pathogens from the invader’s native range—to restore ecological balance.

Conclusion

The enemy release hypothesis provides a compelling explanation for why invasive species often grow and propagate more successfully in new environments than in their native habitats. By escaping their natural enemies, these species gain a significant advantage, leading to rapid population growth and ecological dominance. Recognizing the role of enemy release is essential for developing effective strategies to manage invasive species and protect native biodiversity.