Enemy impact hypothesis

Definition

An ecological hypothesis proposing that natural enemies—predators, parasitoids, or pathogens—are the primary limiting factor on herbivore population density and therefore exert stronger top-down control over plant-herbivore dynamics than do bottom-up forces such as plant quality or resource availability. The hypothesis predicts that herbivore populations remain below levels that would otherwise deplete their host plants because mortality from enemies prevents overexploitation.

Etymology

Coined in ecological literature distinguishing top-down versus bottom-up regulation of trophic cascades; 'enemy' refers broadly to any natural antagonist of the herbivore.

Example

In temperate forests, the enemy impact hypothesis predicts that outbreaks of gypsy moth (Lymantria dispar) populations are checked primarily by viral epizootics and parasitoid wasps rather than by food limitation, with moth densities crashing before defoliation can fully exhaust oak foliage.

Synonyms

• top-down hypothesis
• natural-enemy hypothesis

Related Terms

• trophic cascade
• bottom-up control
• apparent competition
• Parasitoid
• density-dependent mortality
• Janzen-Connell hypothesis

Usage Notes

Contrasts with the plant stress hypothesis and resource limitation models that emphasize bottom-up control. Often tested by excluding enemies with pesticides or cages and measuring resulting herbivore population growth; results are frequently mixed, with enemy impact varying by system, herbivore guild, and spatial scale. Not to be confused with the 'enemies hypothesis' of biological control, which focuses on enemy diversity in complex versus simple habitats.