Quick Answer
Large predators regulate prey through both killing and behavior. If they disappeared, some prey populations could increase, causing heavier grazing and browsing that alters vegetation and other species. The outcome would vary by ecosystem because prey, plants, diseases and human hunting would all influence the response.
Predators influence ecosystems through direct predation and by changing where and when prey feed. This means removing predators can affect vegetation and landscapes even before prey numbers become extreme.
The trophic cascade
If predators disappear, some herbivores may become more abundant or less cautious. More grazing can reduce vegetation, which then affects insects, birds, soil and water systems.
Not every ecosystem reacts the same
A predator-free forest, coral reef and grassland would not respond identically. Some prey have other predators, some are controlled by disease, and some are already limited by food or habitat.
Humans could become the replacement predator
People might increase hunting or management to control prey populations. That could partially replace some predator functions, but human harvest is rarely identical to natural predation.
Could ecosystems collapse?
Some food webs could experience severe instability, while others might settle into a new equilibrium. The phrase “collapse” is therefore too broad without specifying the ecosystem.
Established scienceEcologists have documented trophic cascades in multiple systems. Their strength and direction depend on the species and environment involved.
The bottom line
Removing large predators would not simply make prey thrive. It would change the structure of food webs, vegetation and animal behavior, with effects spreading far beyond the predators themselves.
A note on our approach: Every article on WhatIfLab separates what current science establishes from what remains genuinely speculative. Where we cite a figure or finding, it reflects published, mainstream research at the time of writing — not a prediction dressed up as fact.