Quick Answer
Many prey populations would initially increase, but that growth would be constrained by food, disease, competition and habitat. Herbivore overpopulation could damage vegetation, alter landscapes and trigger secondary declines. The exact outcome would differ between ecosystems because predators occupy many different ecological roles.
The first effect would be prey release
Animals that normally face predation would experience lower mortality. Their populations could rise, especially where food is abundant and reproduction is rapid.
Then resources become the limit
Predators are not the only population control. Food shortages, parasites, disease, competition and weather still constrain prey. A population that grows beyond the ecosystem's carrying capacity can later crash when resources become scarce.
Vegetation could change dramatically
In ecosystems with large herbivores, predator loss can increase grazing pressure. Forest regeneration, grassland composition and riverbank vegetation could shift as herbivores consume more plants.
The effects could cascade
Removing one group can alter many others. More herbivores can reduce plants; fewer plants can affect insects and birds; changed vegetation can alter soil and water movement. These cascading effects make the final ecosystem difficult to predict.
Established scienceEcologists have documented trophic cascades in real ecosystems where changes in predator abundance alter prey behavior, vegetation and other parts of the food web.
Humans would not simply replace predators
People could hunt or manage prey populations, but doing so would require continuous intervention across huge landscapes. Some predators also perform ecological roles that are difficult to reproduce mechanically, such as removing weak or diseased individuals.
The bottom line
A predator-free world would initially look richer in prey but could become less stable as food webs reorganized. Predators are part of the architecture of ecosystems, not just consumers sitting at the top.
Predator removal can produce rapid changes in prey abundance, behavior and vegetation, but ecosystems do not respond in one direction indefinitely. Food limitation, disease, competition and habitat constraints eventually become important. The trajectory would differ between ecosystems because predators occupy very different ecological roles.
Why prey populations would not simply grow forever
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.