Because predators survive by preying on other animals, they greatly affect the food web, keeping in check the negative impacts other animals have on natural systems if too abundant. When wolves, bears, or mountain lions prey on elk or deer, this keeps the ungulates moving around (in fear) and prevents population explosions. This predation (or risk) limits the affect ungulates have on plant biomass; thus more trees, shrubs, bushes, and grasses can grow, which then preserves or creates habitat for many other species, from insects and reptiles to beavers and birds.
Image courtesy of Yellowstone National Park
In this way, predators contribute to preserving biodiversity, which humans rely on for survival. Healthy natural systems provide us with clean water, trees and forests, seed dispersal, natural pest control, climate regulation, healthy amounts of vegetation, pollination, soil fertility, regulation of disease, and many other "ecosystem services."
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