How is it that deer and other herbivores like insects and rabbits don’t eat allllll of the plants on earth? As they eat plants, individuals become more fit to reproduce, allowing their populations to grow. Bigger deer populations could eat even more plants!
Hairston, Smith, and Slobodkin proposed that predators keep herbivore populations in
check. This is a top-down control. Predators can alter prey populations by impacting the prey population size and behavior. Prey such as deer or rabbits may shift habitat to escape from the risk of wolves.
However, plants themselves limit consumption by herbivores. Armed with physical structures like thorns and chemicals like tannins, plants defend against herbivore attack. These defenses suggest that bottom-up mechanisms limit the ability for herbivores to eat all of the plants.
So which is it? Why is the world green? In most ecosystems, both top-down and bottom-up forces combine to keep plants around for the long-haul.