A negative feedback loop is a self-stabilizing mechanism in which an increase in a system's output reduces or halts the processes that led to that increase, thereby diminishing the increase or restoring output to its previous level. A simple example is the turning off of a heater by its thermostat when the temperature gets too high and then turning it back on when the temperature gets too low.
This contrasts with a positive feedback loop, which occurs when an increase in a system's output stimulates the creation of still more output and thus leads to exponential growth. Positive feedback loops are much less common than negative feedback loops because such systems will eventually run out of resources for their accelerating expansion.
Negative feedback loops play an important role in the maintaining of stability in biological and ecological systems. A simple example is the sensation of hunger, which prompts eating. As food is consumed, the hunger subsides, thereby reducing the desire to eat until a balanced state is restored. Likewise, when a human's body temperature rises, sweating occurs to cool it, and conversely, when its temperature drops, shivering occurs to increase body heat.
Negative feedback loops are essential for maintaining stability within ecosystems and providing resilience for them to recover from disturbances and thereby maintain the complex interactions that support biodiversity. For example, they can help prevent any single species from dominating an ecosystem because an increase in a prey populations leads to an increase in the predator population, thereby reducing the former and thus allowing the balance between species to recover. They can also help stabilize environmental conditions such as temperature, water levels and nutrient availability.