Answer:
a stimulus that causes stress
Explanation:
Start with your producers, or plants (grasses, flowers, etc.). Then your 1st level consumers (herbivores), like grasshoppers, mice, etc. Then your second level producers (carnivores and omnivores) that eat the herbivores (like birds). And keep going, knowing each of the levels get eaten by each other. 10% of energy is transferred in each level. An example might be...
Grass > Mice > Hawk > Vulture
A phrase to describe a negative feedback loop could be: self-regulating (though I'm not sure the exact word you're looking for).
