Answer:
enzymes i think but i might be wrong.
Explanation:
The population size of a predator species is directly controlled by the population size of it's prey. If the predator has no prey the population size will go down because they have no food.
Jet streams are fast moving air currents much like how a river is a fast moving water current so the answer is jet streams.
I haven't taken biology since my freshman year of high school, so this is kind of a guess. But I know for sure it isn't a solid. So I would go with a gas.
