Why is an ecosystem never constant?

Lots of factors play different kinds of roles on ecosystems.
Ecosystems are never constant because there are so many levels and layers of animals to each one. It's impossible for the headcount of each species to stay the same within an ecosystem when instances such as flooding, drought, poisoning, and hunting can take part. 
Let's say the hawk, who is over the bunny on the pyramid, gets poisoned by water. Now, there's fewer hawks, which means there's less predators to hunt the bunnies, hence causing more bunnies and even increasing the chance of overpopulation. 
The animals within ecosystems keep each other in check, but numbers are constantly going up and down as things happen. Natural disasters, pollution, and anything such as, effects the ecosystem, therefore it cannot remain constant. 

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