A perturbation small enough so that it not affects sinificatively any other variable in the complex system...

Difficult to be sure but generally an order of magnitude analysis should be enough...so that, at least, it is just small as hundreds of other variable that already present in nature and can affet the system....
Not an easy think to compute but some people were trying.. the human footprint and all that stuff

A pleasure

I therefore claim to show, not how men think in myths, but how myths operate in men's minds without their being aware of the fact. Levi-Strauss, Claude