In this model food web the only feeding links are these: tiny floating animals eat phytoplankton; small fish eat the tiny floating animals; seabirds eat the small fish. The small fish are the only predator of the tiny floating animals in this web, and phytoplankton remain plentiful, so food is not a limiting factor here. If all the small fish are removed and nothing else in this web changes, what happens to the population of tiny floating animals?
Explanation
The tiny floating animals lose the one predator this web gives them, so fewer of them are eaten and their numbers rise. The claim that they stay the same because the phytoplankton are unaffected fails, since their food supply was never the thing that changed.