Every creature and plant on the planet is in a food chain and a food web. A food chain is like this:
Phytoplankton is eaten by Mussels, which are eaten by Starfish, which are eaten by Crabs.
From now on = means is/are eaten by.
Food webs are more like:
Phytoplankton = Zooplankton = Herring = Tuna = Killer Whale.
Phytoplankton = Zooplankton = Herring = Shark.
Phytoplankton = Zooplankton = Squid = Shark.
Phytoplankton = Zooplankton = Humpback Whale.
Phytoplankton = Zooplankton = Mussel = Starfish = Crab.
Phytoplankton = Mussel = Starfish = Crab.
This is only part of the vast Food Web. If, for instance, one of those animals disappeared, it would have a huge consequence on the Food Web. For instance: If Phytoplankton disappears, the Zooplankton will die, causing the wildlife in the ocean to go extinct. If Crabs disappeared, then Starfish would overpopulate the sea, eating all of the pearls, causing their food source to go extinct and Starfish themselves.
So, we must do what ever we can to prevent a Food Web catastrophe happening.