Why is antarctica food web fragile?

I believe that it's called fragile because of how the ice is melting. If the ice melts, all animals that live on it or rely on it for survival will become extinct. Polar Bears for example, eat fish, so do seals. If they aren't around anymore the fish population will grow and grow. Then all of those fish will make the plankton population decline because they will out number the plankton and eat them all. And last, the fish will become extinct due to their food being gone.