how can overfishing and decimation of forests prove harmful?

The overfishing will hamper the ecosystem of water body(Sea, Ocean and river). Which is harmful...as if we will finish the fishes there will no fish left for the hunters of the sea or ocean and it will kill them because of the lack of food also these hunter fishes like Shark can became aggressive due to starvation and can attack the boats or humans. Also these fishes also maintain the ecosystem sea without them there will no ecosystem. also decimation of forests will result in soil erosion, increasing global warming, and increasing Carbon dioxide level in atmosphere and decreasing Oxygen level in our atmosphere. which in result will increase threat to mankind.