There are over 450 species of sharks! With sizes that range from as big as a school bus to as small as your hand, sharks have roamed the ocean for over 400 million years. Sharks are a diverse group; however, most share a common role in marine ecosystems, they are apex predators at the top of the food chain. This means that sharks have no natural predators of their own. They feed on prey at lower levels in the food chain, including fish that are weak, old, and unhealthy. This helps stop disease from spreading through fish populations and is just one way that sharks keep marine ecosystems healthy. But many shark populations are rapidly shrinking. Reasons for this decline include both natural factors (slow growth rate, small number of offspring) and human causes (overfishing, bycatch, shark finning). However, growing global awareness about the vulnerability of shark species and international cooperation are helping protect these key predators needed for a healthy ocean.