I found this video on Youtube. It is wonderful that children are learning how important sharks are to our own survival and the survival of other life in the oceans of this planet!
Here are some links to some websites which discuss why sharks are important to the ocean ecosystem and this planet:
http://ohia.org/ohia/roadshows/ocean/tony/foodchain.htm
http://www.sharksavers.org/education/why-sharks-are-important/710-sharks-role-in-the-oceans.html
http://www.oceanicresearch.org/education/wonders/sharks.html