Trophic cascades are changes in lower trophic levels as a result of a change in the abundance of predator populations. One example is the removal of wolves from an ecosystem might result in an increase in the number of elk and a decrease in the vegetation the elk feed on.(See Yellowstone National Park’s famous example of a trophic cascade: http://www.missionwolf.org/page/trophic-cascade/ ) It is important to consider the potential effects of trophic cascades because while a predator alone may not be an important natural resource, the removal or addition of predators from an ecosystem can have consequences for ecosystem services such as nutrient cycling.
Image Sources: http://wyofile.com/deb-donahue/trophic-cascade-the-case-for-wolves/
http://www.cof.orst.edu/wolves/
For more information see the Encyclopedia Britannica article: http://www.britannica.com/EBchecked/topic/1669736/trophic-cascade