A study in Science put in perspective the current decline of predators and the affects of their loss on the environment. The study was conducted by a international team of scientists whom looked at predators in terrestrial, freshwater and saltwater ecosystems. The study concluded that "the loss of apex consumers is arguably humankind's most pervasive influence on the natural world." and claims that humankind has only seen the beginning changes that are occurring in ecosystems. See why below!
Basically, the delcine of apex predators (sharks, lions, killer whales and wolves) causes a large change in the ecosystem in which they belong, this is called a "trophic cascade" where a chain of effects continues down the food web. This can be seen in many ecosystems around the world, for example the decline of sea otters in kelp forests have cause a increase in sea urchin populations which in itself causes a decline in kelp forests.
This just shows how humans can change their environments without realzing the total affects which are very apparent in these studies. I urge everyone to read the entire article here (and more trophic cascade examples) and read how these studies can tell the future of our world.
*Picture from: http://www.greatwhiteadventures.com
*Cited from: University of California - Santa Cruz. "Loss of large predators has caused widespread disruption of ecosystems." ScienceDaily, 14 Jul. 2011. Web. 14 Jul. 2011.