A: This question has been at the forefront of ecological thinking for decades.
The answer depends on our appreciation of the complexity of life. A quick look around us reveals there are lots of roles that plants and animals play in the web of life. Plants make energy from sunlight. Some animals eat these plants to grow and to reproduce. Other animals eat these animals, preventing populations from getting too large. Finally, bacteria and fungi decompose the waste that each type of plant and animal creates, recycling this waste into nutrients for plants to again turn into new energy and tissue.
Without each of these species serving its unique role, the cycle of life would be interrupted. Without plants, for example, we would not get the food that we eat or the oxygen that we breathe. Only with each of the components of life remaining active and intact can we be assured of a healthy and fully functional environment.
Unfortunately, we frequently learn how important each species is to our lives only after we have lost the species. For example, a mass mortality in 1983 killed more than 99 percent of the long-spined sea urchins in the Caribbean. Only later did we realize they serve a critical role in eating algae, preventing these seaweeds from overgrowing corals. Today, algae, not corals, dominate most of the reefs of the Caribbean because the urchins no longer control the algae.
Our job as ecologists is both to understand what each species does and to protect each one, assuring that the web of life continues to function well.
--Stuart Sandin, marine ecologist, Center for Marine Biodiversity and Conservation