Damage to the ecosystem may cause loss of several valuable species and may result in increase in the numbers of parasite or predatory communities and may have several adverse effects. therefore, the conservation biologists focus on conservation of entire ecosystem protecting several species rather than conservation of a single species. cite source

True, several ecosystems have been damaged thus conserationalist focus on protecting several species.

Explanation:

As the ecosystem is not only a part of land found on the earth it involves the entire system of the mountain to oceans to the atmosphere to the food web and even the humans. Thus ecologists and biologists are aware of the fact that man for his selfish needs has destroyed the natural carrying capacity of the ecosystem beyond repair. Thus conservationists focus on protecting the entire chain of species as all are linked to the same biotic community and hence live in harmony and coexistence of each other. By keeping these species protected the balance of energy flow in the ecosystem will remain stable and population of these species will continue to grow thus they focus on the making biological areas for their protection, reserved forest and protected forest areas are solely created so that the species get their chances to survive.

Source guardian paper.