Let me answer this question by using bullet points for the sake of clarity and succinctness.
Conservation of single species (advantages):
- It is much easier to conserve a single species than a whole ecosystem. There is much more control.
- From a financial point of view, the preservation of single species is possible. Funds are always limited.
- You are able to pin point the most endangered species and help it.
Conservation of single species (disadvantages):
- The species may only survive in the long run in that particular ecosystem.
- The ecosystem might need that species to function properly.
Conservation of ecosystem (advantage):
- It is always best to preserve ecosystems, because it is the optimal place of that species to survive.
- There are many other species in the ecosystem that will benefit if it is preserved.
- Later in time, we might find out that something in the ecosystem is vital for our lives as well.
Conservation of ecosystem (disadvantage):
- There might be issues outside of our control. For example, acid rain from mining companies can cause changes in the ecosystem. We might not be able to do anything about this.
- Cost will be great. Who will pay for it all?