This answer is exactly the reason I asked what does it mean.
Serious question: why keeping polar bear alive makes planet sustainable? Who kept dinosaurs alive? Or any other species which went extinct before we even were there? I think we vastly overestimate our influence on the earth (just compare total mass of humans to that of bacteria, not to mention plants) and our responsibility for it.
If human brings some alien seeds and the new plant starts to dominate the "locals" we see that human as criminal. Would winter or water do the same it's natural then.
We are not the children of nature. We are the part of nature, and the small one. Sure, we have an ability to reflect on our actions and inflict disproportional changes to environment, but still.
Just for some context. Fukushima nuclear disaster: nobody killed, 39 injured.
Japan earthquake and tsunami: more than 15 000 killed, more than 5000 injured, more than 7000 missing. The sentiment is still "get rid of nuclear, save the planet". Who is killing whom? Indian Ocean tsunami? Haiti earthquake?
George got it right: no matter what we do planet is here to stay, it is not going anywhere. We cannot in any way affect the sustainability of it. Only of our own lives.