The problem with ecosystems is that they conjour up ideas of rare species of - I don't know - wasps, or something in the Amazon Rain Forest, and it's probably hard for many of us to feel much for any wasps, and we wonder if the ecosystem can do without them. Even larger organisms - say polar bears - though of tremendous aesthetic value don't seem to many of us to have much connection with our survival as a species. Species have become extinct, and we're still here.
None of this changes the fact that the biosphere - the overall ecosystem, I suppose, sustains human life. Even if scientists don't know which organisms are vital to this task and which are irrelevant, it is ceratinly the case that we will die if we don't conserve what sustains us. Never mind what looks pretty, or what seems pointless; we know that there is a vital minimum.