That is a serious "quotation needed" statement. How can you even know you know about all relevant ecosystems when you are missing 90% of species? Besides, just knowing about the "keystones" of an ecosystem is required but not sufficient.
As an analogy, if you knew the top 10% keystone chemicals that make up the human body but did not know the long tail of the other 90%, you'd likely miss out on a lot of the details that make it all work like immune system cells and vitamins. Just because a molecule (or species) is rare does not make it non-essential.
I hope we are building a statistical model of which one is eating which to map where are the missing pieces of the puzzle and direct our focus of research.
As an analogy, if you knew the top 10% keystone chemicals that make up the human body but did not know the long tail of the other 90%, you'd likely miss out on a lot of the details that make it all work like immune system cells and vitamins. Just because a molecule (or species) is rare does not make it non-essential.