Are there species which are ecological backbones? That is, are there species which are irreplaceable in a particular ecology without the whole system collapsing? Or does evolution ensure that there is enough diversity in each niche to ensure some degree of redundancy? 

A related question: If such such backbone species do exist, do high profile conservation cases like the Panda or the Tiger  belong to this group? If not, should we not be concentrating on them to rather than these high profile cases? Thanks