Which keyword means: a measure of the variety of different species (in an ecosystem)

Which keyword means: different species rely on each other (there is a complex network of relationships between different organisms in the ecosystem)?

Which keyword means: the place where an organism lives?

Which keyword means: an organism with essential roles in the community (if this species becomes rare or absent then the whole ecosystem is dramatically altered and the biodiversity is likely to decrease)?

Which keyword means: all of the populations of different species living together in an ecosystem

Which keyword means: the interactions between a community and the (abiotic) environment 

Which is best definition of a 'species'?

Which of the following ecosystems have the highest biodiversity?

True or False? Ecosytems with high biodiversity are more stable (than those with lower biodiversity)

What is the best subject?