Hi,
A considerable number of species (outnumbering the positive trend species) are in real danger of extinction, while the places they live and communities that are valuable to biological diversity face great pressures.
This seems like the kind of sentence that is a lot easier to understand when you see it in context.
the positive trend species This phrase needs to have been defined in an earlier part of the text. I guess that it means 'species whose numbers are increasing'.
I don't know what is meant by the word 'communities'. Does it mean 'groups of animals that live together'? So it wouldn't include, for example, solitary animals like tigers?
Clive