is or are should be used in this sentence?
For this sentence: Native species for which trading are allowed were also represented by large numbers, should I use "is" or "are"?

Question

is or are should be used in this sentence?
For this sentence: Native species for which trading are allowed were also represented by large numbers,  should I use "is" or "are"?

Answer

Native species, for which trading (is) allowed, were also represented by large numbers.

It's "is" instead of "are" because it applies to (trading), which is singular,  and not to (Native species), which is plural. 
That way it reads alone as "trading is allowed," not "trading are allowed."