Here is the sentence which is being questioned.
The gray wolf was present in Yellowstone when the park was established in 1872. However, their population thoughout the lower 48 had been steadily declining since the 1800's.
It has been reviewed and the suggested correct is that since gray wolf is singular, their should be singular (its). However in this paper and in this context gray wolf never refers to a singular wolf but to the species, which is very clear throughout the paper.
Who is correct and if I am correct, what is the rule?