Decreases in membership at the club are affecting the ability of the club to maintain its staff.

decreasing in membership at the club is affecting

affecting the decrease in members are

the decrease in members at the club were affecting

the decrease in members from the clubs are affecting

no improvement or correction is required.

I believe the right correction for the sentence would be "The decrease in members from the clubs are affecting". The first option wouldn't be correct because the word "decreasing" doesn't fit right grammar wise in the sentence. Option two wouldn't make any sense put together with the sentence, it would just sound completely out of place. Option three has the word "were" as in it was affecting the club, not presently affecting the club. And obviously, this sentence needs improvement so option five is out of the question.