In the following sentence, should it be effort or efforts . . .

Competition has heightened with the development of a regional open heart program, declining market volumes, and area cardiology groups’ continued effort to develop additional office-based revenue streams.
Either one works, but because 'groups' is plural, 'efforts' might be better. Basically, it depends on whether the writer is thinking about many individual efforts of different sorts, or of a general effort to solve the problem.

'It will take much effort to solve global warming.'
'It is the many little efforts of each citizen that makes a country great.'

It should be efforts.Because the sentence is talking about continued attempts.