Yes, there are, with a few exceptions. Buses directly connect all parks except the Magic Kingdom, where they operate out of the TTC instead (use monorail or ferryboat between MK and TTC). Also, to/from Epcot and the MK is by monorail connection, also at the TTC. For instance, going from Studios to AK you have a direct bus. Going from MK to Animal Kingdom requires monorail to the TTC, where you catch the bus on to AK.
As an alternative, since buses to Disney resorts do directly operate to the Magic Kingdom, you have the option of taking a bus to a resort (not necessairily your own) and transferring there. This usually takes longer, but it can be a good way to reach World Showcase (via the Boardwalk) from AK or MK.