The greatest competition a colony of honeybees has must be with other
colonies of the same species.  Read Dr Leslie Bailey on the effects of
colonies being placed at at density greater than desirable from the bees'
point of view.
On the other hand I have noticed that bees can to some extent enhance their
own pastures through improved pollination.  The paddock next to my first
apiary has had a great increase in wild white clover since I started keeping
bees there.  This extra nectar is also available for other species of bees.
Chris Slade