  It is a major open question which physical processes stop the accretion of
gas onto giant molecular clouds (GMCs) and limit the efficiency at which gas is
converted into stars within these GMCs. While feedback from supernova
explosions has been the popular feedback mechanism included in simulations of
galaxy formation and evolution, `early' feedback mechanisms such as stellar
winds, photoionisation and radiation pressure are expected to play an important
role in dispersing the gas after the onset of star formation. These feedback
processes typically take place on small scales ($\sim 10-100$ pc) and their
effects have therefore been difficult to constrain in environments other than
the Milky Way. We apply a novel statistical method to $\sim 1$" resolution maps
of CO and Ha emission across a sample of nine nearby disc galaxies, in order to
measure the time over which GMCs are dispersed by feedback from young,
high-mass stars, as a function of the galactic environment. We find that GMCs
are typically dispersed within $\sim$ 3 Myr after the emergence of unembedded
high-mass stars, showing no significant trend with galactocentric radius.
Comparison with analytical predictions demonstrates that, independently of the
environment, early feedback mechanisms (particularly photoionisation and
stellar winds) play a crucial role in dispersing GMCs and limiting their star
formation efficiency in nearby galaxies. Finally, we show that the efficiency
at which the energy injected by these early feedback mechanisms couples with
the parent GMC is relatively low (a few tens of per cent), such that the vast
majority of momentum and energy emitted by the young stellar populations
escapes the parent GMC.
