  UV radiation feedback from young massive stars plays a key role in the
evolution of giant molecular clouds (GMCs) by photoevaporating and ejecting the
surrounding gas. We conduct a suite of radiation hydrodynamic simulations of
star cluster formation in marginally-bound, turbulent GMCs, focusing on the
effects of photoionization and radiation pressure on regulating the net star
formation efficiency (SFE) and cloud lifetime. We find that the net SFE depends
primarily on the initial gas surface density, $\Sigma_0$, such that the SFE
increases from 4% to 51% as $\Sigma_0$ increases from $13\,M_{\odot}\,{\rm
pc}^{-2}$ to $1300\,M_{\odot}\,{\rm pc}^{-2}$. Cloud destruction occurs within
$2$-$10\,{\rm Myr}$ after the onset of radiation feedback, or within
$0.6$-$4.1$ freefall times (increasing with $\Sigma_0$). Photoevaporation
dominates the mass loss in massive, low surface-density clouds, but because
most photons are absorbed in an ionization-bounded Str\"{o}mgren volume the
photoevaporated gas fraction is proportional to the square root of the SFE. The
measured momentum injection due to thermal and radiation pressure forces is
proportional to $\Sigma_0^{-0.74}$, and the ejection of neutrals substantially
contributes to the disruption of low-mass and/or high-surface density clouds.
We present semi-analytic models for cloud dispersal mediated by
photoevaporation and by dynamical mass ejection, and show that the predicted
net SFE and mass loss efficiencies are consistent with the results of our
numerical simulations.
