with molecular gas fractions of $\lesssim 10^{-3}$ [@Young2011]. What is the fate of the CO-traced gas reservoirs in these post-starburst galaxies? We find in French et al. (2018, submitted to ApJ) that the CO-traced molecular gas to stellar mass fraction declines with the time elapsed since the starburst ended, implying post-starburst galaxies should reach early-type levels of molecular gas in 700-1500 Myr. Thus, post-starbursts become gas poor as their stellar populations, color gradients, morphologies, and kinematics start to resemble early-types.

The CO-traced gas undergoes a dramatic transition as the galaxy evolves from starbursting to the post-starburst phase over $\lesssim$ 1 Gyr. Starburst galaxies have enhanced CO-traced SFEs compared to normal star-forming galaxies. After the starburst ends, we observe post-starburst galaxies to have suppressed CO-traced SFEs relative to normal star-forming galaxies. Over the $\sim$ Gyr of evolution between these two phases, the dense gas mass ratio also evolves from high to low, but the HCN luminosity tracks with the SFR throughout this process. Thus, despite the fact that collapse to star formation is not guaranteed at the $n \gtrsim 8\times10^3$ cm$^{-3}$ densities traced by HCN, the processes which drive the starburst, the end of the starburst, and the dramatic change in 