Evolution of total gas density (Σ_HI + Σ_H2) over 8 Gyr for a $M_{\ast }={10}^{9.5}{\mathrm{M}}_{\odot }$ galaxy undergoing simple starvation. The threshold gas density for star formation (explained in Appendix D) is shown as a dashed red line. By the t = 8 Gyr curve, all H I gas has been consumed and as such the H₂ is not being replenished, causing the total gas density to decrease at all radii.