Contribution of HUDs to the galaxy stellar mass function (green dashed line and error bars). Error bars are 1 − σ and only account for Poisson uncertainties. Bins that contain only one object have been removed. The double Schechter function fit to the GAMA SMF (Wright et al. 2017) is shown for comparison (smooth orange line), as well as the Santa Cruz SAM parent sample SMF (red line), the SAM central UDGs SMF (purple dashed line), and the SAM HI-rich central UDGs SMF (blue dash-dotted line).