We investigate the change in ionizing photons in galaxies in the range 0.2\lt z\lt 0.6 using the F2 field of the SHELS complete galaxy redshift survey. We show, for the first time, that while the [O iii]/Hβ and [O iii]/[O ii] ratios rise, the [N ii]/Hα and [S ii]/Hα ratios fall significantly over the 0.2\lt z\lt 0.35 redshift range for stellar masses in the range 9.2\lt {log}(M/{M}⊙ )\lt 10.6. The [O iii]/Hβ and [O iii]/[O ii] ratios continue to rise across the full 0.2\lt z\lt 0.6 redshift range for stellar masses in the range 9.8\lt {log}(M/{M}⊙ )\lt 10.0. We conclusively rule out active galactic nucleus contamination, a changing ISM pressure, and a change in the hardness of the EUV radiation field as the cause of the change in the line ratios in the range 0.2\lt z\lt 0.35. We find that the ionization parameter rises significantly with redshift (by 0.1-0.25 dex depending on the stellar mass of the sample). We show that the ionization parameter is strongly correlated with the fraction of young-to-old stars, as traced by the Hβ equivalent width. We discuss the implications of this result on higher redshift studies, and we consider the implications on the use of standard optical metallicity diagnostics at high redshift.
