  We present an analysis of the luminosities and equivalent widths of the 284 z
< 0.56 [O II]-emitting galaxies found in the 169 square arcmin pilot survey for
the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). By combining
emission-line fluxes obtained from the Mitchell spectrograph on the McDonald
2.7-m telescope with deep broadband photometry from archival data, we derive
each galaxy's de-reddened [O II] 3727 luminosity and calculate its total star
formation rate. We show that over the last ~5 Gyr of cosmic time there has been
substantial evolution in the [O II] emission-line luminosity function, with L*
decreasing by ~0.6 +/-0.2 dex in the observed function, and by ~0.9 +/-0.2 dex
in the de-reddened relation. Accompanying this decline is a significant shift
in the distribution of [O II] equivalent widths, with the fraction of high
equivalent-width emitters declining dramatically with time. Overall, the data
imply that the relative intensity of star formation within galaxies has
decreased over the past ~5 Gyr, and that the star formation rate density of the
universe has declined by a factor of ~2.5 between z ~ 0.5 and z ~ 0. These
observations represent the first [O II]-based star formation rate density
measurements in this redshift range, and foreshadow the advancements which will
be generated by the main HETDEX survey.
