  Although the stellar initial mass function (IMF) has only been directly
determined in star clusters it has been manifoldly applied on galaxy-wide
scales. But taking the clustered nature of star formation into account the
galaxy-wide IMF is constructed by adding all IMFs of all young star clusters
leading to an integrated galactic initial mass function (IGIMF). The IGIMF is
top-light compared to the canonical IMF in star clusters and steepens with
decreasing total star formation rate (SFR). This discrepancy is marginal for
large disk galaxies but becomes significant for SMC-type galaxies and less
massive ones. We here construct IGIMF-based relations between the total FUV and
NUV luminosities of galaxies and the underlying SFR. We make the prediction
that the Halpha luminosity of star forming dwarf galaxies decreases faster with
decreasing SFR than the UV luminosity. This turn-down of the Halpha-UV flux
ratio should be evident below total SFRs of 10^-2 M_sun/yr.
