Dwarf galaxies tend to have a large spread in metal abundances and overall lower metallicites than large Spiral or Elliptical ones. Here we propose that if all stars are formed from an invariant canonical IMF within each star cluster which are distributed according to an embedded cluster mass function (ECMF), then the integrated stellar IMF over the whole galaxy (the integrated galaxial IMF, IGIMF) must be steeper than the canonical IMFs within the clusters, and additionally they must depended on the stellar mass of the galaxy. In particular, dwarf galaxies and low-surface brightness galaxies show steeper galaxial IMFs which vary strongly with the star-formation rate. The resulting number of Supernovae II per low-mass star, and the chemical enrichment history of these galaxies therefore vary substantially depending on the galaxy assembly history.