Originally Posted by Dogbertday
does it simply imply a different mode if you start a counterpoint line in the bass on the 5th scale degree (or anything except the unison/octave)?
Yes, it would imply a different mode or key - you don't want to do that. Always the tonic in the bass if you are supplying the lower line.
NB: Species counterpoint is often studied in the modal context of the Renaissance but it needn't be. One of the main reasons to study it is that it's an important foundational element of music composed in the Baroque through Romantic eras.