another science phd student here, with lots of teaching experience. The program I am in actually required a minimum of one semester teaching, but most of the students in the programme were supported by teaching assistantships and I taught every semester. This was at a big R1 university but although research comes first, most profs in the programme understand that teaching is an important part of the job. I guess this is the exception in science. Maybe a reflection of the low level of grants available in ecology as opposed to other science fields. Must admit I really enjoyed teaching as well as research and would like a job that involves both. Unfortunately those seem to be hard to find now that I'm graduating and looking for work.