I guess the non-invasive samples are feces. Starch (as well as BSA) reduces the quantity of bile acids and bile salts, that inhibits PCR. Sometimes it works, sometimes it does not. There are kits to specifically extract DNA from feces, that use to work well. Whatever you follow for DNA extraction, my advise is to do PCR (if you have to) with Hemo Klen Taq (New England Biolabs). I use Hemo Klen Taq when working on feces. This enzyme is not inhibet by bile, heglobin or heparine.