Q:"Can straight ironing your hair all the time cause spilt ends?"
A: Yes, continual use of any heat on your hair can cause dry or split ends. Remember, the ends of your hair are "furthest from the source (follicles)" and therefore are more prone to being dried out already. Ad heat that robs the hair of moisture and straightening tension plus heat that depletes the hair chaft and the long short is "yep, that can totally cause split ends". The only advice I can offer is a. stop straightening your hair (you knew I was gonna say that) or b. stop straightening so often and c. make sure to moisturize your ends regularly and d. make sure to trim your ends regularly or e. try alternating your straight styles with a protective style like braids or buns (anything where the ends are resting, out of the elements and not being manipulated) and also you could f. lower the heat setting you use.
Hope that helps and Happy Nappying!