The IDers believe in Platonic Forms (often without realizing it), and view species as Forms. They believe natural processes like evolution cannot produce new Platonic Forms, since the Forms preexist and reside in some other timeless dimension our minds somehow contact. Space aliens (an alternative proposed by the IDers in my earlier post Did the First Giraffe have a Navel?) are also unlikely to be able to add new Platonic Forms. The only viable alternative is a deity, the same one who set the mathematical and physical constants (like Pi, and the speed of light C) before the universe was created. The species Forms would have also all been predefined before the universe started.
[*] Note this summary also slides towards a common trap, not unique to IDers. My linked post also asserts species don't really exist, but are an arbitrary man-made construct. So in an important sense evolution doesn't produce species, only individuals. The IDers are asserting nature is bounded by preexisting species Forms, while I say nature in generating new individuals does not recognize or respect any such boundaries.