I use a lot of spring bulbs too, but have been a bit confused by the longevity of our species tulips.
Bed below is April 10, 2011, the snowdrops and earlier crocuses are done.
The red here is species tulips.
The pink is Chionodoxa 'Giant Pink'/'Pink Giant'.
I find that the species tulips don't last more than a couple of years.
Thinking it's either our summer watering of beds (we have a lot of perennials) or voles (meadow mice).
The latter can mine out edible bulbs.
Since the Chionodoxa do last and spread and I keep needing to replant, especially crocuses, I'm thinking it's voles.
However, I've never seen any evidence of them in that particular bed.
Note of caution re Chionodoxa: the blue one (Chionodoxa forbesii/lucilliae) is (to me) a terrible spreader.
It doesn't stay in clumps like the bigger pink one, but seeds all over the place.
The voles don't seem to eat it (or it tolerated our summer watering - that is, until I finally managed to eradicate it).