Clever. Interesting to not top speed is not best (those of us who don't venture out into the real world often don't know these things off the tops of our heads, though). The fighting while switching is cute.
Great idea, especially to promote the van. Has a lot of potential, but you definitely need more room between the bike and the van. Not everyone has super reactions times. That said, great idea with the groupies.
I like the game. A nice difference to other "food chain" games. You may want some kind of meter that tells you how close you are to having other organisms turn blue, or how close you are to the end of a level.
Reaction time is a bit slow. Cheesed me off that a monster walked up and started attacking me when I was doing a treasure maze. You may want to look into that.