Few things here:
1. Belly fat is not necessarily the 1st place fat leaves your body as someone said above. Maybe it was for you, but that may not be the case for the next person. The order that we store and lose fat is predetermined completely by our genetics.
2. I agree that cardio should be in the form of an activity that you enjoy. I also believe that you should switch things up as you get used to them. Maybe you jog this month, and next month you cycle, and next month you use the elliptical, and next month you swim, etc. etc. What type of biking are you talking about. I find that most people who ride a real bike on the trails or roadways normally don't hit that aerobic level. There is a lot of coasting and breaks involved. For instance, I went on a 20 mile bike ride last weekend. However, I was with my little cousin and we kept a slow to moderate pace, coasted some of the time which conflicts with the idea of "aerobic" exercise which needs to be consistent, sustained movement with a large muscle group. If you are talking about riding a stationary bike, then by all means, that is a great method of aerobics.
3. Somebody said they horseback ride for a cardio program. I don't see how this can possibly keep your heart rate up and oxygen uptake constant over long periods of time, i.e 20-60 minutes. I have never ridden a horse, but I doubt that it can be considered cardiovascular.
4. To the original poster, how often are you doing crunches. I hope you are not doing crunches in hopes of "toning" your belly. As someone said before, you can't pick a part of your body and focus on it by doing exercises to target the fat on that area. Human physiology does not work like that. The only way to successfully reduce fat permanently is to use aerobic activity to burn calories and fat, as well as proper, clean eating habits. You can do crunhces until you are blue in the face, but if you are eating like crap, and not doing cardiovascular training, your success is going to elude you.