What could be controlling the patchiness of tadpole density in the wetland?  There are areas, such as those pictured below, where tadpole congregate in large numbers and appear to move about as a school.  In a couple of cases, I notice one or two dead tadpoles being cannibalized in the mass, but here there was no obvious food source that had attracted them.   In the surrounding area (maybe within a 5 m radius), there were very few tadpoles; 2-3 per sqaure meter.

On a related note, how can I measure tadpole density?