Tropic transfer studies on wild radish (Raphanus raphanistrum) and ragweed (Ambrosia artemisiifolia) both indicated variations in the time of P32 uptake by herbivore species. This variation resulted from rapid temporal changes in plant organ availability with subsequent changes in consumer feeding patterns. Specific trophic positions of major herbivores were determined only by correlating phenological changes in plant organ availability with the uptake curves, abundances and feeding adaptations of consumer species. It was concluded that P32 uptake curves cannot be used alone in determining trophic positions unless a long—term stability in producer—consumer relationships is demonstrated.