Interference and Exploitation Competition Among Tadpoles of Rana Utricularia

The relative importance of exploitative and interference mechanisms for intraspecific competition among tadpoles of the Southern Leopard frog was evaluated by raising sibling tadpoles in 16 experimental environments designed to alter the costs and benefits of the two mechanisms. Growth of tadpoles in water conditioned by the 16 treatments was used to assay the level of interference chemical in each treatment. Body weights of tadpoles subjected to manipulations of density, food level, food dispersion and access to their feces increased with increasing food level and access to their feces. The largest tadpole in each replicate grew as though increased density also increased the food supply. The mean weight of tadpoles in each replicate indicated that there was less food at high densities. The largest tadpole in each replicate appears to outcompete the others for food. This food is fecal material and other particular material rather than the solid food source. Body weights of single tadpoles raised in water conditioned by their siblings increased as the food level in the conditioning treatment increased. Greater dispersion of the food source in the conditioning treatment decreased the growth of single tadpoles. Other factors affected the growth of the single tadpoles by altering the food level in the conditioning treatments. The decreased growth in conditioned water is interpreted as resulting from chemical interference competition. It may account for 5—6% of the variance in the density experiment. Interference mechanisms appear to supplant exploitative mechanisms as the relative food level decreases. I suggest that this is a general phenomenon and present evidence for its occurrence in other taxonomic groups.