Host‐Parasite Interaction as a Potential Population‐Regulating Mechanism

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Abstract
Candida humicola, a parasitic yeast occuring in the gut of Green frog (Rana clamitans) tadpoles, enhanced the growth of tadpoles of this species under certain conditions. Low concentrations of C. humicola (1—10 cells per millilitre) increased the growth rate of tadpoles relative to siblings raised under identical conditions but without C. hunicola cells. Furthermore, the impact of a given concentration of C. humicola cells varied inversely with the size of the tadpole. Candida humicola appears to act as a mutualistic symbiont of the larger tadpoles and as a parasite of the small ones. Because larger tadpoles are more likely than their smaller siblings to metamorphose and reproduce, the mutualism benefits those individuals which are most likely to contribute to the continuation of the Green frog population. For this reason, resistance to the parasitism is not likely to develop. The parasite may act to regulate the adult Green frog population size by altering the probability of metamorphosis of individual tadpoles.