EFFECTS OF CHANGING DENSITY AND FOOD LEVEL ON METAMORPHOSIS OF A DESERT AMPHIBIAN, SCAPHIOPUS-COUCHII

Amphibians that breed in temporary ponds provide a good opportunity to study the ecological and evolutionary consequences of environmental variability. Ephemeral aquatic habitats provide larval amphibians a transient and highly variable opportunity for growth. In the desert ponds used by Couch's spadefoot toad (Scaphiopus couchii), tadpole density varies considerably among ponds and often increases within a pond as it dries. Models of optimal size and timing of metamorphosis predict that, relative to constant high resource environments, metamorphosis should occur at a smaller size in constant low resource environments, and smaller and earlier in declining resource environments, assuming all else is equal. Considerable evidence supports the first prediction, but few studies have examined the effect of decreasing resource availability. I conducted an experiment to examine the effects of increasing density and decreasing food level on metamorphosis in Couch's spadefoot toads. Tadpoles at constant high per capita food metamorphosed at the largest sizes. At low or decreasing per capita food, tadpoles metamorphosed at a uniformly small size, but varied in the time required to reach that size. Tadpoles from both increasing density and decreasing food treatments metamorphosed earlier than tadpoles from constant density, constant food treatments. These results support the idea that tadpoles can respond adaptively to resource variation.