Abstract

Spadefoot Toads, Spea bombifrons and Spea multiplicata, are often the most abundant amphibian species in playa wetlands in the semi-arid Southern Great Plains. Tadpoles of these species are morphologically indistinguishable, but both can develop omnivore and carnivore morphotypes. We tested the influence of species and morphotype on body mass of premetamorphic Spea tadpoles and the influence of land use (cropland vs. native grassland) and desiccation stress (water depth and daily rate of water-depth loss) on body mass of S. bombifrons carnivores and omnivores and on body mass of S. multiplicata and S. bombifrons omnivores. Spea bombifrons carnivores were larger than omnivores only in relatively deep playas. A decrease in water depth resulted in less body mass for carnivores, but omnivore body mass did not change. Spea bombifrons omnivores had larger body mass than did S. multiplicata omnivores at low and average playa depth. Spea bombifrons omnivores did not alter body size, whereas S. multiplicata omnivores were smaller when depth decreased. Generally, in average and deep playas, tadpoles were larger in cropland than grassland playas, but in shallow waters, cropland and grassland tadpoles had similar, smaller body mass. Also, cropland tadpoles had reduced body size, whereas grassland tadpoles had similar body mass when depth decreased. Our study documents body size divergence within the same morph in the two sympatric Spea species and species divergence within the omnivore morph depending on desiccation stress. Future studies should investigate whether these species may be mediating competition through both morph development and body size.