Ecosystem Engineering by a Tropical Tadpole: Density-Dependent Effects on Habitat Structure and Larval Growth Rates

Abstract

Tadpoles of the web-footed frog, Rana palmipes, are epibenthic consumers that are widely distributed in the Neotropics. Rana tadpoles feed on algae and sediments and potentially act as ecosystem engineers by modifying habitat structure via their foraging activities. We conducted two experiments in the Andean piedmont of Venezuela to examine the following questions: (1) Can Rana tadpoles influence benthic sediment distributions? (2) Are growth rates of sediment-feeding tadpoles density dependent? and (3) Are sediments viable nutritional sources for tadpoles? A field enclosure experiment was conducted in which sediment accrual and daily growth rates were compared among four tadpole density treatments ranging from 1 to 10 individuals per m2. Tadpoles had highly significant effects on stream sediment accrual that were inversely related to tadpole density; thus, benthic sediments rapidly accumulated when tadpole density was low (one tadpole per m2), whereas substrata were thoroughly cleared of sediments when tadpole densities were high (10 tadpoles per m2). Furthermore, over the course of the experiment, daily growth rates of tadpoles were strongly affected by tadpole density, and individuals from low-density treatments displayed greater than five times the daily growth of tadpoles from highdensity treatments. A second experiment was conducted in wading pools to assess the importance of benthic stream sediments as a nutritional source for tadpoles. Tadpole daily growth and development were measured in pools in which diets were supplemented with stream sediments and compared to tadpoles from pools with no sediment addition. Tadpoles displayed net positive growth and significantly higher development rates when stream sediments were added to wading pools. In contrast, tadpoles lost weight in the absence of sediment supplements. Our results suggest that tadpoles can act as ecosystem engineers by reducing sediment accrual rates;