Ecosystem engineering by a tropical tadpole: Density-dependent effects on habitat structure and larval growth rates

Tadpoles of the web-footed frog, Rana palmipes, are epibenthic consumers th at are widely distributed in the Neotropics. Rana tadpoles feed on algae an d sediments and potentially act as ecosystem engineers by modifying habitat structure via their foraging activities. We conducted two experiments in t he Andean piedmont of Venezuela to examine the following questions: (1) Can Rana tadpoles influence benthic sediment distributions? (2) Are growth rat es of sediment-feeding tadpoles density dependent? and (3) Are sediments vi able nutritional sources for tadpoles? A field enclosure experiment was con ducted in which sediment accrual and daily growth rates were compared among four tadpole density treatments ranging from 1 to 10 individuals per m(2). Tadpoles had highly significant effects on stream sediment accrual that we re inversely related to tadpole density; thus, benthic sediments rapidly ac cumulated when tadpole density was low (one tadpole per m(2)), whereas subs trata were thoroughly cleared of sediments when tadpole densities were high (10 tadpoles per m(2)). Furthermore, over the course of the experiment, da ily 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 high-density treatments. A second experim ent was conducted in wading pools to assess the importance of benthic strea m sediments as a nutritional source for tadpoles. Tadpole daily growth and development were measured in pools in which diets were supplemented dth str eam sediments and compared to tadpoles from pools with no sediment addition . Tadpoles displayed net positive growth and significantly higher developme nt rates when stream sediments were added to wading pools. In contrast, tad poles lost weight in the absence of sediment supplements. Our results sugge st that tadpoles can act as ecosystem engineers by reducing sediment accrua l rates; however, low availability of sediments can negatively feed back on tadpoles by reducing their daily growth rates.