Top-down effects of a terrestrial frog on forest nutrient dynamics

Abstract.

Many studies have found top-down effects of predators on prey, but few studies have linked top-down effects of vertebrate predators to nutrient cycling rates in terrestrial systems. In this study, large and significant effects of a terrestrial frog, Eleutherodactylus coqui (coquí), were recorded on nutrient concentrations and fluxes in a subtropical wet forest. In a manipulative experiment, coquís at natural densities were contained in or excluded from 1 m³ enclosures for 4 months. Chemistry of leaf wash (throughfall), foliage, and decomposed leaf litter in the enclosures were measured as indicators of coquí effects on nutrient cycling. Coquí exclusion decreased elemental concentrations in leaf washes by 83% for dissolved organic C, 71% for NH₄ ⁺, 33% for NO₃ ^–, 60% for dissolved organic N, and between 60 and 100% for Ca, Fe, Mg, Mn, P, K, and Zn. Coquí exclusion had no effect on foliar chemistry of plants transplanted into the enclosures. However, coquí exclusion decreased nutrient availability in decomposing mixed leaf litter by 12% and 14% for K and P, respectively, and increased C:N ratios by 13%. Changes in nutrient concentrations that occurred with coquí exclusion appear to be due to concentrations of nutrients in coquí waste products and population turnover. The results supported our hypothesis that coquís have an observable effect on nutrient dynamics in this forest. We suggest that the primary mechanism through which they have this effect is through the conversion of insects into nutrient forms that are more readily available for microbes and plants. The potential for higher trophic level species to affect nutrient cycling through this mechanism should not be overlooked.