SEASONAL VARIATION IN TERRESTRIAL INSECT SUBSIDIES TO TROPICAL STREAMS AND IMPLICATIONS FOR THE DIET OF RIVULUS HARTII

Authors

David C. Owens, University of Nebraska at LincolnFollow

Date of this Version

Spring 5-2010

Document Type

Article

Comments

A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Natural Resources Sciences, Under the Supervision of Professor Steven A. Thomas. Lincoln, Nebraska: May, 2010
Copyright (c) 2010 David C. Owens

Abstract

Terrestrial invertebrates subsidize fish diets in lotic ecosystems. Seasonality strongly influences terrestrial invertebrate abundance in temperate regions and alters their delivery to streams. Seasonal changes in the tropics are characterized by distinct wet and dry periods, with marked variation in invertebrate abundance. However, little is known about how these seasonal changes affect invertebrate subsidies and their ecological consequences for tropical streams. We measured the effect of rainfall and canopy density on terrestrial invertebrate falling input, as well as seasonal variation in falling input, benthic and drifting invertebrate, and Rivulus hartii (Hart’s Rivulus) diet composition during both the wet and dry seasons at three stream sites in Trinidad. Rates of input of terrestrial invertebrates showed seasonal trends in biomass and abundance. Rainfall magnitude and canopy density were directly correlated with falling input. The delivery of terrestrial invertebrates increased from an average of 52 mg m^-2 day^-1 to 72 mg m^-2 day^-1 from wet to dry season. Conversely, average benthic invertebrate abundance and biomass decreased from 382 mg m^-2 in the dry season to 130 mg m^-2 in the wet season, presumably due to displacement and mortality resulting from severe flow conditions. A 75% increase in drifting invertebrate biomass was driven by a terrestrial invertebrate biomass that more than doubled during the wet season. Prey selectivity in Rivulus diets mirrored this seasonal variation in prey invertebrate availability, as percent composition of terrestrial invertebrate volume in Rivulus guts also doubled during the wet season. We conclude that terrestrial invertebrates are a substantial energetic subsidy for tropical river ecosystems, and the spatial and temporal variation in delivering these resources from wet to dry season have profound effects on consumer-resource dynamics.