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Seasonal Variation in Terrestrial Insect Subsidies to Tropical Streams and Implications for the Diet of Rivulus Hartii

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Seasonal Variation in Terrestrial Insect Subsidies to Tropical Streams and Implications for the Diet of Rivulus Hartii University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Dissertations & Theses in Natural Resources Natural Resources, School of Spring 5-2010 SEASONAL VARIATION IN TERRESTRIAL INSECT SUBSIDIES TO TROPICAL STREAMS AND IMPLICATIONS FOR THE DIET OF RIVULUS HARTII David C. Owens University of Nebraska at Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/natresdiss Part of the Natural Resources and Conservation Commons, and the Terrestrial and Aquatic Ecology Commons Owens, David C., "SEASONAL VARIATION IN TERRESTRIAL INSECT SUBSIDIES TO TROPICAL STREAMS AND IMPLICATIONS FOR THE DIET OF RIVULUS HARTII" (2010). Dissertations & Theses in Natural Resources. 8. https://digitalcommons.unl.edu/natresdiss/8 This Article is brought to you for free and open access by the Natural Resources, School of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Dissertations & Theses in Natural Resources by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. SEASONAL VARIATION IN TERRESTRIAL INSECT SUBSIDIES TO TROPICAL STREAMS AND IMPLICATIONS FOR THE DIET OF RIVULUS HARTII By David C. Owens 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 SEASONAL VARIATION IN TERRESTRIAL INSECT SUBSIDIES TO TROPICAL STREAMS AND IMPLICATIONS FOR THE DIET OF RIVULUS HARTII David Christopher Owens, M.S. University of Nebraska, 2010 Advisor: Steven A. Thomas 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. i ACKNOWLEDGEMENTS I would like to thank Carl Fitzjames for help in the field as well as spiritual wisdom passed down, as well as the Fitzjames family, Kelly, Kahlil, and Kamala. Natasha, Albert, Andrew, Murdoch, Trey, for providing love and fresh produce in the bush. And wild meats. Friends and family in Tennessee. Tom Weathrington Fairchild Heatherly, II, Tyler Kohler, and Chris Pracheil for being brothers in a cold, barren arctic Midwestern climate. Steve Thomas for insightful advising, Kent Eskridge and Mark Pegg for acting as wise, nurturing committee members. Benda Pracheil, John Quinn, Alex Flecker, Eugenia Zandonà, Mike Marshall, David Reznick, Brad Lampier, Doug Fraser, Jim Gilliam, Catherine Pringle, and Pat Shea for their technical assistance and intellectual insight. Joey Churill for help with processing invertebrates. I also thank the Ronnie Hernandez and the Asa Wright Nature Center for providing housing and logistical support, Mary Alkins-Koo and the University of the West Indies for logistical support. ii TABLE OF CONTENTS ACKNOWLEDGEMENTS……………………………………………………………..i TABLE OF CONTENTS………………………………..………………………………ii LISTOF TABLES……………………………………………..…………………………v LISTOF FIGURES…………………………………………..…………………………vii INTRODUCTION…………………………..…………………………………………...1 METHODS……………………………………………………...………………………..5 Study Area………………………………………………………………………...5 Invertebrate Sampling…………………………………………………………......7 Falling Terrestrial Input…………………………………………………..9 Benthic Invertebrates………………………………………………….…..8 Drifting Invertebrates……………………………………………………..8 Precipitation……………………………………………………………………….9 Canopy………………………………………………………………………….…9 Rivulus Diet Analysis…………………………………………………………....10 Sample Analysis………………………………………………………………….11 RESULTS…………………………………………...…………………………………..13 Environmental Conditions……………………………………………………….13 Falling Input……………………………………………………………..……….13 Season……..………….……………………………………………………13 Canopy.…………………………………………………………………..14 Precipitation…………………………………………………….……….14 Precipitation*Canopy………………………………………....................15 iii Precipitation*Season.................................................................................15 Canopy*Season..........................................................................................15 Rain*Canopy*Season................................................................................15 Benthic Invertebrates……………………………………………………….……16 Season........................................................................................................16 Stream........................................................................................................16 Habitat.......................................................................................................16 Stream*Habitat..........................................................................................17 Habitat*Season..........................................................................................17 Stream*Habitat*Season.............................................................................17 Drifting Invertebrates.............................................................................................17 Season……………………………………………………………………17 Stream……………………………………………………………………18 Stream*Season……………………………………………..…………….18 Rivulus hartii gut analysis………………………………………………………..18 Season……………………………………………………………………18 Presence/Absence………………………………………………..19 # Individuals in Gut………………………………………...……19 % Individuals in Gut…………………………………………..…19 Volume in Gut……………………………………………………19 % Volume in Gut……………………………………………...…19 Stream……………………………………………………………………19 iv Stream*Season……………………………………………………..................….19 DISCUSSION...................................................................................................................20 Falling Input…………………………………………………………………......21 Benthic Invertebrates…………………………………………………………….24 Drifting Invertebrates………………………………………………………...….24 Rivulus Diet Analysis………………………………………………………...…..26 Conclusion……………………………………………………………………….27 LITERATURE CITED...................................................................................................29 TABLES AND FIGURES...............................................................................................39 APPENDIX………………..…………………………………………………………….70 v LIST OF TABLES Table 1. Characteristics of the study sites………………..…………………………...….39 Table 2. Mean terrestrial invertebrate biomass (mg m-2 day-1) and abundance (# m-2 day-1) falling inputs for all streams……………………………………………………..40 Table 3. Analysis of covariance (ANCOVA) output for significant comparisons of terrestrial invertebrate biomass (mg m-2 day-1) and abundance (# m-2 day-1) inputs to the streams. Comparisons were made by season (Se), precipitation (R) and canopy density (C)………………………………………………………………41 Table 4. Dry vs. Mean seasonal benthic invertebrate biomass (mg m-2) and abundance (# m-2)……………………………………………………………………………….42 Table 5. Analysis of covariance (ANCOVA) output for significant comparisons of benthic aquatic invertebrate biomass (mg m-2 day-1) and abundance (# m-2 day-1) in the Miguel and Tapana streams. Comparisons were made by habitat (H), transect (T), stream (S), season (D)……...............................................................43 Table 6. Means and SE of invertebrate biomass (mg m-3 hr-1) and abundance (# m-3 hr-1) comprising drift in the Miguel, Tapana and Taylor streams……………………..44 Table 7. Analysis of covariance (ANCOVA) output for significant comparisons of drifting invertebrate biomass (mg m-3 hr-1) and abundance (# m-3 hr-1) inputs to the streams. Comparisons were made by stream, season, site within stream for the Taylor River………………..…………………………………………………….45 vi Table 8. Means and SE of invertebrate biomass (mg m-3 hr-1) and abundance (# m-3 hr-1) comprising drift in each the Miguel, Tapana and Taylor streams……………….46 Table 9. Seasonal means of invertebrates comprising Rivulus gut volume (mm2) and
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