Monday, August 2, 2010: 4:20 PM
320, David L Lawrence Convention Center
Steve A. Thomas1, Alexander Flecker2, Rana El Sabaawi3, Thomas N. Heatherly4, Michael C. Marshall5, Tyler Kohler6, Catherine M. Pringle5 and David N. Reznick7, (1)University of Nebraska-Lincoln, (2)Cornell University, (3)Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, (4)University of Nebraska, Lincoln, Lincoln, NE, (5)Odum School of Ecology, University of Georgia, Athens, GA, (6)Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO, (7)University of California Riverside, Riverside, CA
Background/Question/Methods: Controls on ecosystem structure and function are often divided into bottom-up and top-down forces. Bottom-up forces include a wide array of physical and chemical factors that influence basal trophic positions while top-down forces control community structure and function through biotic interactions. To explore the relative importance of these forces in structuring tropical stream ecosystems, we conducted whole stream
15N tracer experiments in which light and species composition were serially manipulated. This research was conducted in 2 streams in Trinindad’s northern mountain range and is part of a broader study that examines feedbacks between evolution and ecological structure and function. We conducted 10 day
15N-NH
4 tracer experiments in two streams (UPL and LOL) during the dry season in 3 consecutive years (March, 2007-2009). In June 2007, the middle and lower tier of the forest canopy was removed from 1 stream (UPL) doubling incident light at the stream surface. Following the tracer additions in 2008, guppies (
Poecilia reticulata) from high predation environments were introduced each stream reach. Spatially explicit sampling of dissolved DIN, epilithon, benthic organic matter pools, invertebrates and fish were multiple times during and following the tracer addition.
Results/Conclusions:
Nitrogen uptake and fate were very similar between streams in 2007 prior to any experimental manipulation. Ammonium uptake lengths ranged from 35 - 50 m and were slightly longer in UPL. Following the light manipulation, the relationship between these streams reversed such that the light manipulated stream had a shorter uptake length (76m vs. 109m) and a higher uptake rate (1.2 vs. 0.8 mg N m-2 hr-1) than those observed in the low light stream. Significantly more tracer was routed into epilithon and grazing invertebrates following the light manipulation. By the 2009 tracer addition, guppy populations had become established and diverged between the light treatments. Populations within the high light stream were approximately twice those observed in the low light stream (~1700 vs. 800 individuals). Preliminary modeling of our results indicates that both light and guppy activity influence N routing with these foodwebs. Future experiments are scheduled to examine how evolution from high predation to low predation phenotypes influences ecosystem structure and function and how light availability influences that evolutionary change.
