Are you what you eat? Effects of trophic discrimination factors on estimates of food assimilation and trophic position with a new estimation method |
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| Institution: | 1. Marine Science Institute, The University of Texas at Austin, 750 Channel View Drive, Port Aransas, TX 78373, USA;2. Department of Marine Biology, Texas A&M University, 1001 Texas Clipper Road, Galveston, TX 77553, USA;3. Department of Marine Science, Texas A&M University, 1001 Texas Clipper Road, Galveston, TX 77553, USA;4. Department of Ocean Sciences, Memorial University of Newfoundland, St. John''s, NL A1C 5S7, Canada;5. Departamento de Oceanografía Biológica, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), 3918 Carretera Tijuana — Ensenada, Ensenada, Baja California 22860, Mexico;6. Department of Life Sciences, Texas A&M University, 6300 Ocean Drive, Corpus Christi, TX 78412, USA;1. Laboratório de Icteiologia, Instituto de Oceanografia, Universidade Federal do Rio Grande (FURG), Rio Grande/RS 96203-900, Brazil;2. School of Environment and Sustainability and Toxicology Center, University of Saskatchewan, Saskatoon/SK S7N 5B3, Canada;3. Laboratório de Piscicultura Estuarina e Marinha, Instituto de Oceanografia, Universidade Federal do Rio Grande (FURG), Rio Grande/RS 96203-900, Brazil;4. Laboratório de Nutrição e Alimentação de Organismos Aquáticos, Instituto de Oceanografia, Universidade Federal do Rio Grande (FURG), Rio Grande/RS 96203-900, Brazil;1. Department of Ecology and Conservation Biology, Texas A&M University, College Station, TX 77843-2258, USA;2. Grupo de Ecologia Aquática, Universidade Federal do Pará, Belém, PA, Brazil;3. River Studies, Inland Fisheries, Texas Parks and Wildlife Department, San Marcos, TX 78667, USA;4. U.S. Geological Survey, South Carolina Cooperative Fish and Wildlife Research Unit and Department of Forestry and Environmental Conservation, Clemson University, Clemson, SC 29634, USA;5. Núcleo de Ecologia Aquática e Pesca da Amazônia, Federal University of Pará, Belém, Pará, Brazil;6. Grupo de Investigación en Zoologá, Universidad del Tolima, Tolima, Colombia |
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| Abstract: | A key factor for estimates of assimilation of resources and trophic position based on stable isotope data is the trophic discrimination factor (TDF). TDFs are assumed based on literature reviews, but may vary depending on a variety of factors, including the type of diet. We analyzed effects of alternative TDFs on estimates of assimilated resources and trophic positions for an omnivorous fish, Jenynsia multidentata, that reveals dietary variation among locations across a salinity gradient of a coastal lagoon in southern Brazil. We also compared estimates of foods ingested vs. foods assimilated. Food assimilation was estimated using carbon (δ13C) and nitrogen (δ15N) stable isotope ratios of food sources and consumer muscle tissue and an isotopic mixing model (SIAR); consumer trophic position (TP) was estimated from consumer and production source δ15N values. Diet was estimated using an index of relative importance based on frequency of occurrence and volumetric and numeric proportions of food items from stomach contents. The effect of variation in TDF on food assimilation and TP was tested using three alternative TDFs reported in review papers. We then created a new method that used food source-specific TDFs (reported separately for herbivores and carnivores) weighted in proportion to estimated assimilation of resources according to mixing model estimates to estimate TP (hereafter TPWAR). We found that plant material was not assimilated in a proportion similar to its importance in the diet of fish at a freshwater site, and the new method yielded best assimilation estimates. Animal material made greatest contributions to fish biomass irrespective of TDFs used in the mixing model. The new method produced TP estimates consistent with differences in estimated food assimilation along the salinity gradient. Our findings support the idea that food source-specific TDFs should be used in trophic studies of omnivores, since the method improved our ability to estimate trophic position and resource assimilation, two important ecological indicators. |
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| Keywords: | Carnivory Diet shift Herbivory Isotopic fractionation Omnivory Trophic enrichment |
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