Trophic discrimination among stable hydrogen isotopes : implications for stable isotope ecology

The hydrogen isotope composition of precipitation water varies with latitude and elevation. By comparing the isotopic composition of animal tissues to that of precipitation water, ecologists are able to estimate the location at which the tissue was grown. This method depends on the assumption that the isotopic composition of precipitation water is faithfully incorporated into animal tissues. To test this assumption, I watered four groups of cabbage plants with irrigation water treatments of varying hydrogen isotope compositions and raised caterpillars and moths on these plants. By analyzing the isotopic composition of moths, caterpillars and cabbages, I determined that the hydrogen isotope compositions of plant and animal tissues greatly differ from that of the water supporting the food web. Thus studies that trace animal movements using stable hydrogen isotopes are subject to large errors unless this difference is accounted for. I will discuss the implications that my results have for the future of the use of stable hydrogen isotopes in ecology.