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I remember the first time I discovered the power of stable isotopes. It was by accident. It was thirty years ago, when I was a beginning graduate student along the south Texas coast. That summer I helped a visiting professor collect rodents (mice, rats, and ground squirrels) in a coastal sand dune community. Yes, I worked with the rodent traps, but I also got bored and wandered off during the hot afternoon hours, collecting plants and grasshoppers from the dunes. One evening later in that summer of 1976 we were at the mass spectrometer, watching the chart recorder display the isotope results for our collections. It was fascinating. One sample was very enriched in the heavy carbon stable isotope, ^13C, and the next sample was depleted in ^13C. A great divide was evident in the isotopes of the sand dune community.We watched the chart recorder for hours as sample after sample showed the basic ^13C distinction, or variations on this ^13C isotope theme.

This chapter gives an introduction to isotope notation, calculations, and measurement.The beginner should probably read only the first section, 2.1, then skip on to Chapter 3 which reviews ecological applications of these isotope tracers. Reading Section 2.1 should allow you to understand the rest of the book, and reading the remaining sections 2.2 to 2.4 of this chapter should deepen your understanding as you read the wider isotope literature. There are also three detailed technical supplements for this chapter on the accompanying CD.

Isotopes are forms of an element that differ in the number of neutrons. Isotopes function as natural dyes or colors, generally tracking the circulation of elements. Isotopes trace ecological connections at many levels, from individual microbes to whole landscapes. Isotope colors mix when source materials combine, and in a cyclic process that ecologists can appreciate, the process of isotope fractionation takes the mixed material and regenerates the sources by splitting or fractionating the mixtures. Elements and their isotopes circulate in the biosphere at large, but also in all smaller ecological plant, animal, or soil systems. Chapter 3 reviews this circulation for each of the HCNOS elements, then gives four short reviews that may stimulate you to think about how you could use isotopes in your own ecological research.

This chapter introduces a modeling approach to understand the interplay between isotope mixing and fractionation in the biosphere. The approach is named Isotope Chi (like Tai Chi) to represent the power in this new approach. An initial example deals with imaginary chocolate isotopes, to stimulate your appetite and to help you get in tune with how fractionation and mixing work together in isotope cycling. The next four sections use a more realistic example of photosynthesis and respiration in the sea to fully introduce the I Chi modeling. A final section switches to a terrestrial example with cows. The different examples develop the appreciation that isotope distributions in natural systems reflect the net balance between fractionation and mixing.

As elements circulate in the biosphere, mixtures arise when two or more sources contribute materials. Isotopes are excellent tracers for mixing processes and indicate which sources dominate the mixtures. This chapter considers isotope mixing in ecological systems.

Ecologists can add isotopes to field and laboratory experiments. This chapter considers what is good about these addition experiments, and what might go wrong. For those who actually perform one of these addition experiments, two technical supplements are included on the accompanying CD to help you in your detailed planning.

This chapter starts with atoms and ends with the whole biosphere, showing how isotope fractionation works in theory and practice. Fractionation starts with atomic-level considerations, but usually starts to make sense in larger ecological contexts only when you grasp the idea of mass balance. Mass balance is an accounting idea that masses and isotopes entering a reaction must equal masses and isotopes exiting the same reaction. This sounds simple, but it forces us to budget several things at once, masses and isotopes, in a kind of multitasking consciousness. This demands a juggling skill that takes practice to learn, so be patient and take time to practice, especially using workbook 7.2 of in the Chapter 7 folder on the accompanying CD. Sections 7.1, 7.2, 7.6, 7.7, and 7.10 contain the more theoretical sections, and may need rereading several times for full comprehension. Sections 7.3 to 7.5 and 7.8 and 7.9 provide examples.Technical Supplements 7A and 7B on the accompanying CD are reference sections for advanced and interested readers.

This chapter briefly considers the future of stable isotope ecology. The future may hold advances in technology, a more routine use of multiple chemical markers in ecological investigations, and more of that essential scientific ingredient, imagination.