| That one simple phrase forms the basic tenet of a powerful application of stable isotopes that has become known as isotope ecology. In the beginnings of accurate stable isotope measurements (about 50 years ago), researchers (mostly geologists) were busy making measurements on every geologic material they could lay their hands on. They were laying the groundwork for our understanding of how the stable isotopes are distributed in nature. |
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As the literature developed and the measurements got better, distribution patterns began to emerge and point the way to further areas of study. In particular, several studies noted the large difference between the isotope ratio of carbon in the atmosphere (as carbon dioxide) and the ratios in plants who derive their carbon from photosynthetic fixation of atmospheric carbon dioxide. In addition, it appeared that there were at least two isotopically distinct types of plants and that their differences arose from the earliest steps of photosynthesis. |
| Studies of simple communities demonstrated that as carbon is transfered from one trophic level to the next (ie plant -> herbivore -> carnivore -> etc), the isotopic ratio of the consumer was by and large a weighted average of the ratios of its food sources. So that a grasshopper that was picky in its eating habits and stuck to one type of plant, ended up with an isotope ratio much like that plant. A consumer that ate equal amounts of two isotopically distinct plant types ended up with a ratio in halfway in between the two food sources. Hence, You are what you eat!, isotopically speaking. |
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This one effect makes stable isotopes a powerful tool for examining the structure and dynamics of ecosystems. Now you can see who's eating what (or whom) just by looking at the isotope ratio of the consumer's tissue. And it gives a long term weighted average of the diet - something you don't get from gut contents. But the story gets even more interesting when you look at nitrogen and sulfur and get complimentary information about the flow of organic matter through a salt marsh, or an alpine lake, or a polar bear. It's as though mother nature has already performed millions of tracer experiments and they are waiting for us to find them. |
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