You are what you eat

Going through my literature I came across an interesting but slightly macabre paper with the innocent title “Seasonal water availability predicts the relative abundance of C3 and C4 grasses in Australia” (by Murphy and Bowman, 2007).  In short, the paper discusses the species distribution and optimal growing conditions of C3/C4 grasses. Differences in the photosynthetic pathway of C4 grasses allows them to function at temperatures exceeding those optimal for C3 grasses. However, not only temperature limits C3 or C4 grass growth but other factors such as the timing of precipitation influences the growth and relative abundance of either group at a given location. Disentangling this relative abundance is required to eliminate potential biases when estimating carbon uptake by the terrestrial biosphere.

The authors tackled the question of relative C3/C4 abundance and diversity across a large geographical extent, by using a common technique in ecology namely transects of quadrats scattered across the Australian continent. Sadly, this is time intensive to establish and measure these quadrats. However, a macabre twist to this story helps to scale these measurements. Dead kangaroos to be precise.

Kangaroos eat a ton of grass. These grasses, depending on their photosynthetic pathway (C3 or C4), will show differences in their 13C stable isotope composition.

Isotopes are atoms of the same element but with a different atomic mass, due to additional neutrons at it's nucleus. When these atoms do not radioactively decay we call them stable isotopes. Carbon (C) in it's most abundant form has 6 neutrons and 6 protons at it's nucleus, hence called carbon-12 (12C).  Several isotopic forms of carbon exist with carbon-13 (13C) having one and carbon-14 (14C) having two extra neutrons in their nuclei. Of these 14C is unstable, decaying into more stable components. The 13C isotope is stable and will therefore persist.

And, since you are what you eat (on a molecular level - and organismic level as well I guess) these differences will be reflected in the tissues of the kangaroo. Assuming that kangaroos don’t prefer a particular kind of grass. Both authors used collagen tissue samples of 779 road-killed individuals to calculate the relative abundance of C3/C4 grasses around the road-kill locations. Combining these measurements with climate data, it allowed them to scale the relative abundance of grasses across the whole of Australia. Hence, they potentially removed some bias in estimates of terrestrial biosphere carbon uptake by these grasslands.

Although my main interest in this paper was the relation between climate and abundance of C3/C4 grasses I think the paper teaches some valuable lesson in science outside it’s original scope, mainly “the proxy measurement “ (and a rather macabre one).  Furthermore, it can be used as a fun example to show people the unexpected part of science. I assume few Australians would have imagined that a lot of road-killed kangaroos could tell a story about the kind of grasses it once ate and where they grow!

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