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Grass, fire and kangaroos: can stable isotopes reveal Australia's past and future ecology?

Murphy, Brett P. (2006). Grass, fire and kangaroos: can stable isotopes reveal Australia's past and future ecology?. PhD Thesis, Charles Darwin University.

Document type: Thesis
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Author Murphy, Brett P.
Title Grass, fire and kangaroos: can stable isotopes reveal Australia's past and future ecology?
Institution Charles Darwin University
Publication Date 2006
Thesis Type PhD
Subjects 0599 - Other Environmental Sciences
0602 - Ecology
Abstract The stable isotopic analysis of ancient herbivore remains is emerging as an important tool for resolving poorly known aspects of Australia's ecological history, but for the approach to be successful, an understanding of the isotopic variation in modern herbivores, such as kangaroos, is required. I examined sources of variation in the isotopic composition of grass foliage (δ13C, δ15N) and kangaroo bone collagen (δ13C, δ15N) and tooth enamel (δ13C, δ18O) collected throughout Australia. The δ13C of foliage of both C3 and C4 grasses was related to water availability; in C3 grasses, the relationship was negative and in C4 grasses the relationship was positive. Despite this variation, the δ13C of kangaroo bone collagen and tooth enamel was very closely related to the relative abundance of C4 versus C3 grasses (C4 relative abundance), with ecological factors such as vegetation woodiness and the presence of a forest canopy having little discernible effect. Water availability explained a considerable proportion of the variation in the δ15N of both grass foliage and kangaroo bone collagen, and the slopes of these negative relationships were very similar, with a near-constant δ15N offset between grass foliage and bone collagen. This suggests that dietary δ15N is the main cause of the relationship between kangaroo bone collagen δ15N and water availability, with metabolic factors having little discernible effect. This finding suggests that the δ15N of kangaroos, and possibly other herbivores, closely reflects ecosystem δ15N. Consistent with previous studies, relative humidity explained a large proportion of the variation in enamel δ18O. We also found a previously unreported interaction between mean annual temperature and relative humidity, with the negative relationship between δ18O and relative humidity much steeper at low temperatures. While carbon isotopic analysis of kangaroo remains appears to be a powerful tool for reconstructing past C4 relative abundance, nitrogen and oxygen isotopic analysis appear to be less useful, primarily due to the complexity of the sources of δ15N and δ18O variation, and our poor mechanistic understanding of them.


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