[R-sig-eco] Questions about stable isotope analysis in simmr

[Brian A. Gill]

Hi R-sig-eco.

I would like to use the simmr package to estimate percent contributions of
C4 v. C3 plants in herbivore diets based on fecal δ13C measurements.

I had a few questions, I was hoping someone might be able to help with:

1) Is it correct that observed fecal δ13C values do not need to be adjusted
to diet δ13C prior to running simmr if using a correction factor in the
model?

To me it seems like adjusting and then running the model with a correction
factor would be doing it twice and incorrect, but I'd like to be sure.

2) Based on diet–feces fractionations from a feeding study (Table 2 in
https://doi.org/10.1139/z03-066), we would like to use a single value of
-0.77 (standard deviation: 0.34) for both C3 and C4 sources. For diet–feces
fractionation the sign should be input into the model as negative, correct?
To implement a single value in simmr, can I simply input the same value for
both sources?

Thanks for any help!


Brian

-- 
Brian A. Gill, Ph.D. (He/Him)

Postdoctoral Research Associate
School of Natural Resources and the Environment
University of Arizona, Tucson, Arizona, USA
www.BrianGillPhD.com

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