Variation in δ15N and δ13C values of forages for Arctic caribou: Effects of location, phenology and simulated digestion

Lindsey L. Vansomeren; Perry S. Barboza; David D. Gustine; M. Syndonia Bret-Harte

doi:10.1002/rcm.7849

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Variation in δ15N and δ13C values of forages for Arctic caribou: Effects of location, phenology and simulated digestion

Lindsey L. Vansomeren·Perry S. Barboza·David D. Gustine·M. Syndonia Bret-Harte

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Abstract

Rationale The use of stable isotopes for dietary estimates of wildlife assumes that there are consistent differences in isotopic ratios among diet items, and that the differences in these ratios between the diet item and the animal tissues (i.e., fractionation) are predictable. However, variation in isotopic ratios and fractionation of δ 13 C and δ 15 N values among locations, seasons, and forages are poorly described for arctic herbivores especially migratory species such as caribou ( Rangifer tarandus ). Methods We measured the δ 13 C and δ 15 N values of seven species of forage growing along a 200-km transect through the range of the Central Arctic caribou herd on the North Slope of Alaska over 2 years. We compared forages available at the beginning (May; n = 175) and the end ( n = 157) of the growing season (September). Purified enzymes were used to measure N digestibility and to assess isotopic fractionation in response to nutrient digestibility during simulated digestion. Results Values for δ 13 C declined by 1.38 ‰ with increasing latitude across the transect, and increased by 0.44 ‰ from the beginning to the end of the season. The range of values for δ 15 N was greater than that for δ 13 C (13.29 vs 5.60 ‰). Differences in values for δ 13 C between graminoids ( Eriophorum and Carex spp.) and shrubs ( Betula and Salix spp.) were small but δ 15 N values distinguished graminoids (1.87 ± 1.02 ‰) from shrubs (−2.87 ± 2.93 ‰) consistently across season and latitude. However, undigested residues of forages were enriched in 15 N when the digestibility of N was less than 0.67. Conclusions Although δ 15 N values can distinguish plant groups in the diet of arctic herbivores, variation in the digestibility of dietary items may need to be considered in applying fractionation values for 15 N to caribou and other herbivores that select highly digestible items (e.g. forbs) as well as heavily defended plants (e.g. woody browse).

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Source-reported bounding extent: 68.2042121888185° to 71.48308562053703° latitude; -166.728515625° to -141.0205078125° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Lindsey L. Vansomeren, Perry S. Barboza, David D. Gustine, M. Syndonia Bret-Harte. 2017-04-04. Variation in δ15N and δ13C values of forages for Arctic caribou: Effects of location, phenology and simulated digestion. https://doi.org/10.1002/rcm.7849

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