Variation in δ N and δ C values of forages for Arctic caribou: effects of location, phenology and simulated digestion.

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 C and δ 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 C and δ 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 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 N was greater than that for δ C (13.29 vs 5.60 ‰). Differences in values for δ C between graminoids (Eriophorum and Carex spp.) and shrubs (Betula and Salix spp.) were small but δ 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 N when the digestibility of N was less than 0.67.

Conclusions: Although δ 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 N to caribou and other herbivores that select highly digestible items (e.g. forbs) as well as heavily defended plants (e.g. woody browse). Published in 2017. This article is a U.S. Government work and is in the public domain in the USA.