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 δ¹³C and δ¹⁵N values among locations, seasons, and forages are poorly described for arctic herbivores especially migratory species such as caribou (Rangifer tarandus).

Methods

We measured the δ¹³C and δ¹⁵N 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 δ¹³C 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 δ¹⁵N was greater than that for δ¹³C (13.29 vs 5.60 ‰). Differences in values for δ¹³C between graminoids (Eriophorum and Carex spp.) and shrubs (Betula and Salix spp.) were small but δ¹⁵N 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 ¹⁵N when the digestibility of N was less than 0.67.

Conclusions

Although δ¹⁵N 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 ¹⁵N to caribou and other herbivores that select highly digestible items (e.g. forbs) as well as heavily defended plants (e.g. woody browse).