Variation in stable isotope ratios of carbon and nitrogen in Hokkaido sika deer Cervus nippon during 1990–2000: possible causes and implications for management

Abstract

Sika deer Cervus nippon populations in eastern Hokkaido, Japan, increased rapidly during 1990–1998. This increase appeared to have halted in 1999–2000, probably due to increased hunting and nuisance control. The period of rapid increase was associated with a disproportionately rapid increase in compensation paid for deer damage to crops. We studied changes in diet during 1990–2000, as reflected by stable isotope ratios of C and N in tooth collagen. We hypothesised that isotope ratios would demonstrate dietary shifts related to population levels and/or time, and that shifts in isotope ratios would be consistent with increasing individual consumption of pasture grasses at higher population levels. δ13C isotope ratios of tooth collagen in 3-year-old sika indicated a diet dominated by C3 plants throughout the period, and that forage species other than pasture grasses and dwarf bamboo Sasa nipponica (the main crop and woodland understory plants, respectively) were important elements. There was a significant decline in the δ13C isotope ratio during 1990–2000 in both males and females. δ15N values showed no trend with time for males, but increased over time in females. Indices of population (Sightings per Unit Effort, SPUE) were negatively correlated with female δ13C, and positively correlated with female δ15N, values, indicating a shift in diet over the period, especially among females. This shift may be related to population and/or offtake levels, in particular the rapid increase in female offtake for nuisance control and hunting during the period. The data are consistent with a relative increase in pasture grass consumption per individual at higher population levels; however, other explanations of the data are equally plausible. Possible dietary changes, and other factors, influencing the observed shift in isotope ratios are discussed. Although statistically significant, the magnitude of dietary shifts nevertheless appeared small, and did not provide evidence which would justify modifying the current policy, of limiting crop damage through managed population reduction to about 25% of peak levels.