Index of deer abundance based on tree browsing

Background/Question/Methods To avoid degradation of vegetation by overabundant deer, deer population controls should be implemented before population levels are high enough to cause clear damage to vegetation. To this end, identifying deer population levels in low-density areas is crucial. In Hokkaido, Japan, sightings per unit effort and spotlight surveys along 156 survey routes (~10 km each in length) are used as the primary indices of sika deer Cervus nippon yesoensis abundance. However, estimating deer abundance by direct observation when densities are low is rather difficult. Therefore, we examined alternative indices of sika deer abundance based on observations of plants. We established 200 to 400-m² study plots at 8 - 10 sites in each of four areas where deer abundance varied. Saplings of hardwood species taller than 50 cm and less than 1 cm in diameter at breast height, and trees with twigs at heights of less than 2 m (ca. the reach of sika deer) were identified. The occurrence of twigs browsed by deer was observed on saplings and trees. The density of saplings and the occurrence of browsed twigs on saplings and trees were compared to spotlight survey data collected near each plot.

Results/Conclusions Browsed twigs were found in areas where no deer had been observed in spotlight surveys. The percentage of browsed trees increased with the deer density recorded by spotlight survey, although the variance was large. The percentage of browsed trees was less than 20% in areas with less than 10 deer/10 km (as determined by spotlight survey) and 20 - 60% in areas with 20 - 50 deer/10 km. In areas where deer density was high, however, the density of saplings and trees with twigs at heights of less than 2 m was low, which impeded data collection. Sapling density was largely affected by the density of overstory trees and competing understory plants, especially dwarf bamboo. These results suggest that observations of browsed twigs on trees provide a useful index of deer density when density is not high.