Decline in Japanese sika population serves as a counter to alarmism

Scientists in Japan have observed that the population of the Yakushima sika deer has declined due to natural factors, suggesting that the population can be regulated without culling.

In an article in Australian Deer Magazine a few years ago, Australian wildlife researcher Neal Finch gave this explanation of how carrying capacity works:

“The size of the population decreases through intra-specific competition almost as quickly as it initially grew. As the herd decreases less of the vegetation is eaten and the plant communities are eventually able to recover. After a few years the food resources of the habitat are sufficient for the deer herd to once again grow. A population expands and contracs according to the available resources over the course of a long period of time. In terms of a deer population a century would be an appropriate length of time for this pattern to stabilise. The carrying capacity of the habitat is an imaginary line somewhere between the highs and the lows of the fluctuating population.”

These observations in Japan mirror similar observations internationally over a century or more and should give pause to the alarmists in Australia who are peddling a myth that wild deer populations will increase exponentially and indefinitely.

The Yakushima sika deer (yakushika: Cervus nippon yakushimae), a subspecies of the Japanese sika deer (Cervus nippon), evolved without natural predators on the island of Yakushima, in Kagoshima Prefecture, Japan. It inhabits the forests on the island which were declared a World Heritage Site in 1993. Within the site, the yakushika has not been hunted in the past 50 years; however, since 2014, their population has been decreasing.

A group of three scientists, including Hokkaido University’s Associate Professor Naoki Agetsuma, have discovered indications that the reduction in the population of the yakushika is due to natural regulation. Their findings were presented at the 68th meeting of the Ecological Society of Japan in March 2021, and were published in the Japanese Journal of Conservation Ecology.

The team conducted onsite research every summer between 2001 and 2018 by designating two areas-the Hanyama and Kawahara districts, each encompassing about 80 hectares-for surveys, and counted the deer present while they trekked 5-kilometer census routes. Further, they tracked 19 marked deer in the Hanyama district for four years from 2014 to determine whether they have moved out of the district.

The scientists found that the number of deer increased at an average annual rate of 9 percent from 2001 through 2014 but began to decrease at an annual rate of 15 percent after then. The number of deer to have migrated to areas outside Hanyama district is estimated at no more than 3.5 percent of the population, annually. The team has frequently witnessed deer dying of unknown causes since 2016.

Changes in the yakushika population index in the World Natural Heritage Area of Yakushima from 2001 to 2018. The population density index peaked in 2014 and has been on a downward trend. Follow-up surveys confirmed that the downward trend is continuing, as of the summer of 2020 (solid triangles are numbers from Hanyama, open squares are numbers from Kawahara; Naoki Agetsuma, Yoshimi Agetsuma-Yanagihara, Hideki Sugiura. Japanese Journal of Conservation Ecology. April 20, 2021).

The overall decline is attributed to natural factors, the first time that a natural decline of sika deer has been observed in Japan. The exact causes have yet to be determined, and may require monitoring the population for several decades while prohibiting culling.

Currently, most sika deer populations in Japan, including other areas of Yakushima, are controlled by culling, in line with the government’s deer management policy. 

Original Article:

Naoki Agetsuma, Yoshimi Agetsuma-Yanagihara, Hideki Sugiura. Increase and decline in the density index of Japanese sika deer (Cervus nippon) over 18 years in an evergreen broad-leaved forest with no hunting pressure in the Natural World Heritage Area of Yakushima, Japan. Japanese Journal of Conservation Ecology. April 20, 2021.

DOI: 10.18960/hozen.1923

Funding:

This research was supported by the the Wildlife Research Center (Cooperation Research Program) and the Primate Research Institute (Cooperation Research Program), Kyoto University, the Fujiwara Natural History Foundation, the Japan Society for the Promotion of Science (JSPS) KAKENHI (14704013, 16780107, 17K00645, 18H02218), The Nature Conservation Society of Japan’s “Conservation and Management Scheme through Long Term Assessment of Natural Ecosystem in the Yakushima World Heritage Area, Southern Japan”, and the Research Institute for Humanity and Nature’s “Evaluation of Sustainable Forest Use Options and their Perspectives”.