Biodiversity, short for biological diversity, is a word that describes the many kinds of life forms on Earth. It also describes the balance of life in a particular climate or terrain, as well as the variations within a group or species. Biodiversity is essential not only for the health of the planet, but also for our own species.

Today the Earth’s biodiversity is being threatened. More than 25,000 species around the world are in danger of extinction, including about 12,000 species of plants. If steps are not taken to reverse these trends, one species after another will be lost in a chain reaction, and eventually it will be impossible for us to survive.

To better understand the cause-and-effect relationships involved in nature, let us look at the biologically diverse island of Yakushima, an island in Kagoshima Prefecture in the southernmost part of Kyushu. This island of roughly 500km² is covered with dense forest. It is home to many unique and endangered species. Some of them exist today only on this small island.

One reason for Yakushima’s exceptional biodiversity is its range of elevations. The island has more than 40 mountains that are over 1,000 meters high. It also receives huge amounts of rainfall every year, ranging from 4,000 millimeters in lower elevations to as much as 10,000 millimeters in the mountains. Some residents joke that it rains “35 days a month.”

For many years, people and nature have coexisted on Yakushima. One example is the yakushika, one of Yakushima’s native animals that has lived alongside the human population since long ago. This type of deer, the smallest in Japan, can be found throughout the island’s forests. There are somewhere between 17,000 and 28,000 yakushika on the island. They move in herds and eat the trees, ferns and other flora found in the island’s forests.

But today, as in many other areas around the world, biodiversity on the island is being threatened. The vegetation of Yakushima is disappearing. One of the reasons is believed to be the damage caused by the yakushika. There are concerns that these deer may eat up all of the island’s precious vegetation.

The problem has greatly concerned scientists who study the island. Researchers conducted a study to better understand what is happening. The researchers noticed several kinds of damage occurring on Yakushima.

First, trees and ferns are disappearing along the paths where yakushika are most often seen.

It was also found that the bark of trees in the forest is peeling and withering, making it difficult for the trees to grow. Deer love to eat tree bark, which explains the peeling trees. The plants most favored by the yakushika are disappearing most rapidly, while those the deer dislike are disappearing more slowly.

Furthermore, the yakushika seem to especially like the bark of citrus fruit trees, such as ponkan mandarin and tankan, which provide Yakushima’s major agricultural products. This harms not only the vegetation, but also the business and industry of the island.

Why have the yakushika done such damage to the environment of Yakushima? One reason is that the deer have begun to use the paths cut by humans to get through the forest. Doing so makes it easier for the deer to reach vegetation in areas of the island they would not have visited before.

The open paths allow more sunlight, which should help the vegetation regrow quickly. However, the wide spaces and lush growth have caused the paths to become feeding grounds for the deer. As a result, deer are treating the areas as all-you-can-eat buffets, and the trees and ferns along the paths are disappearing. The changes made by humans to the natural environment have had an unintentional but noticeable effect on the island’s biodiversity.

Another reason so much damage has occurred is the enormous population of yakushika on the island. Part of the reason for their large numbers is that Japan put the yakushika on its protected species list when it was thought they faced extinction. As a result, the number of hunters on the island decreased and fewer deer were killed. In addition, an increase in tourism to Yakushima has helped the yakushika thrive. Although the visitors are told not to feed the deer, many are charmed by the animals and like to give them food.

One more reason often given for the population is the mild winters. Due to warmer temperatures during the winter months, more deer are surviving the cold season and the survival rate of baby deer has also increased.

The actions of humans have helped increase the population of yakushika and the damage the deer have caused to the island’s vegetation. So humans have a responsibility to consider what to do about this problem. In fact, the residents of the island have been working on solutions in cooperation with the Ministry of the Environment for many years.

But our attempts to solve the problems can lead to unintended consequences. Controlling the number of yakushika is one way, but their role in the island’s ecosystem must also be protected. If too many yakushika are eliminated, they may face extinction, which would have even more harmful effects on the ecosystem. Therefore, population control must be considered very carefully.