Sunday 22 March 2015

Changes in the diet of Brown Bears on Hokkaido.

Expanding Human populations have been the major factor affecting almost all of the Earth’s ecosystems since the end of the last glaciation. Human activity has altered food webs on every continent, led to the extinction of many species of animals and plants, with larger animals particularly adversely affected, and altered climates on a regional and increasingly global scale. Brown Bears, Ursus arctos, are highly adaptable large omnivores found across much of the Northern Hemisphere. They are extremely flexible in their dietary habits, and able to change their diet in response to Human or other environmental pressures in ways that few other large animals can manage. Reconstructing the diets of past populations of Brown Bears therefore offers the potential insight into wider impacts on the environment and food chain.

In a paper published in the journal Nature Scientific Reports on 17 March 2015, a team of scientists led by Jun Matsubayashi of the Center for Ecological Research at Kyoto University and the Laboratory of Forest Ecosystem Management at Hokkaido University describe the results of a series of tests on stable isotope ratios from Brown Bear collagen extracted from bone samples from archaeological sites on Hokkaido Island.

A Brown Bear eating a Salmon on Hokkaido. Masaaki Kadosaki/Hokkaido Wildlife Laboratory.

To establish a comparison Matsubayashi et al. established stable isotope levels for potential food sources for Brown Bears on Hokkaido; Herbs, Fruits, Corn, Terrestrial Animals (primarily Sika Deer, Cervus nippon) and Salmon (Chum Salmon, Oncorhynchus keta, and Pink Salmon, Oncorhynchus gorbuscha). These levels were then compared to stable isotope levels from the Bear collagen, in order to try to reconstruct the diets of the living Bears.

The Bear samples were split into two groups, for East and West Hokkaido, where development has proceeded at different rates and where environmental conditions are somewhat different. West Hokkaido is closer to the other islands of Japan and has been intensively settled for longer. It is home to Chum Salmon, which migrate upriver from October to February, and has had a low Deer population throughout the study period. East Hokkaido has been intensively settled and developed only since the mid nineteenth century. It is home to both species of Salmon, with the Chum Salmon migrating upriver from October to February and the Pink Salmon migrating upriver from August to October as well as having had higher Deer populations throughout most of the study period.

The locations of Hokkaido Island and each archaeological sites. Matsubayashi et al. (2015).

The Bear samples were also split into three time periods; Predevelopment (before 1890 in the west and before 1920 in the east), Early Development (1931-1942) and Post Development (after 1996), and the contribution of each dietary source to the isotope ratio in the Bear collagen from each period was estimated.

Bears on the west of Hokkaido consumed high levels of Deer and Salmon in the Predevelopment phase, but had switched to a diet comprising almost entirely Herbs and Fruits by the Early Development phase. Bears on the east of the Island also began with a diet rich in Deer and Salmon, and had largely dropped Salmon from their diet by the Early Development phase. These Bears were still consuming relatively high levels of Deer in the Early Development phase, but again had largely switched to a diet of Herbs and Fruits by the Post Development stage.

Salmon is a highly prized dietary component for Brown Bears, and the Bears would be predicted to use this food source for as long as it is available. Salmon populations are known to have crashed due to overfishing in the late nineteenth century, but recovered during the twentieth century. The failure of Bears to re-adopt Salmon as a major dietary component with recovering populations suggests that the loss of this item from their diet was driven by some factor other than the Salmon population. Matsubayashi et al. suggest that the main factor in determining Bear access to Salmon might be levels of development along river systems, particularly the lower parts of these systems, which are the preferred Salmon-fishing grounds for Brown Bears, but also the first places settled and developed by Humans. Many of these areas were covered by environments that Bears tend to avoid, such as urban areas, paved roads and intensive farmland early in the development of the island, potentially excluding the Bears from their former feeding grounds.

Deer populations also underwent a crash at the end of the nineteenth century, driven by overhunting and two periods of severe snowfall, but again recovered during the twentieth century. Unlike Salmon, Deer occupy essentially the same terrestrial environment as Bears, and therefore human development, unless completely excluding either or both species, cannot be directly responsible for the loss of this food source from the Bear’s diet. Instead Matsubayashi et al. suggest that this might be connected to the extinction of the Hokkaido Wolf (Canis lupus hattai), which died out at the end of the nineteenth century, due to a combination of overhunting and the crash in the population of Sika Deer, its main food species. While Bears will occasionally hunt and bring down Deer, they are not well adapted to this behaviour, and studies in other part of the world have suggested that most of the meat they consume comes from chasing other predators, particularly Wolves, off their kills. If this relationship also formerly existed on Hokkaido, then the loss of Deer from the Diet of Brown Bears there can be directly attributed to the extinction of the Wolves.

See also…

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