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.
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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