Japanese black bears are ecosystem engineers, breaking off branches in tree canopies to allow more light to penetrate and increase the growth of fleshy fruits according to research in Japan’s central forests.
A study led by Kazuaki Takahashi from Nagano University has found that tree canopy disturbance by the bears is critical to improving light conditions in forest layers and accelerating the growth of fruits and berries.
The results are published online in Plos One, and include a list of fruits that benefit from these bouts of ursine engineering. The paper’s co-authors are Kaori Takahashi from Shinshu University and Izumi Washitani from Tokyo University.
Japanese black bears (Ursus thibetanus japonicas) climb trees to eat fruits in the forest canopy, often breaking off branches that bear fruit.
They frequently place these broken branches across the forks of other branches, and these signs of bear activity are used by ecologists to study feeding habits and estimate population numbers.
“Because these broken branches look like shelves, they are known as “bear shelves” or “Kuma-dana” in Japanese,” the research paper says.
A Japanese black bear’s diet can include around 10 types of nuts or acorns from trees such as the Mongolian oak and Japanese beech, and around 70 different species of fruits and berries.
The research was carried out over 12 months in a deciduous forest in Nagakura-yama national park, near Mt Asama in central Japan.
It found that the bears created around 26 “bear shelves” per hectare in the forest canopy during autumn, while foraging in Mongolian oak trees (Quercus crispula).
“Our results suggest that Japanese black bears directly increase the availability of light resources to fleshy-fruited plants by causing physical state changes in the canopies of Q.crispula trees,” the paper says.
“Canopy gaps created by black bears improve light conditions, which facilitates fruiting of adult fleshy-fruited plants located beneath the gaps.”
The study looked at five forest layers beneath canopy gaps created by the foraging bears and found the gaps increased availability of soil moisture and nutrients. They also affected germination of seeds and growth of seedlings and saplings.
“Our results provide evidence that Japanese black bears have high potential as ecosystem engineers that increase the availability of resources to other species,” the study says.
“It suggests that black bears contribute to the provision of food resources for various frugivorous animals by indirectly increasing fruit abundance in forests.”
The foraging bears were also found to play a role in improving forest pollination.
All of the 28 fruiting plant species included in the study were insect-pollinated, but pollution success depended on the occurrence of neighbouring plants that shared the same pollinators.
“Plants that occur at low densities receive low pollinator visitation, whereas increases in the density of neighboring plants lead to increases in pollinator visitation,” the study says.
It found that “abundant flowers” on woody lianas and trees that benefited from canopy gaps created by the bears attracted a high number pollinators.