How bears engineer Japanese forests

If you were to look up when walking through a forest in Japan, you might see “Kuma-dana,” or “bear shelves,” high in the trees. These patches of broken branches and dead leaves are created by Japanese black bears when they climb high to find fruit. That’s why climbing a tree won’t save you from a Japanese black bear.

These bears are a subspecies of the Asian black bear (Ursus thibetanus), and they are quickly disappearing. Now found on only two islands — Honshu and Shikoku — they probably number fewer than 10,000. About two-thirds of Japan is forested, but much of the native forest has been replaced by managed groves where bears are not welcome.

Where the bears remain, though, the animals are “ecosystem engineers,” a new study finds. Their presence helps plants to fruit more, producing more seeds, and it’s because of those bear shelves.

Kazuaki Takahashi of the University of Nagano in Japan and colleagues studied the effects of black bears on plants in the Nagakura-yama national park near Mount Asama in Nagano Prefecture, Japan. They focused on Mongolian oak trees, because the bears like to bulk up on the trees’ acorns in the fall. They measured light availability in the canopy below trees that had bear shelves and others that didn’t and quantified fleshy fruits produced by the plants in that canopy. The results  of their study appear July 24 in PLOS ONE.

In the closed canopies, two species of trees bore fruit. But in those that had gaps, seven species fruited. Six shrub species bore fruit in closed canopies; 10 species had fruit when there were gaps. Similar results were seen with woody liana plants.

“Light conditions improved beneath bear-disturbed trees with large canopy gaps,” the researchers note, and “fruiting of fleshy-fruited plants was facilitated by the improvement of light conditions.” The plants create more flowers, more fruits and — most importantly — more seeds as a result.

The increase in the number of flowers on the plants that grew beneath the bear shelves may also help pollinators, the team notes, since a higher density of flowering plants can support a great number of pollinators, such as bees or other insects.

The creation of gaps that let in light and have a host of effects on other species in the ecosystem qualifies the bears as “ecosystem engineers,” the researchers say. These are species that “directly or indirectly modulate the availability of resources (other than themselves) to other species by causing physical state changes in biotic or abiotic materials.” Beavers are a classic example; others include African elephants and badgers. But this would be the first bear species to receive the label.