When Japanese black bears, Ursus thibetanus japonicas, climb high up into trees to eat acorns and other nuts, they often leave a bunch of broken branches and dead clusters of leaves in their wake. They're also known to place broken branches across the forks of intact tree branches. These so-called bear shelves, or “Kuma-dana” in Japanese, create gaps in the canopy, allowing light through to help fruits grow. Like dam building beavers and soil burrowing earthworms, these bears are ecosystem engineers in the deciduous forests of central Japan where they live, according to work published in PLoS One last month. They’re the first bears to receive this title!
These black bears feed on the fruits of about 10 nut-producing tree species and about 70 plant species with fleshy fruits, from grapes to mulberries to gooseberries. When they break a lot of branches, they leave behind small canopy gaps about 6.9 square meters (74 square feet) on average, which is much smaller than gaps created by falling trees. These larger, well-studied canopy gaps are known to let light down into lower forest layers, promoting the growth of seedlings and saplings as well as adult shrubs and trees.
To see how bear-induced canopy disturbances impact light conditions, a trio led by Nagano University’s Kazuaki Takahashi investigated plants in five forest layers beneath both black bear-created canopy gaps and closed canopies in Nagakura-yama national park near Mt. Asama. They focused on bear shelves created in Mongolian oaks, Quercus crispula, since the bears especially like to feed on its acorns in the fall before hibernation. To the right you can see bear shelves made of broken branches of Japanese white oak, Quercus serrate (above), and Japanese chestnut, Castanea crenata (below).
Light conditions, they found, improved beneath bear-disturbed trees with canopy gaps. The effect was the biggest for the top canopy layer, as light availability decreased with descending forest layers. These improved lighting conditions accelerated the fruiting of fleshy-fruited plants and climbing plants called woody lianas below. All three of the woody liana species that occur beneath bear-created gaps had ripe fruit, while most of them living under closed canopies lacked ripe fruit. And there are 12 shrub species in these forest layers: 10 beneath bear-created canopy gaps bore ripe fruit while only six bore fruit beneath closed canopies. The team observed similar results with trees.
Ecosystem engineers are defined as organisms that either directly or indirectly help modulate the availability of resources (that aren’t themselves) to other species by changing the physical state in both biotic and abiotic materials. In other words, they modify, maintain, or create habitats. Until now, there’s never been an ecosystem engineering study that focused on the roles of the planet’s eight bear species. Japanese black bears, the team concludes, directly increase the availability of light resources to fruiting plants by changing the canopies of oak trees – enhancing seed production.
Small canopy gaps created by black bears in Mongolian oak (Quercus crispula) canopies. K. Takahashi et al., 2015 PLoS One
[H/T: Science News]
Images: K. Takahashi et al., 2015 PLoS One