Japanese Bears are Moving (and Conserving) Cherry Trees with Poop

We know Earth’s climate is changing rapidly, but we don’t fully know what those changes will mean for our planet and its inhabitants. Every organism interacts with those around it, making it hard to trace or predict the effects of pollution, species loss, and global warming. For example: a recent report published in the journal Current Biology shows that bears and their poop are slowly moving the cherry trees up mountains near Tokyo, Japan, and out of the damaging heat of lower altitudes. 

Plants, of course, don’t have legs (stick with us, this is going somewhere). Instead, they rely on the wind and on organisms with legs, wings, and fins to pollinate their flowers, eat their fruit, and carry the seeds to new locations. 

A team of Japanese researchers wondered how the behavior of these seed-dispersing animals might affect a plant’s ability to withstand climate change. They focused their study on the wild cherry tree (Prunus verecunda) for two reasons: first, botanists have predicted that the tree, which grows in temperate regions, will be especially vulnerable to rising temperatures; and second, the tree’s fruit are a favorite of local animals, especially the moon bear (Ursus thibetanus). 

To track the plants’ movement, the scientists compared stable oxygen isotopes in their seeds. Every tree has these isotopes, which it gets from its parent tree while it’s still a seed. The isotope ratios vary by altitude, which makes them useful in geotagging. If a pooped-out seed on a mountain has a ratio associated with a lower altitude, it means the seed must have been carried up. (Unlike some species, P. verecunda seeds are exclusively dispersed by animals.) 

The researchers found that mountain-climbing bears brought cherry trees with them, often transporting seeds several hundred meters above their parent trees. The trees took root at new altitudes, in climates just slightly cooler than those down below. But this slight difference in temperature, around 3°F, might be enough to keep the trees safe. 

This is good news for the wild cherry; not all trees will be so lucky. P. verecunda happens to be a spring-fruiting tree, which means that bears stop and snack on its fruit on their journey up the mountain. The reverse would be true for autumn-fruiting trees: their seeds would be carried to lower, less tolerable altitudes. 

"The most important implication of our study on a warming planet is that seed dispersal direction can be asymmetric," lead author Shoji Naoe of the Forestry and Forest Products Research Institute said in a press statement. "Most previous studies have predicted future plant distributions under global warming based on the simple relationships between present plant distribution and environmental factors there, assuming that there are no seed dispersal limitations and no bias in dispersal direction. However, our study indicates that predicting future plant distributions can be very uncertain without considering the seed dispersal process that determines plant movement."