Vegetarian Bears?

The remains of a bromeliad, eaten by an Andean bear.

May 16 to 22, 2011, is Bear Awareness Week, and we hope you’ll join us in celebrating these amazing animals. Today we focus on Andean bears.

What is the role of the Andean bear, also called the spectacled bear, in its environment? That is a good question! The answer is not well known, and it definitely depends on which Andean bear population you’re talking about.

Andean bears use a wide range of habitats, ranging from alpine habitats above treeline through humid cloud forests and transitional montane forests to dry tropical forests. Because each of these habitats varies in its complexity, and because the bears’ range extends from the border of Columbia and Panama south to northern Argentina, the bears’ role in its environment also varies. Wherever they live, though, they’re thought to be primarily vegetarian.

Because the bears forage on a variety of forest fruits when they’re available, and because each bear might travel a long distance each day, Andean bears have often been thought to be important seed dispersers. That means that they ingest the fruits and seeds in one place, then transport them to another as they go about their bear business, before depositing them with a little bit of fertilizer. This would generally be thought of as beneficial to the plants, because the seeds would have a better chance of growing up without competing with their parents. The importance of Andean bears as seed dispersers has not often been well tested, but if they are important seed dispersers it is probably for plants whose fruits and seeds are too large and heavy for other animals to transport.

Andean bears are known to forage on a wide variety of bromeliads that live on the branches of trees, and on the ground above treeline. When they eat bromeliads, bears tear them apart to get at the fleshy core, similar to how humans eat artichokes. Now, being torn apart is not likely to be any more beneficial to a bromeliad than it is to an artichoke, but no one has yet investigated the impact of bear foraging on bromeliad populations. In places in the cloud forest, most tree limbs are liberally covered with bromeliads, so perhaps the bears are not in danger of running out of food.

A fruiting sapote tree offers tasty treats for Andean bears.

In the dry tropical forest of northwest Peru, the role of Andean bears as seed dispersers, and the role of bears as consumers of plants, may be enhanced because in the dry forest there are not many alternative foods for the bears to eat. Bears congregate every year when the fruit of the sapote tree is ripe and focus their energies on consuming these large fatty fruits filled with seeds. These fruits and their seeds are also consumed by a wide range of birds and other mammals, including the Sechuran desert fox, but each bear can consume more fruit and travel farther than each fox, making it possible that the bears play a vital role in distributing the seed of the sapote tree.

Here are the remains of a pasallo tree after an Andean bear has shredded its trunk.

It’s a little harder to evaluate the impact of the dry forest bears on the pasallo tree, which they turn into woodchips. The bears don’t just peel back the outer bark to feed on the inner bark, or to feed on insects—the bears actually eat the wood itself! Amazingly, it appears that a pasallo tree that has been exploded by an Andean bear is often not killed. The pasallo resprouts and regrows. This must be an energetically expensive process for the pasallo trees, and it seems as though there must be a negative effect on the pasallo population, but apparently healthy populations of pasallo survive.

We’re in the process of planning research to investigate more closely the interactions between the bears and their primary food sources, so hopefully in a few years we’ll better understand how the bears benefit from their various food sources and how the plants are impacted by the bears. We’ll keep you posted on our progress.

Read more about San Diego Zoo Global’s conservation work with bears…

Russ Van Horn is a senior researcher for the San Diego Zoo Institute for Conservation Research. Read his previous post, Andean Bears: 30 Years Later.

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