The Pine Seed War

Both Clark's nutcrackers (Nucifraga columbiana) and chickarees (Tamiasciurus hudsonicus) (also known as red squirrels, or pine squirrels) eat the seeds produced by limber pine (Pinus flexilis) and whitebark pine (Pinus albicaulis.) They compete with each other for this food source. However, the result produced by chickarees and nutcrackers on the pines themselves is very different. Researchers believe both pine species have evolved over time to have their seeds dispersed by birds, based on their seeds being wingless (don't disperse well on their own,) the large size of the seeds, and the the presentation of the cones on the trees.

In the northern Rocky Mountains the large seeds from limber and whitebark pine provide a nutritious meal for Clark's nutcrackers who appear to depend on these seeds for their primary food source. The nutcrackers have a special pouch that allows them to carry several seeds at a time to locations where they cache them for future use. Generally, the birds only eat a third to a fifth of what they cache, allowing the remainder a chance to germinate in a new location far from the parent tree. This type of arrangement where both species benefit is called mutualism.

Chickarees use limber and whitebark pine seeds in quite a different way. They collect pine cones and cache them for winter use. They usually bury the cones too deeply for the seeds to germinate if the squirrels do not return to eat them. Also, they often cache the cones near the parent tree so they are not nearly so good at dispersing the few seeds that are able to germinate. This means they either eat or effectively kill the seeds they harvest and are thus considered seed predators. In areas where squirrels occur, pinecones of these two pine species are heavier and seedcoats thicker to better protect the seeds. This makes it harder for both squirrels and birds to use the seeds.

Researchers have worked in Rocky Mountain National Park and other northern Rockies sites to study how limber and whitebark pines are evolving to foil chickarees and encourage nutcrackers to use their seeds in a three-way interaction probably most simply described as the "pine seed war."

Last updated: March 31, 2012

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