The Squirrel-Conifer-Fungi Connection

The evolutionary divergence of the northern flying squirrel (Glaucomys sabrinus) from the southern flying squirrel (G. volans) is an excellent example of speciation resulting from environmental change.  Genetic studies suggest both of these American species of flying squirrels diverged from Eurasian flying squirrels between 4-6 million years ago.  Eurasian flying squirrels are much more diverse and include 44 species, most of which live in southeast Asia–evidence this part of the world is where they originally evolved.  During the late Miocene about 5 million years ago, a forested landbridge connected Asia with America, explaining how the ancestor of both American species of flying squirrels colonized this continent.  Genetic evidence suggests the 2 American species of flying squirrels diverged from each other early during the Pleistocene between 1-2 million years ago when Ice Ages began to become more severe.  Boreal spruce forests expanded during Ice Ages, growing as far south as middle Georgia and Alabama.  In the middle south spruce forests grew in higher elevations while deciduous oak forests still occurred in adjacent lower elevation.  Oak forests are rich in mast such as acorns and nuts, but spruce forests offer less food for squirrels–seeds from spruce cones are only available for 2 months of the year.  However, underground fungi, also known as truffles, are available year round in spruce forests.  For most species of squirrels, fungi is a minor component of their diet, but truffles and other fungi make up 85% of the northern flying squirrel’s diet whereas southern flying squirrels eat more acorns, nuts, berries, and animal matter.  The ancestors of the northern flying squirrel were those individuals from the parent population best able to subsist on a diet of mostly fungi.  These individuals were able to colonize spruce forests, while the rest of the parent population remained in oak forests.  Eventually, this habitat partition resulted in a divergence between the 2 American species.

Photo: Northern Flying Squirrel, Glaucomys sabrinus.

Northern flying squirrels eat mostly fungi which is a minor component in most squirrel’s diet.  The ability to subsist on a diet of mostly fungi enabled this species to colonize spruce forests.  Eventually, they evolved into a different species than southern flying squirrels because of this capability.

Elaphomyces or truffle–favorite food of the northern flying squirrel.

 

 Red Spruce (Picea rubens)

Red spruce (Picea rubens).  Red spruce, truffles, and northern flying squirrels are beneficial and interdependent to each other.

Fossils of both species of flying squirrels have been found at Ladds and Kingston Saltpeter Cave in Bartow County, Georgia.  This is evidence that patches of spruce forest grew near patches of oak forest in this region during some climatic stages of the Pleistocene.  Northern flying squirrels are confined to the former; southern flying squirrels require the latter.

There is an interesting ecological interdependence between northern flying squirrels, red spruce, and several species of fungi.  Truffles grow intertwined with the red spruce roots, and they exchange nutrients.  The squirrels eat the truffles and spread their spores throughout the forest in their droppings.  A healthy spruce forest requires an abudance of truffles.  Many red spruce forests have been logged, and without the squirrel’s help, trees such as oak, maple, beech, and cherry are replacing them.  In West Virginia the U.S. Forest Service has successfully re-established red spruce forests.  Foresters discovered that red spruce seedling grow best in ground ripped apart by bulldozers and strewn with woody debris.  Some of these young spruce forests are on land reclaimed from strip mining. 

 Report fox squirrel sightings in Florida Sherman's Fox Squirrel

Fox squirrel.  This species may play a role in distributing fungi in longleaf pine savannah. 

Rhizopogon nigrescens–a fungi common to longleaf pine savannahs and likely an item in the diet of the fox squirrel.

Virgin stand of longleaf pine (Pinus palustris) in east Texas (circa 1908).

Although fox squirrels (Scirius niger) have a much more varied diet than northern flying squirrels, they occasionally eat fungi and may play a role in the health of longleaf pine savannahs.  Certain kinds of fungi that grow in the soil of savannahs also exchange nutrients with longleaf pine trees, and fox squirrels spread these spores in their dung as well.  Fox squirrels and longleaf pine savannahs were formerly common in the south, particularly on the coastal plain, but today both are rare.  The changes man has wrought have really sickened the natural communites of the world.

Reference:

Arbogast, Brian

“A Brief History of the New World Flying Squirrel: Phylogeny, Biogeography, and Conservation Genetics”

Journal of Mammalogy 88 (4) 2008

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One Response to “The Squirrel-Conifer-Fungi Connection”

  1. P Says:

    Interesting. I didn’t think anything could subsist on fungi.

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