Posts Tagged ‘Ice Age forest composition’

Pleistocene Pack Rat Middens

October 15, 2012

The Neotoma genus includes 22 species of rodents known as packrats in the west and woodrats in the east.  The eastern woodrat (Neotoma floridana) looks like an overgrown field mouse with a brown back and white belly.  They’re entirely vegetarian, feeding on acorns, nuts, and such common woodland plants as Virginia creeper and greenbrier–foods so abundant in the average woodlot that they don’t have to forage far from the safety of their bulky nests.  Unlike the invasive Norway rat (Rattus norvegicus) and the house mouse (Mus musculus), both of which were accidental stowaways on colonial ships, woodrats cause little damage to agriculture.

Eastern woodrat.  Reportedly, they make good pets and do almost no damage to agriculture.  I’ve never seen one.  For some unknown reason they’re absent from the Augusta, Georgia region but live in most the rest of the state.

In Georgia woodrat bones have been found in just about every Pleistocene fossil site where small mammals accumulated, proving they’ve been common residents of eastern deciduous forests here for millenia.  It’s unfortunate, however, that none of the nests they built during the Pleistocene have been preserved in the east.  All species in the Neotoma genus build very large nests out of sticks upon which they urinate.  The sugar in their urine is sticky, and it acts like cement glue that holds the sticks together.  It’s also a preservative that turns the wood into a substance known as amberat.  Amberat preserves the wood for as long as 50,000 years.  Eastern woodrats build their nests in forests under tree stumps or even in trees.  Rain and moist soil eventually destroy these nests.  But western packrats often build their nests in caves and rock shelters where they remain intact for tens of thousands of years.  These ancient nests provide a treasure chest of data for paleoecologists.

Bushy-tailed  packrat (Neotoma cinerea) nest.  The next is so bulky, large predators probably don’t want to waste the energy trying to tear them apart to get such a small meal.  Rats can escape through a back entrance when fleeing smaller predators.  The sticks are cemented together with sugary rat urine.

Fossil packrat nest in a cave.  The rat urine preserves them for tens of thousands of years as long as they are unexposed to rain.

Scientists have analyzed over 2,000 western packrat nests from caves found from Mexico to British Columbia and from west Texas to east California.  By carbon-dating the wood and identifying the species of tree from which it originated, scientists can determine the forest composition at the time the packrat built the nest.  Moreover, packrats have a curious habit of collecting odd objects, and they often carry fossils, feces, and insect remains back to the nest with them.

Diagram showing changes in forest composition over a 25,000 year period based on evidence from packrat middens.  During the Ice Age some associations of tree species were distributed 700 meters lower in elevation than they are today, and many species compositions have no modern analogue.

The evidence from fossil packrat middens suggests certain associations of woody plants were distributed about 700 meters lower in elevation during the Last Glacial Maximum (~28,000 BP-~15,000 BP) than they are today.  Fir, spruce, and limber pine grew together in vast forests below subalpine meadows which in turn were also lower in elevation than they are today.  Juniper, pinyon pine, and oak grew at elevations between 300 Meters -1700 Meters where highland desert prevails today.  Desert vegetation was nearly absent then and so was ponderosa pine which today comprises the largest zone of forest in the Rocky Mountains.  Evidence from packrat middens suggests this commercially important species didn’t dominate western forests until just 500 years ago.  Much of the composition of Ice Age Rocky Mountain environments have no modern analogue.  The association of plant species everywhere is truly random and transient.

On a humorous sidenote I came across this historical anecdote:  In the 19th century starving miners discovered a packrat midden in a cave.  Some of them mistakenly thought it was a kind of candy they considered manna from heaven.  Packrat middens glisten and have a sweetish resiny odor, explaining the delusion.  A few of them dared to eat it.  Needless to say, they became “nauseated” after eating the 10,000 year old wood cemented together with rat urine.

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