EPOW - Ecology Picture of the Week

Each week a different image of our fascinating environment is featured, along with a brief explanation written by a professional ecologist.

23-29 August 2004

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Nurse Erratics in Chile

Glacial erratic boulder with native vegetation, 
Puyehue National Park, Chile

Credit & Copyright: Dr. Bruce G. Marcot

Explanation:   Here is a common phenomenon that has never been given a name.  Until now.  "Nurse erratic."  

Note, the fellow in the photo is not a nurse, nor is he (usually) erratic.  He is friend, colleague, travel partner, author, and master botanist Andy MacKinnon from British Columbia, Canada. 

We are hiking in the tremendously diverse temperate rainforests of the southern hemisphere, in Puyehue (pronounced locally as "poo-JHAY-way") National Park in southern Chile. This is a land scoured and shaped during the last ice age by alpine glaciers that emanated from the Andes Mountains.  As the glaciers passed, they carried huge boulders which got deposited as the ice melted and receded.  The boulders are known as "erratics" because they are found in erratic and odd locations, often well beyond the geologic formation from which they were originally plucked by the moving ice.  

In this temperate rainforest, the boulders have served the ecological function as a "nurse" substrate on which grow a distinctive flora ... a profusion of lichens, bryophytes, and vascular plants including forbs, shrubs, and even trees ... with secondary communities of spiders, insects, and other animals.  

It is well known that, in such forests, stumps and down trees often serve this "nurse" function -- and in fact, in an earlier EPOW we discovered that even termite mounds in Africa can serve as tree "nurses."  But there has never been a name for this glacial boulder function, so we propose "nurse erratic."  

Not to be confused with erratic nurses, nurse erratics can provide substrates for uncommon species of cryptogams (especially some lichen and moss species) that require bare-rock surfaces on which to take hold and proliferate.  

In this forest, we found several species of shrubs and trees, including species of southern beech (Nothofagus) entwined and clinging to the boulder surface and rooted in its crevices.  Such ecological roles of nurse erratics seem to have been poorly studied, however.   

New Information:
     Carson, W. P., J. A. Banta, A. A. Royo, and C. Kirschbaum. 2005. Plant communities growing on boulders in the Allegheny National Forest: evidence for boulders as refugia from deer and as a bioassay of overbrowsing. Natural Areas Journal 25:10-18.

Next week's picture:  Cryptic Band-backed Wrens of Ecuador


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