Island Biogeography of the Great Basin

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The theory of island biogeography is one of the explanations as to why speciation occurs.  The term “island biogeography” is not specific to land masses surrounded by water but rather it refers to any area of habitat that is surrounded by unsuitable habitat for the species in question; this can be isolated springs, mountain peaks, deserts, or even habitat isolated by development.  The areas of suitable habitat become islands, surrounded by areas inhospitable to those species living in that area.

In the Great Basin, these habitat islands are formed though the Basin and Range topography in which north-south trending mountain ranges (some peaks in excess of 10,000 feet) are separated by low and broad desert valleys (as low as 282 feet below sea level, in Death Valley).  In the Great Basin, suitable habitat is based predominantly on elevation changes rather than changes in latitude.

Location of the Great Basin on a map of North American drainage basins / watersheds. Source; Wikimedia Commons, 2006.

Location of the Great Basin on a map of North American drainage basins / watersheds. Source: Wikimedia Commons, 2006.

Climate change can significantly alter the habitat of these island mountains.  Dryer and warmer conditions will cause the treeline and associated arboreal habitat to retreat up the mountain while the desert habitat expands in the valleys.  At the terminal Pleistocene, global warming had significant impacts to mammalian species; many became isolated on mountain peaks and some even became extinct.

Mammals, especially small sized species, are particularly subject to genetic isolation and extinction.  An example of extinction caused by isolated populations and global warming is that of the noble marten (Martes americana nobilis). The nobel marten was survivor of the Pleistocene-Holocene transition but became isolated to mountain tops in the Great Basin during the Holocene due to the warming conditions and the animal’s retreat to seek cooler temperatures and suitable habitat.  Recent studies from Hughes (2009) suggests that the noble marten was adapted to open grasslands in mountain foothills, commonplace during the Ice Age, but the loss of this habitat during the Holocene and the isolation of the populations through island biogeography is likely the cause of this species extinction.

A notable survivor to the global warming since the Pleistocene and an adaptor to elevation-controlled habitat is the Great Basin Bristlecone Pine species (Pinus longaeva).  Today, this species of tree is relatively rare and only occurs at the highest elevations of mountain peaks in Utah, Nevada, and eastern California but during the last glacial maximum this species was rather prevalent and occurred at much lower elevations.  However, once the last ice age ended and the western United States began to warm and dry, the cold-adapted bristlecone retreated to the highest elevation mountain peaks.  These stands of bristlecone pine groves are now isolated on the mountain peaks and are surrounded by inhospitable hot desert valleys.

Great Basin Bristlecone Pine species (Pinus longaeva). Source: Jim Gordon, Biloxi, MS.

Great Basin Bristlecone Pine species (Pinus longaeva). Source: Jim Gordon, Biloxi, MS.

Bristlecone genetics of these disjunct distribution species is of high interest to biogeographyers. Great Basin bristlecone pine is closely related to two other pine species, foxtail pine (P. balfouriana) of California and Rocky Mountain bristlecone pine (P. aristata) of Colorado and New Mexico areas.  Crossbreeding between the Great Basin bristlecone pine and these other species is entirely possible.  In a laboratory setting, foxtail pine has been combined with Great Basin bristlecone pine and the union has produced fertile hybrids suggesting a relatively recent common ancestor.  But, these hybrids are yet to be found in the wild which strongly suggests that genetic drift of these isolated pine populations, due to island biogeography, may be the reason for speciation.

When discussing bristlecone pines and geography, one cannot ignore the side story of Dr. Donald R. Curry and Prometheus, the oldest known (at the time) single organism.  Don Curry (deceased) is a highly respected geographer and geomorphologist who is most well known for mapping and describing the major shorelines and events of Pleistocene Lake Bonneville; but in the summers of 1963 and1964 he was a graduate student at the University of North Carolina and was studying the paleoclimate of the Little Ace Age.  He was looking for evidence of the Little Ice Age in the rings of ancient trees and decided to take a sample core from a promising Great Basin bristlecone pine specimen at the elevation of 10,700 feet on Wheeler Peak in what is now Great Basin National Park in Nevada.   Due to its longevity, this tree had been named Prometheus.

According to Curry’s own recount of the event, his borer became stuck in Prometheus and he was unable to retrieve his dendrochronology sample or his borer.  With permission and assistance from the Forest Service District Ranger, the tree was sectioned (cut down) with a chainsaw.  Only after counting the rings of the sectioned tree did Curry and the Forrest Service Ranger release that Prometheus was likely the oldest living single organism; Curry dated the tree to be at least 4,900 years old but others since have suggested it may be more than 5,100 years old.

Curry received a lot of criticism for cutting Prometheus; however, at the time it was a perfectly acceptable scientific sampling technique and one of the sections of Prometheus continues to be available for researchers to study.

Sources:

Donald K. Grayson (2011)  The Great Basin: A Natural Prehistory, Revised and Expanded Edition.  University of California Press

Susan S. Hughes (2009) Noble Marten (Martes americana nobilis) Revisited: Its Adaptation and Extinction. Journal of Mammalogy: February 2009, Vol. 90, No 1 Pages 74-92