Images (c) by James L. Reveal, Norton-Brown Herbarium, University
of Maryland, unless otherwise credited.
LECTURE 35 - TERRESTRIAL BIOMES
The North American continent is the home of a diverse array of plants and animals all occupying a multitude of ecological sites in a variety of climatic regimes. The biota of North America - the sum total of its living creatures - when considered in relation to climatic, soil and physiographic features allows biologists to recognize grand associations that are termed biomes.
The purpose of the two lectures on biomes is to provide the student with an awareness of life from the arctic to the tropics. Our goal is to make you, the young men and women who are the future, cognizant of the living creatures, the land, and the environment of the planet.
To understand the present flora, it is useful to understand in broad terms the fate of the flora over the last 18,000 years. The flora of North America has changed significantly insofar as its distribution is concern during this short period of time in the earth's history. A large mass of ice dominated much of the northern third of the continent. Unglaciated regions in Alaska allowed not only numerous plants and animals to survive in refugia, but provided a broad avenue for people to enter onto a new land. Much of the native flora was compressed in front of the ice sheet, the vast taiga of today was little more than a narrow strip along the front and down the backbone of the Appalachian Mountains.
The eastward expansion of the eastern coast of the United States onto the Atlantic ocean shelf, and especially around Florida, proved to be an escape route for species to survive. In the West, deserts were more southerly, and the grasslands more compact.
By 8,000 years ago, a vast warming trend had significantly reduced the size of the ice sheet, with the tundra and taiga expanding rapidly northward as the ice retreated. On the Great Plains, the grasslands moved northward, the southern tip being equally rapidly replaced by desert conditions. In the West, cold desert vegetation expanded northwardly to a degree, but mainly elevation up slope and, in the Great Basin, onto the floors of the now dried pluvial lake beds.
Present-day biomes were rapidly establishing starting some 5,000 years ago as the climate cooled to its present norm. The tundra continued to move northward, expanding onto the Arctic slope as the ice continued to melt. The taiga proved to be an effective barrier to the northward expansion of both the grasslands of the Great Plains and the deciduous forest in the East.
In the west, the Chihuahuan and Mojave deserts became established in their present position, and the northward expansion of the Sonoran Desert was halted by a combination of cooling climates and topography. The rapid desertification in the West, coupled with the survival of plants and animals in small refugia or nunataks resulted in a diverse modern flora.
Diversity in terms of species is far greater in the American West and the Deep South, where the sheer force of the ice sheet was less compared to that in the Northeast and most of Canada. The geologically ancient flora of the Southeast survived by virtue of its position on the continent; that of the West underwent rapid and profound speciation resulting in a multitude of kinds.
Biome map of the world from Missouri Botanical Garden
For a map of the biomes of North America, see this site
Other sites of interest
Biomes of the world: Excellent summary
Biogeography of North America by Jonathan Adam
Critical ecoregions from the Sierra Club
Land Use History of North America: A major review, see in particular the report on the land-use history of the Baltimore-Washington area by Timothy Foresman
Baltimore Ecosystem Study
Desert USA: Excellent links
Prairie Restoration Links
Description of the Ecoregions of the United States by Robert G. Bailey
Ecological Subregions of the United States by W. Henry McNab & Peter E. Avers
"Effects of climate change on biological diversity in western North America: Losses and Mechanisms" by D.D. Murphy & S.B. Weiss (1992): Technical but most informative
Endangered Ecosystems of the United States: A Preliminary Assessment of Loss and Degradation: A technical report by Reed F. Noss, Edward T. LaRoe III & J. Michael Scott
Desert Ecosystems Resource Page
Rainforest Action Network
Conservation and Planetary Survival: An essay
Expanded PBIO 100 biome notes - extensive discussion and pictures of each biome- particularly good for deserts
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