A biome is a particularly useful biogeographic unit that results from large-scale climatic patterns. A portion of a biome, such as the tropical rain forest found in the Amazon basin, may cover thousands of hectares. It will haverecognizable vegetation features that impart a sameness that defines the unit. The fact that this region differs from others, such as the desert biome, is evident to even the most untrained observer. Frederic Clements (1874-1945) and Victor Shelford (1877-1968) observed in 1939 that not only was the vegetation characteristic, but that these were plant-animal formations with associated fauna. In the 1960s an ecological research effort, the International Biological Program, sought to characterize the structure and function of major biomes through intense study. American botanist and ecologist Robert Whittaker (1920-1980) summarized some characteristics of the biomes in his book Communities and Ecosystems, in which he arranged them graphically according to interacting gradients of mean annual temperature and moisture availability (see accompanying figure).
Mean annual precipitation and temperature of biomes. Source: Reprinted from Aber and Melillo, 1991, p. 16.
The Influence of Rainfall
You can bring this model to life though observation as you travel within the continental United States. The Interstate Highway System allows easy movement across the country along latitudes, parallel to the equator, or the longitudes, perpendicular at the equator. Travel on Interstate 80 from New York City to San Francisco during the summer growing season reveals an obvious gradient of change in vegetation. The deciduous forests of central New York State are lush green in most years, with trees predominating where human development has not occurred. In the Midwest, traveling from Illinois into Iowa and Nebraska, the trees are absent except along rivers, and grasses and broad-leaved nonwoody forbs cover the landscape like a carpet.Much of this region is now dominated by fields of grain such as corn and wheat, domesticated replacements of the native vegetation that once was predominant in the grasslands. (Remnants of the native prairie are rare.) In August, the green prairies begin to show some signs of the golden color of mature fields as the various annual grasses produce seeds that will ultimately produce plants for the following year. Meanwhile the perennials will die back to ground level by late fall, and then start growth again the following year.
The farther west one travels, the sparser the vegetation and the less luxuriant the crops, until dry short-grass prairies merge into deserts and badlands with more widely spaced plants and soil that is frequently bare. The predominant color throughout the year in the deserts tends toward earth tones of beige and brown with small green leaves being less evident. Two major north-south mountain barriers, the Rocky Mountains and the Sierra Nevada range in California, interrupt the gradual transition. In these areas, elevation-dependent differences produce a complex mosaic of vegetation that does not fit the overall pattern. Finally, on the West Coast, moisture-laden air deposits rain on the windward sides of the coastal mountains and abruptly changes the pattern. Lush temperate rain forest vegetation forms a margin along the western edges of California north of San Francisco, just as in coastal Oregon and Washington. Southward, the forests are replaced by coastal shrub lands because rainfall is not sufficient to support extensive forests in most places, and the inland deserts are even closer to the ocean shores. This east-west gradient across the country parallels a decline in moisture availability, the first of two major climatic factors that determine the character of biomes.
The Influence of Temperature
A similar trip along Interstate 75 in January from the Canadian border at Sault Ste. Marie, Michigan, to Florida provides evidence of the second major climatic factor gradient. In northern Michigan, it is likely that the ground will be snow covered most of the month. Ohio and Kentucky can present occasional periods with persistent snow cover; by the time you reach Georgia, however, snow generally disappears shortly after it falls, and Florida has only rain. This gradient is not so much a result of differences in volume of precipitation (the amount of rain or snowfall), but the prevailing temperatures. When the north to south temperature gradient overlays the east to west moisture gradient, the type of biological community that will be supported is determined. Similar interactions of temperature and moisture availability determine the presence of other biomes globally as indicated in the map of their distribution. Toward the poles, Canadian boreal forests change over to tundra and illustrate the results of extending the temperature gradient.
Tropical rain forests occur where the temperature is greater near the equator and daily tropical rains maintain high moisture levels. This tropical region along the equator forms a boundary with the temperature gradients in the Southern and Northern Hemispheres being mirror images of each other. In the tropical forest biomes, there is little seasonal change; the prevailing conditions are wet and warm. Seasonal temperature variation is minimal in regions adjacent to tropical forests, but ultimately at increasing distance from the equator, alternating wet and dry periods produce fluctuations in growth rates in subtropical seasonal forests. This moisture-dependent alternationis different from the temperate biomes covering the United States and Canada where the seasons are produced by the alternation of a cold winter and relatively warmer summer.
The classification of biogeographic areas into biomes is only one of several different methods of organizing terrestrial environments. Michael Barbour, in Terrestrial Ecosystems, gives a brief historical review of other classification systems, and Robert Bailey, in Ecosystem Geography, updates various ways of classifying the Earth's surface. Barbour uses the term vegetation types to classify landscape patterns and points out significant differences between the various subdivisions that can be identified, while Bailey prefers the term ecoregion as the unit of subdivision at the level of assemblages of landscapes.
Since the distribution of biomes is dependent on global climate patterns, one may question what impact global climatic change could have on these biogeographic units. A continuation of global warming would change the boundaries by moving warmer conditions closer to the poles.
In addition, the climatic changes do not just involve changes in temperature; they also involve changes in air movement patterns, alteration of temperatures (known as El Niño and La Niña) within the oceans, and storm distributions that change the annual growth of both plants and animals. This is not a new phenomenon on Earth. Cold periods have existed in the past, and biome boundaries have shifted in response. This is known from the pollen record left in lakes and bogs over many thousands of years. The challenge to plants and animals is not so much the extent or duration of expected temperature changes, but the rate of those changes. Vegetation can respond to changes that take hundreds or thousands of years to occur by gradually dispersing into new areas or evolving. It cannot move or reproduce fast enough to adapt to the same changes if they occur over tens or hundreds of years. The total ecologic impact of these potential rapid changes is not currently known.
