Chapter 6: Biogeography: Climate, Biomes, and Terrestrial Biodiversity
6-1: Weather: A Brief Introduction
The physical properties of the atmosphere that occur at a particular place and time are the weather. These properties include temperature, humidity, precipitation, wind speed, etc.
Tornadoes are one example of extreme weather. Tornadoes typically form over land, at times when warm and cold air masses interact. The United States is the most tornado prone country in the world, especially Kansas and Oklahoma. About 5 or 6 years ago a small tornado touched down around here, knocking down the scoreboard on the baseball field and ripping part of the roof off one of the hotels.
Cyclones are large tropical storms that form over warm, open water, but can sometimes move over coastal areas and cause severe wind damage and flooding. Cyclones that form in the Atlantic Ocean are called hurricanes. The Southeast and Gulf coasts of America are particularly susceptible to hurricanes. The Gulf of Mexico in particular has very warm, shallow water, which is perfect for the generation and intensification of hurricanes.
6-2: Climate: A Brief Introduction
Climate is the term for the general pattern of weather conditions in an area over a long period of time. The two main factors which determine climate are temperature and precipitation.
Why do temperature and precipitation patterns vary in different places?
1. The sun strikes the area around the Equator at a much more direct angle than the poles, causing it to be much hotter there.
2. The Earth is tilted on its axis, meaning that at certain times, parts of the Earth are either tilted toward or away from the sun. This causes the progression of seasons.
3. The Earth is continuously rotating on its axis. This causes air and water currents to swirl in big circular motions instead of traveling in a straight line. This circular motion is called the Coriolis Effect. The end result of this is that the heat and water of the Earth tend to get a lot more mixed together than they otherwise would.
Ocean currents especially have a large influence on climate. The Gulf Stream is a warm water current that carries heat from the Caribbean along the East Coast of the U.S. all the way up to Northern Europe. The climatic conditions in England, Ireland, etc. are much warmer that they would otherwise be without the influence of the warm water of the Gulf Stream.
Conversely, a cold water current called the California Current works to cool the water off the West Coast of the U.S. This water circles down from the North Pacific near Alaska and causes the ocean water off the California coast to be rather cold year round.
An upwelling occurs when cold, nutrient rich water from near the ocean floor replaces the warm low-nutrient water on the surface. This leads to areas with extremely high NPP.
The El Nino-Southern Oscillation (ENSO) is the single biggest factor that has been observed to influence short-term global climate patterns. The occurrence of an El Nino is based on variation of water temperature in the Central Pacific Ocean. If the temperature of the water becomes higher than normal, an El Nino pattern occurs, whereas if the temperature of the water becomes lower than normal, a La Nina pattern occurs instead.
Both El Nino and La Nina patterns cause global changes in temperature and precipitation patterns. These changes are felt most strongly in the Southern Hemisphere, but are also very noticeable in the United States. The exact effects of El Nino or La Nina depend on your location and the season of the year. Consult the map diagrams for more detailed information.
Upwellings are also disturbed by El Nino conditions, causing normally nutrient-rich water to become completely non-productive. This can devastate the fishing industry of the coastal nations of Southern Hemisphere.
The last El Nino pattern began in September 2006 and lasted through early 2007. The El Nino of 1997-1998 was especially strong and had worldwide weather consequences that brought the subject to a wide audience.
6-3: Biomes: Climate and Life on Land
Biomes are determined by climate, particularly temperature and precipitation. The community of plants and animals in any biome is well-adapted to the particular climatic conditions found there.
A climate graph is a shows the average temperature and precipitation for each month of the year in a particular location. You should be able to read a climate graph and determine what type of biome it is by its distinctive temperature and precipitation patterns.
Make sure you are familiar with the generalized biome patterns of the United States (map found on p. 123).
Latitude and altitude are two factors which directly affect temperature and so determine what kind of biome is located in a particular area. The temperature becomes colder with increasing latitude and altitude. As the temperature changes, the biome also changes, as a new community is adapted to the different conditions.
Plant life in each biome has evolved traits to match the climatic conditions found there. Examples include:
1. There are no trees in the tundra because the cold, windy conditions would cause them to lose too much heat.
2. Many plants in the desert have no leaves to prevent dehydration.
3. Trees in tropical rain forests have wide, flat leaves to collect sunlight and allow for transpiration.
4. Trees in areas with cold or dry seasons often drop their leaves for a time to conserve energy.
5. Trees in areas with long winters and a short summer have needle-shaped leaves to slow down heat loss.
6-4: Desert Biomes
A desert is basically any area with very low precipitation, typically less than 10 inches per year. The temperature in a desert often fluctuates widely between day and night, but has no direct impact on the classification of an area as a desert.
Tropical deserts have high temperatures year round and extremely low rainfall. Examples include the Sahara and Kalahari in Africa, the Sonoran in Arizona, and the Outback of Australia.
Temperate deserts have hot summers and cooler winters with a little more rainfall than a tropical desert. Examples include the Atacama in South America and the Great Basin (parts of Utah, Nevada, and California) in the U.S.
Polar deserts have much colder winters, but can still reach high temperatures in the summer. An example is the Gobi Desert of China and Mongolia.
Deserts are a very difficult biome in which to survive, primarily due to the lack of water, but also the high temperatures. Plants and animals there must have very specific adaptations to deal with the harsh conditions there.
Deserts typically have a very low species biodiversity because so few plants and animals are adapted to survive in the extreme conditions presented there.
6-5: Grassland, Tundra, and Chaparral Biomes
Grasslands are areas with enough rainfall to support lots of grass and a few isolated trees. Drought and fire limit tree growth in these areas.
Tropical grasslands have consistently high temperatures with distinct wet and dry seasons. Tropical grasslands are also known as savannas. The largest area of tropical grassland is in Africa; there is no savanna in the United States. The long dry seasons are a tough test of adaptation.
The principle of resource partitioning can be seen here by the way in which the herbivorous animals use the vegetation in different ways so that there is little direct competition for the food resource.
Temperate grasslands experience hot summers and cold winters; the precipitation is more evenly spread out compared to a savanna and there is no prolonged dry season. Temperate grasslands are also known as prairies. The interior of the United States and Canada is a large area of temperate grassland. Typically called the Great Plains, it includes states like Iowa, Kansas, and Nebraska.
Temperate grasslands have the most fertile soil of any biome and most of them have been converted into agricultural areas to take advantage of this. This “breadbasket” in America has provided the country with a tremendous advantage over the years, providing us with an independent food supply.
Polar grasslands are very cold most of the year with only a brief growing summer growing season; most of the precipitation is snow. Polar grasslands are also known as the tundra. There are no trees at all in the tundra, only grasses, mosses and very small woody plants.
Arctic tundra occurs in northern Alaska, Canada and Siberia, just south of the polar ice. The ground here is a frozen mixture of soil and ice called permafrost. Only the top of the permafrost melts in the summer; the lower portions remain frozen year round. This leads to temporary marshes and bogs in the summer as the melted ice water is blocked from percolating by the permafrost underneath it. These pools are a breeding ground for all kinds of flies and mosquitoes. If the ten months of freezing cold winter weren’t bad enough, the two months of summer are bug infested nightmares. Needless to say, the arctic tundra is not heavily populated by humans.
Alpine tundra occurs just above the tree line on mountains. Conditions there are similar to the arctic tundra except for the lack of permafrost.
Temperate shrubland is a type of grassland found along certain coastal areas. The summers are long, hot, and dry and the winters are mild and somewhat rainy. Temperate shrublands are also known as chaparral. Much of Southern California (Los Angeles, San Diego) fits this description.
Chaparral often experiences large fires during the long dry season. The vegetation found there is adapted to survive these fires and quickly recover. However, problems have arisen because the climate of the chaparral is so appealing to people that large populations now live in these areas with regular fires. Every summer and fall news stories show firefighters battling to save homes in Southern California from brushfires.
6-6: Forest Biomes
Forests are areas with rainfall significant enough to support large numbers of trees.
Tropical rain forests have consistently hot temperatures and high rainfall. The trees there are broadleaf evergreen, not dropping their leaves. They are found near the Equator in places such as the Amazon of Brazil, Costa Rica in Central America, central Africa, and Southeast Asia.
Tropical rain forests have the highest level of species biodiversity of any biome. Covering about 2% of the Earth’s land surface, they are home to over 50% of the world’s total species. Tropical rain forests have nutrient-poor soils because almost all of the available nutrients are absorbed and contained within the bodies of the thick vegetation found there.
Tropical deciduous forests have consistently hot temperatures and significant rainfall, but with a noticeable dry season. The trees in this area are mostly deciduous, losing their leaves in the dry season to conserve energy. They can be found along the west coast of Mexico.
Temperate deciduous forests have warm summers and cool winters with fairly even abundant rainfall year round. Deciduous trees that drop their leaves in the winter (oak, hickory, maple, etc.) dominate in this biome. Most of the Eastern United States is some type of temperate deciduous forest, including the area where we live. Much of Europe, China, and Japan also share this biome.
The specific type of temperate deciduous forest we live in here is called an oak-hickory-pine forest after the three most common types of trees. Although pine trees are not deciduous, they are a minority in comparison to the many deciduous species that signify this biome.
Evergreen coniferous forests have long cold winters and short warm summers. Precipitation is relatively constant throughout the year, but generally not as abundant as in other forests. Evergreen coniferous forests are also known as taiga and boreal forests. They occur across large areas of the Northern U.S., most of Canada and Russia.
Boreal forests are dominated by cone-producing trees with needle shaped leaves that never drop off (pine, fir, spruce, cedar, etc.) Diversity within an evergreen coniferous forest is the lowest of all forests due to the relatively tough conditions, but the total land area they cover is huge, making them the largest biome (along with deserts).
Temperate rain forests are found in areas with conditions similar to temperate deciduous forests, but with extra precipitation. Large coniferous trees are the dominant vegetation here. Temperate rain forests are found along the west coast of North America, from southern Canada down to northern California. The extra moisture in this case comes from the proximity to the Pacific Ocean.
Isolated areas of temperate rain forest also exist in western North Carolina, in areas just west of the Appalachian Mountains, like Cashiers and Highlands. The prevailing westerly winds cause storms to always dump their rainfall on the same side of the mountains, meaning these particular areas get extra precipitation.
6-7: Mountain Biomes
Mountains cover 20% of the Earth’s surface. They are important ecologically because the type of biome present changes the higher you go up the mountain, as the temperature decreases. For this reason, mountains often have very high levels of species diversity.
For example as you climb Mount Kilimanjaro in Africa, you start in a tropical rain forest, which changes to temperate deciduous forest, which goes into an evergreen coniferous forest, then alpine tundra, and eventually ends in a summit of just rock and ice.
Also, because they are often isolated, mountains tend to have lots of species that are found only in that one specific location. These are called endemic species.
6-8: Lessons from Geographic Ecology
The main lesson is that the climate of an area (as determined by temperature and precipitation patterns) determines the type of biome. The plants and animals in each biome have evolved adaptations designed to fit the climate encountered there.