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Chapter 3: Climatic determinants of global patterns of biodiversity

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Edited by Peter Moyle & Douglas Kelt
By Douglas A. Kelt, September 2004

Wonderful Biodiversity

The number of species residing on spaceship Earth is staggering. Current estimates include over 4,600 species of mammals, about 9,000 birds, over 6,000 reptiles, more than 4,000 amphibians, and over 26,000 fishes – a total of some 50,000 types of vertebrates. The known number of invertebrate animals, such as clams, worms, octopus, spiders, lobsters, beetles, and butterflies, tops one million. And let's not forget plants, with at least 250,000 known species. Most specialists predict that these numbers merely touch the surface of the proverbial iceberg, and that upwards of 5-30 million or more different species may exist. Of course, all of these species aren't found in all parts of the globe, and some are quite restricted in their distribution. The objective of this chapter is to introduce you to the environmental factors that influence the contemporary distribution of all these species. The study of the geographical distribution of life is called biogeography, and those who study this are biogeographers.

It is well known that certain patterns in the distribution of species follow some simple rules. For example, monkeys and their relatives are generally found in tropical areas, and kangaroos are limited to Australia and some nearby islands. Elephants occur in Africa and parts of southern Asia, and polar bears and walrus are found only in Arctic areas of northern North America and Asia. Based on the distribution of species and groups of species, we can perceive the world as consisting of a series of biological regions, or biomes (Fig. 1). Biomes are largely defined in terms of climatic patterns, as we will soon discover.

Fig. 1. The world distribution of major terrestrial biomes

Understanding the factors that produce the major biomes of the world can provide important insights into the factors that have led to the incredible diversity of life that surrounds us, and is the focus of this chapter.

Two general classes of factors have led to the observed distribution of life. Historical factors include such events as the advance and retreat of glaciers, the lifting of mountains, formation of islands, and the slow but inexorable shifting of the continents across the surface of the globe. These are interesting in their own right, and constitute a major area of scientific inquiry. However, in this chapter we will focus on the second class of factors, which are ecological factors, and include such things as the timing and distribution of rainfall, annual (and extreme) temperatures, the influence of latitude, and proximity to oceans or other large water bodies, and elevation, to name a few.

Earth's Seasons
We will begin our discussion by reviewing the seasonal changes in the Earth's position relative to the sun. The orbit of the Earth is not in the same plane as its orbit around the sun. Rather, the Earth spins like a top that is tilted slightly off of this plane. To be precise, the Earth is tilted by 23.5° (see Fig. 2). This simple observation has profound implications. As the Earth rotates around the sun, this tilt is retained, such that the sun appears to shift north and south with the changing seasons. In the summer in California, the sun is located relatively far north. But, in the fall the sun gradually “moves” further south, and in the spring the sun appears to slowly shift northward again. This endless progression results in the seasons that characterize life in many parts of the world. Of course, when it is summer in California, it is winter in Australia, and vice versa.

Fig. 2. Seasonal variation in day length with latitude is due to the inclination of the earth on its axis. At the equinoxes, the sun is directly overhead at the equator, and all parts of the earth experience 12 hours of light and 12 hours of darkness each day. At the summer solstice, however, the 23.5° angle of inclination causes the sun to be directly over the Tropic of Cancer, while the Arctic Circle and areas farther north experience 24 hours of continuous daylight; at the same time all regions in the Southern Hemisphere experience less than 12 hours of daylight per day, and the sun never rises south of the Antarctic Circle.

Since ancient times, humans have marked the movements of the sun with various names and ceremonies. The Summer Solstice occurs on 22 June, and marks the day when the sun has made its greatest progression northward. The Winter Solstice, on 22 December, marks its southernmost progression. The half-way points are also marked by the Autumnal (Fall) Equinox (22 September) and the Vernal (Spring) Equinox (21 March). Historically, the year was believed to begin after the Autumnal Equinox, when the good times were finished, and the long, cold nights of winter were approaching. Because winter brought uncertainty and fear, it also was associated with frightful and ugly creatures – bats, cats, mice, goblins and ghouls, etc. – which ultimately lead to the tradition we now call Halloween, or the Dia de los Muertos (Day of the Dead).

Also related to the movements of the sun are specific latitudinal markers. Of course, the equator marks the Earth's midpoint; at both the Vernal and Autumnal Equinox the sun lies directly over the equator. On the Summer Solstice, when the sun is at it's most northern extent, it lies directly over the Tropic of Cancer, at 23.5° N; and, on the Winter Solstice it lies directly over the Tropic of Capricorn, located at 23.5° S. The tropics are often defined as the band of the Earth that lies between these two latitudes. When the sun moves far enough south, its light no longer shines on areas in the extreme north. The Arctic Circle is located at 66.3° N, and marks the latitude above which the sun never rises in the deepest of winter. Of course, in the summer the reverse is true – the sun never sets above this latitude. And there is an Antarctic Circle that shares these seasonal characteristics, although remember that summer in the northern hemisphere is winter in the southern hemisphere, and vice versa. As simple as these observations are, they form the basis that underlies much of meteorology and the global distribution of biomes.

The Solar Constant is the amount of solar energy that impacts the surface of the Earth's atmosphere. Solar energy forms the basis of most of life on earth, as it is central to photosynthesis, in which plants convert simple oxygen and carbon dioxide to sugars, which are then combined to form more complex carbohydrates. The Solar Constant is called a constant because it is essentially unchanging – every part of the atmosphere receives an equal amount of the sun's energy. However, because the Earth is round, the energy is absorbed over a larger area at higher latitudes (see Fig. 3). Additionally, solar energy must pass through a greater amount of atmosphere at higher latitudes, such that less total energy reaches a given area on the surface of the earth at higher latitudes. One consequence of this that is known to everybody is that higher latitudes generally remain cooler than lower latitudes.

Fig. 3. Average input of solar radiation to the earth's surface as a function of latitude. Heating is most intense when the sun is directly overhead, so that incoming solar radiation strikes perpendicular to the earth's surface. The higher latitudes are cooler than the tropics because the same quantity of solar radiation is dispersed over a greater surface area (a as opposed to a') and passed through a.

Table of Contents

1. Roots of the modern environmental dilemma: A brief history of the relationship between humans and wildlife
2. A history of wildlife in North America
3. Climatic determinants of global patterns of biodiversity
4. Biodiversity
5. Natural selection
6. Principles of ecology
7. Niche and habitat
8. Conservation biology
9. Conservation in the USA: legislative milestones
10. Alien invaders
11. Wildlife and Pollution
12. What you can do to save wildlife

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