Chapter 3: Climatic determinants of global patterns of 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.
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.
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.
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.
Why does it rain so much in the Tropics?
Now we are ready to evaluate the consequences of the Earth’s tilt and its round shape. On average, the sun spends more time directly above the equator than any other part of the earth. And, when the sun is directly over the equator, there will be more solar radiation striking a given area of the surface at the equator than at higher latitudes (either north or south). This leads to our first important observation – the air immediately over the equator will tend to heat up and, because warm air rises, equatorial air tends to rise. As it rises, however, it also escapes the gravitational pull of the earth, and expands. A basic law of physics holds that expanding gasses will cool (this is the basis of the modern refrigerator and will be explained in detail when you take a physics course – for now, I ask that you accept this as fact). Therefore, this warm and rising air begins to cool. Another basic law of physics states that warm air holds more water than colder air (again, take this on faith for the moment). Of course, water can exist in either a gas or a liquid state, and humid air simply contains much water in gas form, with individual water molecules bouncing around but not grouping as water droplets. When this air is cooled, however, there is a temperature at which these molecules condense to form droplets; the temperature at which this occurs is called the “dew point.” As air rises above the equator, it is heavily laden with moisture (we all know that the tropics are humid). However, as this air rises it cools and reaches its dew point, after which the water molecules condense to form droplets, and clouds begin to form. These continue to rise, the droplets coalesce to bigger drops, and soon rain drops are produced. Thus, the tropics are characterized by their high amounts of rainfall.
You have probably heard meteorologists talk about high pressure zones and low pressure zones. High atmospheric pressure is generally associated with good weather, whereas low pressure is associated with cloudy or stormy weather. The reason for this is simple. Air consists of molecules of oxygen, nitrogen, carbon dioxide, and other gases, and thus it has mass. Any mass is subject to the inexorable pull of gravity, so even a mass of air puts some pressure on the surface of the earth. However, when air rises the pressure it exerts is reduced, and we refer to this as a region of low pressure. So, in areas of low pressure, air is rising and it generally follows the pattern we just described for the tropics – air rises, cools, water condenses to form clouds, and we often get rain. Now we’ll discuss a portion of the earth that is characterized by high atmospheric pressure.
Why are deserts generally located at about 30° latitude?
Air rising in the tropics can’t rise forever. If it did, then it would escape the Earth entirely and we would be left with a barren, lifeless planet. Rather, as it rises it dissipates somewhat, it cools, and as it cools it begins to condense again, and becomes heavier. But, air below continues to rise. The air has to go somewhere, and at a high altitude it diverts towards the north and south (see Fig. 4). Air continues to move north (or south), but it is cooling now, and tends to begin falling back towards the surface of the Earth. This air generally manages to reach about 30° latitude before subsiding towards the earth. As it drops in altitude, this air begins warming again, but remember that it has already lost most of its water before moving north of the tropics. Because warm air holds more moisture than cool air, this warming air acts as a sponge, literally drawing moisture from the environment around it. This is our second important observation – as this air descends at about 30° latitude, it literally pulls moisture from the environment. If you refer back to the map of global biomes you will see that this corresponds to the latitude at which most of the world’s deserts are found.
To complete one cycle then, the air that descends at about 30° latitude then moves north or south along the Earth’s surface (Fig. 4). That which moves towards the equator again becomes captured in the cycle of rising air that we discussed above. This cycle was initially described by a meteorologist named Hadley, and the “cell” of air movement is now known as the Hadley cell.
There are two other cells, although we won’t worry about their names. Air moves towards the poles from the 30° region, and rises again at about 60° latitude. This air rises, cools, forms clouds, and spreads both north and south. Finally, at the poles we have another mass of air that descends towards the surface. Thus, there is a generally predictable pattern of air movement over the earth, and it is largely responsible for the distribution of tropical areas as well as of the world’s major deserts. This basic pattern can explain why the tropics are moist and lush whereas regions about 30° N and S are relatively xeric (dry).
What are the “horse latitudes” and the “doldrums”?
Imagine that you are at the north pole – if you stood directly over the axis of the earth’s rotation you would slowly turn a complete circle. This would require about 24 hours. If you walked south a little bit, you would find that a 24 hour cycle would move you a certain distance – how great that distance is would depend upon how far from the pole you walked. The circumference of the earth is about 4,000 km at the equator, so that if you were standing on the equator, you would travel about 4,000 km every 24 hours. This seems a little esoteric, and might make a useful question for “Jeopardy.” But now imagine you were in a weather balloon, and you flew north from the equator with the north-bound air in the Hadley cell. You would start your trip with a certain velocity – about 4,000 km/day towards the east – but as you move north, the ground below you is traveling east at a slower and slower rate. Another tenet of physics is referred to as the law of conservation of angular momentum. It is a mouthful, but it means that you don’t simply lose this 4,000 km / day velocity as you move north. You keep moving east at this rate.
However, the Earth beneath you moves east more slowly. Thus, rather than traveling north, you will appear to begin veering towards the east, which is towards your right (Fig. 5). The further north you travel, the more rapidly you will veer east. The same thing happens to the air in the Hadley cell – as it moves north it becomes shifted eastward. The reciprocal situation would involve an air mass moving towards the equator (see Fig. 5). As the air mass approaches the equator, the earth beneath it begins moving faster, and the air mass appears to veer westward, which again is towards the right of the direction the air mass is traveling. This intriguing pattern is called the Coriolis Effect. The important thing to understand is that in the northern hemisphere the Coriolis effect results in air shifting to the right when it moves to either higher or lower latitude. You can probably work out the dynamics to realize that the reverse is true in the southern hemisphere – air masses shift to their left.
Now, recall that air generally rises at the equator, descends at about 30°, and then moves either south towards the equator or north towards the rising air at about 60°. Air moving south from 30° will veer westward, whereas air moving north from 30° will veer eastward (in both cases, the air veers to the right of its line of travel). Now, many global features were given names during the days when sailing ships surveyed the earth, and trading ships carried supplies between Europe and the Americas. When these ships traveled from Europe towards the New World they would travel south to intercept westward flowing winds (“easterlies“) – as a result, these became known as the tradewinds to reflect their importance in commerce. These ships would return to Europe by a northern route, capturing eastward winds that were subsequently called westerlies. Of course, the southern hemisphere has a similar set of winds at comparable latitudes.
In the vicinity of the equator, however, air is generally moving up, and there is no lateral movement. Here, winds are poor and many early sailing ships became stranded for weeks or more. These areas are called the equatorial doldrums, and this is where the phrase “in the doldrums” arose. Similarly, at about 30° air is descending but not providing much lateral motion. Many ships transporting soldiers became stranded here as well, and many soldiers were forced to resort to eating their horses as food supplies grew thin – thus arose the term “horse latitudes.” You may have heard that the quickest route from Europe to North America in these times was not the most direct. Because of the Coriolis effect, ships could make this trip most rapidly by sailing south from Europe, then west with the trade winds, and then north along the coast of North America.
The major patterns of ocean circulation
Wind drives the surface waters of the oceans, and therefore is largely responsible for the major patterns of oceanic circulation (compare Fig. 5 with Fig. 6). This results in the clockwise circulation patterns observed in the northern hemisphere, and the counter-clockwise patterns in the southern hemisphere. However, there is one very important exception to this, and it results in a particularly important phenomenon. We will use the coast of northern California to exemplify this, as it is a regional event and has important ramifications for both the ecology and the economy of our state.
If we look at currents along the coast of California, we note that they run parallel to the coast of the northern third of the state. However, the southern two thirds of the state begin to curve eastward, while ocean currents continue southward. As ocean water moves south along the coast of northern California, it is subjected to the Coriolis Effect, and it is shifted to the right (which is towards the west), resulting in surface waters being moved offshore (Fig. 7).
This water has to be replaced, however, and the water that replaces it wells up from deeper levels off the continental shelf. This deeper water has two important characteristics. First, it is colder than the surface water. Cold water holds more oxygen than warm water (another physics fact that you need to accept at the moment) so it provides a basic nutrient needed by plankton. Second, it is very rich in nutrients; this is because it comes from deeper regions where light does not penetrate, and where photosynthesis does not occur. Therefore, all of the nutrients that are needed by plankton are in relatively high abundance. As a result, areas characterized by upwelling waters also are characterized by having cool and nutrient rich near-shore water. How does this affect these areas?
First, the high concentrations of oxygen and nutrients make these areas very productive. Plankton develop and occur at very high concentrations, feeding fishes which in turn feed larger fishes, birds, and marine mammals. The famous anchovy fisheries off the coast of Peru are a direct result of upwelling. In the Peruvian case, waters are moving north along the coast, and are deflected westward by the Coriolis Effect. The same is true along the northeast Pacific, where upwelling provides rich waters that support tremendous populations of salmon.
Second, when cold water comes in contact with warm air, fog develops. Thus, along the coast of northern California we have very productive forests that are largely dependent upon summer fog to provide moisture through the dry season. Since this region is particularly interesting to us, let’s spend a bit of time on this.
Climate in California
California is hailed internationally as a haven for sun worshippers. We have wonderful weather along much of our southern coast, and the beaches are a haven for bikinis, muscle-bound show-offs, and many marine mammals as well. However, the coast of northern California calls a very different image to our minds – one of fog, lush forests, and rain. Additionally, the climate of California is very seasonal, with relatively dry air and clear skies throughout the summer and fall, and clouds and rain during the winter. A famous song proclaimed “it never rains in California, but girl don’t they warn ya, it pours, man it pours.” When it pours in California, it can really pour. But, what causes the rather substantial change in weather between summer and winter?
To understand this, we need to digress briefly to understand a couple of additional features of climate in the North Pacific. At about the latitude of Hawaii there is a zone of high pressure (air is subsiding, causing the famous weather characterizing Hawaii). This area is called the Hawaiian High Pressure Region, or the Hawaiian High. Air that descends here then moves either south or north. Air that moves north reaches about the Aleutian Islands before encountering polar air that is moving south. When these meet, two things happen. First, they have nowhere to go but up, so we find air rising in this region. This creates a low pressure region that is referred to as the Aleutian Low, and the associated clouds and storm development. However, because we have warm southern air meeting cold northern air, there is an additional degree of turbulence, so that the storms here also tend to be somewhat violent. Because mid to upper troposphere winds generally move eastward (the so-called “westerlies”), any storms that form along the Aleutian Low tend to be displaced eastward, and they often drop a lot of rain and snow on the North American continent. These storms also are shifted south by a high pressure region in Canada, so these storms usually end up hitting North America somewhat south of their origin.
Now, this all gets interesting (and particularly relevant) when we superimpose the seasonal shifting of the earth relative to the sun onto the position of the Aleutian Low and the related weather patterns. Recall that the position of the sun relative to the earth shifts seasonally, from as far north as the Tropic of Cancer to as far south as the Tropic of Capricorn. As the sun shifts north and south, the major bands of regions of rising and subsiding air masses also shift north and south, although not as much as the sun does. However, the major storm generator for the Pacific northwest, the Aleutian high pressure region, shifts from about 60° N in the summer to about 50° N in the winter. So, in summer this low pressure region lies sufficiently north that the storms generated there collide with North America fairly far north. However, winter brings a double whammy to this scenario.
First, the air that is descending from the Polar regions is colder in the winter than in the summer. When this air meets the air moving north from the Hawaiian High, the greater difference in their temperatures results in even stronger storms than in the summer.
Additionally, the low pressure zone itself has shifted south in response to the suns southerly migration, and the storms that are generated plow right into the northern coastal regions of our fair country. This is why northern California gets so much rain in the winter while staying warm and dry in the summer.
Fortunately for people in southern California, the storm tracks generally reach no further south than the middle of the state. Of course, these storms also are responsible for the multi-million dollar skiing industry.
Finally, to fully understand the distribution of biodiversity in California we need to appreciate local dynamics that operate on relatively small scales. We discussed coastal upwelling already. This is responsible for the lush nature of northern California forests, the existence of the spectacular redwood trees, and the surreal beauty of the Old Man’s Beard (often erroneously called Spanish moss, it is not really a moss) that hangs from many trees in the coastal range. Three other local influences are particularly important.
If you looked carefully at Fig. 1 you noticed that not all deserts occur right around 30° latitude. In fact, if you have ever traveled through much of Nevada, Utah, or eastern Oregon, you probably recall that this region seems very desert-like. Indeed, this is the Great Basin Desert, one of four deserts in North America (the others are the Sonoran, Chihuahuan, and the Mojave). But the Great Basin Desert extends from about southern Nevada (about 35°) to southern Alberta (about 55° N), and is not a product of air subsiding at about 30° N. The Great Basin Desert lies in the zone of westerly winds, which roll from the Pacific Ocean full of moisture and then begin crossing California. Before reaching Nevada, however, these air masses cross two mountain ranges, the Coast Range and the Sierra Nevada (Fig. 9). As air climbs up these mountains it cools with increasing altitude. As it cools, the air reaches its dew point, and rain forms. By the time this air crests the Sierra Nevada, much of the moisture that it carried from the ocean has been lost. This air then descends the eastern slope of the Sierra Nevada, and warms. Just like the subsiding air at 30° latitude, warming air holds more water, and results in a very arid region. The Great Basin Desert owes its existence largely to its geographic position in the “shadow” of the Sierra Nevada. Regions lying to the lee of a mountain range are therefore called rainshadows, and deserts that are products of such patterns are called rainshadow deserts. Some other rainshadow deserts are Patagonia and the Monte Desert of South America (constituting much of the country of Argentina), and much of inner Asia such as the Gobi Desert, which lies in the rainshadow of the Himalaya Mountain Range.
In the northern hemisphere the sun warms south-facing slopes much more than it does north-facing slopes, resulting in greater temperatures and more rapid desiccation there. A common consequence of this is that very different plant formations characterize the southern and northern flanks of hills or mountains. This is particularly true and apparent in relatively dry regions (chaparral regions of southern California, for example), but is also apparent in temperate and even boreal areas. The greater moisture availability on north-facing slopes often leads to more lush and dense stands of plants. Animals that require dense vegetation, either for protection from predators or for food resources, often may be restricted to north-facing slopes. A drive along Berryessa Reservoir, not far from Davis, provides many opportunities to see this type of pattern.
Finally, the microclimate found at different points along a mountain may be quite different. Most campers know that cold air settles in the depths of valleys, and that a camp set up at the bottom of a ravine, while attractive during the heat of the day, may become quite cool at night. This is because when air cools at night, it descends the slopes of a ravine to meet cool, descending air from the other side of the ravine. These cool air masses merge and continue down the valley. If you are camped in the center of such a ravine you will have colder air at night, and you also will have stronger breezes than if you had camped slightly upslope. This has implications for the types of plants and animals you will find in these areas. If you have ever camped in such a spot, you may have noticed that in the evening, air descended the valley. Now you can see why. You also may have noticed that breezes reverse in the morning, and run up the valley. This is because the reverse dynamic occurs when the sun warms the landscape and the air, causing the air to rise and to flow uphill.
In the Santa Ana Mountains of southern California, fog funnels through passes and condenses on the needles of knob-cone pine trees. This adds as much as 10 cm of precipitation annually, allowing this tree to extend its growth into rainless periods. It also favors the local occurrence of Coulter pine and other species that require more moisture than would otherwise characterize this area.
Consequences of global climate patterns
We have learned much about the global distribution of climate in this chapter. What can this tell us about the distribution of plant and animal life? Because climate strongly influences soil production we might expect to find very different soils under different climatic regimes. This is the case and it obviously influences the distribution of plants. Additionally, plant growth often is limited by water availability, so the plant abundance might be expected to relate to climate – and it does; tropical trees and vines are highly unlikely to survive in a desert environment. On the other hand, many plant species that do quite well in deserts have very poor defenses against fungal pathogens, and these fare poorly in moist climates. Additionally, understanding climate gives an indication of where you would find animals that are particularly good at surviving on little water or animals that require high humidity and/or frequent rains. If you are studying animals that live at high latitudes, understanding the climatic regime there greatly improves your understanding of their natural history. If you work with marine systems this will give you some important insights into the transport of the eggs of your focal species (consider the global circulation of water), or of their food items (think of upwelling). At smaller scales we often see differences in plant and animal communities across very short distances. The three local influences discussed above have strong influences on where animals move in their daily travels. Think of the redwood trees that make California’s coast so renowned. Think of the salmon populations that are well fed off our coasts. Look at a map of California and consider the distribution of deserts, grasslands, oak woodlands, conifer forest, alpine and tundra, and consider the daily and seasonal changes in temperature and precipitation that organisms face in these different habitats.
However, on a simpler level it is useful to recall that through process of evolution animals become adapted to the environment in which they live. The large variations in local climate that we have discussed here – from large scale differences such as the difference between tropical and boreal habitats to small scale differences such as the foggy redwood forests of northern California versus the warm and dry Sierra foothills to the hot and very dry Mojave and Colorado Deserts – provide a seemingly endless array of environments to which species may adapt. Thus, we see different lizard species in Palm Beach than we do in Eureka, and different plants as well. We even see different plant communities in the coastal and Sierra mountains of the state, and on different faces of a single hillside. Understanding local and regional climate and the factors governing these can provide important insights into the influences that have led to such tremendous diversity in California as well as elsewhere.
Finally, the general similarities in climate at very distant parts of the globe often lead to superficially similar habitats, and it is not uncommon to find similar adaptations to these environments by unrelated taxa. When animals find evolve similar means of coping with their environments in different parts of the world we say that they have converged on a similar strategy. Some examples of ecological convergence in mammals include (Fig. 10) capybaras (the world’s largest rodent; South America) and hippopotamus (Africa), kangaroo rats (North America) and jerboas (Asia and Africa), wolves (North America and Eurasia) and the Tasmanian wolf (really a marsupial; Australia), and flying squirrels (North America) and flying phalangers (another marsupial; Australia). Numerous examples also exist for birds, reptiles, fishes, plants, and other groups.
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
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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