3.1 INTRODUCTION
The loss of biodiversity has been called by E. O. Wilson the "folly for which our descendants are least likely to forgive us." This chapter will review the concept of biodiversity and its development and then describe general patterns in the global distribution of vertebrates. The purpose of this chapter is to show how the distribution of life on Earth is influenced by climate and geography. This means that protection of the diversity of life on Earth requires both a grand global strategy and intensely local and regional strategies for day to day conservation.
3.1.1 Definitions
Biodiversity is a term used to describe the whole of biological diversity, i.e., all living things: plants, animals, and microorganisms and all their interactions with each other and their environment. The term includes three levels of organization:
1) Ecosystem diversity refers to areas with sets of interacting populations of plants, animals, and microbes. Each place in the world has its own set, which are often defined by humans in terms of the most distinctive organisms and processes: a hardwood forest ecosystem, a temperate lake ecosystem, a Pacific coast rainforest. Implicit in the recognition of ecosystem diversity is that natural systems are more complex and species-rich than human dominated systems. A forest, with its myriad species of trees, birds, mammals, insects, and other organisms is more diverse than a neighboring agricultural field with its single dominant plant species and near lack of wildlife.
2) Species diversity refers to the number of different species of all types in a given area or region. Because it involves counting things (a uniquely human obsession), species diversity is what is most commonly thought of as biodiversity. It is particularly useful for comparing differents areas. The number of bird species in a hectare of tropical moist forest is much higher than the number in a hectare of temperate forest and, hence, the tropical forest is said to have a more diverse bird fauna.
3) Genetic diversity refers to the amount of genetic information (variation) carried within a population of organisms. It is genetic variation that allows a species to evolve. Evolution is constantly occuring in response to the constantly changing climatic and habitat conditions of planet Earth. Thus, in the long run, those species with a higher degree of genetic variation are thought to be less vulnerable to population collapse or extinction than those with a lower degree of genetic variation.
3.2 BACKGROUND: HOW MANY SPECIES ARE THERE?
Historically, humans lived with little knowledge of the world outside their immediate environs. Early peoples had an intimate relationship with local plants and animals they relied on for their food, clothing, and housing needs. Although this relationship required an appreciation for the seasonal rhythms and habits of local plants and wildlife, this knowledge did not reflect an understanding of the world on a larger scale. Europeans of several centuries ago believed that the organisms they experienced in their everyday lives were all that existed on the earth. The total number of species of plants and animals was thought to be in the hundreds, so an interested individual could know them all in colloquial terms. It was not until the mid-eighteenth century that a system for naming and classifying organisms was even developed, in response to the voyages of exploration that revealed to Europeans how diverse the world really was. Alfred Russell Wallace and Charles Darwin, who embarked on such trips, were especially important in demonstrating the rich diversity of life on earth and in explaining why llamas occurred only in South America, orangutans only in southeast Asia, and marine iguanas only on the Galapagos Islands.
A. R. Wallace was a British naturalist who from 1848-52 lived in South America and from 1854-62 in Indonesia for the sole purpose of collecting and describing plants and animals not found in the museums of Europe. Charles Darwin was a young British geologist who from 1831-36 sailed aboard the H.M.S. Beagle as the ship's naturalist on a mapping expedition to South America. Both men made extensive collections of previously unknown organisms and took comprehensive notes on plants and animals in native habitats. In an attempt to explain patterns in the distributions and appearances of animals they saw, both men, independently, "discovered" the mechanism of evolution, natural selection. Their travels also documented the rich diversity of life, vastly increasing our appreciation for the number and variety of animals on earth.
Today, approximately 1.4 million organisms (including 400,000 beetles) have been described, named, and classified (Wilson 1988), but biologists do not know even within an order of magnitude (i.e., 10 times) the total number of species. Estimates of the total number of species range from 3 to 50 million (Erwin 1988) and the difficulties involved in arriving at an accurate estimate have been the subject of investigation and conjecture for decades (May 1988). Even 50 million species may be a low number, as we are just beginning to appreciate, for example, the diversity of life in the soil (a mass of animals less than 5 mm long), life on the sea floor, and life in the forest canopy. Indeed, it can be argued that whatever number it hit upon, it should be doubled based on nematodes alone. Nematodes are small worms that parasitize all organisms (even you!) and it is likely that each non-nematode species supports at least one distinct nematode species!
The nematodes, as a group, are largely undescribed, reflecting the fact that all groups of organisms have not been equally surveyed. Some large, conspicuous groups (e.g., birds, mammals) are thought to be 90-99% described (Wilson 1988). Even with these groups, though, we are still discovering new species and even new genera (groups of related species). For example, a new species of antbird (a small, dark, forest-dwelling songbird) was described from Peru in April, 1990; a new species of monkey was described from Brazil in 1990 and a new parrot from Colombia was described in September, 1989; a new genus and species of Hawaiian honeycreeper was discovered on Maui in 1973. This latter species is already in imminent danger of extinction. Expeditions to poorly documented parts of the world are still turning up many new species of vertebrates; a biologist (James Patton) visiting the Andes Mountains in Columbia in July 1995 discovered, in two weeks, six new species of mammals: four rodents, a shrew, and a marsupial. Often even local diversity is overlooked. For example, the "red crossbill" is a favorite forest bird of bird watchers because of its bright color and ability to break apart pine cones with its unusual beak. However, the crossbill is not one species, as had long been thought, but seven species that can be told apart by eye with great difficulty (Benkman 1993). The birds, of course, have no problem telling each other apart; each has a distinctive set of calls. And just as advances in the technology of sailing ships allowed faunal surveys of many areas of the globe two centuries ago, new technologies, especially deep-diving submersibles, are allowing biologists access to regions previously unexplored. These surveys have discovered entire ecosystems with hundreds of new species in the deep sea near hydrothermal vents.
The effort to estimate the number of species is an interesting exercise, if for no other reason than to remind us that we humans are just one species among the millions. However, we will never really know how many species there are because the very term implies species are fixed entities. The founder of modern systematics, Carolus Linnaeus, indicated in his massive work Systema Naturae (1758) that species could be classified because each was an individual creation of God, fixed in it characters. Modern biologists more or less extended this concept by thinking of species as groups of interbreeding organisms with distinct morphological, physiological, behavioral, and ecological characteristics, each created in geographic isolation from other similar populations through a long evolutionary process.
Increasingly, we are realizing that species can be as dynamic as the landscapes in which they live. They can change through time and one species can give rise to hundreds of distinct local forms, each evolving in a few generations. For example, the threespine stickleback (Gasterosteus aculeatus), a small fish, has been called by evolutionary biologist Michael Bell "a superspecies made up of semispecies!" What we know as the species (the superspecies) is found in coastal regions throughout Eurasia and North America. Throughout its wide distribution it is easily recognizable to us as a threespine stickleback. The stickleback is a very widely distributed form because it can live in both salt and fresh water. The saltwater form can invade isolated coastal streams and lakes, where it settles down and becomes non-migratory ( a genetic characteristic, indicating natural selection at work). In each of the lakes and streams the new form can develop distinctive characteristics and will not interbreed with any marine sticklebacks it might encounter. In a few hundred years (or less), in essence, a new species has evolved (the semispecies). If a natural disaster occurs (e.g. , the eruption of Mt. St. Helens), the local species may go extinct in a wink of a lava flow, and the process starts again. A conventional taxonomist could spend a lifetime decribing each of the hundreds of isolated stickleback populations as a species, but there seems little point in doing so. More important is understanding and appreciating the beauty of the evolutionary process - and making sure it is allowed to continue.
The rest of this chapter will be concerned primarily with the results of the process of speciation: the distribution patterns of vertebrates (their biogeography). Although a relatively small proportion of the total number of described species (48,000 or about 3%), from our point of view, they are dominant over much of the earth's surface and comprise all the species that we commonly refer to as "wildlife."
3.3 DIVERSITY OF THE WORLD'S VERTEBRATES
3.3.1 Fishes
Fishes constitute the oldest group of vertebrates, with origins dating back to the Ordovician some 400 million years ago (Figure 4.1). The 24,600 species of extant (living) fish are usually subdivided into 3 groups (classes) based upon anatomical characteristics. The jawless vertebrates (lampreys and hagfishes), with many characteristics of the ancestral vertebrates, include about about 80 species. The Chondrichthyes or cartilaginous fishes are the sharks, rays, skates, and relatives and are represented by about 830 species. The Osteichthyes, or bony fishes, are the familiar perch, catfish, bass, trout, and relatives and are by far the most diverse group with around 23,700+ species (Moyle and Cech 1996). Although the oceans cover 70% of the Earth’s surface and contain 97% of the Earth’s water, only 58% of the fishes are marine. The rest are in fresh water, in the many isolated lakes and streams on our continents.
3.3.2 Amphibians
The class Amphibia consists of animals that are dependent upon water for reproduction but are otherwise quasi-terrestrial and live part of their lives on land. They descended from fishes about 350 million years ago and today are represented by about 9800 species such as the familiar frogs, toads, and salamanders. To see newts (salamanders) and tree frogs in their annual "Rite of Spring" seeking water for breeding, visit Stebbins' Cold Canyon Reserve, one of the field trip sites, in late February or March.
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Figure 3.1. Examples of the diversity of fishes. On the upper right is a hagfish, which lacks jaws. Below the hagfish is a cowshark, a 4 m-long representative of the evolutionary line of cartilaginous fishes. The evolutionary line of bony fishes is represented by the Australian lungfish (top center), and seahorse, moray eel, deep sea angler fish, and Sacramento perch (bottom, left to right). From Moyle (1993).
3.3.3 Reptiles
Reptiles, which evolved from amphibian ancestors, were the first truly terrestrial vertebrates. Some, such as marine tortoises, have since "reinvaded" the sea. Once the dominant vertebrates on earth (the Mesozoic, 210-65 million years ago, is known as the "Age of Dinosaurs"), there are approximately 6,300 species of reptiles today. Reptiles are represented by such animals as snakes, lizards, and turtles.
4.3.4 Birds
Birds are thought to have arisen from reptilian ancestors about 150 million years ago. Birds, the class Aves, are now represented by approximately 9,100 species and are well-adapted for their largely aerial existence, although a few forms (ostriches, rheas, several others) have lost their powers of flight and are now completely terrestrial. Others, such as the penguins, have decided to mimic fish and have become aquatic.
3.3.5 Mammals
Mammalia, the class to which we (Homo sapiens) belong, is thought to have arisen during the Triassic Period, over 200 million years ago, and contains some of the smallest (shrews and some bats weigh less than 4 g) and largest (blue whale, over 160,000 kg) vertebrates (Vaughan, 1972). The mammals include aerial (bats), marine (whales, dolphins, porpoises), and terrestrial forms. There are approximately 4 700 described species.
3.4 LATITUDINAL TRENDS IN VERTEBRATE DIVERSITY
In addition to the analysis of numbers of vertebrates in each of the vertebrate classes, it is of interest to know how these animals are distributed over the globe. There are several patterns to the way in which vertebrates are distributed, the most prominent of which is the latitudinal trend in diversity, such that diversity (number of species) is highest near the equator and lowest near the poles. Such a pattern was recognized over a century ago by Wallace (1876). In an attempt to simplify what could otherwise be a complex, confusing analysis, we will consider the world as being broken into 3 major latitudinal regions: the polar regions, areas greater than 67o (north and south) latitude; the temperate regions, between 23o and 67o (north and south) latitude; and the tropics, between the equator and 23o (north and south) latitude. We will also only consider the distributions of the five major groups of vertebrates: fish, amphibians, reptiles, birds, and mammals.
3.4.1 Polar Regions
For most of us, the arctic and the continent of Antarctica evoke images of barren, windswept, inhospitable land. For much of Antarctica, this is indeed the case. For the arctic, however, much of the land is clothed in vegetation, traversed by rivers, and dotted with lakes and ponds. Still, for animals to survive the rigors of the long arctic winter, they must possess adaptations that allow them to endure months of sub-freezing temperatures in total darkness when food is extremely scarce. Alternatively, they must be able to survive for months without eating. As you might expect, relatively few animals have evolved the means by which to cope with such conditions. The majority of vertebrates that live in the arctic are migratory, living there at times when conditions are favorable and leaving before severe conditions return.
For a terrestrial vertebrate, a prerequisite for survival in winter at high latitudes is the ability to thermoregulate; that is, to regulate body temperature, as to fail to do so would result in death due to freezing. Not surprisingly, therefore, reptiles and amphibians are absent from these areas. However, marine fishes are abundant in Polar regions. There is only one fish species that has truly adapted to life in fresh water in the Arctic, the blackfish. It is a small (up to 20 cm), very fat fish that lives in shallow water. It possesses the amazing ability to survive for long periods while partially frozen! Other freshwater fish, e.g., pike perch, trout perch, and suckers, have distributions which include limited arctic regions (Moyle and Cech 1996) but the blackfish is the only species whose distribution is exclusively arctic.
Marine environments in the polar regions have low biodiversity because of their low temperatures year-round. Even though the entire Arctic marine fish fauna may comprise about 125 species, the vast majority of these species are found at lower latitudes as well. The Antarctic, however, presents a different picture. Here, more than 90% of the roughly 100 species of marine fish are endemic (i.e., are restricted to this area and found nowhere else; DeWitt 1971). However, although species are few in polar regions, their abundance is great, thanks to the enormous abundance of food organisms that are able to grow during the brief polar summers. This abundance of food is what attracts whales to the polar regions. They spend their summers feeding, creating enormous fat stores (blubber) that allow them to live for months without feeding when they move to warmer waters for the winter.
Birds, although found in appreciable numbers at high latitudes, have many more species in other regions. Perhaps the best-known examples of antarctic residents are penguins, the oldest extant avian group, having evolved sometime before the Miocene (more than 30 million years ago). Penguins, however, are not restricted to this region; the Galapagos penguin lives at the equator. No bird family (group of related genera) is restricted in its distribution to the arctic, although several species migrate to and breed there. An indication of the paucity of species found in polar regions is provided by Irving (1972) who reported 14 species of birds from Ellesmere Island, northern Canada (82o N.), and by Ogilvie and Taylor (1967) who reported 28 species from Spitzbergen, an island in the Arctic Ocean (78o N).
Mammalian distributions are similar to those of birds. No family of mammals is restricted to arctic areas and no terrestrial mammal inhabits the Antarctic continent. Some species of mammals are restricted, however, to polar regions; these include terrestrial species such as the musk ox, polar bear, and arctic hare (all exclusively arctic) and marine mammals such as the walrus, beluga, and narwhal (in the arctic) and Weddell, crabeater, and leopard seal (in the antarctic).
3.4.2 Temperate Regions
In temperate zones, those regions between 23o and 67o north and south latitude, the picture changes dramatically. Whereas only one species of freshwater fish is endemic to the arctic, about 300 freshwater fish species are found in the Mississippi-Missouri River system alone and roughly 420 species of freshwater fish are found in the Soviet Union and Europe combined (Moyle and Cech, 1996). California has over 500 described species of fish, freshwater and marine.
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Figure 3.2. Diversity vs. latitude plots for three groups of terrestrial poikilotherms, showing what appear to be latitude-related anomalies in the region 15-30° that are probably a response to the less favorable conditions prevailing in the desert regions often found in those latitudes. Data showing highest number of genera in 5° latitude classes. Solid curve smoothed through points indicating highest diversity, dotted curve following the points suggestive of persistent anomaly. A: Genera of amphibians. B: Genera of lizards. C: Genera of snakes.
Amphibians, absent from arctic regions, are well represented in the mid-latitudes (Figure 3.2A). Amphibians, being poikilothermic, are dependent upon benign ambient conditions for reproduction, though they can survive long periods of inclement conditions such as a North American winter. Forty-seven species of amphibians are found in California (Laudenslayer and Grenfell, 1983).
Reptiles, too, are represented by more species in the temperate latitudes. The diversity of lizards is shown in Figure 3.2B and for snakes is shown in Figure 3.2C. Both of these figures show slight decreases in diversity for these groups between 15o and 30o latitude. These are the latitudes at which most of the world's deserts are found. There are 77 species of reptiles in California (Laudenslayer and Grenfell, 1983).
Birds really increase in diversity in temperate latitudes. For example, at least 88 bird species breed on the Labrador Peninsula of northern Canada (55o N.), 176 species breed in Maine (45o N.), and more than 300 species can be found in Texas (31o N.; Peterson, 1963). The total number of bird species found in California exceeds 540 (Laudenslayer and Grenfell, 1983); the total for all of North America is roughly 700 (Welty, 1982).
An indication of the latitudinal trend in mammalian diversity was provided by Simpson (1964) for continental North American mammals (Figure 4.3). Here again, species diversity is apparent with decreasing latitude. This analysis also shows that, superimposed on the latitudinal trend, is an effect due to elevation such that mountainous regions have more species of mammals than lowlands. There are 214 species of mammals in California (Laudenslayer and Grenfell, 1983).
4.4.3 Tropical Regions
The tropics, between 23o north and 23o south latitude, have the greatest diversity of life. It has been estimated that at least 75% of all species (plants, animals, microorganisms) exist in the tropics (Raven, 1988). Lest we conjure up images of steamy jungles filled with colorful birds and swarms of insects, it is instructive to recall the differences among the tropics of Africa, Asia, and South America.
In Africa, much of the region between 23o north and south latitude is the most inhospitable desert on earth. The Sahara stretches for 3,000,000 square miles and covers 25% of the continent. Rain forest is restricted to the west central part of Africa and covers less than 9% of the continent.
The Asian tropics are mostly lowland rain forest and richly deserve the "steamy jungle" image. A significant feature of southeast Asia is the preponderance of islands. Islands serve to isolate populations of organisms and facilitate speciation (the process of formation of new species).
The tropics of Central and South America are an extremely complex mosaic of lowland wet and dry forest and ecosystems that change with elevation such as high-elevation shrublands (paramo) and grasslands (puna). Rain forest covers about 32% and savanna about 38% of the South American continent.
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Figure 3.3. Species density contours for recent mammals of continental North America. The contour lines are isograms for numbers of continental (nonmarine and noninsular) species in quadrats 150 m (240 km) square. The "fronts" are lines of exceptionally rapid change that are multiples of the contour interval for the given region (after Simpson 1964).
A majority of all fish species are found in tropical waters. We can get an indication of the diversity of fish in the tropics by a consideration of two examples, one freshwater and one marine. Our first example is that provided by the dazzling array of coral reef fish. Something on the order of 30-40% of all marine fish species are in some way associated with tropical reefs and more than 2,200 species can be found in a large reef complex (Moyle and Cech 1996). Second, the Amazon River of South America, huge in comparison to most other river systems (3,700 miles long, drains a quarter of the South American continent), has over 2,400 species of fish. The Rio Negro, a tributary of the Amazon, contains more fish species than all the rivers of the United States combined!
As might be expected given the warmth and humidity of much of the tropics and the inability of amphibians to thermoregulate, this group reaches its greatest diversity here (Figure 4.2A). In fact, one of the three orders (groups of related families) of the class Amphibia, called caecilians (160 species of worm-like creatures), is restricted in its distribution to the tropics.
The two major groups of terrestrial reptiles, lizards (Figure 4.2B) and snakes (Figure 4.2C), are represented by more species in the tropics than in higher latitudes. The pattern is even more pronounced for turtles (Figure 4.4).
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Figure 3.4. Diversity of recent turtle genera. (data from Wermuth and Mertens, 1961).
Bird diversity is highest in the rain forests of the South American tropics where 86 families and over 2,700 species are found (Welty, 1982). Costa Rica, a tiny (50,700 km2 , 19,600 sq. mi.) Central American country, has over 750 species of birds and Colombia has well over 1,500. Mammals, too, are most diverse in the tropics. For example, Venezuela has 304 species, Bolivia 327 species, East Africa 351 species, and Zaire (central Africa) 427 species (Eisenberg, 1981). Much of this increase in diversity is due to a single order of mammals, the order Chiroptera: bats.
CONCLUSIONS
Historically, the distribution of life on earth was largely determined by interactions through evolutionary time with climate, geography, and other organisms. Now we humans have become the dominant "other organism" that all creatures have to contend with, as we drive some species to extinction, introduce other species to remote parts of the globe, change whole ecosystems, and even change climates. We humans generally assume we can exist independently of climate and geography - and are finding to our sorrow that is it not true.
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LITERATURE CITED
Brown, L. R. 1988. And Today We're Going To Talk About Biodiversity...That's Right, Biodiversity. pp. 446-449 in E. O. Wilson and F. M. Peter, eds., Biodiversity. Natl. Acad. Press, Washington, D.C.
Eisenberg, J. F. 1981. The Mammalian Radiations. Univ. Chicago Press, Chicago.
Erwin, T. L. 1988. The Tropical Forest Canopy: The Heart of Biotic Diversity. pp. 123-129 in E. O. Wilson and F. M. Peter, eds. Biodiversity. Natl. Acad. Press, Washington.
Frankel, O. H. 1974. Genetic Conservation: our evolutionary responsibility. Genetics 78:53-65.
Frankel, O. H. and M. E. Soule. 1981. Conservation and Evolution. Cambridge Univ. Press, Cambridge.
Laudenslayer, W. F. Jr. and W. E. Grenfell Jr. 1983. A list of amphibians, reptiles, birds and mammals of California. Outdoor Calif. Jan-Feb:5-14.
May, R. M. 1988. How many species are there on Earth? Science 241:1441-1448
Moyle. P.B. 1993. Fish: an enthusiast’s guide. University of California Press, Berkeley.
Moyle, P. B. and J. J. Cech, Jr. 1996. Fishes: An Introduction to Ichthyology, 3rd Ed. Prentice-Hall, Upper Saddle River, N.J.
Simpson, G. G. 1964. Species density of North American recent mammals. Syst. Zool. 13:57-73.
Vaughan, T. A. 1972. Mammalogy. W. B. Saunders, Philadelphia.
Wallace, A. R. 1876. The Geographical Distribution of Animals (2 volumes). Harper, N.Y. (reprinted in 1962 by Hafner Publ. Co., Inc., N.Y.).
Welty, J. C. 1976. The Life of Birds. Saunders, N.Y.
Wilson, E. O. and F. M. Peter. 1988. Biodiversity. Natl. Acad. Press, Washington, D.C.
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