Chapter 4: Biodiversity
DIVERSITY OF THE WORLD'S VERTEBRATES
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 2004). 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.
Amphibians 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 (salamanders) and tree frogs in their annual "Rite of Spring" seeking water for breeding, visit Stebbins' Cold Canyon Reserve near Winters, California, one of the field trip sites, in late February or March.
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.
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.
Mammals, the hairy group of animals to which we humans belong, are thought to have arise, over 200 million years ago and spend most of their history skulking in the bushes. Mammals include the smallest (shrews and some bats weigh less than 4 g) and largest (blue whale, over 160,000 kg) vertebrates. The mammals include aerial (bats), marine (whales, dolphins, porpoises), and terrestrial forms. There are approximately 4,700 described species.
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. Chapter 3 (Biogeography) provides much of the detail as to why there are profound differences in the wildlife in different parts of the world. This section describes some of the patterns of biodiversity that result from the climatic and geologic patterns on planet Earth. There are several ways to look at how vertebrates are distributed, the most prominent of which is the latitudinal trend in diversity, such that species richness (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 consider the world as divided into 3 major latitudinal regions: the polar regions (areas greater than 67° north and south latitude); the temperate regions (areas between 23° and 67° north and south latitude); and the tropics, (the area between the equator and 23° north and south latitude). We also only consider the distributions of the five major groups of vertebrates: fish, amphibians, reptiles, birds, and mammals.
For most of us, the arctic and the continent of Antarctica evoke images of barren, windswept, inhospitable lands. 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, because to fail to do so would result in death due to freezing. Not surprisingly, therefore, reptiles and amphibians are absent from these areas. There are all sorts of wonderful adaptations for thermoregulation possessed by mammals and birds. The most obvious of these are good insulation (fat, fur, feathers) and an ability to migrate to warmer climates (many birds).
Despite their apparent inability to thermoregulate, marine fishes are abundant in Polar regions, taking advantage of the fact that deep water does not freeze, even if it slightly less than 0°C. 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, mainly in deep lakes or large rivers but the blackfish is the only species whose distribution is exclusively arctic (Moyle and Cech 2004).
Marine environments in the polar regions have low species richness because of their low temperatures year-round. Even though the entire Arctic marine fish fauna may comprise about 110 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 over 300 species of marine fish are endemic (i.e., are restricted to this area and found nowhere else; Moyle and Cech 2004). 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. 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 indicated by the presence of just 14 species of birds on Ellesmere Island, northern Canada (82° N), and just 28 species from Spitzbergen, an island in the Arctic Ocean (78° 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 seals (in the Antarctic).
In temperate zones, those regions between 23° and 67° north and south latitude, the picture changes dramatically, as the number of species increases dramatically. Whereas only one species of freshwater fish is endemic to the arctic, about 375 freshwater fish species are found in the Mississippi-Missouri River system alone and roughly 350 species of freshwater fish are found in the Soviet Union and Europe combined (Moyle and Cech 2004). Marine temperate areas have even higher species richness. At least 1200 species of fish are known from the Atlantic coast of North America, while probably twice that number are found along the Pacific coast (Moyle and Cech 2004). California alone has over 550 described species of marine fish.
Amphibians, absent from arctic regions, are well represented in the mid-latitudes. Amphibians, being poikilothermic (cold-blooded), are dependent upon benign ambient conditions for reproduction, though they can survive by hibernating through long periods of inclement conditions such as a North American winter. Fifty-one species of amphibians are found in California (CDFG 2003).
Reptiles, too, are represented by more species in the temperate latitudes. The diversity of lizards and snakes show slight decreases in richness between 15° and 30° latitude. These are the latitudes at which most of the world's deserts are found. There are 84 species of reptiles in California (CDFG 2003), but concentrated in desert regions.
Birds really increase in diversity in temperate latitudes. For example, at least 88 bird species breed on the Labrador Peninsula of northern Canada (55° N), 176 species breed in Maine (45° N), and more than 300 species can be found in Texas (31° N). The total number of bird species found in California exceeds 600 (CDFG 2003); the total for all of North America is roughly 950, of a worldwide total of about 10,000. 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 197 species of mammals in California (CDFG 2003)
The tropics, between 23° north and 23° 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 23° 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.
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 2004). 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 regulate their internal temperatures, except through behavior, this group reaches its greatest richness here. 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 and snakes, are represented by more species in the tropics than in higher latitudes.
Bird diversity is highest in the rain forests of the South American tropics where 86 families and over 2,700 species are found. Costa Rica, a tiny (50,700 sq km, 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.
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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