Setting aside our focus on increasing complexity for now, we should remind ourselves of the splendid richness of life on land. In this chapter, we take a look at how terrestrial biodiversity is distributed around the planet, and in the next chapter we explore significant patterns in those distributions. Certainly biodiversity is not evenly distributed around the globe.1 Some regions are graced with a multitude of species, while other areas have few visible residents. Terrestrial species numbers are determined by two primary factors: temperature and moisture.
Where temperatures are warm and rainfall is plentiful, biological diversity reaches its full expression. Where temperatures are frigid or moisture is scarce we find wind-swept ice fields and barren desert. Extreme conditions constrain the exuberance of life and have a global signature. Our planet is marked by several distinctive climatic zones.
At the poles, where sunlight is less direct, freezing temperatures predominate. The South Pole is in the center of a single frigid continent—Antarctica—surrounded by the turbulent Southern Ocean. In contrast, the North Pole is at the center of an ocean, much of it frozen. Neither pole provides a friendly environment for complex terrestrial life, though the cold waters do support a wealth of marine creatures. Our planet's polar biotas are antipodal, widely separated by temperate and tropical zones. In the natural world, polar bears, emblematic of the Arctic, have never encountered wild penguins, confined to southern seas.
The world's most severe deserts also have a global pattern; they are clustered around the Tropic of Cancer (at 23.5° north latitude) and the Tropic of Capricorn (at 23.5° south latitude). Here, longer days are coupled with intense sunshine during the summer months. This is where Earth's highest temperatures are often recorded. Tropical rains do not reach these latitudes, and zones of high atmospheric pressure, hovering around 30° north and 30° south latitudes, result in broad bands of limited rainfall. The world's largest desert region ranges across the entire northern tier of Africa (the Sahara), continuing eastward across Arabia into western India. Similarly, severe dry areas are found in the subtropics of the Southern Hemisphere, especially in southern Africa and across central Australia.
In addition to polar frigidity and subtropical belts of aridity, our planet features broad zones of tropical and “temperate” vegetation. All told, our planet supports many different biomes, all contributing to the variety of species richness around the globe.2 Unfortunately, defining and delimiting different biomes or vegetation types is often a matter of convenience: more art than science. As rainfall diminishes or temperatures change across hundreds of miles, local floras change gradually in stature and species composition. Distinguishing North America's tall-grass prairie from the short-grass prairie or semi-desert steppe can be quite arbitrary. These biomes all intergrade, and their varied habitats share many species of plants and animals as well. It is only over longer distances or over sharp changes in elevation that one can clearly see that the vegetation has changed. However arbitrary, dividing the natural world into biogeographic realms, biotic provinces, ecoregions, biomes, or plant communities helps us understand the natural world. Our land surfaces have been divided into as many as 867 ecoregion units.3 Similarly, 426 freshwater ecoregions have been delineated.4 Though useful on a local scale, such smaller units are too detailed an inventory for our brief survey. On the other hand, zoological geographers have recognized as few as six major faunal realms around the world: too broad a brush for our discussion. Instead, let's survey the larger geographical regions by their major vegetation zones. Since green plants capture the energy of sunlight and provide much of the community's physical structure, vegetation is an appropriate focus for our overview.
More significantly, vegetation is usually the primary factor in determining overall regional biodiversity, both in terms of numbers of species and variety of lineages.5 For example, a recent study found that the high numbers of herbivorous insects in tropical rain forests is due to the high number of different plant species, not to more restricted feeding patterns on the part of the insects.6 Across Australia, bird diversity is closely correlated with vegetation and evapotranspiration rates (a measure of water loss for plants).7 Examining vegetation around the world, let's begin at the “top” of the world, where the numbers are low.
THE FRIGID NORTH
Modern cartographers place the North Pole at the top of their maps, and from this perspective we see that a large majority of our planet's land surfaces are located in the Northern Hemisphere. Both the Eastern and Western hemispheres have huge land areas bordering the Arctic Ocean. In dramatic contrast, the Southern Hemisphere has wide oceans covering much of its breadth, and harbors many isolated islands as well. Thus, with the great majority of land surfaces at the “top” of our planet, we begin our journey in the north.
Bordering the frigid Arctic Ocean, we find a circumpolar belt of vegetation called the tundra. Ranging from a few centimeters to less than a meter (3.3 ft.) in height, this vegetation is composed of hardy and wide-ranging plant species, but there are no trees. Northernmost Russia differs little in its species listings from northernmost Canada and Alaska. Here, many species of mosses, liverworts, and lichens contribute to the ground cover. They survive the harshest conditions, on soils permanently frozen just a few inches below the surface. Members of the blueberry family, sedges, fescue grass, and a number of colorful flowers are significant elements of the tundra vegetation. In more protected sites, stunted little shrublets of willow, larch, spruce, and alder can be found. Because of the climate's severity, tundra plants are perennials, hanging on to the landscape by the growth of previous years. With a growing season of less than fifty days, flowers often bloom in one year and form their seeds the following summer. With the unfortunate exception of blood-sucking mosquitoes, insects are few. Larger mammals of the tundra are the caribou (called reindeer in Eurasia), hares, a few rodents, and arctic foxes. Small passerine birds are few, with larger, warmly insulated geese, ptarmigans, and owls active during the summer. Larger land animals survive the long winter by hibernating or migrating southward into the boreal forest. In contrast, many larger birds, seals, walrus, and polar bears live along the edge of the cold northern sea, feeding on marine life. They border but are not part of the tundra ecosystem.
Surely, one the most fortunate qualities of our planet is that it spins at a slight angle in respect to the plane of our orbit around the Sun. Consequently, summery days in the Arctic north may be few in number but long in hours. Without this extended summertime sunshine, land bordering the Arctic Ocean could not support a tundra vegetation. But the winters are long and cold, with soils thawing only a few inches above the permafrost in summertime. And because roots are unable to absorb moisture from a frozen substrate, trees cannot sustain evaporation from their leaves in this environment.
Overall, the northern tundra ecosystem supports fewer than one thousand species of higher plants. That's not a lot of biodiversity over almost one-fifth of the world's land surface. We'll be using higher plants or flowering plant numbers in many of our comparisons. (Higher plants—vascular plants—have an internal plumbing system. They include flowering plants, conifers, and other seed plants, as well as ferns and fern allies. The “lower,” nonvascular plants include mosses and liverworts, while lichens are part of the fungal kingdom.) Importantly, tundra-like vegetation is not limited to polar areas. Even in the tropics, high windswept mountain tops support tundra-like meadows, alpine grasslands, and rocky surfaces decorated only with lichens and mosses.
NORTHERN (BOREAL) FORESTS
Leaving the tundra and moving south, we enter the northern boreal forest. This biome is also called the northern conifer forest, evergreen needle-leaf forest, or by the Russian designation taiga. One of the largest ecosystems on our planet, boreal forest covers about 29 percent of the world's forested land.8 Toward the north, scattered little trees intergrade with the tundra, becoming more numerous and taller toward the south. With soils often frozen only a few feet below the surface, drainage is limited and lakes and bogs are common. Cone-bearing trees with needle-like leaves dominate the boreal forest; these include the spruces (Picea), firs (Abies), pines (Pinus), and larches (Larix). Slender conical shapes help these trees survive long fierce winters, as wide lower branches brace the short upper branches when burdened with ice and snow. Flowering plants are represented among the trees by willows (Salix), birches (Betula), aspen (Populus), and alders (Alnus). Members of the rose, blueberry, composite, and other families contribute to the shrubby layer. Grasses and sedges, as well as mosses, liverworts, and lichens, are significant in the ground cover. Lichens, in fact, are a primary food for the caribou here, as they are in the tundra. (We'll discuss lichens in chapter 6). Conifer seeds nourish voles, lemmings, and a variety of birds. It has been estimated that these extensive forests of the north include a third of all the world's trees. But that's a tree count, not a species count. These cooler biomes of the north are not rich in species. Alaska and the adjacent Yukon are home to about 1,560 vascular plants, while Norway numbers 1,715. (These numbers include species of both the tundra and the boreal forest.)
Though similar in overall form and stature, the boreal forests of North America and northern Eurasia differ slightly in species composition. Among larger mammals, the populations may differ only enough to be called subspecies. Both regions are home to wolves, foxes, beavers, elk, lynx, badgers, bears, and the world's largest deer: the moose. Because of this, the northern boreal forest zone is usually divided into two: the American, or Nearctic, and the Eurasian, or Palearctic realms. Biogeographic terms like Nearctic and Palearctic reflect the impact of Columbus's discovery; for Europeans the Americas were new and unexpected! Traveling further south, we encounter more temperate climates and a vegetation much richer in species.
THE NORTHERN TEMPERATE ZONE
Below about 50° north latitude, we find “temperate” regions encompassing a great variety of biomes over a large portion of the world's land surfaces. In North America, and beginning in the west, we find dense evergreen conifer forests bordering the Pacific coast, ranging from Alaska to northern California, where we find the world's tallest trees. In fact redwood forests may comprise as much as 3,500 tons of biomass per hectare (2.47 acres), fully four times as weighty as a tropical rain forest. And though these coastal conifer forests are resplendent in lofty tree trunks, they lack the biodiversity found in many other temperate forests.
Moving eastward from the coast, we encounter the results of massive tectonic convulsion: the sharply sculpted sierras, high intermountain basins, and older Rocky Mountains. Beginning in southern Alaska and northwestern Canada, this rugged topography extends southward into western Mexico. Conifer forests and grasslands dominate these rugged landscapes, thanks to limited rainfall, higher elevations, and frequent fires. Moving farther eastward, the sharply rising Rocky Mountains mark the western edge of North America's broad interior plains. Ranging from Alberta in the north to Texas in the south, the Great Plains experience restricted rainfall, frigid winters, and hot summers. Before their slaughter in the late nineteenth century, sixty million bison roamed North America's broad interior plains, in addition to mule deer, elk, and the fleet pronghorn antelope. Drier in the west and moister toward the east, the Great Plains support short-grass steppe in the west and tall-grass prairies in the east. Partly as a result of fire, trees are often restricted to river valleys and moist protected slopes. By setting fires regularly, Native Americans expanded tall-grass prairies eastward into more densely forested Illinois and Indiana.
With cold winds from the north and warm moist air from the Gulf of Mexico, North America's central plains are subjected to a rare weather phenomenon: tornadoes. When warm moist air from the Gulf of Mexico clashes with cold dry air coming down from Canada, we have the possibility of storms with limited area but horrendous power. America's central plains have something no other place on Earth can match—“Tornado Alley”!
Eastern North America, with more reliable rainfall, milder temperatures, and the Appalachian Mountains, supports evergreen coniferous forest, broadleaf deciduous forest, and mixtures of the two. Such woodlands once carpeted the region from the western Great Lakes southward to the Gulf of Mexico, and from eastern Canada to Florida. Ecologists recognize about ten different forest assemblages in this region.9 The flora is especially rich in the southern Appalachians, home to the world's highest concentration of salamander species! Again, though we call them “temperate,” the eastern United States are subject to long and frigid winters. Only southern Florida escapes freezing temperatures, thanks to the Atlantic's nearby Gulf Stream.
For an overview of species richness, the multi-volume Flora of North America covers all the seed plant species of North America, north of Mexico. All biomes are included, from Arctic tundra, evergreen conifer forests, and deciduous forests, to grasslands and deserts. Overall, this flora numbers about 19,500 species of native vascular plants. As one would expect over so vast an area, species richness varies greatly. The richest single biome of this large region is the broadleaf deciduous forest of the eastern Appalachians. The northeastern United States, from east of the Great Plains and north of Virginia to southern Canada, numbers 4,500 species of higher plants. North and South Carolina have around 3,500 native species (together with about 1,000 introductions). In contrast, and with a fifth of the area of all lower forty-eight states, our drier interior Great Plains number only around 3,100 species of native plants. California, with a great variety of biomes over a large and mountainous topography, numbers about 5,700 species of higher plants. Differing patterns of rainfall and altitude allow California to support vegetation ranging from seasonally parched chaparral in the south to tall evergreen conifer forests along the north coast, and a variety of montane and subdesert vegetation as well. Here live the world's largest living things—Giant Sequoias—and the oldest—Bristlecone Pines, some more than four thousand years in age.
Despite a fine diversity of habitats and large areas, North America's biodiversity does not compare well with species numbers in the tropics. Returning to the Midwest again, but shifting to butterflies, we find that the state of Michigan supports only 134 species of butterflies over its 150,780 km2. Compare that to tropical Panama, which numbers 1,550 species of butterflies over its 78,200 km2 area. Clearly the temperate zone is a not an easy place to make a living. Remaining at similar latitudes, we'll move to the world's largest temperate region: Eurasia.
NORTHERN EURASIAN BIOMES
The world's largest single land area covers much of the Eastern hemisphere's northern half; arbitrarily separated into Europe and Asia. Northern Europe's biotas are dramatically separated from their southern neighbors by the Pyrenees, Alps, and Balkans. These mountain chains—running west to east—inhibit interchange between Europe's northern flora and the drier and warmer Mediterranean vegetation to the south. France's flora of higher plants numbers around 4,600 species, while more northerly Germany has only 2,700 species. Though poorer in species than its eastern American equivalent, the flora of northern Europe is dominated by many of the same genera, such as oaks, maples, birches, and a variety of conifers. Europe's deciduous forests intergrade with boreal forest, both to the north and eastward across Russia.10
Western and central Asia's huge interior is also bounded along its southern edge by mountains, from the Caucasus to the Himalayas. Situated between the Black and Caspian Seas, the Caucasus region harbors a robust 6,300 species of vascular plants, with 1,600 of them found nowhere else. These high numbers have led to this region being designated a “biodiversity hotspot.”11 (We will discuss “hotspots” in the next chapter). Western Asia's broad interior supports montane conifer and deciduous forests in moister areas, and grasslands intergrading with semi-desert steppes in the high plateau north of the Himalayas. Unlike North America, western Asia's interior is drier to the south and moister to the north.
Resembling eastern North America, eastern Asia has more plentiful rainfall and was once covered with broad-leaved deciduous and evergreen forests. Higher elevations, from the Himalayas into China, are dominated by conifer forests. Eastern Asia's broad-leaved forests once ranged, uninterrupted, from the boreal north to the tropical forests of Southeast Asia. Despite dense human populations, China supports an estimated flora of 32,870 species.12 This number far exceeds the 19,500 species in all of North America north of Mexico! Varied and complex mountains in southern China and the eastern Himalayas are part of the reason, but history has also played a role.
Why might China be so much richer in species and genera of vascular plants than either eastern North America or Europe? The answer seems to involve recurrent glacial advances over the last two million years (the ice ages or Pleistocene epoch), coupled with each region's specific topography. While the Himalayas are oriented east-west, a majority of mountains in southwestern China are oriented north-south. Here, plants and animals could migrate in response to warming or cooling climates, north or south. Simply stated, eastern Asia suffered less extinction than occurred in the floras of northern Europe and northeastern America. In these two regions, plant migration was restricted by the mountains of southern Europe, and the Gulf of Mexico; today's species numbers reflect that history.
THE WORLD'S “MEDITERRANEAN” FLORAS
In contrast to its northern neighbors, the floras of southern Europe, the Mediterranean area, and southwestern Asia are unusually rich in species. This Mediterranean Flora has hot and dry summers, followed by a dry fall. Rains begin during the cold of winter and extend into spring. As temperatures rise in springtime, a majority of plants bloom together in one grand spectacle. Half the plant species are annuals, while shrubs and small trees bearing small stiff evergreen leaves dominate the landscape. In addition, highly dissected mountains provide a wide range of habitats around the Mediterranean Sea and southwestern Asia. In spite of limited rainfall, the Mediterranean flora is estimated to harbor 25,000 vascular plant species, a number that easily exceeds the flora of all of North America!
Greece, though relatively small in area, is home to about 5,000 species of higher plants, while larger Turkey numbers around 8,600 species. Complex mountain topography and isolated islands are major factors in sustaining biological diversity in this part of the world. However, the Mediterranean and Middle East are not the only parts of the world to support this special biome.
Our planet is graced with four additional “Mediterranean floras.” All are subtropical, located between twenty and thirty degrees from the equator, and mostly found near the western flanks of large continental land masses. In addition to the Mediterranean, they occur in southern California, central Chile, southwestern Australia, and at the southern tip of Africa. All are open woodlands or shrub lands with drought-tolerant plant communities. Summer and fall are hot and dry, while a colder winter and early spring bring the rainy period. And, with a long dry season, all are fire prone. Despite being widely distant, these biomes have a number of similarities. With limited rain fall, animal life is not especially rich. However, there are unusual similarities in the faunas. Southern California's chaparral supports 235 species of non-marine birds, while the matorral of central Chile supports 230 non-marine bird species; a striking similarity.
Each of the Mediterranean floras is famous for the spectacle of their springtime flowering. In late September, five-day tours allow tourists to survey wildflowers near Perth, Western Australia. Current estimates claim that this Mediterranean-type flora numbers 7,380 native species of vascular plants. That's a terrific number for so dry a corner of the world. South Africa's Mediterranean-type flora is home to an incredible 9,000 native vascular plant species (something we'll come back to in the next chapter). More generally, the world's Mediterranean floras are home to about 48,000 species of higher plants. That's close to 20 percent of the world's higher plant species on only 5 percent of its land surface and explains why these floras have given us so many useful food sources and ornamental plants!13
THE AMERICAN TROPICS
Excepting high mountain-tops, life in the tropics does not experience the challenge of a killing frost. Here, with sufficient moisture, species reach their greatest numbers. Biogeographers refer to the American tropics as the Neotropics (again, referring back to a world that was new for Europeans). Moving from north to south, the Neotropics begin in Mexico. Rich in floristic diversity and with complex topography, Mexico has thirty-two distinctive vegetation types, supporting an estimated 26,000 species of higher plants.14
Central America, embroidered with complex mountains and many volcanoes, is flanked by a moist Caribbean slope on the east, and a seasonally dry Pacific coast. We botanists define the Mesoamerican floristic region as ranging from the narrowed Isthmus of Tehuantepec in Mexico, south to the Panama-Colombia border. With about 17,000 species of flowering plants, the Mesoamerican flora approximates the species numbers for all of North America north of Mexico.
In designating Mesoamerica a “‘biodiversity hotspot” Russel Mittermeier and his associates estimated a fauna of 2,859 non-fish vertebrate species in this region, with 1,159 (40.5 percent) found nowhere else.15 Reptiles contribute 685 species to these totals. These many species live in a variety of Mesoamerican biomes, ranging from seasonally dry deciduous woodlands and highland conifer forests to true lowland rainforests, montane evergreen forests, and high montane alpine formations.
SOUTH AMERICA
South America supports the world's most biodiverse region. Running along this continent's entire western edge are the Andes Mountains. Not only the largest mountain range to be found anywhere in the world, this system runs from north to south and across the equator. Thanks to the north-south orientation, cold-requiring plants and animals were able to migrate along the Andes’ highest elevations and survive through glacial cycles. Even more important, the Andes’ grand variety of elevations and complex topography affords many varying habitats; allowing Colombia, Ecuador, and Peru to support floras of more than 16,000 species of vascular plants each. The number of recorded bird species in Colombia and in Peru is around 1,800, more than twice the 770 recorded for all of the United States and Canada.
After the Andes, the Amazon basin is the second most obvious physiographic feature of South America. Covering 7,050,000 km2 (2,722,000 sq. mi.), the Amazon drains about one-fourth of the continent. With over a thousand tributaries, the world's largest river system contains about one-fifth of the world's flowing fresh water. The overall number of freshwater fish species for the Amazon basin is estimated to be 3,000, more species than live in the Atlantic ocean! Despite its vastness, the basin is relatively flat. Combining low relief with strongly seasonal rainfall, some places in the basin have river levels rising and falling as much as ten meters (thirty-three feet) each year! These annual cycles result in large areas of seasonally flooded forest (varzea) and continuously flooded swamp forest (igapo). The highest species numbers, however, are found in the non-flooded forests on higher ground (terra firme)—especially along the eastern slopes of the Andes Mountains.
Other areas of South America support distinctive floras as well. The northeastern Guiana Shield is marked by isolated table mountains harboring many unique species. In Brazil, dry scrubland in the northeast, remnant evergreen forests along the southern Atlantic coast, and a rich deciduous vegetation in the south support many endemic species. These floras, together with the Amazonian basin, give Brazil an estimated 56,000 higher plant species! And that number keeps growing; 2,875 new flowering plant species were described from Brazil between 1990 and 2006!16 The cerrado, a seasonally deciduous woodland, is especially rich in both species numbers and endemics. Around 6,400 higher plant species grow in this biome, together with 837 species of birds and 1,000 species of butterflies. Endangered by development, Brazil's cerrado has been designated another biodiversity hotspot.17 Grasslands are also important to South America's species richness, especially in the Orinoco basin, in the Andean highlands, and over much of Argentina. Cooler temperate forests at the southern tip of the continent and coastal Pacific deserts add other distinctive biomes.
Regional ecological provinces across South America can be divided in a variety of ways, with the Andes and lowland Amazonia the largest. However, the Andes can be subdivided along the north-south axis, near the Ecuador/Peruvian border, where there is a depression in the mountains. A more important division is elevation itself: above about one thousand meters elevation, moist eastern slopes support montane cloud forests. On the far drier Pacific slope, subdesert, grassland, and scrub abound. The highest elevations, between three thousand and five thousand meters, have their own specific biomes; here is where the potato was first cultivated. All together, these many biomes over huge areas give South America its high species numbers.18
TROPICAL SOUTHEAST ASIA AND NEARBY ISLANDS
As in the Neotropical world, the Paleotropics contribute greatly to the world's biodiversity. Biomes in India range from parched desert to evergreen rain forests, with the subcontinent numbering about 25,000 higher plant species. Southeast Asia tends to have greater rainfall, and this region includes many island archipelagos. Malaysia, a long narrow peninsula, is home to about 15,500 species of higher plants. The island of Borneo has about 13,000 flowering plant species, with Mount Kinabalu (4,101 m; 13,455 ft.) supporting many endemic species.
Not only are the rain forests of Southeast Asia rich in species, some are unusual in physical structure and composition. Many of these forests are dominated by a single family of flowering trees, the Dipterocarpaceae. These trees have two unusual features. The first is well-known: unpredictable flowering after several years of not producing any flowers or fruits. Not only do a variety of species and genera join together in these unpredictable flowering extravaganzas, they fruit synchronously! (El Niño weather oscillations may be the cause of these unpredictable cycles). The resulting pattern of huge fruit production, followed by several years with little to eat, is a challenging environment. Consequently, the forests of this part of the world do not have quite as many animals living on the forest floor as do Neotropical rain forests.
A second feature of these Dipterocarp forests is the height of their trees. American rain forests may have emergents reaching 50 meters (165 feet) in height, but rarely do they reach higher. In contrast, trees reaching 70 m (230 feet) are not uncommon in the forests of Southeast Asia. The dipterocarps, with their slender trunks and high crowns, give the forest canopy a tall and more open, spindly, appearance. Local animals have responded to this unique canopy architecture. These are the only forests where you can find flying frogs, flying snakes, and a flying lizard. Of course, they're not really flying; these creatures glide from tree-to-tree with flaps of skin along bodies and tails and webbed fingers. No other forests in the world support such a variety of gliding animals. What puzzled me was that these tall slender trunks carried a crown of relatively few leaves. A recent study explains this phenomenon: Dipterocarps appear to photosynthesize more efficiently than trees in other families; they simply don't need as many leaves!
Also, there is history! Two remarkably different faunas abut here, along what is called “Wallace's Line.” Running roughly from the Indonesian island of Lombok northward between Borneo and the Celebes, this imaginary line has animals such as elephants, deer, leopards, and monkeys on its western side, and marsupials, a greater number of lizards, and birds of paradise to the east. Zoogeographers use Wallace's Line to separate the Oriental Realm from the Australian Realm. (We'll analyze “Wallace's Line” in the next chapter.)
AUSTRALIA: A WORLD APART
Australia is an odd continent with a strange biota. Just about flat, only 5 percent of its surface rises more than 100 m (328 ft.) above sea level. The lack of recent geological activity—there are no volcanoes—has resulted in weathered nutrient-poor soils. In addition, recent tectonic displacement has shifted Australia from a moister, more southerly location northward into hotter and drier subtropical latitudes. Fossils indicate that Australia has lost much of its earlier flora, with severe drying over the last twenty-five million years. Today, two-thirds of Australia has an annual rainfall of less than 500 mm (19.5 in.). Poor soils, together with unpredictable rains and frequent fires, make this the world's least productive continent.19
Isolated in the Southern Ocean over many millions of years, island Australia's fauna lacks the more advanced placental mammals, such as monkeys (primates), deer (ungulates), and cats (carnivores). (Dog-like dingoes arrived more recently with people.) Not only does Australia sustain marsupials, such as kangaroos, wallabies, wombats, and koalas, it harbors an even more ancient mammalian lineage: the monotremes. With skeletal features resembling reptiles, these are the only living mammals giving birth by laying leathery eggs! The semi-aquatic duck-billed platypus (eastern Australia) and several species of spiny echidnas (Australia and New Guinea) are the only survivors of this ancient group. In addition, the bird and reptile faunas are rich in species. All told, Australia has the most unusual land vertebrate fauna in all the world.
Despite the dry conditions and poor soils, Australia's higher plants number around 15,000 species over an area of about 7,899,850 km2. Many landscapes are dominated by trees in the genera Eucalyptus, Acacia, and Casuarina—trees and shrubs that tend to remain green even through severe drought. Intense fires have resulted in many woody plants having seeds protected by thick woody structures. Hakea, a genus of woody shrubs, has solid woody fruits about the size of a walnut that split down the middle to release a single winged seed. Banksia inflorescences are thick colorful, upright spikes with many flowers densely arrayed around a columnar center. That central axis becomes a thick woody cone, with seeds developing inside; odd, lip-like openings on the surface provide exit for the deeply hidden seeds. Eucalyptus trees protect their seeds within a little urn-like woody fruit, topped with a circular lid. These woody fruits all open after a hot burn to release the protected seeds. Concordant with both fire and drought, Australia has few trees with fleshy fruit and, consequently, very few frugivorous birds or mammals. Australia, however, is not the only continent that has been sharply affected by drought.
AFRICA SOUTH OF THE SAHARA
Bordered along its north by both the Sahara and Arabian deserts, biogeographers delineate the African realm as Africa South of the Sahara. Celebrated for a rich and spectacular mammalian fauna, Africa is a continent well-endowed in biodiversity. Here are the world's only grass savannas where large grazing animals still roam freely by the thousands. The Serengeti plains have been estimated to support four million zebras, wildebeests, and gazelles, with about three thousand lions culling them. Rich rain forests persist along coastal West Africa and over the Congo River's wide basin. Our closest relatives—chimps, bonobos, and gorillas—still live in some of these moist evergreen forests.
Eastern Africa differs greatly from the western half; it is marked by the Great Rift Valley system, one of our planet's most distinctive geological features. Beginning in Mozambique, this system of elevated highlands and north-south oriented valleys extends northward through Tanzania, Kenya, Uganda, and Ethiopia into the Red Sea. Geological uplift and volcanism along the Rift have given eastern Africa diverse topography, rich soils, and a grand variety of habitats, ranging from lowland semi-deserts and grasslands to acacia woodlands and higher mountains bedecked with evergreen forests. It was in this rich and diverse landscape where a two-legged primate became human.
Africa's fauna includes many striking endemic mammals, such as hippopotamuses, giraffes, elephant shrews, naked mole rats, and bush babies (small nocturnal primates). The pig-sized and termite-eating aardvark once ranged over all sub-Saharan Africa and resembles nothing else. Colorful and often large, the birds of Africa are another glorious aspect of the fauna. However, and in strong contrast to the fauna, Africa's tropical flora is relatively poor in species numbers.
When counting plant species, tropical Africa doesn't measure up to either the American tropics or tropical Asia. For example, little Singapore island, at the end of the Malay Peninsula, is home to eighteen genera and fifty-one species of native palms. That's close to the fifteen genera and seventy-two species of palms native to all of Africa! The native higher plants of Ethiopia probably number around 7,000 species, despite Ethiopia's ample area (at 1,127,130 km2 it's bigger than Texas). Though twenty times the area, Ethiopia's species numbers are far fewer than those of little Costa Rica, with around 10,600 species on about 51,100 km2. Both countries are tropical, sit about ten degrees north of the equator, get their primary rains between April and October, and both have elevated montane habitats. Yet Ethiopia doesn't come close to Costa Rica's floristic richness. An astounding 1,300 species of orchids have been found in little Costa Rica. In contrast, Ethiopia's native flora contains fewer than 200 orchid species!
On the western side of Africa, the flora of Nigeria, with an area of about 923,770 km2, is home to only 4,715 higher plant species—again, much less than little Costa Rica. The Democratic Republic of Congo, with an area of 2,345,410 km2 and vast areas of rain forest, sustains a flora estimated at around 11,000 species. A few animal lineages are also poorer in Africa than in tropical Asia and the Americas. Comparing the butterflies of three tropical countries with similar areas, we find that Liberia in western Africa has 720 species, the Malay Peninsula has 1,031, while Panama has 1,550 species. Liberia with 111,370 km2 has fewer than half the butterfly species of Panama with about 78,200 km2.20 These are significant differences!
However, species numbers are very different at the southern end of Africa, where a more temperate flora numbers an astonishing 23,400 higher plant species.21 What can explain why the huge central core of Africa—tropical Africa—numbers only around 27,000 species?22 This is less than the flora of China and similar to individual countries in the northern Andes, all with smaller areas. Why is Africa's tropical region so poor in plant species?
Several factors may have played a role in reduced floristic species richness in Africa's tropical midsection. First of all, there are no large mountain chains in tropical Africa, although areas along the eastern rift are elevated. Also, central Africa is bordered with many harsh deserts. The Sahara sweeps across the entire northern fifth of the continent. Sudan, with 2,505,800 km2, has a flora of only around 3,200 native plant species. The Namib and Kalahari are large deserts covering much of southwestern Africa. In addition, even moister areas of Africa are often seasonally stressed for rainfall. Nowhere in Ethiopia does the rain fall as reliably and as generously as it does on the Caribbean slopes of little Costa Rica. However, the primary reason for tropical Africa's paucity of plant species is most likely historical. Africa probably suffered one or several crises of drought in the not-too-distant past.23 Also, it is still dry. Annual leaf-area indices—as measured from satellites—are much lower for Africa than they are for South America. Consider those African plant genera that are rich in species numbers; they are grasses, acacias, cactus-like euphorbias, and members of the hardy thorn-bush flora. An unusual aspect of Africa's flora is that it is not particularly poor in families or genera of flowering plants, compared to the American and Asian tropics, suggesting that widespread drought decimated the vegetation, leaving fewer species but having a lesser effect on higher ranks of plants.24
Biogeographers once placed Africa-south-of-the-Sahara together with Madagascar into a single “Ethiopian realm.” However, the biota of the island of Madagascar is so unusual that it is now considered its own zoogeographic realm.
MADAGASCAR AND OTHER ISLAND BIOTAS
Moored just east of southern Africa, in the Indian Ocean, we find one the world's most distinctive biotas. Island Madagascar is the home of an endemic and ancient primate lineage: the lemurs. With pointed dog-like faces, these monkey relatives resemble fossil primates from earlier times. Despite being close to Africa, this island has not a single ungulate species (pigs, antelopes, and their kin). Likewise, there are no true cats, no native dogs, and only one lineage of rodents. Of some 209 native and regularly breeding species of birds, a little over half are found nowhere else! (Since most birds fly, this is an impressive percentage.) This island was also the home of giant predatory flightless birds—the elephant birds—as well as a number of larger lemurs, small hippos, and a giant eagle. Climate changes, beginning 2,500 years ago, and the arrival of humans, about 1,200 years ago, resulted in their extinction.
Geologists estimate that Madagascar first ripped loose from Africa about 160 million years ago, then separated from India about 80 million years later. Madagascar has major mountain ranges running along its entire eastern flank. Once blanketed by lush evergreen forests, these mountains produce a rain shadow to the west, where the vegetation is seasonally dry. The island's arid southwest supports an unusual spiny shrubland, with odd succulents and thick-trunked baobab trees. Despite the proximity to Africa, the flora has ten families not found elsewhere. By latest count, Madagascar may have as many as 17,000 species of higher plants.25 Of these, ferns number over 600 species, compared to only 500 species in all of Africa! Palms are well represented, with sixteen genera having 170 species; 165 of these are found nowhere else in the world. In fact, 96 percent of Madagascar's trees and shrubs are species found nowhere else.
Long isolation has also resulted in a very unusual fauna. With an area similar to California or France, Madagascar is home to 191 native terrestrial species of mammals, 346 species of reptiles, and over 300 species of frogs and toads.26 While these numbers may not seem unusually high, all the non-flying mammal species and nearly all the amphibian species are found nowhere else! Clearly, Madagascar is distinguished by having one of the most unique biotas on the planet.
New Zealand is another island nation with a unique fauna and flora. Part of an active undersea ridge, formed where two tectonic plates abut, both the north and south islands have high mountains of recent vintage. Warm to cool-temperate, the islands are largely covered with evergreen forests and a flora whose relationships lie with Tasmania, Australia, and southernmost South America. Coniferous trees are especially well-represented in the forests of New Zealand. Indigenous flowering plants number about 2,400 species, with 86 percent endemic. Despite its isolation, New Zealand does not have a single endemic family of flowering plants; perhaps the flora developed too recently. New Zealand's fauna has a few outstanding endemics. Exterminated on the mainland, but still surviving on rocky islands off the coast, is the Tuatara (Sphenodon), an iguana-like reptile whose lineage has been distinct for almost two hundred million years. As in Madagascar, New Zealand was home to giant flightless birds, the moas. They vanished after humans arrived about eight hundred years ago. Today, a related chicken-sized flightless bird, the nocturnal kiwi, survives in dense forests.27 All together, the native birds of New Zealand number 287 species, 74 of them endemic.
Isolation has resulted in the distinctive biotas of both New Zealand and Australia, but further on out, in the Pacific, we find the world's most isolated biota, that of Hawaii. Formed by a “hot volcanic vent” under the ocean floor, the Hawaiian archipelago has been affected by plate movement over more than forty million years. The most westward of the islands, Kauai, is also the oldest at approximately six million years. The youngest and most eastward of the islands, Hawaii, continues to grow with newly erupting lava. Because new islands formed over the “stationary volcanic hotspot,” progressively younger islands are formed to the east as the plate moves westward. Thus, while today's oldest large island may be only six million years old, species could have spread from earlier islands—islands now eroded into the sea. Because of this historical progression and remote isolation, Hawaii is a unique laboratory for the study of dispersal, adaptation, and recent speciation.28
Isolated by more than 1,000 miles (1,609 km) from larger land areas, about 90 percent of Hawaii's thousand native plant species are endemic. Surprisingly, and despite the family's minute and easily dispersed seeds, Hawaii has only two native species of orchid! Similarly, and since they don't travel well across large ocean distances, there were no native freshwater fish, amphibians, reptiles, or mammals on the Hawaiian Islands! Unfortunately, isolation has resulted in vulnerability. New invaders, brought in by human activity, have led to the loss of many native species. Around sixty species of native Hawaiian plants survive with wild populations of fewer than a dozen individuals; they are not likely to be with us for long. Recent discoveries suggest that Hawaii was once the home of several smaller flightless bird species. Again, all vanished shortly after human colonization. Today, Hawaiian birds number 112 species, with 32 endemic and 54 recently introduced; the rest are oceanic travelers. Continuing introduction of plants, animals, and their diseases further threatens the native biota of Hawaii. No other part of the United States has been so vulnerable to invasive species.
Interestingly, after arriving on an island lush with vegetation and with few competitors, some groups expanded explosively. The fruit fly genus, Drosophila, has done exactly that: with approximately two thousand species around the world, almost one thousand are found only in Hawaii! This grand diversification, coupled with the fact that Drosophila melanogaster has been the subject of intense genetic analysis, has made Hawaiian fruit flies especially informative regarding both speciation and diversification. Zoogeographers place Hawaii and other isolated Pacific islands into a separate realm they call Oceania.
Clearly, the world is rich in faunal realms, biomes, and varied kinds of vegetation, as well as island biotas with many species found nowhere else. All contribute to a planet resplendent in biodiversity. However, and as we've seen, some areas are especially blessed with high species numbers, while others support a more restricted diversity. In the next chapter we move on to more specific patterns of how both numbers and lineages are arranged around our globe.