Chapter 4: Biodiversity
Edited by Peter Moyle & Douglas Kelt
By Peter B. Moyle, Anitra Pawley, and Robert J. Meese, last revised September 2004
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 then describe general patterns in the global distribution of vertebrates and the reasons why we are losing biodiversity so rapidly. The purpose of the chapter is to give some indication of what we humans stand to lose if we let present trends continue. We start by giving some definitions of biodiversity, then discuss species diversity, focusing on vertebrates, followed by a discussion of species numbers in relation to biogeography. The final section of the chapter is devoted to extinction, the loss of biodiversity.
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 geographically defined areas with sets of interacting populations of plants, animals, and microbes. Each ecosystem has its own set of organisms and interactions, 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 and that by protecting natural ecosystems we are protecting many organisms and processes about which we know little. 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 different 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 occurring in response to the constantly changing climatic and habitat conditions of vulnerable to population collapse or extinction than those with a lower degree of genetic variation because they can adapt to changed conditions.
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 (Chapter 1). 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 Wallaceand 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 lived, from 1848-52, in South America and then, 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 naturalist who in 1831-36 sailed aboard the H.M.S. Beagle 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 distribution 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; 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 (Moyle 2002). 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 describing 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.”
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.
Figure 4.1. 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), drawings by Chris M. van Dyck.
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.
Figure 4.2. The bird pictured here is the Calayan rail, a new species of bird discovered on a remote island in the Philippines in 2004. One or two species of birds are still being discovered every year. Photo by Des Allen, BBC News.
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.
Figure 4.3. Species density contours for recent mammals of continental North America. The contour lines are isograms for numbers of continental (nonmarine) mammal species. The “fronts” are lines of exceptionally rapid change that are multiples of the contour interval for the given region (from Simpson 1964).
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.
EXTINCTION: THE LOSS OF BIODIVERSITY
One of the best reasons for understanding the patterns of species diversity is that we are losing species on a daily basis and many of our most spectacular mammals and birds may soon be lost forever: giant panda, California condor, white rhinoceros, whooping crane, blue whale. These animals are the symbols of the rapid loss of biodiversity that our planet is experiencing, an extinction spasm that may be as large or larger than any the Earth has experienced in the past. If we are to reduce the size of this extinction event, if for no other reason than to protect the species that we value in particular, then we must understand extinction, its causes, and its impact on humans.
There are those who wonder what all the fuss is about because extinction is a natural process which has occurred since life began. We have a long fossil record of species that once existed and no trilobites, an extinct phylum of invertebrates, and 300 million years later, dragonflies with wingspans nearly a meter across flew through primitive forests. In fact, it has been estimated that today’s species account for only 2% of all those that have existed over the millennia. Indeed Charles Darwin recognized the extinction process in his 1869 book, On the Origin of Species: “…as new forms are continually and slowly being produced, unless we believe that the number of specific forms goes on perpetually and almost indefinitely increasing, numbers inevitably must become extinct.”
As Darwin noted, the species that inhabit the earth are a product of a long sequence of speciation (the evolution of new species) and extinction events. These events are driven by physical or biotic changes in the environment. Physical changes include climatic changes due to falling meteorites, movement of land masses, or to more local events such as volcanic eruptions, drought, or fires. Biotic factors include the invasion of species causing extinction through competition, predation, and diseases. Usually multiple factors are involved in extinction events. One factor may decrease population levels to such an extent that another seemingly harmless factor causes the final blow. Thus, when studying extinction, it is often difficult and unrealistic to look for one cause. In modern extinctions, natural events, such as a flood or a drought, may be the final blow to a species driven to low numbers by human actions. We will study these problems as we review examples of extinction and search for clues to prevent future extinctions.
Speciation is generally a slow process, happening in a single evolutionary line or from the splitting of lines. Yet as slow as this process is, it is on the average faster than the rate of extinction. As a result there are probably more species on Earth today than have existed at any time in the past. So, why is there so much concern over extinction? It is because of the rate at which extinction is presently occurring. The expansion of human populations has caused extinction rates to skyrocket; the balance now has tipped in the opposite direction; extinction rates are outstripping speciation rates.
NATURAL CAUSES OF EXTINCTION
Extinction is a natural process. Natural extinctions can be divided into two general categories: normal extinctions, those that have occurred gradually throughout time, and mass extinctions, those that have occurred on a global level in relatively short geological time due to catastrophic events. Normal extinction occurs as the result of localized, gradual environmental changes working on the variation among species. For example, pupfish left behind by receding waters of large lakes that once occupied Death Valley (California and Nevada) evolved into a number of separate species in isolated springs, while the original lake fishes became extinct. If the water feeding the springs should dry up, the various pupfish species would also become extinct. Mass extinction, on the other hand, is almost always seen to be caused by rapid widespread environmental change. Meteorites, glaciation, continental drift, and massive volcanic eruptions are large-scale phenomena that invoke large-scale changes. Although both normal and mass extinctions are usually tied to physical changes in the environment, biotic interactions also play an important contributing role. Competition, predation, parasitism, and disease all have their effects. Thus, South America was once inhabited by many marsupial mammals, much like Australia is today. When North America and South America became joined, North American mammals invaded South America, causing widespread extinction of the marsupials, presumably through predation and competition. The rapidity with which such extinction can occur is amply demonstrated by the success of so many introduced species today, and their frequently devastating effects on native species.
Paleontologists have recorded at least six episodes of mass extinctions, relatively short intervals (lasting from 1 million to 10 million years-short in geological time), in which a significant portion of the Earth’s taxa became extinct. Five of these episodes are predominately marine and one episode is exclusively terrestrial. The most significant mass extinction occurred 250 million years ago at the end of the Permian period. Some 77 to 96% of the species then alive are believed to have become extinct (Raup 1979). In the last 250 million years there appear to have been 9 different periods when extinction rates have increased. Only two of these were drastic enough to be termed mass extinctions. As noted above, a plethora of reasons have been proposed for mass extinctions. For example, for marine mass extinctions, meteorites, massive volcanic eruptions, extraterrestrial radiation, changes in temperature, salinity, and oxygen, and the shortage of various resources or habitats have all been suggested.
The most well-known of the mass extinctions is the most recent, occurring about 65 million years ago at the end of the Cretaceous period. At this time marine reptiles, flying reptiles, and both orders of the dinosaurs died out. There have been numerous hypotheses as to why this occurred. A gradual cooling of the earth’s climate may have brought an end to these creatures. The rise and dominance of flowering plants to replace the giant ferns and horsetails that comprised the dinosaurs’ diet may have played a role. The appearance of mammals may have led to the dinosaurs’ downfall. Yet recently these factors have been suggested to be coincident with or caused by an extra-terrestrial force, a huge meteorite. Walter Alvarez, a geologist at Berkeley, came upon this idea in a rather round-about way, not atypical for scientific endeavors. He was trying to find a tool for determining depositional rates in sedimentary rock (limestone, which is formed by the deposition of marine shells). Because meteorite dust contains a rare element, iridium, and is deposited on the earth at a fairly constant rate, Alvarez reasoned that quantifying the amount of iridium in sedimentary rocks could facilitate the determination of sedimentation rates. In testing this hypothesis on sediments in northern Italy, Alvarez found abnormally high levels of iridium, which indicated a huge influx of meteorite dust. The layer with abnormally high levels of iridium (also known as the Cretaceous-Tertiary boundary) has now been found in at least 50 other sites across the globe. This layer coincides with the end of the “Age of the Reptiles,” so Alvarez hypothesized that a huge meteorite fell to the earth causing dinosaur extinctions as well as the extinction of many other members of the flora and fauna. There is evidence that numerous marine forms also became extinct during this period. More recently, support for this theory has come from discovery of a large meteor crater from the right time period near the Yucatan Peninsula in Mexico.
EARLY HUMAN-INDUCED EXTINCTION
Human-induced extinction is often viewed as a modern event, yet there is evidence that our ancestors were responsible for extinctions as well, both indirectly by changing the landscape and directly through hunting, as indicated in the introductory essays. The discovery of the use of fire inhabited. Its use has been documented for at least a quarter million years. Not only did ancient humans abandon campfires which started conflagrations, but they deliberately started fires in order to manipulate the landscape for their us including to rouse and drive game during hunting. Fires opened up pasture for large herbivores, improved yields of certain plants, and later was used to clear land for agriculture. The Mediterranean and much of Europe has been substantially altered by humans.
“Denudation of the forests made such inroads upon the wood supply of Italy that by the fifth century Roman architectural technique had become modified to meet the growing scarcity and increased price of wood… Fires were often started, either intentionally or accidentally, by the herdsmen who ranged the mountain forests with their sheep and goats in the dry season. Burning improved the pasturage, because the ashes temporarily enriched the soil and the abundant shoots from the old roots furnished better fodder. The forests once destroyed were hard to restore.” (Semple, 1931).
When the forests declined, so did forest-dependent animals. As Europe became more densely settled, many large mammals disappeared from the landscape. The lions and leopards that were still present during the rise of ancient Greek civilization were presumably hunted to extinction, as were the ancestors of modern cattle, the aurochs. The extinction large mammals in parts of Europe is analogous to the disappearance of large mammals in North America around 11, 000 B.P, coincident with the invasion of humans (see Chapter 2). Likewise the loss of giant lemurs in Madagascar and moas in New Zealand followed the invasion of humans, as did the extinction of many species of flightless birds on oceanic islands as they were colonized by Polynesians. Despite evidence that ancient humans may have caused a substantial number of extinctions, it is modern humans that have played a key role in the demise and decline of most species. A comparison of the Earth’s population curve and the extermination of mammals reveals a striking conformity (Figure 4.2.1). Some projections indicate that if extinction rates continue to increase at current rates, we will lose roughly 1/5 of the earth’s species by the end of the century. Myers (1981) predicted that the destruction of moist tropical and temperate forests is proceeding so fast that they “may be reduced to degraded remnants by the end of the century, if they are not eliminated altogether. This will represent a biological debacle to surpass all others that have occurred since life first emerged 3.6 billion years ago.” A similar, but more hidden crisis, is taking place in temperate freshwater environments, where at least 20% of the fauna is in danger of extinction worldwide (Moyle and Leidy 1992).
Figure 4.2.1 (a) The increase in human population over the last three hundred years. (b) The number of species of mammals (white bars) and birds (black bars) eliminated over the last three hundred years. Each bar represents a 50-year period.
Is the situation really so dire? The unfortunate answer appears to be yes, unless we change our present practices. It has been calculated that if present rates continue, the loss of tropical species will rival that of every recorded mass extinction event except for that at the end of the Permian.
WHAT MAKES SPECIES SUSCEPTIBLE TO EXTINCTION?
When humans change the landscape, many species nevertheless manage to persist. Some actually become more abundant. So what makes some species so vulnerable to extinction? Although there are no universal answers to this question, there are key characteristics that seem to increase species vulnerability. What immediately comes to mind, of course, is low population size, natural or human induced. The Devils Hole pupfish, with 200-600 individuals in a single cave pool is clearly very susceptible to extinction. Yet some species can be very abundant and still highly vulnerable to extinction. Flocks of passenger pigeons once darkened the skies of eastern North America; their abundance was overwhelming, counted in the billions. Now they are but a memory.
Susceptibility to extinction is tied to many factors including trophic position, distribution, habitat and life history characteristics. For example, an animal may be rare because it is a top predator depending on large numbers of prey at lower trophic (feeding) levels which requires lots of land as well, so they are both relatively uncommon and often regarded as competitors with humans. Timber wolves, Bengal tigers, and bald eagles are examples of species that are rare partly due to their presence on the apex of the trophic pyramid. Such species may be rare even if they occupy a wide geographic range. On the other hand, a species (no matter what its trophic position) that is abundant but is confined to a small area can be extremely vulnerable if the area is altered by humans or a natural catastrophe (such as a volcano blowing up). Many plants and invertebrates fit this scenario including many species in the highly diverse tropics. For example, the world’s largest butterfly, Queen Alexandra birdwing, is confined to a tiny area in the lowlands of New Guinea. Likewise, many fish and other aquatic species are threatened because they occur in isolated lakes, streams, or springs, where the water is desired for use by humans. In California, about half the native fish species are endemic to the state and, of these, two thirds are either threatened, endangered, or have declining populations, mostly because of limited distributions.
Vulnerability to extinction can also be tied to habitat specialization. Species that specialize on certain types of patchily distributed habitats or resources that are infrequently available tend to be rare. Whether this is a symptom of habitat destruction or a cause of their sensitivity is not always apparent. Examples include: the spotted owl, which nests only in old growth forests; the whooping crane, which depends on marshes for food and nesting; and the green sea turtle, which requires specific beaches for egg laying. Species that exhibit specialized feeding habits are also at risk. The black footed ferret, which survived mainly on prairie dogs and pocket gophers, is an endangered species partly because rodent control programs have destroyed its prey base.
Life history characteristics that can play a role in increasing vulnerability to extinction are low reproductive rate, large size, and fixed migratory or behavioral patterns. The passenger pigeon is an example. This species was considered the most numerous of all bird species in 1800 and millions of birds were harvested for food, including young in nests. In 1880 there were still several thousand pigeons left, and these were so scattered that it was unprofitable to hunt them. Passenger pigeons had low reproductive rates, migrated in dense flocks, and formed breeding communities of a several thousand individuals. It is believed that they needed the stimulus of a large flock to breed, which may explain why they never successfully bred in captivity. Thus when the total population fell below a size that to us seems large, they were still unable to reproduce. In the jargon of ecologists, their numbers had fallen below the minimum viable population size.
ENDANGERED SPECIES: ON THE ROAD TO EXTINCTION
The previous section emphasizes the biological characteristics that are likely to make a species prone to extinction. Indeed, most extinct species and many species in danger of extinction have these characteristics. Unfortunately, the causes of extinction increasingly have much more to do with human activity and much less to do with the characteristics of the species. Given the scope of human activity, virtually any species can be prone to becoming extinct if it happens to be in the wrong place at the wrong time. This can be seen by examining the causes of species becoming threatened or endangered, the final steps towards extinction. Major causes examined briefly here are habitat change, contamination, introduced species, and exploitation. Usually, the decline of a species, however, has multiple causes.
In the last few decades modification of habitat by humans has clearly become the most severe threat to wild organisms and ecosystems. Drainage schemes, reservoir and dam construction, urban, industrial and agricultural development, and deforestation are examples of human induced habitat modification. The view from the window of any jetliner flying over almost any country in the world will reveal the extent to which humans have altered the landscape, with our endless fields, urban sprawl, and straightened rivers. Two examples in California of species endangered primarily due to habitat modification are winter run chinook salmon and spotted owl.
Winter run chinook salmon are unique to the Sacramento River, adapted for spawning in the cold, spring-fed water of the upper Sacramento, McCloud, and Pit rivers. When Shasta Dam was built in the 1940s, they were cut off from their historic spawning grounds, much of which were flooded by the reservoir as well. Below the dam, flows were greatly altered but increased flows in the summer duplicated the cold-water conditions the salmon needed to rear their young, so they survived the dam building. In fact, it was estimated that winter run chinook salmon populations in the Sacramento River numbered well over 100,000 fish in the mid-1960s. However, after the Red Bluff diversion dam was built in 1966, the population fell to around 2,000 fish and in subsequent years to only a few hundred fish. The new dam blocked the passage of spawning fish upstream despite the presence of salmon ladders (which were badly designed, so the fish had a hard time finding them). In addition, Shasta Dam was increasingly degrading the spawning habitat: dam operations caused the water to become warmer and the immense Shasta Reservoir prevented any gravel, needed for spawning, from washing downstream, to replace gravel removed by natural river processes. The Sacramento River below the Shasta Dam was becoming a warm stream with a solid bed of large rocks, conditions unsuitable for spawning of the few fish that made it over Red Bluff Diversion Dam. Once the salmon was listed as an endangered species, the following steps were taken: (1) the gates of Red Bluff Diversion Dam were raised to allow direct passage of the migrating fish, (2) thousands of tons of gravel were dumped into the river for spawning habitat, and (3) a multi-million dollar device was installed on Shasta Dam so cold water could be sucked from the bottom of the reservoir to lower water temperatures in the river. As a result of these actions, and others, the winter-run Chinook salmon populations seem to be increasing once again. At the present time, Battle Creek, a spring-fed tributary to the Sacramento River, is being restored as winter-run spawning habitat, largely through the removal of small dams that have prevented access to spawning areas.
Spotted owls largely depend upon large unbroken stands of old growth forest to feed and reproduce. Each pair of birds needs a large amount of this habitat to survive and reproduce, in part because they prefer to feed on a mouse that lives in big trees, the red-backed tree vole. The forest with the appropriate habitat was once widespread along the Pacific coast, from northern California to southern Alaska, where there are forests with trees up to 1,000 years old, and in the Sierra Nevada. These are uneven aged forests; ancient trees stand beside young ones. The complex ecological interactions present in these old growth forests are necessary for the survival of other many species of birds and mammals as well, for which the spotted owl stood as a surrogate. Unfortunately for the owl, old growth timber is extremely valuable and has been cut rapidly in the past century. The cutting of old growth forests resulted in the loss and fragmentation of spotted owl habitat to the point where the owl became listed as an endangered species. In addition, habitat change across the country has allowed the larger and more aggressive barred owl from the eastern USA to move into spotted owl habitats and displace them.
Figure 4.3 Spotted owl. Photo by Gerald and Buff Orsi © 1999 California Academy of Sciences
Pollution is a pervasive and insidious problem facing humans and all other species on this planet. There is no place on Earth that is free of contaminants. Nevertheless, it is unusual to find an example of a vertebrate species that have become extinct or endangered as the direct result of pollution, although many species have their ranges severely restricted by contaminants (e.g., fish that are absent from polluted waters). There is growing consensus, however, that many pollutants have subtle deleterious effects, weakening animals to make them more susceptible to disease or mimicking hormones to reduce reproduction. The best know contaminant problem in wildlife came close to eliminating some of our most spectacular birds. The osprey, peregrine falcon, bald eagle and brown pelican were all victims of the widespread use of DDT as a pesticide. As DDE, a derivative of DDT which cannot be further broken down, passes through the food chain it accumulates, reaching higher and higher concentrations at each step (See Chapter 10). Top predators thus receive the heaviest doses, which in this case almost led to their extinction. DDE accumulated to such high levels in these birds that it caused a hormonal imbalance resulting in eggshell thinning. Shells became so thin that they broke under the weight of the incubating parents. In some cases the eggs were produced with no shell at all. The recovery of these predatory birds can be attributed to the ban in 1972 of the use of DDT in this country.
The invasion of alien species can greatly disrupt ecosystems through predation, competition, and spread of disease (See Chapter 9). Such invasions typically go hand in hand with habitat change because humans alter habitats in ways that favor non-native species such as Norway rats, house mice, common carp, cockroaches, and starlings. They can finish the job quickly that habitat alteration has started. Thus, in the Colorado River, the unique native fishes had their populations greatly reduced by habitat changes caused by major dams. These large, long-lived (20-30 years) fishes are capable of living as adults in reservoirs behind the dams, but they can’t reproduce because small alien fishes congregate in spawning areas and eat all their eggs and young. Thus extinction of some species in the wild is likely. Introduced species can be especially devastating on islands. In the absence of natural checks that keep their populations regulated on the continent, alien species often flourish on islands at the expense of native island fauna, many of which have evolved in the absence of major predators and consequently lack protective avoidance behavior. An example is the introduction of the brown tree snake to Guam which is a very effective predator on birds. The snakes, which probably arrived on Guam hidden in ship cargo from the Papua New Guinea area, have virtually wiped out the native forest birds of Guam. Nine species of birds, some found nowhere else, have disappeared from this island, and several others are close to extinction.
Figure 4.4 Brown tree snake © 2004 Geordie Torr, California Academy of Sciences
Even in protected natural areas alien species can create serious problems. Thus alien burros are a serious threat to the remaining populations of desert bighorn sheep in California. Burros are descendants of the African wild ass and are well adapted to arid areas. When burros escaped or were turned lose at the end of the mining era, they quickly established large populations in California, Nevada, and Arizona. Feral burros are in direct competition with the bighorn sheep for the limited amount of water available in desert habitats. The more aggressive burros often drive bighorn sheep away from prime drinking and grazing areas. The behavior of feral burros at water holes so greatly disturbs these sites that bighorn sheep can no longer use these areas to acquire water. Burros congregate around springs thereby polluting the water with feces and urine, reduce the plant cover by rapidly eating the most succulent plants in the area, and muddy the water by simply standing in it. Burros also compact the soil, which prevents plant growth and causes erosion. The resulting scarcity of water and forage weakens the bighorn sheep making them especially susceptible to disease spread by domestic animals.
Exploitation, such as the killing of bison and passenger pigeons, was a major source of extinction and endangerment in North America in the 19th century, but is less of a problem here today. Unfortunately, this is not true for the rest of the world, where increasing human populations confine animals and plants to limited areas which makes them increasingly vulnerable to killing for various reasons. Improved technology and weaponry also makes even large animals like whales, tigers, and rhinoceros vulnerable to extirpation, especially as their value as dead animals increases with their rarity. A growing problem is the overexploitation of fish populations in the seas by highly mechanized fishing gear, which is changes ecosystems (as fish populations collapse and as huge nets rip up habitats) and endangers marine mammals and birds through the loss of their food supply.
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 “force” 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. Biodiversity is disappearing so fast that we often do not know what we are missing until it is too late. There is growing realization that protection of biodiversity has enormous benefits to humans and that we need to find ways to support, rather than eliminate, the ecosystems and species that provide these benefits. The protection of the diversity of life on Earth, however, requires both a grand global strategy and intensely local and regional strategies for day to day conservation (Wilcove 1999). This is what Conservation Biology (Chapter 8) is all about.
Figure 4.5. The Condor’s Shadow by David Wilcove (1999) is a well-written, easy to read book that makes a good follow-up to this chapter for those with a deep interest in conservation of vertebrates in North America.
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