Chapter 4: Biodiversity
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).
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.
Table of Contents
1. Roots of the modern environmental dilemma: A brief history of the relationship between humans and wildlife
2. A history of wildlife in North America
3. Climatic determinants of global patterns of biodiversity
5. Natural selection
6. Principles of ecology
7. Niche and habitat
8. Conservation biology
9. Conservation in the USA: legislative milestones
10. Alien invaders
11. Wildlife and Pollution
12. What you can do to save wildlife
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