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Even a completely degraded environment can be successfully exploited by certain species but others are sure to perish.




  Be fertile and multiply, fill the Earth and instill fear and terror into all the animals of the Earth and birds of the sky.
GOD, in a conversation with Noah

In the now far-off decade of the 1960s a famous bumper sticker graced many a Volkswagen bus: Reunite Gondwanaland. In those days the theory of plate tectonics (also known as continental drift) was still in its infancy, and the waggish slogan was a cry to bring back all of the southern continents into the single continental landmass that existed at the end of the Paleozoic era, some 250 million years ago. In a strange way, that call has been heeded: Gondwanaland has been reunited. Not in any physical way Africa is not measurably closer to Australia or South America than it was thirty years ago. But functionally they have been brought together as barriers to biotic exchange between them have been eliminated. The common travel of boats and ships across the oceans has shrunk those oceans by giving the animals and plants of the now separated continents access to their age-old corridors of dispersal. When Gondwanaland existed, it was a time of greater global homogeneity, of fewer ecological niches, of fewer and lower mountain ranges, of more uniform global climate and, because of these factors, it was characterized by far lower planetary biodiversity than the present era. The functional reuniting of Gondwanaland may take us back to a lower global biodiversity reminiscent of that bygone age. This renewed homogenization of the worlds biota may set the current mass extinction


apart from all such previous events, for after this event there may not be a subsequent diversity increase. The planet may well stay at the low levels typical of a single landmass, rather than the higher diversity of numerous separated continents.
Biological diversity is so commonplace to us that it is taken for granted. Yet the factors leading to diversity are still great biological enigmas. Since the Cambrian Period more than 500 million years ago, the diversity of species on Earth has been fluctuating, but increasing overall. Will it continue to do so? Here I will argue that the new mass extinction, which is causing a dramatic decrease of diversity on Earth, will not be followed by a renewed burst of diversification, or even a return to pre-extinction diversity values until, perhaps, many millions of years have gone by.

Trends in Diversity

What controls the diversity of a given region? How can a coral reef be so rich in life and a sand bed beside it so poor? And if we change scale, how can a large region be species-rich and a neighboring province species-poor? If we define diversity as the number of species present in any given area, can we arrive at some rough mathematical rule governing diversity? There are no simple answers to these questions. Many factors enter into the equations, such as nutrient availability, habitat type, and amount of water; there are also numerous factors affecting the formation of species, such as rates of barrier formation, rates of genetic change, and, especially, rates of extinction.
Biologists have long recognized that diversity appears to be roughly related to habitat size, and this makes good sense: the larger the area of habitat available, the more animals and plants, and at the same time, the more different kinds of animals and plants, can be accommodated. But is extinction rate also related to habitat size, in some inverse way? Do larger habitats or only larger population sizes protect individual species from extinction?
Some rough rules of thumb about this relationship were first formulated in the 1960s by two famous ecologists, Robert MacArthur and E. O. Wilson, who proposed a new theory relating diversity to habitat area. MacArthur and Wilson called their idea the equilibrium theory of island biogeography. In essence, it relates the area of habitat to the number of species present: as habitat area increases, so too do species numbers, and they do so in a predictable way. Similarly, as habitat area decreases, species numbers fall. Because the number of species bears such a predictable relationship to the area available, we can analyze the way in which defor-


estation, for example, leads to the shrinking of habitat and thus to the loss of species. This influential model was one of the seminal theories about the regulation of biodiversity through time. While it was originally designed to examine diversity on islands, models patterned on the theory of island biogeography have now been scaled upward to encompass continental and even global scales of community and evolutionary diversity.
MacArthur and Wilsons equations can be used to predict rates of extinction. They found, for example, that an island always has fewer species than a mainland or continental habitat area of the same size, even if the habitats are otherwise exactly identical. The implications of this finding are that parks and reserves, which essentially become islands of habitat surrounded by disturbed areas, will always suffer a loss of species. It also means that subdividing any sort of larger habitat into smaller patches of disturbed and undisturbed regions will increase the rate of extinction.
With these implications in mind, paleontologist Michael McKinney of the University of Tennessee has recently summarized the general traits of global diversity:

1. Diversity (which can be defined here as the number of species in the habitat being examined, be it an island, a given community, or the Earth as a whole) fluctuates around some mean equilibrium value when viewed over a time scale we might call ecological time: tens to at most hundreds of years. Sometimes it drops, sometimes it rises, but generally it can be considered stable.

2. Although this mean equilibrium value of diversity remains approximately constant, the component species can and do change. Local extinction, immigration, and the formation of new species drive these changes.

3. If the same system is viewed over geologic time (thousands to millions of years), the mean equilibrium value of diversity changes as forces such as continental drift or mass extinction alter the major habitats on Earth.

4. The equilibrium value of diversity is determined by competition for available and finite resources. As the total number of species increases, this competition increases, reducing the rate of formation of new species and increasing the extinction rate.

The causes of change in diversity have been debated for over a century. Generally, the proposed causes fall into two categories: abiotic factors (those brought about by nonliving aspects of the environment, such as climate change) and biotic


factors (those brought about by life itself, such as competition, predation, and disease). Not surprisingly, ecologists have stressed the importance of biotic factors, viewing the world in short time spans and at the limited geographic scales of individual habitats and ecosystems. Those who examine global biodiversity from the perspective of a larger and longer framework (such as paleontologists) have long believed that abiotic changes are the most important factors determining diversity. According to this view, the two most important mechanisms regulating diversity are the rate of origination of new species and the rate of species extinction. These two competing factors are affected by abiotic factors and by each other.
In terms of the shorter ecological time scale, speciation is always a relatively slow process, while extinction can be either fast or slow. In the present-day Age of Humanity, it appears that the large-scale environmental changes causing the observed rise in extinction are abiotic climate change and changes in landscape and vegetation yet their ultimate cause is biotic the actions of humans. These circumstances have no precedent on Earth.
Our understanding of the rate of diversification relies on the concept of niche saturation. For many decades ecologists have used the concept of a niche to describe how a particular species lives and interacts in its ecosystem. The niche is somewhat analogous to the profession of a species: what it eats, where it lives, what it does in its community. As more and more species either evolve in or invade a given community with finite energy resources, more and more of the available niches are filled. It may be that the overall carrying capacity of a given habitat, community, continent, or even the Earth, limits the number of available niches, and that these niches can become saturated with species, thus limiting the potential for new speciation. Human activities appear to be reducing the number of niches available, at least in terrestrial habitats. The replacement of a forest with a field, or a field with a city, reduces niche availability. Suddenly the world is a less heterogeneous place just as it was during the time of Gondwanaland, 200-300 million years ago.

Disturbance and Diversity

Since the actual number of species on Earth today is so important, knowing what controls that number is also important. Why are there not twice as many, or half as many, plants and animals? Why are there more now than during the time of Gondwanaland? Although there is an enormous scientific literature on diversity, this is a question that has perplexed biologists for nearly two centuries, and it appears that


there is no easy answer. The most famous book on the topic Charles Darwins On the Origin of Species does not even address the issue. Darwin was concerned with the transformation of individuals, rather than how and why new species form. Most of the more recent treatises on diversity, such as Yale ecologist Evelyn Hutchisons famous paper Homage to Santa Rosalia, or Why Are There So Many Kinds of Animals? do not examine the mechanisms leading to the origin of species, but simply describe the maintenance of species once they have evolved.
Nevertheless, this problem was reexamined recently in a thoughtful essay by paleontologists Warren Allmon, Paul Morris, and Michael McKinney, who attacked it in a different way. They asked how short-term environmental changes, or perturbations, as well as more severe and longer-lasting changes, which they called disturbances, affect evolution and diversity. Because humans are producing both perturbations and disturbances in copious quantities around the globe, this particular question is highly relevant to understanding and predicting possible future trends in diversity.
All organisms encounter perturbations in their daily lives. Fluctuations in temperature, food availability, rates of predatory attacks these and a thousand other environmental changes are part of the everyday lives of all organisms. Sometimes, however, one or several of these changes are severe enough to kill off or otherwise remove a species or group of species from a given geographic area, creating a patch in space from which these organisms are now absent. Of course, what constitutes a perturbation or disturbance varies from species to species a disturbance for a protozoan may not even be noticeable to a fish. Disturbances are thus species-specific. They can also be thought of as acting at many environmental scales, as well as many scales of time. Perhaps the most interesting for our purposes are time scales ranging from a thousand to a hundred thousand years the intervals of time necessary for the speciation of large animals and plants.
Ecologists have long understood that there is a relationship between the degree of disturbance and the ability of nature to maintain diversity. Many studies of marine intertidal zones have shown that in areas of either too little or too much disturbance, few species occur. The disturbances can be both abiotic such as a violent storm and biotic such as the incursion of a new predator. Both types of disturbance create patches of open space or habitat. By reducing the numbers of abundant species, they allow rare species to maintain their existence or allow new species to gain a foothold in the environment. In environments with little disturbance diversity drops as a few species outcompete all the others and dominate the


environment. In high-disturbance areas, diversity also stays low, since only a few species can maintain viable populations in the face of high mortality. Maximum diversity is found in areas that can be considered to have intermediate levels of disturbance. Such conditions allow many species to survive, but do not allow any particular species to take over through predation or competition.
On the other hand, there have been virtually no studies trying to link disturbance with speciation, or the creation of diversity. Allmon and his colleagues have suggested that, like the maintenance of diversity, the creation of diversity through the formation of new species may occur in regions of intermediate disturbance. Paleontologist Steven Stanley has postulated a similar model, noting that high rates of speciation are actually promoted by less severe environmental deterioration deterioration severe enough to elevate extinction rates to a moderate degree but not so severe as to cause wholesale extinction.
This idea has some interesting implications. It predicts that endemic species those restricted to specific and hence small geographic regions will encounter relatively higher levels of disturbance than more broadly adapted species, and therefore experience both higher levels of origination and extinction. These species the specialists and types found in restricted ranges are those that produce the largest number of new species. Yet they also have the highest extinction rates.
Global diversification remains a simple equation: origination minus extinction. The highest net rates of diversification seem to occur in animals with small body sizes, short generation times, wide distributions, and high abundances beetles and rodents, for example. Although two of these traits wide distribution and high abundance seem to negate new speciation, they retard extinction even more. The net result is higher diversification than extinction.

Compounded Disturbance and Ecological Surprise

All species have evolved in the presence of disturbance. Thus, disturbances that happen within a particular range of intensity not too extreme result in little long-term change in the nature, composition, and energy flow of a population, or even an ecosystem. But what of compounded disturbances, when major disturbances occur repeatedly at higher than normal frequencies?
This was the question posed by ecologist Robert Paine and his colleagues in a 1998 article. Paine has spent his entire research career studying intertidal organisms and has contributed fundamental discoveries about the architecture of ecosystems and diversity. According to Paine, disturbances, ranging from small-scale and


Deforestation and fragmentation are the future and bane of post-industrial ecosystems.


frequent perturbations to large and infrequent catastrophes, occur from time to time in any habitat. It is these cycles of disturbance that led to ecologys first paradigm, that of ecological succession. Disturbances often cause widespread mortality, leaving a residual assemblage of flora and fauna, which provides a legacy that subsequent processes and populations use to rebuild. Even large, infrequent disturbances do not appear to override the biotic mechanisms that structure the eventual recovery. Paine and his colleagues used the example of the catastrophic 1988 Yellowstone National Park fire, which burned nearly 40% of the park and was an order of magnitude larger than previous fires in the park region. Even a decade after this major event, there have been no ecological surprises; the ecosystems returning are similar to those present prior to the fire. But what if the park underwent another such fire ten years after the first, and then another a year after that? If such major catastrophes are compounded, will ecosystems return to their previous state? Paine and his colleagues argue that they will not.
Compounded disturbance can be portrayed in two ways. First, it can occur in the manner proposed in the Yellowstone fire example, in which a normal community undergoes a second (or multiple) disturbance before recovery from the first is completed. Second, a major stress can be superimposed on a community altered by some significant disturbance. Examples of the this second type of compound disturbance can be seen when fish stocks are depleted by overfishing and then subjected to some other type of large-scale disturbance. In such a case their recovery is markedly delayed, if it occurs at all. Climate change may produce the same effect: a series of major storms one after another may markedly alter ecosystems that have evolved under lower storm frequency regimes.
Paine and his colleagues note that the prime cause of compounded disturbance is human activity. The prime result is lowered diversity a return to Gondwanaland.

Plate Tectonics and Diversity

The studies above (and many others as well) suggest that compounded disturbance produced by humanity may have caused the equilibrium level of world biodiversity to drop. Yet there is a second and equally important factor that is taking us back to Gondwanaland: the functional removal of barriers to migration. In a way, to borrow from another hoary bumper sticker, we have indeed stopped continental drift.
One of the major influences on the equilibrium value of global biodiversity is continental configuration. When the various continents were united, there was obviously easy faunal interchange around the globe. When the continents are


widely separated, however (as they are today), there is greater heterogeneity in environments, less faunal interchange, and many more species. Two hundred fifty million years ago, all the major continents were merged, and biodiversity was far lower than it is today. But by introducing non-native species across ecological boundaries and continents, humanity has found a way to functionally reunite the various continents, as least as far as gene flow is concerned.
Since the majority of the Earths biodiversity today is found on continents (and there is no reason to believe that this relationship has changed over the last 300 million years), the processes of plate tectonics are especially important for life and its ecosystems. As continents have shifted their positions through time, they have affected global climate, including the overall albedo (the planets reflectivity to sunlight), the occurrence of glaciation events, the pattern of oceanic circulation, and the amounts of nutrients reaching the sea. All of these factors have biological consequences that affect global biodiversity. Moreover, continental drift can help augment diversity by increasing the number and degree of separation of habitats (which promotes speciation).
Plate tectonics also promotes environmental complexity and thus increased biotic diversity on a global scale. A world with mountainous continents, oceans, and myriad islands is far more complex, and offers more evolutionary challenges, than a planet dominated by either land or ocean. James Valentine and Eldredge Moores first pointed out this relationship in a series of classic papers written in the 1970s. They showed that changing the positions and configuration of the continents and oceans would have far-reaching effects on organisms, causing increases in both diversification and extinction. Changes in continental position would affect ocean currents, temperatures, seasonal rainfall patterns and fluctuations, the distribution of nutrients, and patterns of biological productivity. Such changing conditions would cause organisms to migrate out of the new environments, and would promote speciation. The deep sea would be affected least by such changes, but the deep sea is the area on Earth today with the fewest species: over two-thirds of all animal species live on land, and the majority of marine species live in the shallow-water regions that would be most affected by plate tectonic movements.
The most diverse marine faunas on Earth today are found in the tropics, where communities are packed with vast numbers of highly specialized species. Not only are there fewer species at higher latitudes, but species composition is different from that in the tropics as well. Most species have fairly narrow temperature limits imposed by physiological adaptation, and since temperature conditions change rapidly with latitude, its not surprising that the north-south coastlines of continents show a continuously changing mix of species.


In 1996, biologist P. Vitousek and three colleagues used mathematical modeling to project the number of mammalian species that would be expected on Earth if all of the continents were reunited into the configuration present at the end of the Paleozoic Era, some 250 million years ago. They concluded that the world would contain about half of the nearly 4,000 mammalian species present if we reunited Gondwanaland. These same authors speculated that the current transport of mammals from continent to continent is leading to an extinction rate of mammals that will yield approximately this same global biodiversity: 2,000 mammalian species.

Aliens Among Us

Whenever a species arrives in an area where it was not previously found, there is a potential for biological change. Such invasions of new species have occurred throughout geologic time, yet the rate of invasion has vastly increased during the Age of Humanity. Today, no area on Earth is immune to such biological invasions. It is estimated that about 11% of all species now living in France have been introduced; in Australia the proportion is 10%, in Hawaii 18%, and in New Zealand more than 40%. These biological invasions are particularly marked in floral communities. There are records of about 1,200 native plant species in New Zealand, but there are now over 1,700 non-native plants living there as well. Although it could be argued that the introduction of so many non-native plant species has more than doubled the diversity of plant life in New Zealand, this is only a transitory result. Over time, many non-natives will inevitably drive the natives to extinction, causing world biodiversity to decline.
Biological invasions aided by humans have come in three major pulses. Over a period starting several thousand years ago until about 1500 A.D., human movement and migration caused the transport of plant and animals mainly in the Old World. Beginning in about 1500, however, a second phase of invasions began with the increasing contact between the Old World and the New due to European exploration and conquest, during which many Old World species were transported to the New World. The final phase began about 150 years ago with the globalization of species movement due to the vastly increased efficiency of human transport.
There have been many reasons for these species introductions. In some cases the introduced species were purposely brought to a new location to become animal or plant crops. Some were brought to serve as ornamentals or pets, while others were introduced for sport or hunting. Still others were introduced to control


pests, only to have the introduced species become even more destructive than the species they were brought in to control. Ironically, some introductions have occurred for purposes of either biotechnology or scientific research. Finally, there have been many accidental introductions from ship ballast and airplane holds, of hitchhiking seeds escaped from either wild or agricultural areas, and simply as side effects of habitat alteration.
The majority of introduced species do not survive. It is estimated that of a hundred introduced species, approximately ten will successfully colonize or naturalize, and only two or three will become pests. But those that do often become major problems, especially in fragile, endangered, or rare ecosystems, such as early successional habitats, ecosystems with few species, and ecosystems that traditionally have a low number of predators or grazers. The pest invaders often show a suite of similar characteristics: they have a high reproductive potential, many offspring, and generalized habits and food requirements. They can thus be characterized as pioneer species, in that they can colonize and flourish in a wide variety of ecosystems. They are often human commensals species that thrive in the presence of humanity.
While the greatest consequences of these invasions are biological, their economic impact is not trivial. In the United States alone, it is estimated that the Russian wheat aphid causes as much as $130 million in crop damage each year, the Mediterranean fruit fly as much as $900 million, and the gypsy moth about $750 million. The boll weevil may have caused as much as $50 billion in damage to cotton crops during the twentieth century.
The ultimate effect of many invasions is extinction of native species, and examples of such extinctions abound. In 1959, in the Rift Valley of Africa, British colonials introduced a northern African fish called the Nile perch into Lake Victoria for sport fishing. The Nile perch is a voracious predator on smaller fishes. Prior to its introduction, the lake was home to over 300 species of endemic cichlid fishes. Yet by the early 1980s, when the problem was finally recognized, over half of the cichlid species in Lake Victoria had gone extinct because of the Nile perch.
Of all the factors causing the translocation of species, the exchange of ballast water may be among the most important and the most difficult to stop. Thousands of species are transported around the globe in ships ballast water. When a ship takes on ballast water, it takes up the plankton of a given region, which often contains the juvenile stages of marine animals and plants. These organisms are



1. Feral pig
2. Norway rats
3. European exploration
of remote oceanic
4. Coconut palm
5. Hawaiian honeycreeper
and mosquito
6. California condor
7. Easter Island
8. Mexican grizzly bear
9. North America
10. Human migration
through Beringia
11. Barredwing rail
12. Aukland Island slate-
breasted rail
13. Laysan rail
14. Ponape crake
15. Samoan wood rail
16. Chatham Island banded rail
17. Lord Howe wood rail
18. Wake Island rail
19. Hawaiian rail
20. Tahitian rail
21. Rhea
22. Rio de Janeiro
23. Tapir
24. Haitian solenodon
25. Piping plover
26. Alaskan pipeline
27. House sparrow
28. Feral pigeon
29. Caiman hunting
30. North Sea oil industry
31. Gorilla
32. EuroDisney
33. Big game hunting
34. The Sahara
35. Origin of modern human
36. Quagga
37. Research whaling
38. Eurasian starling
39. Ostrich
40. Walia ibix
42. Brookesia peyrieresi
43. Human migration to
44. Dodo and dodo tree
45. Thylacine
46. Feral rabbits
47. Human migration brings dingo to
48. Giant land snail
49. Ceylon elephant
50. Snow leopard
51. Traditional Chinese medicine
(bear gall bladder, tiger paw, and
rhinoceros horn)
52. Eurasian bison
53. Cretan deer
54. Tiger
55. Kamchatkan bear
56. Shistosoma
57. Hunting woolly mammoth
58. Stephen Island wren
59. Human migration from Asia into the Pacific
60. Human migration to New Zealand
61. Brown tree snake
62. Ocean-borne garbage
63. Stellars seacow
64. Sandwich tern

A recent history of the world, from an ecological perspective.



then discharged when the ship reaches its destination port. One such invader was the infamous zebra mussel, which made its way into the Great Lakes of North America. The zebra mussel, which originated in Europe, is an extremely efficient filter-feeding organism, straining plankton from the surrounding water so efficiently that it outcompetes native species, which then starve to death. It multiplies rapidly, attaching itself to pipes, boats, and the shells of other mollusks. Zebra mussels clog water intake pipes, thus affecting public water supplies, irrigation, sewage treatment plants, shipping lanes, and recreational activities.
Governments around the world are trying to monitor the alien species arriving in ballast water. A recent study conducted on Japanese ships entering Oregon ports discovered the presence of over 350 alien species being discharged into Oregon waters. Among the most undesirable of such invaders are predatory crabs, which are capable of wreaking havoc on shellfish beds. Such an invasion began in the 1990s with the appearance of the green crab in Washington State. The green crab feeds on small clams, and is capable of decimating local populations of clams and snails.
Plants also suffer a great deal from biological invasions. Because plant seeds are usually small, they are easily transported long distances, and often can colonize and take over new ecosystems quickly. In different areas of the United States, introduced plants make up between 7% and 48% of the total plant diversity. Many of these non-natives, such as kudzu, were deliberately introduced to control soil erosion. Others were introduced as agricultural crops. On rangelands, invasive plants such as cheatgrass crowd out more nutritious native plants, cause soil erosion, and pose threats to native wildlife.
Even more deleterious than these plant invasions has been the transport of plant pathogens from one part of the world to another. Dutch elm disease decimated elm trees in both England and the United States after it was accidentally imported. The introduction of the chestnut blight from Asia to America in 1890 drove the American chestnut tree to the verge of extinction in less than fifty years. In Australia, native Jarrah forests have been destroyed due to the introduction of a root fungus imported from Eastern Australia.

Winners and Losers

Predicting winners and losers in the future can be as perilous for biologists as it is for stockbrokers. In both cases, however, there are some clear signs of what may prosper (and even diversify) and what may die out.


One clear insight into predicting whether a species will flourish or not comes from the size of its geographic range. In the late 1980s, biologists J. Brown and M. Maurer showed that species of North American birds with small geographic ranges almost always had low population densities within those ranges. In other words, there are virtually no species of birds in North America that are both narrowly distributed and abundant within their small geographic ranges. On the other hand, birds with widespread geographic ranges are usually abundant in most regions within their ranges. Although we see this all around us, it is not an obviously intuitive finding; but it is a generalization that has the utmost importance for picking winners as well as losers among species in the coming years.
The correlation between range size and abundance can be understood by looking at the geometry of species ranges. Geographic ranges exist because they encapsulate all the areas where a species can exist. Thus the outer limits of a geographic range tend to be less favorable areas for the species than the interior of the range. If the size of the less favorable perimeter is large relative to total area of the range, it becomes limiting to the overall population, and small geographic ranges have higher perimeter-to-area ratios than large ones. Not surprisingly, then, when a large geographic range is suddenly broken up into many smaller ranges, the abundance of the species will drop. Geographic range fragmentation can thus influence the likelihood of extinction by affecting the rate of extinction of local populations that find themselves confined to habitat islands.
This correlation between range size and abundance is the single greatest nightmare of conservation planners. The urbanization and agriculturalization of the world has been fragmenting the ranges of most wild species while greatly expanding the geographic ranges of agricultural species. This effect will in essence spell doom for a majority of the worlds rare species, many of which are megamammals. Once again, the conclusion of the Age of Megamammals is the functional reuniting of Gondwanaland.






Biological Futures
Niles Eldredge
PREFACE   xiii
INTRODUCTION The Chronic Argonauts 1

The Deep Past: A Tale of Two Extinctions

TWO The Near Past: The Beginning of the End of the Age of Megamammals 37
THREE Into the Present 47

Reuniting Gondwanaland


The Near Future: A New World


The First Ten Million Years: The Recovery Fauna


After the Recovery: A New Age?


The Future Evolution of Humans


Scenarios of Human Extinction: Will There Be an After Man?


Deep Time, Far Future



INDEX   183

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