Why is biological diversity important? Many arguments can be made from scientific, philosophical, economic, ethical, and aesthetic perspectives. Scientists argue that much remains to be learned about many species and ecosystems around the world and that the loss of these species would foreclose that opportunity. Certain rare species are singled out as worth saving for their sake; the loss of the Sumatran tiger, for example, or the rhinocerous would be mourned by many who have never seen these animals in the wild. There is great beauty in forests, coral reefs, savannahs, and other landscapes that is worth preserving for future generations, as well as our own, to appreciate.
Natural resources also provide critical ecosystem services. Forests retain moisture in the soil and prevent erosion; hillside areas can be subject to mudslides where forests are cleared, resulting in loss of life and property. Of the approximately 1.4 million species that are known, almost one million are insects and other invertebrates, and these are, as E.O. Wilson has said, “the little things that run the world” by breaking down plant and animal matter and making it available as nutrients. Crop and forest lands provide food and wood for shelter and sustenance. The interactions between the living and the non-living parts of the environment provide essential ecosystems services of soil formation, climate control, and water recycling. In one study, published in 1997 in the science journal Nature, researchers estimated the value of these ecosystem services at between $16 trillion and $54 trillion a year.
Arguments for increased international efforts to conserve habitats and ecosystems often emphasize the value of biodiversity to humans: the "un-mined riches" that we may discover in plants and animals and the potential of new food sources. For example, approximately 25 percent of all prescription drugs in the United States are derived from plants. The rosy periwinkle from Madagascar is the source of a drug used in the treatment of Hodgkin's disease and leukemia. The Pacific yew tree is the source of Taxol, a drug which has been found effective in treatment of ovarian cancer. Some drugs found have been identified through native folklore. Peruvian Indians, for example, treated malaria with an extract from the bark of the Cinchona tree. Study of this extract led to discovery and use of quinine. the first effective treatment for malaria.
The potential for discovering medicines is often cited as an argument for international cooperation in preserving tropical forests, but also as a means for doing so, by finding useful products that can be extracted profitably from the plants and animals of the rainforest, through bioprospecting. For example, Glaxo Wellcome, a British pharmaceutical company, funds the Centre for Natural Products Research in Singapore, which surveys species in Asia for medicinal purposes. Conservation International has initiated an agreement between Bristol-Meyers Squib, Suriname, and the National Institutes of Health.
There are concerns about bioprospecting, however. Some developing countries maintain that they will not receive a sufficient portion of the profits from drugs developed from plants found within their borders. On the other hand, the costs of isolating useful species, developing drugs and other products, and testing them for use is enormously expensive, and those costs are borne by the drug companies.
Monday, September 24, 2007
What Lies Ahead?
The population of the Earth will likely double by the year 2050, resulting in a world of at least 10 billion people, the largest number of whom, by far, will live in tropical and subtropical Asia, Africa, and South America. These are as well the regions in greatest need of economic development, and the twin pressures of population growth and economic expansion can only increase the demands on biological resources. We can anticipate an ever-increasing competition among different uses of the available land, and the maintenance of biodiversity may not rank high in the face of other, more obvious demands.
Many of the existing policies of our own country that have been enacted to preserve biodiversity have been focused on threatened species, or to preserve striking or unique ecosystems, such as Yellowstone National Park. The Endangered Species Act, the Convention on International Trade in Endangered Species, and our system of National Parks will continue to help in preserving biodiversity. But there are other areas of public policy that are as useful and important. In fact, it may well be that lands and waters that are necessarily exploited for their natural resources will hold the key for practical strategies to maintain biodiversity, for parks and preserves, alone, are inadequate for the task.
In truth, much that happens to preserve or decrease biodiversity arises through secondary effects of policies that are enacted for other reasons. Fisheries policies that aim to maintain fishery harvests; forestry policies that seek to maximize the economic yield of marketable timber; agricultural policies that maintain subsidies for keeping land in production that might be used for other beneficial purposes; and policies for the management of public lands that encourage overgrazing by maintaining artificially low grazing fees all have important negative effects on biodiversity, although not by design.
Other existing policies have impacts that work in the other direction. But unless the impacts on biodiversity of private acts or public policies are understood, and until there exists a broader consensus regarding the relative value of biodiversity, there is little hope, in this or any country, of holding the line at the levels that are needed for almost any use or service. We all need to be more aware of the direct benefits, indirect benefits, services, and future potential that biodiversity offers for both private gain and public benefit. We need greater awareness and coordination of policies that affect biodiversity, and national goals that go deeper than the protection of endangered species and the preservation of public parks.
From an economic perspective, much more work needs to be done to put a fair and meaningful valuation on biodiversity. The service aspects of biodiversity must be understood, and market mechanisms put in place to include these very real factors in both policy and business decisions.
From a scientific perspective, we need to learn more, and more quickly, about the role that biodiversity plays in the working of ecosystems. Gaps in our present knowledge of these connections now limit our assessments of the risks imposed when biodiversity declines, and preclude more complete economic evaluations.
In all of this, calls will be heard to defer action until we have in hand a more complete and reliable inventory of the present extent and variety of life on Earth, in terms of the number of species of plants and animals. Although counting must go on, it is now clear that waiting to learn the full extent of biodiversity before acting to stem so precipitous a decline is not a prudent choice, for both ecological and economic reasons.
Last, but certainly not least, are the issues of stewardship and ethics. In the long run, we must be concerned about maintaining the capability of the biological world to adapt, through adjustment and evolution, to changes in the physical environment. In addition, many would agree that as a society we bear the ethical obligation to protect the habitability of the planet, and to act as responsible stewards of its biological riches for the present and future welfare of the human species. To do that requires an appreciation of the value of biodiversity--both what it provides for the natural world and the ways that we can use it--and a commitment to preserve it so that our children and their children will continue to realize the benefits of a biologically rich Earth. Surely such a challenge demands the attention of scholars and policy-makers alike.
Many of the existing policies of our own country that have been enacted to preserve biodiversity have been focused on threatened species, or to preserve striking or unique ecosystems, such as Yellowstone National Park. The Endangered Species Act, the Convention on International Trade in Endangered Species, and our system of National Parks will continue to help in preserving biodiversity. But there are other areas of public policy that are as useful and important. In fact, it may well be that lands and waters that are necessarily exploited for their natural resources will hold the key for practical strategies to maintain biodiversity, for parks and preserves, alone, are inadequate for the task.
In truth, much that happens to preserve or decrease biodiversity arises through secondary effects of policies that are enacted for other reasons. Fisheries policies that aim to maintain fishery harvests; forestry policies that seek to maximize the economic yield of marketable timber; agricultural policies that maintain subsidies for keeping land in production that might be used for other beneficial purposes; and policies for the management of public lands that encourage overgrazing by maintaining artificially low grazing fees all have important negative effects on biodiversity, although not by design.
Other existing policies have impacts that work in the other direction. But unless the impacts on biodiversity of private acts or public policies are understood, and until there exists a broader consensus regarding the relative value of biodiversity, there is little hope, in this or any country, of holding the line at the levels that are needed for almost any use or service. We all need to be more aware of the direct benefits, indirect benefits, services, and future potential that biodiversity offers for both private gain and public benefit. We need greater awareness and coordination of policies that affect biodiversity, and national goals that go deeper than the protection of endangered species and the preservation of public parks.
From an economic perspective, much more work needs to be done to put a fair and meaningful valuation on biodiversity. The service aspects of biodiversity must be understood, and market mechanisms put in place to include these very real factors in both policy and business decisions.
From a scientific perspective, we need to learn more, and more quickly, about the role that biodiversity plays in the working of ecosystems. Gaps in our present knowledge of these connections now limit our assessments of the risks imposed when biodiversity declines, and preclude more complete economic evaluations.
In all of this, calls will be heard to defer action until we have in hand a more complete and reliable inventory of the present extent and variety of life on Earth, in terms of the number of species of plants and animals. Although counting must go on, it is now clear that waiting to learn the full extent of biodiversity before acting to stem so precipitous a decline is not a prudent choice, for both ecological and economic reasons.
Last, but certainly not least, are the issues of stewardship and ethics. In the long run, we must be concerned about maintaining the capability of the biological world to adapt, through adjustment and evolution, to changes in the physical environment. In addition, many would agree that as a society we bear the ethical obligation to protect the habitability of the planet, and to act as responsible stewards of its biological riches for the present and future welfare of the human species. To do that requires an appreciation of the value of biodiversity--both what it provides for the natural world and the ways that we can use it--and a commitment to preserve it so that our children and their children will continue to realize the benefits of a biologically rich Earth. Surely such a challenge demands the attention of scholars and policy-makers alike.
The Economic Value of Ecosystem Services
The economic value of ecosystem services is difficult to calculate, and this raises several important problems when we look at biodiversity in the context of public policy. How can we measure the economic value of ecosystem services such as water purification, or resistance to environmental disturbances? Since the maintenance of biodiversity involves choices and ultimately, costs, how can markets reflect and distribute these values appropriately?
The task may be somewhat easier in the case of new products and materials that are derived from the natural world. Prospecting for new pharmaceuticals is the most publicized, but not the only example. New food crops are also a possibility, although to date there have been very few such introductions that have achieved more than regional importance, either dietarily or economically. More intriguing, perhaps, is the use of genetic engineering to extract biochemical processes from the natural world. Research of this kind has found application in biological clean-up, or bioremediation of toxic waste and oil spills. An even more promising and somewhat more controversial opportunity is found in harnessing processes at the most fundamental levels of biological structure.
The pool of resources hidden in the genetic resources of living things is potentially huge. An example is the polymerase chain reaction (PCR) that is used in genetic research and in commercial applications to manipulate DNA. The ready availability of substances that speed up the rate at which the cells replicate--the catalysts that in living matter are proteins known as enzymes--has literally made genetic engineering practical on industrial scales.
The enzymes used to catalyze PCR were first isolated from bacteria that can survive only in high temperatures, and the source from which they were taken was natural hot springs in Yellowstone National Park. In this case, to say that an entire new industry depended on the diversity of organisms and habitats in the National Park system is no exaggeration. Substantial prospecting is now underway in these and other extreme environments to find enzymes that will catalyze other, industrially-useful reactions
The task may be somewhat easier in the case of new products and materials that are derived from the natural world. Prospecting for new pharmaceuticals is the most publicized, but not the only example. New food crops are also a possibility, although to date there have been very few such introductions that have achieved more than regional importance, either dietarily or economically. More intriguing, perhaps, is the use of genetic engineering to extract biochemical processes from the natural world. Research of this kind has found application in biological clean-up, or bioremediation of toxic waste and oil spills. An even more promising and somewhat more controversial opportunity is found in harnessing processes at the most fundamental levels of biological structure.
The pool of resources hidden in the genetic resources of living things is potentially huge. An example is the polymerase chain reaction (PCR) that is used in genetic research and in commercial applications to manipulate DNA. The ready availability of substances that speed up the rate at which the cells replicate--the catalysts that in living matter are proteins known as enzymes--has literally made genetic engineering practical on industrial scales.
The enzymes used to catalyze PCR were first isolated from bacteria that can survive only in high temperatures, and the source from which they were taken was natural hot springs in Yellowstone National Park. In this case, to say that an entire new industry depended on the diversity of organisms and habitats in the National Park system is no exaggeration. Substantial prospecting is now underway in these and other extreme environments to find enzymes that will catalyze other, industrially-useful reactions
The Services that Biodiversity Provides
Assessments of the economic benefits of biological diversity have been based primarily on our ability to generate revenue from biodiversity, through activities that produce measurable results in current markets, such as pharmaceuticals or tourism. But there are additional benefits from biodiversity that are not so easily included in commercial market analyses, and that come under the name of ecosystem services. These are the end results of natural biological processes that either improve the overall quality of the environment, or provide some benefit to the human users of the landscape--such as improvement of water quality and reduction of flooding. The concept of ecosystem services is unabashedly tilted toward human uses.
The study of ecosystem services is relatively new, but what is known points consistently in one direction: maintaining diversity on a variety of levels of ecological and biological organization--within forests, or among the trees that are there, or even within the genes of a single variety---is critical if services are to be maintained on a sustainable basis.
Ecosystem services can be provided in a variety of forms. One example is the purification of water that generally occurs by flowing through forested ecosystems and wetlands, which is an extremely important function from the standpoint of human populations that live downstream. The presence of living vegetation provides an efficient sink for many atmospheric pollutants as well. The regulation of stream flow by vegetation in the upper reaches of watersheds has long been recognized as an important ecosystem service, and watershed managers manipulate both the amount and type of vegetation in watersheds to help control sedimentation, floods, and sometimes stream flow.
The services that ecosystems provide often depend on the underlying physical structure of the habitat, such as the conditions for feeding and breeding that may be needed for the continued survival of an important animal species. What is often required is a diversity of habitats over an entire landscape. Ecosystem services may also depend on the presence of a particular species, as is the case in highly co-evolved plant-pollinator systems, or in highly managed agroecosystems that rely on specific pollinators, such as honeybees.
Biodiversity also plays an important role in maintaining ecosystem services over long periods of time, through the ups and downs of natural variations. Ecosystems that have lost either genetic or species diversity are less resistant to the effects of environmental perturbations, such as droughts, and are slower to recover when disturbed. Diversity is a form of ecosystem health insurance: those ecosystems that include several species that serve the same or similar functions tend to be more resistant to environmental stress and recover faster from perturbations.
The study of ecosystem services is relatively new, but what is known points consistently in one direction: maintaining diversity on a variety of levels of ecological and biological organization--within forests, or among the trees that are there, or even within the genes of a single variety---is critical if services are to be maintained on a sustainable basis.
Ecosystem services can be provided in a variety of forms. One example is the purification of water that generally occurs by flowing through forested ecosystems and wetlands, which is an extremely important function from the standpoint of human populations that live downstream. The presence of living vegetation provides an efficient sink for many atmospheric pollutants as well. The regulation of stream flow by vegetation in the upper reaches of watersheds has long been recognized as an important ecosystem service, and watershed managers manipulate both the amount and type of vegetation in watersheds to help control sedimentation, floods, and sometimes stream flow.
The services that ecosystems provide often depend on the underlying physical structure of the habitat, such as the conditions for feeding and breeding that may be needed for the continued survival of an important animal species. What is often required is a diversity of habitats over an entire landscape. Ecosystem services may also depend on the presence of a particular species, as is the case in highly co-evolved plant-pollinator systems, or in highly managed agroecosystems that rely on specific pollinators, such as honeybees.
Biodiversity also plays an important role in maintaining ecosystem services over long periods of time, through the ups and downs of natural variations. Ecosystems that have lost either genetic or species diversity are less resistant to the effects of environmental perturbations, such as droughts, and are slower to recover when disturbed. Diversity is a form of ecosystem health insurance: those ecosystems that include several species that serve the same or similar functions tend to be more resistant to environmental stress and recover faster from perturbations.
The Loss of Species
Species extinctions have received the lion's share of the attention in debates regarding biodiversity and the need to sustain it. The loss of individual species in ecosystems, such as frogs in wetlands or ferns in a forest, can certainly affect the ways in which those systems work together to cycle essential nutrients and water and process energy. Since we have only limited ability to predict how ecosystems will respond in terms of replacement or built-in redundancy to the possible loss of a specific species, there is some reason to be concerned when any are threatened by extinction.
At the same time, the same degree of concern should apply to reductions in the populations of species, even if they are not reduced to disappearance altogether. The role that classes of organisms play in ecosystems depends not only on what they do in terms of material cycling and energy flow, but on how many are there to do it. Reductions in abundance of an essential species can clearly affect overall system functioning, and therefore the degree to which ecosystem services will continue to be provided.
Some, known as keystone species, play a role in ecosystems that seems out of proportion to their number, such that even small changes in their abundance may have great impacts on the ecosystems in which they live. A common example is the sea- otter, a marine mammal that lives along the coasts of the northern Pacific Ocean. They dive and prey on sea-urchins that principally feed, in turn, on large seaweed called kelp. When sea-otters are present, the number of urchins is kept sufficiently low that stands of kelp--which are of commercial value as a source of potash and iodine--can become established and thrive. When otters vanish from the scene, the resulting growth in urchin populations effectively prevents the plant's successful regeneration, and eventually leads to the loss of kelp forests.
In time, all classes of living things--like the dinosaurs, or, we must presume, our own species--must face extinction. But the disappearance of any of them is a critical endpoint, marking the end of 3.5 billion years of evolutionary development. In Nature it represents a permanent depletion of biodiversity and a loss of genetic information on which evolution is based. In terms of people and nations, it counts as a loss of potential economic value in terms of services or products. Each species is a reservoir of unique genetic information that cannot be reproduced once it is gone. In this broader sense, any extinction, however trivial it may seem, represents a permanent loss to the biosphere as a whole.
What we need to know for informed policy decisions are the ecosystem services that a threatened species provides; the degree to which it offers opportunities for direct economic benefit; how expected benefits weigh against costs of preservation; and on a more general level, how present or expected rates of extinction compare to what might be expected through natural changes. The telling questions are whether and by how much the present rate of species loss differs from the rate that Nature would exact, were we not here, and whether the species that are lost play important keystone roles. The challenge is that this sort of information is only rarely available. Nor do we have, as yet, a predictive theory of keystone species.
Rates of loss
The UNEP Global Biodiversity Assessment has recently reviewed the methods that have been used in the literature to calculate natural, or background extinction rates and have compared them against current trends. The results, which are intentionally conservative, are sobering. Best estimates are that current extinction rates for well- documented groups of primarily, but not exclusively, vertebrates and vascular (in general, seed-bearing and fern-like) plants are at least 50 to 100 times larger than the expected natural background. There is no good reason to expect these rates to differ very much for plant or animal groups that are less well-studied.
On the basis of recent estimates of land-use change, largely in the tropics, there is a reasonable expectation that extinction rates in the very near future could rise, worldwide, to as much as 10,000 times the natural level. Extinctions of this number and extent would approach, and possibly surpass, the major mass extinctions of the geologic past, as when dinosaurs and other life forms disappeared, about 65 million years ago.
The total number of species that inhabit the planet is unknown. The UNEP Global Biodiversity Assessment uses an estimate of about 13 million, but the range varies from 8 to 50 million or more. Only about 2 million species have been described scientifically, and they are distributed very unevenly among different taxonomic groups (Table 2). While important in its own right, the number need not be precisely known to be concerned about the rates at which the better documented species are now disappearing. In today's world, most extinctions will occur before the species have even been named and described, much less known ecologically.
At the same time, the same degree of concern should apply to reductions in the populations of species, even if they are not reduced to disappearance altogether. The role that classes of organisms play in ecosystems depends not only on what they do in terms of material cycling and energy flow, but on how many are there to do it. Reductions in abundance of an essential species can clearly affect overall system functioning, and therefore the degree to which ecosystem services will continue to be provided.
Some, known as keystone species, play a role in ecosystems that seems out of proportion to their number, such that even small changes in their abundance may have great impacts on the ecosystems in which they live. A common example is the sea- otter, a marine mammal that lives along the coasts of the northern Pacific Ocean. They dive and prey on sea-urchins that principally feed, in turn, on large seaweed called kelp. When sea-otters are present, the number of urchins is kept sufficiently low that stands of kelp--which are of commercial value as a source of potash and iodine--can become established and thrive. When otters vanish from the scene, the resulting growth in urchin populations effectively prevents the plant's successful regeneration, and eventually leads to the loss of kelp forests.
In time, all classes of living things--like the dinosaurs, or, we must presume, our own species--must face extinction. But the disappearance of any of them is a critical endpoint, marking the end of 3.5 billion years of evolutionary development. In Nature it represents a permanent depletion of biodiversity and a loss of genetic information on which evolution is based. In terms of people and nations, it counts as a loss of potential economic value in terms of services or products. Each species is a reservoir of unique genetic information that cannot be reproduced once it is gone. In this broader sense, any extinction, however trivial it may seem, represents a permanent loss to the biosphere as a whole.
What we need to know for informed policy decisions are the ecosystem services that a threatened species provides; the degree to which it offers opportunities for direct economic benefit; how expected benefits weigh against costs of preservation; and on a more general level, how present or expected rates of extinction compare to what might be expected through natural changes. The telling questions are whether and by how much the present rate of species loss differs from the rate that Nature would exact, were we not here, and whether the species that are lost play important keystone roles. The challenge is that this sort of information is only rarely available. Nor do we have, as yet, a predictive theory of keystone species.
Rates of loss
The UNEP Global Biodiversity Assessment has recently reviewed the methods that have been used in the literature to calculate natural, or background extinction rates and have compared them against current trends. The results, which are intentionally conservative, are sobering. Best estimates are that current extinction rates for well- documented groups of primarily, but not exclusively, vertebrates and vascular (in general, seed-bearing and fern-like) plants are at least 50 to 100 times larger than the expected natural background. There is no good reason to expect these rates to differ very much for plant or animal groups that are less well-studied.
On the basis of recent estimates of land-use change, largely in the tropics, there is a reasonable expectation that extinction rates in the very near future could rise, worldwide, to as much as 10,000 times the natural level. Extinctions of this number and extent would approach, and possibly surpass, the major mass extinctions of the geologic past, as when dinosaurs and other life forms disappeared, about 65 million years ago.
The total number of species that inhabit the planet is unknown. The UNEP Global Biodiversity Assessment uses an estimate of about 13 million, but the range varies from 8 to 50 million or more. Only about 2 million species have been described scientifically, and they are distributed very unevenly among different taxonomic groups (Table 2). While important in its own right, the number need not be precisely known to be concerned about the rates at which the better documented species are now disappearing. In today's world, most extinctions will occur before the species have even been named and described, much less known ecologically.
Over Exploitation
Many of the best documented cases of individual species being driven to extinction or near-extinction by humans are those of over- exploitation.
The passenger pigeon--a species that resembled the smaller, mourning dove--was in the early 1800s the most abundant bird in North America, and so plentiful that migrating flocks of a billion or more individuals would darken the skies of parts of the eastern U.S. for days at a time. By the end of the last century it had been hunted to the brink of extinction, and in September of 1914, in a Cincinnati zoo, the passenger pigeon disappeared forever with the death of the last remaining bird. The American bison, or buffalo, of the Great Plains was also nearly hunted out of existence in the same century, and its larger, woods-dwelling relative was driven to extinction.
As many as a quarter of all the bird species in the world may have similarly vanished in the course of the last 1000 years with the expansion of human populations through the islands of the South Pacific. The spread of early people through the New World, about 10,000 years ago, was probably responsible for the extinction of many of the large mammals that were originally here: now-extinct mammoths, sloths, and cave bears are known to have been hunted by those who first walked through North and South America. The same impact was felt by large mammals in Australia, New Zealand, and Madagascar. The current and rapid loss of tropical hardwoods in many regions due to high commercial demand, low rates of successful replacement, and the long periods of growth necessary to produce new, marketable resources has raised concern about over- exploitation of some species, such as rosewood, although none of the trees are known to have been driven to extinction.
Over-exploitation is also a major factor in reducing the natural biodiversity of marine fisheries through major reductions in populations, although again, no extinctions have been documented. During the last two decades, the world has seen the collapse of a number of marine fisheries. Some of these have recovered, but others, such as the cod and haddock fisheries in the North Atlantic, have not. Even for those that recover, the consequences of the original over-exploitation on population dynamics and genetic diversity are now only poorly understood. What is often apparent is a systematic decrease in the size, and hence age, of the individuals that are harvested. The selective loss of larger fish has significant impacts on those that remain. If fertility is strongly related to body size, as is the case for many fish species, over-exploitation not only reduces the abundance of a species, but it may also make recovery more difficult in systematically removing the most fecund individuals. The ensuing consequences for overall ecosystem functioning and biodiversity are as yet not well understood.
Whole ecosystems can also be affected by over-exploitation. For example, a reduction in organic carbon and nutrients, including phosphorus or nitrogen, as may occur in intensively farmed areas, decreases the fertility of soils. When losses are severe, the resulting depletion can lead to either more intensified use by adding more fertilizers and then herbicides and pesticides to control weeds and pests (in the cycle noted above), or to abandonment. If abandoned, the land will probably not recover its original component of plant and animal species because of the depleted nutrients. Through this chain of happenings, an over-exploitation of the soil for agricultural gain can have long term, negative impacts on the biodiversity of the region.
Alien introductions
Introductions and invasions of alien species of plants and animals is a long-recognized problem, as detailed in an earlier issue of CONSEQUENCES. We have only limited ability to predict quantitatively the results of any particular intruder, including its capability of establishing a permanent, reproducing population. What is certain is that some areas are by nature more susceptible. Continental forests are reasonably resistant to newly introduced tree species, except in cases where they have been disturbed by heavy cutting or partial clearing. Native meadows and prairies, when disturbed, have also proven particularly susceptible to intruders, as is the case for the many grasslands around the world that have been converted to pasture or cultivated land. For example, many of the now common grasses in the intermontane western U.S. and southwestern Canada are transported Eurasian weeds. These species were able to invade and become established because the original perennial tussock grasses were unable to support the intensification of grazing from large-scale cattle ranching.
Freshwater lakes and streams have little immunity to invading species. Alien plants or animals seem able to establish reproducing populations relatively easily, and the new species often have significant impacts on biotic composition, and on a variety of ecosystem processes. Two examples of the kind of changes that can result from even well-meant introductions are the purposeful introduction of game fish to many lakes and streams throughout the world that replaced native varieties, and the ecological havoc that followed the introduction of the Nile perch in Lake Victoria in 1960 to benefit commercial fishing. In less than thirty years, the appetite of the Nile perch, whose food is smaller fish, led to the extinction of about thirty species of fish that were native to the lake.
In terrestrial ecosystems, the largest changes occur when the intruder brings quite different traits from those of native species. The best documented example is that of the introduction of the exotic tree, Myrica faga, into Hawaii, which has resulted in large changes in ecosystem dynamics. The significant difference, in this case, was the ability of the introduced tree--a legume like peas and beans and clover--to convert atmospheric nitrogen to ammonia, a characteristic not previously present in those ecosystems. This ability of the introduced tree increases the nitrogen content of soils, and thus alters the raw materials on which many other plant species depend.
Introduced species with characteristics that are not qualitatively different from those of native species, can through force of numbers have large and long-lasting effects on them. About 100 European starlings were released in New York City in 1890-91 by a collector bent on establishing all the birds mentioned in the writings of William Shakespeare. The result, evident throughout the country today, is a diminished number of many native American songbirds, through competition for nest-sites, in which the aggressive and now very abundant starling has been extremely successful
The passenger pigeon--a species that resembled the smaller, mourning dove--was in the early 1800s the most abundant bird in North America, and so plentiful that migrating flocks of a billion or more individuals would darken the skies of parts of the eastern U.S. for days at a time. By the end of the last century it had been hunted to the brink of extinction, and in September of 1914, in a Cincinnati zoo, the passenger pigeon disappeared forever with the death of the last remaining bird. The American bison, or buffalo, of the Great Plains was also nearly hunted out of existence in the same century, and its larger, woods-dwelling relative was driven to extinction.
As many as a quarter of all the bird species in the world may have similarly vanished in the course of the last 1000 years with the expansion of human populations through the islands of the South Pacific. The spread of early people through the New World, about 10,000 years ago, was probably responsible for the extinction of many of the large mammals that were originally here: now-extinct mammoths, sloths, and cave bears are known to have been hunted by those who first walked through North and South America. The same impact was felt by large mammals in Australia, New Zealand, and Madagascar. The current and rapid loss of tropical hardwoods in many regions due to high commercial demand, low rates of successful replacement, and the long periods of growth necessary to produce new, marketable resources has raised concern about over- exploitation of some species, such as rosewood, although none of the trees are known to have been driven to extinction.
Over-exploitation is also a major factor in reducing the natural biodiversity of marine fisheries through major reductions in populations, although again, no extinctions have been documented. During the last two decades, the world has seen the collapse of a number of marine fisheries. Some of these have recovered, but others, such as the cod and haddock fisheries in the North Atlantic, have not. Even for those that recover, the consequences of the original over-exploitation on population dynamics and genetic diversity are now only poorly understood. What is often apparent is a systematic decrease in the size, and hence age, of the individuals that are harvested. The selective loss of larger fish has significant impacts on those that remain. If fertility is strongly related to body size, as is the case for many fish species, over-exploitation not only reduces the abundance of a species, but it may also make recovery more difficult in systematically removing the most fecund individuals. The ensuing consequences for overall ecosystem functioning and biodiversity are as yet not well understood.
Whole ecosystems can also be affected by over-exploitation. For example, a reduction in organic carbon and nutrients, including phosphorus or nitrogen, as may occur in intensively farmed areas, decreases the fertility of soils. When losses are severe, the resulting depletion can lead to either more intensified use by adding more fertilizers and then herbicides and pesticides to control weeds and pests (in the cycle noted above), or to abandonment. If abandoned, the land will probably not recover its original component of plant and animal species because of the depleted nutrients. Through this chain of happenings, an over-exploitation of the soil for agricultural gain can have long term, negative impacts on the biodiversity of the region.
Alien introductions
Introductions and invasions of alien species of plants and animals is a long-recognized problem, as detailed in an earlier issue of CONSEQUENCES. We have only limited ability to predict quantitatively the results of any particular intruder, including its capability of establishing a permanent, reproducing population. What is certain is that some areas are by nature more susceptible. Continental forests are reasonably resistant to newly introduced tree species, except in cases where they have been disturbed by heavy cutting or partial clearing. Native meadows and prairies, when disturbed, have also proven particularly susceptible to intruders, as is the case for the many grasslands around the world that have been converted to pasture or cultivated land. For example, many of the now common grasses in the intermontane western U.S. and southwestern Canada are transported Eurasian weeds. These species were able to invade and become established because the original perennial tussock grasses were unable to support the intensification of grazing from large-scale cattle ranching.
Freshwater lakes and streams have little immunity to invading species. Alien plants or animals seem able to establish reproducing populations relatively easily, and the new species often have significant impacts on biotic composition, and on a variety of ecosystem processes. Two examples of the kind of changes that can result from even well-meant introductions are the purposeful introduction of game fish to many lakes and streams throughout the world that replaced native varieties, and the ecological havoc that followed the introduction of the Nile perch in Lake Victoria in 1960 to benefit commercial fishing. In less than thirty years, the appetite of the Nile perch, whose food is smaller fish, led to the extinction of about thirty species of fish that were native to the lake.
In terrestrial ecosystems, the largest changes occur when the intruder brings quite different traits from those of native species. The best documented example is that of the introduction of the exotic tree, Myrica faga, into Hawaii, which has resulted in large changes in ecosystem dynamics. The significant difference, in this case, was the ability of the introduced tree--a legume like peas and beans and clover--to convert atmospheric nitrogen to ammonia, a characteristic not previously present in those ecosystems. This ability of the introduced tree increases the nitrogen content of soils, and thus alters the raw materials on which many other plant species depend.
Introduced species with characteristics that are not qualitatively different from those of native species, can through force of numbers have large and long-lasting effects on them. About 100 European starlings were released in New York City in 1890-91 by a collector bent on establishing all the birds mentioned in the writings of William Shakespeare. The result, evident throughout the country today, is a diminished number of many native American songbirds, through competition for nest-sites, in which the aggressive and now very abundant starling has been extremely successful
Land Use
Changes in how the land is used are probably the principal contributor to the current decline in biodiversity. About 1 to 2 percent of the land surface of the Earth is now devoted to urban use, but other changes in land cover and land use far exceed the direct impact of the small fraction that is paved or developed for homes and factories and other buildings. Homo sapiens has already converted about a quarter of all the land surface to agricultural uses. By some estimates we now appropriate directly or indirectly about 40 percent of what biologists call the primary production of the Earth's biota (the products of photosynthesis on which all other life depends), and the percentage that comes under our control in this way is increasing.
The pressures on terrestrial resources and land depend very much on population growth and the demands of early stages of economic development. Moreover, land acquisition, especially for agriculture and forestry, focuses initially on those areas with the most fertile soils and equable climates, which are often the areas of greatest biological diversity.
Deforestation in the humid tropics is probably the best-known current example of rapid land-use change. During the decade of the 1970s, vast areas of tropical forest in South America, Africa, and Southeast Asia were cleared and converted to agriculture and other uses. In the middle-to-late 1980s, the rates of deforestation in South America slowed dramatically, largely due to economic and tax policy changes in Brazil, but the pace of cutting in Africa and Southeast Asia, though poorly quantified, remains high. Globally, the rate of loss of tropical forests for the 1980s has been estimated at about 1 percent per year, but there is still considerable uncertainty. The rates of extinction of local species that accompany these rapid changes in land cover may soon be far in excess of what is found today, reaching as high as 10,000 times the natural background rate.
In the industrialized nations of the Northern Hemisphere the most rapid and widespread conversion of forest to other uses took place over the last several hundred years. In this time, much of the northeastern U.S., for example, was deforested at least once, in connection with the rise of agriculture and timber industries. But as regional and national economies changed, many previously cleared areas were left to return to their natural vegetation. As a result, forests have reappeared in parts of the Northeast, and indeed the country as a whole has probably gained forested land over the last several decades.
The current trend of most concern with respect to tree-cover in the U.S. is a shift to smaller parcel sizes. What once were continuously forested landscapes are now a quilt of small patches of trees, criss- crossed with roads, subdivisions, agricultural tracts, and a variety of different land-uses and land-covers: a scene that is familiar to anyone who has looked out an airplane window. The average size of tree-covered parcels is smaller than was the case twenty, fifty, or a hundred years ago, resulting in a landscape that is highly fragmented and partitioned.
The difference in terms of the natural world is great, and several studies now point with concern to the biological impacts of the shift to less continuous landscapes. The known consequences of these changes are reduced numbers of both plants and animals and a greater possibility of the outright loss of some of them--when in effect, they are painted into a corner with nowhere left for them to go. The interweaving of favorable and unfavorable habitats also curtails the ability of organisms to disperse, and makes recolonization of distant areas more difficult.
An analogous pattern of fragmentation can be found in parts of South America where deforestation was previously extremely rapid. Although the amount of new cutting appears to have fallen from that of previous decades, it seems to be increasing again in the rain forest of the Amazon, and the deforested, newly colonized regions now have their own distinctive appearance. Patchworks of active fields, orchards, abandoned fields, second growth forest, and primary forest are the norm. But the scene is ever changing through an interplay of active use by initial colonizers, abandonment, partial recovery through natural processes, and as then often happens, subsequent re-use. Analyses of potential impacts on biodiversity that are based on simple measures of deforested area can provide little more than very general conclusions.
Deforestation is not the only land-use change of interest or concern. Another with broad implications for biodiversity is the intensification of agriculture and grazing on those lands that have been traditionally devoted to these purposes. Of particular importance for biodiversity are the secondary impacts of intensive agriculture. Heavy applications of fertilizers and pesticides have the potential of creating additional environmental problems as well as affecting the abundance and viability of the other plants and animals and micro-organisms in the same or adjoining areas.
The adverse effects of non-point-source pollution due to the run-off of pesticides and herbicides from intensively-used fields are well- known. In addition, because of the understandable tendency to put the best land into production first, the expansion of agriculture into less fertile areas typically requires heavier applications of chemicals, more extensive site preparation, and other forms of more intensive management. The typical result is increased chemical run-off to the landscape, and with ensuing degradation, additional pressure for expansion, and so on. It is such a cycle that has led to widespread desertification in some parts of the world, primarily through overgrazing that can be compounded by naturally occurring droughts.
The pressures on terrestrial resources and land depend very much on population growth and the demands of early stages of economic development. Moreover, land acquisition, especially for agriculture and forestry, focuses initially on those areas with the most fertile soils and equable climates, which are often the areas of greatest biological diversity.
Deforestation in the humid tropics is probably the best-known current example of rapid land-use change. During the decade of the 1970s, vast areas of tropical forest in South America, Africa, and Southeast Asia were cleared and converted to agriculture and other uses. In the middle-to-late 1980s, the rates of deforestation in South America slowed dramatically, largely due to economic and tax policy changes in Brazil, but the pace of cutting in Africa and Southeast Asia, though poorly quantified, remains high. Globally, the rate of loss of tropical forests for the 1980s has been estimated at about 1 percent per year, but there is still considerable uncertainty. The rates of extinction of local species that accompany these rapid changes in land cover may soon be far in excess of what is found today, reaching as high as 10,000 times the natural background rate.
In the industrialized nations of the Northern Hemisphere the most rapid and widespread conversion of forest to other uses took place over the last several hundred years. In this time, much of the northeastern U.S., for example, was deforested at least once, in connection with the rise of agriculture and timber industries. But as regional and national economies changed, many previously cleared areas were left to return to their natural vegetation. As a result, forests have reappeared in parts of the Northeast, and indeed the country as a whole has probably gained forested land over the last several decades.
The current trend of most concern with respect to tree-cover in the U.S. is a shift to smaller parcel sizes. What once were continuously forested landscapes are now a quilt of small patches of trees, criss- crossed with roads, subdivisions, agricultural tracts, and a variety of different land-uses and land-covers: a scene that is familiar to anyone who has looked out an airplane window. The average size of tree-covered parcels is smaller than was the case twenty, fifty, or a hundred years ago, resulting in a landscape that is highly fragmented and partitioned.
The difference in terms of the natural world is great, and several studies now point with concern to the biological impacts of the shift to less continuous landscapes. The known consequences of these changes are reduced numbers of both plants and animals and a greater possibility of the outright loss of some of them--when in effect, they are painted into a corner with nowhere left for them to go. The interweaving of favorable and unfavorable habitats also curtails the ability of organisms to disperse, and makes recolonization of distant areas more difficult.
An analogous pattern of fragmentation can be found in parts of South America where deforestation was previously extremely rapid. Although the amount of new cutting appears to have fallen from that of previous decades, it seems to be increasing again in the rain forest of the Amazon, and the deforested, newly colonized regions now have their own distinctive appearance. Patchworks of active fields, orchards, abandoned fields, second growth forest, and primary forest are the norm. But the scene is ever changing through an interplay of active use by initial colonizers, abandonment, partial recovery through natural processes, and as then often happens, subsequent re-use. Analyses of potential impacts on biodiversity that are based on simple measures of deforested area can provide little more than very general conclusions.
Deforestation is not the only land-use change of interest or concern. Another with broad implications for biodiversity is the intensification of agriculture and grazing on those lands that have been traditionally devoted to these purposes. Of particular importance for biodiversity are the secondary impacts of intensive agriculture. Heavy applications of fertilizers and pesticides have the potential of creating additional environmental problems as well as affecting the abundance and viability of the other plants and animals and micro-organisms in the same or adjoining areas.
The adverse effects of non-point-source pollution due to the run-off of pesticides and herbicides from intensively-used fields are well- known. In addition, because of the understandable tendency to put the best land into production first, the expansion of agriculture into less fertile areas typically requires heavier applications of chemicals, more extensive site preparation, and other forms of more intensive management. The typical result is increased chemical run-off to the landscape, and with ensuing degradation, additional pressure for expansion, and so on. It is such a cycle that has led to widespread desertification in some parts of the world, primarily through overgrazing that can be compounded by naturally occurring droughts.
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