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Our Common Journey: A Transition Toward Sustainability (1999)
4 Environmental Threats and Opportunities
The goals for a transition toward sustainability, as we set them out in Chapter 1, are to meet human needs over the next two generations while reducing hunger and poverty and preserving our environmental life support systems. The activities to approach this goal can only move ahead within the constraints set by resources and the environment. Many people have argued that, unless we make dramatic changes in our human enterprises, the development needed to meet future human needs risks damaging the life-support capabilities of the earthwhich in turn would of course prevent society from meeting its goals. In this chapter, we therefore ask two related questions:
&bull What are the greatest threats that humanity will encounter as it attempts to navigate the transition to sustainability?
&bull What are the most promising opportunities for avoiding or circumventing these threats on the path to sustainability?
Our object is not to predict what environmental damages might be caused by development at particular times and placesa largely futile activity for all but the most specific and immediate development plans. Rather, it is to highlight some of the most serious environmental obstacles that might be met in plausible efforts to reach the goals outlined in Chapter 1 and along development paths such as those explored in Chapters 2 and 3, to take timely steps to avoid or circumvent these obstacles. 1
This chapter begins with a brief discussion of the approaches and issues we considered in scouting the environmental hazards that societies may confront. We then turn to efforts to assess the relative severity of
these hazards for particular times and places. Following the lead of the Brundtland Commission, we next analyze how human activities in a number of crucial developmental sectors might pose important challenges and opportunities for navigating the transition toward sustainability. Finally, we turn to the question of interactionshow multiple developmental activities may interact with complex environmental systems to transform the very nature of the journey before us.
Throughout our discussion, we not only seek to identify potential obstacles to a successful transition, but also to highlight the skills, knowledge, and materials that might be most useful in detecting and understanding the hazards, and in devising solutions or mid-course corrections to address them. We conclude that in any given place there are significant if often place-specific opportunities for societies to pursue goals of meeting human needs while sustaining earth's life support systems. Some of these opportunities are likely to be realized by individual actorsfirms, organizations, and statesin the normal course of their self-interested activities. Others, however, will require integrative planning and management approaches.
One of the most difficult challenges of the Board's exerciseand one that has bedeviled other attempts to evaluate the pitfalls to sustainable developmenthas been to determine which of the many potential problems are truly those that cannot be ignored. Perhaps the easiest approach might be to list as potential concerns for sustainable development every resource limitation or environmental response that can be imagined. Equally clear, however, is that a canoe-steering society that tries to focus public resources on avoiding every possible danger in a river at once will likely be looking the wrong way as it collides with the biggest rock. How can we distinguish those threats that, while not insignificant, are likely to be avoided or adapted to from those with a real potential for sinking the vessel? And how can we devise a system that encourages society to update its priorities among all hazards in light of new information and expertise?
A further difficulty in the analysis arises because hazards have spatial and temporal dimensions and important interactions. However connected the world may be, and however global the transformations humans impose on it, the sustainability transition will be played out differently on a vast number of local stages. Neither population growth, nor climate change, nor water limitations will be the same in Japan as in the Sudan. The environmental hazards that nations and communities find most threatening and the response strategies they look to will continue to be
significantly different in different places in the world and at different times. Moreover, some components of the environmental system have impressive resiliency and ability to recover from human-caused or natural stress. Temporal dynamics and variations in the resiliency of systems confound clear illumination of critical hazards. Identification of hazards must also confront the difficulty of identifying, measuring, and predicting cumulative and interactive effects and discontinuous changes. Many of the activities that humans engage in occur at local scales, but as these activities are repeated around the world, their effects accumulate collectively, local changes can lead to regional and global changes. Many of the worst and of the best-known environmental problems (e.g., stratospheric ozone depletion, anoxia in the Gulf of Mexico) resulted from the slow, day-by-day accumulation of small changes and dispersed activities. Such cumulative effects are only noticed after they have intensified over time, or when nonlinearities in the response of global or regional systems lead to dramatic and unforeseen events. Interactions of multiple changes also lead to surprise. Consequences that are deemed unlikely are often overlooked, yet rare events with extreme or large-scale consequences may influence the sustainability of the global system even more than cumulative effects.
Clearly, uncertainty is rampant and surprise is inevitable. Recent environmental surprises have ranged from the emergence of "new" communicable diseases such as Legionnaires' disease, in a part of the developed world where such things were assumed to be hazards of the past through the devastation of the developing-world town of Bhopal, India, in a very modern industrial accident to the belated discovery that the nontoxic, noncorrosive CFCs that had displaced hazardous refrigerants and propellants turned out to have their own serious risks. 2 More such surprises are likely as the earth system comes under increasing pressure from human activities. One difficulty lies in achieving a balance between falsely declaring certainty to engender action and the fatalistic resignation that societies can never know enough to know when or how to act.
In dealing with these difficulties, the Board has attempted to develop a process for setting priorities and for identifying issues that require top concern. While our analysis builds on numerous national and international "stock-taking" efforts, we ultimately focus our attention on those issues that cut across sectors and that interact to simultaneously threaten human and ecosystem health, urban development, industrial advances, and sustained agricultural production. We conclude that integrative solutions-those aimed at interacting challenges across many sectorswill be key to successfully navigating the transition to sustainability.
Perceptions of risk change with circumstances, as pressures increase, information is collected, technology advances, and surprises occur. The
environmental challenges that local places face as they navigate the transition to sustainability will also differ, because of inherent variations in resource bases and biophysical, social, and political environments. These variations include differences in geochemical and ecological vulnerability to pollution, social capital formation, and countless other details. Together, they make unsatisfactory any global-scale exercise to rank potential hazards. How do we then focus on challenges and opportunities that are relevant at the global scale yet meaningful locally?
We conclude that the most serious threats are those that (1) affect the ability of multiple sectors of almost any society to move ahead toward our normative goals for sustainability (2) have cumulative or delayed consequences, with effects felt over a long time (3) are irreversible or difficult to change and/or (4) have a notable potential to interact with each other to damage earth's support systems. To identify the problems that fit these criteria, we draw on several approaches. First, we use an environment-oriented analysis, 3 in which hazards are ranked on the basis of the breadth of their consequences (e.g., having human health consequences, ecosystem consequences, and consequences for materials and productivity). Secondly, we use the framework of ''common challenges" to development in various sectors proposed by the 1987 Brundtland Commission as the basis for expert group analyses of threats and opportunities for the transition to sustainability. Finally, we identify the threats stemming from the interaction of sectoral activities.
Researchers 4 drew on the UN Environment Program's The World Environment: 1972&ndash1982, the U.S. Environmental Protection Agency's Unfinished Business and a range of other national and international environmental assessments that had been carried out worldwide, to develop a list of 28 potential environmental hazards that included most issues judged important in one or more of these studies. The hazards fell into five broad categories: land and water pollution, air pollution, contaminants of the human environment (e.g., indoor air pollution), resource losses, and natural disasters. Environmental data and explicit value judgments about the relative importance of present versus future impacts and of human health versus ecological impacts were then combined to generate comparative national rankings of the overall hazards list. From their analysis, it is apparent that the availability of high-quality freshwater is a priority concern in the United States, whether the most weight is given to human health, ecosystem, or materials concerns. Also, the more regional to global problems of stratospheric ozone depletion, climate change, acidification, and tropospheric ozone production and air pollution are common
and highly ranked issues of concern across the three areas. Such an approach provides the basis for assigning priorities to environmental threats.
In support of this Board's activities, the list was modified 5 and compared with eight other major efforts to assess environmental hazards, scoring each hazard on the basis of how important the various efforts found them to be (Table 4.1). Looking at Table 4.1 as a whole, some problems such as groundwater contamination and forest degradation stand out as being of nearly universal concern. Others, such as indoor air pollution and contamination, show up less frequently. Over time, there has been a shift from a focus on the depletion of natural resources and contamination of the environment to the loss of particular ecosystems (e.g., forests). In the individual assessments, the environmental threats identified as the most serious are often those most salient to a particular population. For example, the report on India devoted considerable attention to the health hazards of chemicals, both in the workplace and in accidental leakages, largely because at the time of the report the Bhopal disaster was still a major environmental event.
Overall, these analyses suggest that, for most nations of the world, water and air pollution are the top priority issues for most of the more industrialized nations, ozone depletion and climate change are also ranked highly while for many of the less-industrialized countries, droughts or floods, disease epidemics, and the availability of local living resources are crucial. The scored hazards approach 6 shows that sufficient data exist to make some relative hazard identifications for both today and the future. It also makes clear that relative hazard rankingseven of global environmental problems are strongly dependent on the circumstances of the region assessed.
One of the limitations of this approach is its failure to address interactionsfor example, the fact that such issues as water quality, acidification, and climate change are intimately linked, and that change in one will have consequences for change in others. In addition, because the approach focuses on the problem rather than the cause, it is not a good pragmatic tool on its own. Solutions are difficult to develop without knowing causes.
For another type of perspective, we built on the work of the Brundtland Commission's report Our Common Future. 7 In the interests of policy relevance, this effort broke with the tradition of analysis focused on environmental issues. Instead, analysis is directed to the "common challenges" to the environment arising from development activities within particular sectors: population and human resource development, cities,
Table 4.1 Assessments of the Importance of Environmental Hazards
Sources: UNCED (1992) World Bank (1992) WRI (1996) UNEP (1982) Easterbrook (1995) Centre for Science and Environment (1995) Council on Environmental Quality and Department of State (1982) Brown (1956).
agricultural production, industry, energy, and living resources. Using the Brundtland "common challenges" concept, we evaluated potential sector-specific resource and environmental impediments to reaching sustainability goals, along with the opportunities each sector offers to reduce, prevent, or mitigate the most serious threats. In addition, we evaluated progress over the last decade in achieving the measures identified by the Brundtland "challenges."
Human Population and Well-Being
In 1987, the Brundtland Commission framed the issue of human population growth in terms of both the balance between population and resources and the need for increased health, well-being, and human rights to self-determination. Today, these issues are strongly linked, and we recognize that the reduction in poverty, poor health, mortality, and the increase in educational and employment opportunities for all are the keys to slowing population growth and to the wise and sustainable use of resources. Thus, one of the most critical challenges for efforts to navigate a transition to sustainability will be to reduce population growth while simultaneously improving the health, education, and opportunities of the world's people.
Population growth is an underlying threat to sustainability due to the increased consumption of energy and materials needed to provide for many more people, to crowding and competition for resources, to environmental degradation, and to the difficulties that added numbers pose in efforts to advance human development. Today, population growth has ended in most industrialized countries and rates of population growth are in decline everywhere except in parts of Africa (see Chapter 2) yet the population of 2050 is nonetheless predicted to reach about 9 billion. In a classic decomposition of future population growth in developing countries, a researcher examined the major sources of this continued growth: unwanted childbearing due to low availability of contraception, a still-large desired family size, and the large number of young people of reproductive age. 8 Currently, 120 million married women (and many more unmarried women) report in surveys that they are not practicing contraception despite a desire for smaller families or for more time between births. Meeting their needs for contraception would reduce future population growth by nearly 2 billion. At the same time, such surveys also show that the desired family size in most developing countries is still above two children. An immediate reduction to the level of replacement (2.1) would reduce future growth by about 1 billion. The remainder of future population growth can be accounted for by so-called population momentum, which is due to the extraordinarily large number of young
people. This momentum ensures that population growth will persist for decades even if fertility were to drop to replacement level.
Addressing each of these sources of future growth could reduce fertility and future population numbers further and faster than current trends would project. Opportunities include making contraception more readily available to those who desire it (Table 4.2), accelerating trends that lead to lower desired family size, and slowing the momentum of population growth arising from the large number of prospective parents that are alive today. 9 Linking voluntary family planning with other reproductive and child health services can increase access to contraception for the many who want it. Improving the survival of children, their education, and the status of girls and women has been correlated with and may lead to a desire for smaller families. Increasing the age of childbearing, primarily by improving the secondary education and income-generating opportunities for adolescent girls, can slow the momentum of population growth. All of these opportunities, if exploited, could contribute directly to our societal goals for a transition to sustainability at the same time, through these factors' influence on reducing the ultimate size of the population, they would increase the probability of meeting environmental goals.
Threats to human-well being stem from many environmental sources. Environmental factors can affect human health directlythrough exposure to air pollution, heavy metals, and synthetic chemicalsand indirectly through loss of natural biological controls over opportunistic agents and vectors of infectious disease. Because of human introductions nearly
Table 4.2 Projections of the Population Size of the Developing World With and Without Unwanted Births
Projected population size (billions) in year
Standard* (with unwanted births)
Effect of unwanted fertility
*World Bank projection as quoted in Bos et al.
Source: Bongaarts (1994). Courtesy of the American Association for the Advancement of Science.
50 years ago, the global environment now carries a number of synthetic chemicals that can interfere with human physiology, including the endocrine system, the immune system, and neurological function. 10 Additionally, heavy metal deposition in the environment is rising and will continue to increase under development scenarios implicit in meeting our normative goals. Health effects of exposure to heavy metals may be substantial, and include long-term neurological effects on intelligence and behavior. Air pollution is a critical problem of urban systems in many regions of the world, and the increase in air pollution with a rapidly urbanizing world raises serious concerns for human health and the health of crops and natural ecosystems. As described in Chapter 2, over the past several decades, there has been an emergence, resurgence, and redistribution of infectious diseases. The potential eruption of diseases in an increasingly populated world is a serious threat to sustainability goals. These diseases threaten human health, water safety, food security, and ecosystem health.
Fortunately, because of biological and other scientific revolutions and policy reform over the past decades, there are opportunities for addressing the health risks from exposure to environmental threats. Biotechnology holds great promise (for example, in the creation of new medicines and diagnostics, pest-resistant crop species, plants with low-water requirements, and biodegradable pesticides and herbicides). Policies that control the point sources of air pollution, deposition of heavy metals, and disposal of synthetic chemicals help resolve health-related problems for local and regional human populations and can have very significant and long-term payoffs for future generations. Also, the establishment of early warning systems and other predictive capabilities to identify conditions conducive to outbreaks and clusters of infectious disease could be useful for health institutions at all spatial scales.
In addition, a number of opportunities arise via interactions of this human well-being sector with others. For example, reduction in industrial wastes through approaches using industrial ecology would have large advantages for human health, and also for the environment as it is affected by energy and water sectors, through the increased efficiency of these resources' use. Finally, the maintenance of natural ecosystems and the protection of their services can influence human health in many ways, including by providing natural enemies for disease vectors and natural water and air purification and supply systems.
Over the next half century, urban populations are likely to grow from the present 3 billion to perhaps 7 billion people, with most of the growth
occurring in non-OECD (see Chapter 2 and 3). 11 Cities are engines of economic growth and wealth creation, of innovation and creativity, but they are also the sites of extremes of wealth and poverty, unequal access to drinking water and sanitation, pollution, and public health problems. As the Brundtland Commission noted, the growth of urban populations has often preceded development of the housing, infrastructure, and employment needed to sustain that population. In the 10 years from 1985 to 1995, a period during which the Brundtland report was published, the world saw the addition of the equivalent of 81 cities with populations of over a million people. 12 There have been dramatic and successful efforts to improve water, air, and sanitation services in developing world urban centers during this period. But the number of city dwellers without adequate water and exposed to poor sanitation and air pollution has grown as urban population growth has outpaced investments. 13 The health consequences of inadequate drinking water and poor sanitation services are felt most strongly by the poor.
Among the major challenges of urban development is air pollution, produced largely by the interactions of hydrocarbons and nitrogen oxides produced in industrial and transportation processes as well as by heating and cooking. 14 While investments in pollution control in industrialized countries have led to air pollutant reductions in many cities, air pollution is still a major problem in the developed world. In the United States, some 80 million people live in areas that do not meet air quality standards, and in many European cities air pollutant concentrations are also higher than the established standards. 15 At the same time, air quality in the cities of the industrializing world has worsened. Worldwide, the World Health Organization estimates that 1.4 billion urban residents breathe air that fails to meet WHO air quality standards. 16
Access to water and sanitation services also present enormous challenges to rapidly growing cities. Despite concerted efforts during the 1980s, designated the "International Drinking Water Supply and Sanitation Decade" by the World Health Organization, in 1990 about 200 million urban dwellers were without a safe water supply, and around 400 million were without adequate sanitation. 17 In the largest cities of the industrializing world, the poorest populations in the slums and at the city margins tend to have the least access to safe water. For example, in Sao Paulo, nearly 20 percent of the city's population lived in slums (called favelas) in 1993 around 85 percent of the favelas had no sewerage service. 18 Innovative technological opportunitiessuch as condominial sewers, 19 improved ventilated pit latrines, various lower cost sewage treatments, and approaches to reuse of municipal wastewaterare available to provide flexible and cost-effective services and are being used with success in some regions, but have yet to be widely applied. Also, in some areas, such
Box 4.1 Mexico City's Water Supply
The population of Mexico City is approximately 20 million and growing, with much migration from rural areas. The continued growth has placed high demand on an unstable water supply network, designed to extract most of the city's water (72 percent) from the Mexico City Aquifer, which underlies the metropolitan area. Increasing land subsidence, groundwater contamination, and inadequate hazardous waste management have made the aquifer and water supply network vulnerable to contamination, posing risks to public health. A 1995 bi-national study of the problem was jointly undertaken by the Mexico Academy of Science, the Mexico Academy of Engineering, and the U.S. National Research Council. The study made recommendations on management of water supply through metering and pricing mechanisms, needed research, treatment of municipal wastewater prior to disposal, demand management approaches, a comprehensive groundwater protection program, a variety of water reclamation schemes, and possible institutional changes related to applying a new cultural perspective to the value of water in Mexico City. 20 It is noteworthy that this comprehensive study recommended several approaches to improved management and conservation of waterand none involving further resource development.
as Mexico City (see Box 4.1), high-priority attention can be given to treatment of municipal wastewater as part of a comprehensive plan for improving the balance of water supply, water demand, and water conservation.
In 1900, there were only 16 cities with populations of 1 million or more by 1994 there were 305 such citiesand of these, 13 had populations of greater than 10 million. 21 Most of this growth has taken place over the last 50 years. As described in Chapter 2, projections of population growth indicate that there will be nearly 7 billion urban dwellers by 2050. The most rapid expansion of high-density cities will be during the next several decades. This trend presents an opportunity to build modern, state-of-the-art facilities and to provide efficient infrastructure systems for the delivery of services. Maintenance and improvement of the quality, adaptability, reliability, cost-effectiveness, and efficiency of these systems are critical to established and aging cities as well. Realizing these opportunities, of course, depends on the foresight, will, capital, and incentives to take advantage of them. Seizing these chances would help to meet the future needs for housing, while reducing the footprint on the land, and, with increases in efficiency, the needs for energy and materials.
Agriculture and Food Security
The task of feeding an additional several billion people in the next 50 years is an unprecedented challenge, one fraught with biophysical,
environmental, and institutional hazards and roadblocks. Food demand will rise in response to population growth, growth of per capita income, and attempts to reduce the undernutrition of the very poor. By 2050 food demand could almost double to accommodate the projected population depending on the growth of income and the nature of diet. 22 But the paths to meeting these demands are far from clear. The challenge of feeding this population and reducing hunger requires dramatic advances both in food production, which we focus on here, and in food distribution and access. Production of the globally traded staples (maize, wheat, rice, soybeans, poultry, and swine) will be driven by new technologies already in or rapidly moving toward the private sector. 23 The emergence of genetic biotechnologies, protected by intellectual property rights and patenting, is attracting enormous private investment. Global markets and the movement of private capital into processing and marketing have increased handling efficiencies. Market balance among rich and poor countries, monopoly control, and environmental impacts due to the scale of operations all remain major issues. Industrial technologies are major engines for continued growth. Prospects for growth in production of the numerous "minor" or regional staples, such as cassava, yams, potatoes, grain legumes, millet, white maize, sorghum, and other crops critical to food security for a large segment of the world's poor, are not nearly as optimistic. Such growth is not now in progress nor is it projected for the foreseeable future. The Brundtland Commission recognized that a great strategic effort would be required to meet the challenge of feeding a growing population, yet the past 10 years have seen a reduction in resources for the international agricultural research community along with indicator values that increasingly show world capabilities for increasing food production are stagnating. 24
During the last half century, the dramatic gains in crop production that have occurred almost worldwide (except, in particular, Sub-Saharan Africa) have come from four interrelated sources: expansion of cultivated land, increased use of fertilizer and pest control chemicals, expansion of irrigated area, and the introduction of high-yielding crop varieties. The continued gains in agricultural production required in the 21 st century will be considerably more difficult to accomplish than in the immediate past. 25 There are currently difficulties in raising yield ceilings for the cereal crops, despite a history of rapid yield gains in the past. Incremental response to increases in fertilizer use has declined in many areas. Expansion of irrigated land has become more costly and has slowed dramatically in the past two decades. Because of rising demand for water with growing urbanization, water supplies are increasingly less available to agriculture. 26 The loss of soil fertility and degradation of agricultural lands due to inappropriate management, climate change, and other factors
has been reversed in some agricultural areas but at the same time has become an important issue in many other areas. 27 For example, the expansion of irrigated area, combined with the failure to design and implement incentive-compatible irrigation management, has contributed to waterlogging and soil salinity. Reductions in agricultural productivity due to air and water quality changes, some of which emanate from agriculture itself, have also raised concerns. 28 Increasing pest problems because of increasing pesticide resistance stemming from misuse of chemical pesticides, the decimation of natural enemies, and the invasion of new pests are also topics of concern. 29 Any one of these problems alone could impede efforts toward increasing production and yield. Together, these biophysical factors threaten achieving a successful transition toward sustainability.
Perhaps more important still are the threats associated with inadequate investment in the agricultural sector nowfor research, education, technological developments, and transfer of knowledge and information to the developing world. 30 Local agricultural research capacity, local public and private capacity to make knowledge, technology, and materials available to producers, and the schooling or informal education of farmers and farm workers are all required for sustained growth in agricultural production. The international agricultural research system and the private sector research community are important sources of new knowledge and new technology, 31 but these systems are effective only in the presence of viable national and regional research systems capable of adapting new technologies to local agroclimatic conditions. Finally, productivity and sustainability depend on the knowledge that farm people bring to the management of their resources and production education is critical. Institutions must make advances in the technology and management approaches available to farmers, and local financial credit and labor markets must function effectively.
Limitations of institutional capacity may be one of the reasons why Sub-Saharan African countries have failed to realize the gains in productivity that have been achieved by green revolution technology in South and Southeast Asia and Latin America. Institutional limitations, along with political instability, complex land tenure systems, and unique agroclimatic environments may all contribute to the apparent lag in productivity gains there. Understanding the dimensions and factors controlling this failure is critically important because Sub-Saharan Africa is the major region where growth in agricultural production is running behind population growth. One of the major challenges of the sustainability transition will be to develop new and appropriate approaches to improve food production in this region.
If the development of international and national agricultural research
systems is maintained, there are many opportunities to enhance our ability to respond to growing world food demand at the same time that we sustain resources and the broader environment. Improved varieties and better management could lead to increases in yield, at least up to fundamental limits set by plant physiology. Scientific and technological breakthroughs, particularly in the area of biotechnology, could over the long term lead to a lifting of the yield ceilings that have been set by the green revolution technologies. 32 Biotechnology is still in its infancy, and its application is controversial. Nevertheless, both the science and the technology are advancing rapidly, and the development and diffusion of biotechnologies may play an important role in increasing and sustaining agricultural production in many areas of the world.
While biotechnology holds substantial hope for improving crop production and efficiency of resource use, many other opportunities exist to increase and sustain food production while decreasing environmental consequences. Protection and careful utilization of soil, water, and biological resources underlie many of these opportunities, and promising management approaches have already been developed and successfully used in some places. For example, integrated nutrient management, like integrated pest management, takes advantage of the ecological processes operating in soils and crop ecosystems and uses them in combination with industrial inputs to optimize productivity and reduce pesticide and nutrient spread. 33 Ecologically based pest management takes advantage of biological diversity to reduce the need for pesticide use. Increased use of efficient irrigation systems will conserve and maintain water supplies and lessen competition with urban and other uses. 34 In breeding programs, increasing attention to flexibility and genetic diversity of crop plants can increase the ability of the agricultural sector to respond to climate and other environmental ''surprises." 35 The development of management systems and breeding programs for regional staple crops could also enlarge the food security basket for the poor in many regions. For these opportunities to be useful, new knowledge is needed about both the biophysical crop system and the sociological barriers to implementation. Taking advantage of these opportunities will help to provide the food needs for future human populations, while preserving water in areas of scarcity and reducing pressure on the land.
Over the next two generations, the global market for goods and services is likely to increase two- to four-fold (Chapter 2 and Chapter 3 appendix). With that increase will come an enormous demand for materials. Avoiding the waste, pollution, and environmental disruption now
associated with the extraction, processing, and consumption of materials, and reducing energy and water inputs into industrial production, are the foremost issues during the transition to sustainability. In the 10 years since the Brundtland Commission's challenge to industry to produce more with less, there have been substantial improvements in reducing and reusing materials by both industry and consumers. But the trend toward increasing material use efficiency and dematerialization, discussed in Chapter 2, must be accomplished universally and at much faster rates if it is to offset the rapid increases in production forecasts for the next decades.
The demand for materials to meet expanding markets may in some cases be limited by resource shortages. However, given a supply of energy at competitive prices, the increased demand most likely will result in substantial materials substitutions. Absolute materials shortages are unlikely, at least in the next several decades. 36 The materials challenge, instead, is likely to be associated with pollution due to the "leakage" of materials from the manufacturing, processing, and consumption systems. 37 Such leakages include not only those of nontoxic but valuable materials wasted in the production and consumption streams, and also those of a variety of toxic and hazardous substances used in industrial production. More than 12 billion tons of industrial waste are generated in the United States each year and municipal solid wastes, which include consumer wastes, are generated at the rate of 0.2 billion tons per year. 38 Clearly, such residual production must be brought under control, or better yet, prevented.
Again, some of these leakages represent not just loss of valuable materials but of substances presenting specific toxicological and ecological threats. More than 100,000 industrial chemicals are in use today, and the number is increasing rapidly in the expanding agriculture, metals, electronics, textiles, and food industries. 39 Some of the effects of these chemicals are well known, but there are insufficient data for health assessment for the majority of these chemicals. Some, like the persistent organic pollutants, are widely distributed beyond their points of origin and concentrate as they move up the food chain. Human exposure to these pollutants can cause immune dysfunction, reproductive and behavioral abnormalities, and cancer. Also, heavy metals such as lead, copper, and zinc can reside in the environment for hundreds of years human exposure to them can lead to kidney damage, developmental retardation, cancer, and autoimmune responses. Nevertheless, global production, consumption, and circulation of many toxic metals and organics have increased dramatically in the last half century because of their utility in many industrial activities, though production began to level off in the early 1970s and emissions began to decline (Figure 4.1). But numerous opportunities exist to reduce material usage as well as
environmentally harmful leakages. Refurbishing or remanufacturing used products or their parts, changing the nature of the product used to a new condition for accomplishing the same purpose (usually provision of a service instead of the product), 40 and recycling and reuse of used subsystems, parts, and materials in products all generally require much less energy, capital, and labor than the original creation of the materials and products. In addition, such processes minimize environmental damage. There is a clear and obvious case for us to examine what we know about the role of industry in the flow of materials, energy, and products, the effects of market forces (e.g., on recycling), and the possibilities for modifying these flows through the system, for more efficient energy use, decreasing environmental damage, and improving the efficiency of providing goods and services.
In recent years, many industries have moved to increase the efficiency of using materials in processing and to control the loss of scrap and other wastes from the production cycle. For example, one corporate plan for introducing customer return programs (copier machines as well as disposable parts like toner cartridges for copiers) led to remanufactured equipment from 30,000 tons of copying machines, thereby reducing both the load on landfills and the consumption of raw materials and energy. 41 Control of leakage is also a means of cost control for industrial production, and there are precedents for the creation of profitable industrial operations based on recapture of consumer materials. Approaches that control the production of garbage and reduce leakage of materials at the consumer end have also been used in some parts of the world. Product recycling has dramatically increased and design of products to facilitate recycling has become a tenet of "industrial ecology." 42 Despite these successes, there is a worldwide loss of valuable materials because of leakage. Thus, one significant set of challenges rests in the development of incentives for higher efficiency and lower leakage from producer and consumer systems. Among such actions would be (1) the provision of incentives to identify heretofore unrecognized economic value of materials (2) the elimination of historical market distortions (e.g., subsidies) that may interfere with choices that would be more sustainable in the absence of the distortions and (3) the provision of incentives to move to competitively priced energy whose production does not result in the release of carbon dioxide (i.e., through the use of noncarbon sources or carbon sequestration).
Beyond the challenges related to the reduction and elimination of industrial wastes, the rapidly changing industrial trajectory carries with it the general problem of anticipating problems in new industries and of projecting the dynamics of employment into a future with many more people. The past decade has seen a shift to increasing employment and
Global production and consumption of selected toxic metals, 1850-1990. The
figure indicates that within the last 20 years, emissions of lead, copper and zinc
have begun to decline.
Source: Nriagu (1979). Updated in Nriagu (1996). Courtesy of the Macmillan
Magazines, Ltd. and the American Association for the Advancement of Science.
productivity within industry. Nonetheless, the current trends toward production of more by fewer people could lead to persistent unemployment of an expanded population, a spectre not foreseen by the Brundtland Commission. 43
As the preceding paragraphs make clear, industry is faced with many enormous challenges and much responsibility for reducing and preventing environmental problems related to industrial wastes and leakages. At the same time, however, it also faces a tremendous opportunity for massive market expansion, the development of new technologies (and, therefore new product possibilities, even beyond the products for which the technologies were developed), and the creation of totally new markets based on the requirements of new customers in industrializing countries. There is also great potential for the industrializing world to skip over transitional technologies to new, cleaner technologies without experiencing the same environmental degradation as the industrialized world due to the use of more traditional technologies. The capital, barriers, and
incentives to diffusion must be understood and addressed to meet this potential. Meeting the coupled objectives of designing and producing for product competitiveness and for environmental protection and resource conservation is the critical challenge to industry in the next century, and the resulting effects will be felt in all other sectors. Involving industry directly in these challenges and in finding the means to meet them is an opportunity to bring creative actors into the process voluntarily, as well as under incentive and regulatory forces.
Energy is a critical ingredient in most activities of industrialized and industrializing economies. It is required to extract, process, fabricate and recycle materials, to heat and cool homes and places of business, to produce foods, to move people and goods, and to power communications. For a successful transition to sustainability, energy sources must grow at sufficient rates to maintain other energy-dependent activities, yet at the same time must impose few if any environmental costs in the form of local air pollution, carbon dioxide, toxic residuals, and despoiled land. The world will need to find a way that allows 9 billion people or more to enjoy a lifestyle that requires energy while at the same time protects and sustains human health and the health of the biosphere from local to global scales.
Numerous environmental hazards, including climate change, acidification of water and soil, and air pollution, stem from our dependence on fossil fuel energy. Alone or together, these significant and accumulating hazards can influence a transition toward sustainability. These environmental risks, rather than any limitations of fossil fuel energy resources, are the most significant factors facing the energy sector today. In most industrialized nations, emissions controls are beginning to bring local and regional pollution under control. In contrast, in much of the developing world, local and regional pollution poses serious and growing problems. Regarding global atmospheric changes, in the 10 years since the Brundtland report, much of the world has come to acknowledge the threat from greenhouse gas emissions via international conventions and agreements, but with few exceptions serious constraints on emissions have not been implemented (see Chapters 1 and 2).
For years there have been concerns about limited reserves of fossil fuel. Modern estimates, however, suggest that despite extensive past extraction, the world has very large reserves. In the absence of "externality" taxes (taxes imposed on these fuels to cover their environmental costs) or other policy changes, fossil fuels are likely to remain abundant and cheap for decades to come. A number of direct and indirect subsidies
36.3 Environmental Limits to Population Growth
In this section, you will explore the following questions:
- What are the characteristics of and differences between exponential and logistic growth patterns?
- What are examples of exponential and logistic growth in natural populations?
Connection for AP ® Courses
Population ecologists use mathematical methods to model population dynamics. These models can be used to describe changes occurring in a population and to better predict future changes. Applying mathematics to these models (and being able to manipulate the equations) is in scope for AP ® . (Remember that for the AP ® Exam you will have access to a formula sheet with these equations.)
Information presented and the examples highlighted in the section support concepts outlined in Big Idea 4 of the AP ® Biology Curriculum Framework. The AP ® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A learning objective merges required content with one or more of the seven science practices.
|Big Idea 4||Biological systems interact, and these systems and their interactions possess complex properties.|
|Enduring Understanding 4.A||Interactions within biological systems lead to complex properties.|
|Essential Knowledge||4.A.5 Communities are composed of populations of organisms that interact in complex ways.|
|Science Practice||2.2 The student can apply mathematical routines to quantities that describe natural phenomena.|
|Learning Objective||4.12 The student is able to apply mathematical routines to quantities that describe communities composed of populations of organisms that interact in complex ways.|
|Essential Knowledge||4.A.5 Communities are composed of populations of organisms that interact in complex ways.|
|Science Practice||6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.|
|Learning Objective||4.13 The student is able to predict the effects of a change in the community’s populations on the community.|
Although life histories describe the way many characteristics of a population (such as their age structure) change over time in a general way, population ecologists make use of a variety of methods to model population dynamics mathematically. These more precise models can then be used to accurately describe changes occurring in a population and better predict future changes. Certain models that have been accepted for decades are now being modified or even abandoned due to their lack of predictive ability, and scholars strive to create effective new models.
Charles Darwin, in his theory of natural selection, was greatly influenced by the English clergyman Thomas Malthus. Malthus published a book in 1798 stating that populations with unlimited natural resources grow very rapidly, which represents an exponential growth , and then population growth decreases as resources become depleted, indicating a logistic growth.
The best example of exponential growth is seen in bacteria. Bacteria are prokaryotes that reproduce by prokaryotic fission. This division takes about an hour for many bacterial species. If 1000 bacteria are placed in a large flask with an unlimited supply of nutrients (so the nutrients will not become depleted), after an hour, there is one round of division and each organism divides, resulting in 2000 organisms—an increase of 1000. In another hour, each of the 2000 organisms will double, producing 4000, an increase of 2000 organisms. After the third hour, there should be 8000 bacteria in the flask, an increase of 4000 organisms. The important concept of exponential growth is that the population growth rate—the number of organisms added in each reproductive generation—is accelerating that is, it is increasing at a greater and greater rate. After 1 day and 24 of these cycles, the population would have increased from 1000 to more than 16 billion. When the population size, N, is plotted over time, a J-shaped growth curve is produced (Figure 36.9).
The bacteria example is not representative of the real world where resources are limited. Furthermore, some bacteria will die during the experiment and thus not reproduce, lowering the growth rate. Therefore, when calculating the growth rate of a population, the death rate (D) (number organisms that die during a particular time interval) is subtracted from the birth rate (B) (number organisms that are born during that interval). This is shown in the following formula:
The birth rate is usually expressed on a per capita (for each individual) basis. Thus, B (birth rate) = bN (the per capita birth rate “b” multiplied by the number of individuals “N”) and D (death rate) =dN (the per capita death rate “d” multiplied by the number of individuals “N”). Additionally, ecologists are interested in the population at a particular point in time, an infinitely small time interval. For this reason, the terminology of differential calculus is used to obtain the “instantaneous” growth rate, replacing the change in number and time with an instant-specific measurement of number and time.
Notice that the “d” associated with the first term refers to the derivative (as the term is used in calculus) and is different from the death rate, also called “d.” The difference between birth and death rates is further simplified by substituting the term “r” (intrinsic rate of increase) for the relationship between birth and death rates:
The value “r” can be positive, meaning the population is increasing in size or negative, meaning the population is decreasing in size or zero, where the population’s size is unchanging, a condition known as zero population growth. A further refinement of the formula recognizes that different species have inherent differences in their intrinsic rate of increase (often thought of as the potential for reproduction), even under ideal conditions. Obviously, a bacterium can reproduce more rapidly and have a higher intrinsic rate of growth than a human. The maximal growth rate for a species is its biotic potential, or rmax, thus changing the equation to:
Exponential growth is possible only when infinite natural resources are available this is not the case in the real world. Charles Darwin recognized this fact in his description of the “struggle for existence,” which states that individuals will compete (with members of their own or other species) for limited resources. The successful ones will survive to pass on their own characteristics and traits (which we know now are transferred by genes) to the next generation at a greater rate (natural selection). To model the reality of limited resources, population ecologists developed the logistic growth model.
Carrying Capacity and the Logistic Model
In the real world, with its limited resources, exponential growth cannot continue indefinitely. Exponential growth may occur in environments where there are few individuals and plentiful resources, but when the number of individuals gets large enough, resources will be depleted, slowing the growth rate. Eventually, the growth rate will plateau or level off (Figure 36.9). This population size, which represents the maximum population size that a particular environment can support, is called the carrying capacity, or K.
The formula we use to calculate logistic growth adds the carrying capacity as a moderating force in the growth rate. The expression “K – N” is indicative of how many individuals may be added to a population at a given stage, and “K – N” divided by “K” is the fraction of the carrying capacity available for further growth. Thus, the exponential growth model is restricted by this factor to generate the logistic growth equation:
Notice that when N is very small, (K-N)/K becomes close to K/K or 1, and the right side of the equation reduces to rmaxN, which means the population is growing exponentially and is not influenced by carrying capacity. On the other hand, when N is large, (K-N)/K come close to zero, which means that population growth will be slowed greatly or even stopped. Thus, population growth is greatly slowed in large populations by the carrying capacity K. This model also allows for the population of a negative population growth, or a population decline. This occurs when the number of individuals in the population exceeds the carrying capacity (because the value of (K-N)/K is negative).
A graph of this equation yields an S-shaped curve (Figure 36.9), and it is a more realistic model of population growth than exponential growth. There are three different sections to an S-shaped curve. Initially, growth is exponential because there are few individuals and ample resources available. Then, as resources begin to become limited, the growth rate decreases. Finally, growth levels off at the carrying capacity of the environment, with little change in population size over time.
Role of Intraspecific Competition
The logistic model assumes that every individual within a population will have equal access to resources and, thus, an equal chance for survival. For plants, the amount of water, sunlight, nutrients, and the space to grow are the important resources, whereas in animals, important resources include food, water, shelter, nesting space, and mates.
In the real world, phenotypic variation among individuals within a population means that some individuals will be better adapted to their environment than others. The resulting competition between population members of the same species for resources is termed intraspecific competition(intra- = “within” -specific = “species”). Intraspecific competition for resources may not affect populations that are well below their carrying capacity—resources are plentiful and all individuals can obtain what they need. However, as population size increases, this competition intensifies. In addition, the accumulation of waste products can reduce an environment’s carrying capacity.
Examples of Logistic Growth
Yeast, a microscopic fungus used to make bread, exhibits the classical S-shaped curve when grown in a test tube (Figure 36.10a). Its growth levels off as the population depletes the nutrients that are necessary for its growth. In the real world, however, there are variations to this idealized curve. Examples in wild populations include sheep and harbor seals (Figure 36.10b). In both examples, the population size exceeds the carrying capacity for short periods of time and then falls below the carrying capacity afterwards. This fluctuation in population size continues to occur as the population oscillates around its carrying capacity. Still, even with this oscillation, the logistic model is confirmed.
HIPPO- A Threat to Biodiversity
While being an environmentalist, one has to take equal care of the fauna as one cares for the flora. While the plant species are very important for the existence of human beings, the role of animals in the eco-system can not be ignored. In recent years, killing of these animals is widely observed and has [&hellip]
While being an environmentalist, one has to take equal care of the fauna as one cares for the flora. While the plant species are very important for the existence of human beings, the role of animals in the eco-system can not be ignored. In recent years, killing of these animals is widely observed and has surfaced as major issues in many countries. Conservation biologists and scientists who try to stop endangered species from dying out, use the word ‘HIPPO’ to remember the different things that threaten animals and plants, because each letter stands for a different threat. Let’s have a look on it and try to observe how all these become a threat to these species.
H- Habitat Destruction
Forests all over the world are being cut down and burned in the guise of development. Forests in the tropical areas are cleared to make room for farms. But tropical forest soil is very poor, and so farmers have to keep moving on and destroying more and more forests to grow their crops. Companies also cut down forests for their timber, or to make room for other plantations, in which the wild animals end up loosing their homes. Each year, 1% of the world’s tropical forests are destroyed. Does this not threaten you? Well, some hundred years and tropical forests will be vanished. And why wait for hundred more years, every cutting tree brings bad news for native plant and animal species. Wetlands- lakes, ponds, marshes, swamps and rivers are also very threatened habitats, credited to the pollution causing agents.
A brilliant cartoon describing the anxieties of the birds as their habitat gets destroyed.
Invasive or introduced pest species have caused many native animals and plants to become extinct across the world. Introduced species often have a very harmful effect on native species. For example, 24 rabbits were introduced to Australia in 1859 for hunting. Rabbits breed quickly, and, in an environment without any of their natural predators, their numbers increased so quickly that in less than a hundred years there were 600 million across the whole continent! The rabbits took over the resources and habitats of native species, like the bandicoot, which is now endangered.
Pollution contaminates the natural environment with harmful substances produced by human activities. An example of pollution is an oil spill. This happens when oil is released accidentally into the sea from a tanker, pipeline or refinery. The spill forms a thin layer of oil, called a slick, poisoning sea life, and damaging the fur and feathers of seabirds and mammals. Due to the contaminated atmosphere, most animals ran away in search of an appropriate place to live. With the construction and expansion of every new city, the pollution increases, so does the danger to these species.
The growth of the human population is the biggest threat to natural environments today. There are over 7 billion people in the world. Quite simply, there isn’t enough room for natural environments to coexist with all these people, and the land they need to provide them with food and shelter. As a result, in the race of survival of the fittest, animals and plants get crushed under the skyscrappers.
O-Overhunting and Overharvesting
There is a huge demand of animal products like whale oil and whale meat, elephant ivory, and rhino and tiger trophies. Although all of these animals are now protected under various laws from hunting, illegal poaching still continues.
Other species are overharvested – they are used faster than they can be replaced – which is likely to lead to decline and extinction. Cod is now too rare to be caught in many areas off the coast of America and in the North Sea- and the situation is the same for many other types of fish. Plant species can also be easily overharvested- the Brazil nut tree might be in danger of extinction, because not enough of its nuts are being left in the rainforest to grow into new tree.
The huge demands of tusks and other animal products encourages poachers and hunters.
If we all do a little bit to help preserve natural resources, we will help prevent many more species of animals and plants becoming endangered. Museums are probably the only place we can find the remains of extinct animals but that day isn`t too far that our children will visit a museum to see a tiger instead of zoo or national parks. While today, the human race might boast about its existence in the race with the animal and plant species, it is sure that if the latter are extint, not much time will be left with the former. The two are strongly interdependent and should not be separated. Human existence will be nothing without the valuable plants and animals. They belong to the earth as much as we are, their existence is what gives us an identity. Cherish them. They are yours.
Confronting Climate Change
Climate change is exacerbating many of the environment issues we currently face. It poses a significant long-term threat that demands our collective action to prevent its root causes and cope with its impacts.
Pollution from harmful greenhouse gas emissions, most notably carbon dioxide, is the leading cause of climate change. The National Wildlife Federation&rsquos vital efforts include the reduction of greenhouse gas pollution through wildlife-friendly clean energy policies and projects, as well as reducing deforestation both nationally and internationally. Beyond our borders, we promote sustainable production methods through the development of market-based solutions and strategies for important agricultural commodities such as palm oil, soy, and biomaterials.
In addition to deforestation, burning fossil fuels contributes significantly to greenhouse gas emissions. The National Wildlife Federation addresses this issue by reducing the reliance on fossil fuels and advocating for renewable energy sources such as solar and wind power. In addition to preventative measures, the National Wildlife Federation is a leader in "climate-smart conservation," looking ahead and integrating new challenges created by climate change into our conservation efforts.
3: Environmental Threats - Biology
Figure 1. Each of the world’s eight major biomes is distinguished by characteristic temperatures and amount of precipitation. Polar ice caps and mountains are also shown.
There are eight major terrestrial biomes: tropical rainforests, savannas, subtropical deserts, chaparral, temperate grasslands, temperate forests, boreal forests, and Arctic tundra. Biomes are large-scale environments that are distinguished by characteristic temperature ranges and amounts of precipitation. These variables affect the types of vegetation and animal life that can exist in those areas. Because a biome is defined by climate, the same biome can occur in geographically distinct areas with similar climates (Figure 1 above).
Tropical rainforests are found in equatorial regions (Figure 1) are the most biodiverse terrestrial biome. This biodiversity is under extraordinary threat primarily through logging and deforestation for agriculture. Tropical rainforests have also been described as nature’s pharmacy because of the potential for new drugs that is largely hidden in the chemicals produced by the huge diversity of plants, animals, and other organisms. The vegetation is characterized by plants with spreading roots and broad leaves that fall off throughout the year, unlike the trees of deciduous forests that lose their leaves in one season.
The temperature and sunlight profiles of tropical rainforests are stable in comparison to other terrestrial biomes, with average temperatures ranging from 20 o C to 34 o C (68 o F to 93 o F). Month-to-month temperatures are relatively constant in tropical rainforests, in contrast to forests farther from the equator. This lack of temperature seasonality leads to year-round plant growth rather than just seasonal growth. In contrast to other ecosystems, a consistent daily amount of sunlight (11–12 hours per day year-round) provides more solar radiation and therefore more opportunity for primary productivity.
The annual rainfall in tropical rainforests ranges from 125 to 660 cm (50–200 in) with considerable seasonal variation. Tropical rainforests have wet months in which there can be more than 30 cm (11–12 in) of precipitation, as well as dry months in which there are fewer than 10 cm (3.5 in) of rainfall. However, the driest month of a tropical rainforest can still exceed the annual rainfall of some other biomes, such as deserts.Tropical rainforests have high net primary productivity because the annual temperatures and precipitation values support rapid plant growth. However, the high amounts of rainfall leaches nutrients from the soils of these forests.
Tropical rainforests are characterized by vertical layering of vegetation and the formation of distinct habitats for animals within each layer. On the forest floor is a sparse layer of plants and decaying plant matter. Above that is an understory of short, shrubby foliage. A layer of trees rises above this understory and is topped by a closed upper canopy—the uppermost overhead layer of branches and leaves. Some additional trees emerge through this closed upper canopy. These layers provide diverse and complex habitats for the variety of plants, animals, and other organisms. Many species of animals use the variety of plants and the complex structure of the tropical wet forests for food and shelter. Some organisms live several meters above ground, rarely descending to the forest floor.
Figure 2. Species diversity is very high in tropical wet forests, such as these forests of Madre de Dios, Peru, near the Amazon River. (credit: Roosevelt Garcia)
Figure 3. A MinuteEarth video about how trees create rainfall, and vice versa.
Savannas are grasslands with scattered trees and are found in Africa, South America, and northern Australia (Figure 4 below). Savannas are hot, tropical areas with temperatures averaging from 24 o C –29 o C (75 o F –84 o F) and an annual rainfall of 51–127 cm (20–50 in). Savannas have an extensive dry season and consequent fires. As a result, there are relatively few trees scattered in the grasses and forbs (herbaceous flowering plants) that dominate the savanna. Because fire is an important source of disturbance in this biome, plants have evolved well-developed root systems that allow them to quickly re-sprout after a fire.
Figure 4. Although savannas are dominated by grasses, small woodlands, such as this one in Mount Archer National Park in Queensland, Australia, may dot the landscape. (credit: “Ethel Aardvark”/Wikimedia Commons)
Subtropical deserts exist between 15 o and 30 o north and south latitude and are centered on the Tropic of Cancer and the Tropic of Capricorn (Figure 6 below). Deserts are frequently located on the downwind or lee side of mountain ranges, which create a rain shadow after prevailing winds drop their water content on the mountains. This is typical of the North American deserts, such as the Mohave and Sonoran deserts. Deserts in other regions, such as the Sahara Desert in northern Africa or the Namib Desert in southwestern Africa are dry because of the high-pressure, dry air descending at those latitudes. Subtropical deserts are very dry evaporation typically exceeds precipitation. Subtropical hot deserts can have daytime soil surface temperatures above 60 o C (140 o F) and nighttime temperatures approaching 0 o C (32 o F). Subtropical deserts are characterized by low annual precipitation of fewer than 30 cm (12 in) with little monthly variation and lack of predictability in rainfall. Some years may receive tiny amounts of rainfall, while others receive more. In some cases, the annual rainfall can be as low as 2 cm (0.8 in) in subtropical deserts located in central Australia (“the Outback”) and northern Africa.
Figure 5. A MinuteEarth video about the global climate patterns which lead to subtropical deserts.
The low species diversity of this biome is closely related to its low and unpredictable precipitation. Despite the relatively low diversity, desert species exhibit fascinating adaptations to the harshness of their environment. Very dry deserts lack perennial vegetation that lives from one year to the next instead, many plants are annuals that grow quickly and reproduce when rainfall does occur, then they die. Perennial plants in deserts are characterized by adaptations that conserve water: deep roots, reduced foliage, and water-storing stems (Figure 6 below). Seed plants in the desert produce seeds that can lie dormant for extended periods between rains. Most animal life in subtropical deserts has adapted to a nocturnal life, spending the hot daytime hours beneath the ground. The Namib Desert is the oldest on the planet, and has probably been dry for more than 55 million years. It supports a number of endemic species (species found only there) because of this great age. For example, the unusual gymnosperm Welwitschia mirabilis is the only extant species of an entire order of plants. There are also five species of reptiles considered endemic to the Namib.
In addition to subtropical deserts there are cold deserts that experience freezing temperatures during the winter and any precipitation is in the form of snowfall. The largest of these deserts are the Gobi Desert in northern China and southern Mongolia, the Taklimakan Desert in western China, the Turkestan Desert, and the Great Basin Desert of the United States.
Figure 6. Many desert plants have tiny leaves or no leaves at all to reduce water loss. The leaves of ocotillo, shown here in the Chihuahuan Desert in Big Bend National Park, Texas, appear only after rainfall and then are shed. (credit “bare ocotillo”: “Leaflet”/Wikimedia Commons)
The chaparral is also called scrub forest and is found in California, along the Mediterranean Sea, and along the southern coast of Australia (Figure 7 below). The annual rainfall in this biome ranges from 65 cm to 75 cm (25.6–29.5 in) and the majority of the rain falls in the winter. Summers are very dry and many chaparral plants are dormant during the summertime. The chaparral vegetation is dominated by shrubs and is adapted to periodic fires, with some plants producing seeds that germinate only after a hot fire. The ashes left behind after a fire are rich in nutrients like nitrogen and fertilize the soil, promoting plant regrowth. Fire is a natural part of the maintenance of this biome.
Figure 7. The chaparral is dominated by shrubs. (credit: Miguel Vieira)
Temperate grasslands are found throughout central North America, where they are also known as prairies, and in Eurasia, where they are known as steppes (Figure 8 below). Temperate grasslands have pronounced annual fluctuations in temperature with hot summers and cold winters. The annual temperature variation produces specific growing seasons for plants. Plant growth is possible when temperatures are warm enough to sustain plant growth, which occurs in the spring, summer, and fall.
Annual precipitation ranges from 25.4 cm to 88.9 cm (10–35 in). Temperate grasslands have few trees except for those found growing along rivers or streams. The dominant vegetation tends to consist of grasses. The treeless condition is maintained by low precipitation, frequent fires, and grazing. The vegetation is very dense and the soils are fertile because the subsurface of the soil is packed with the roots and rhizomes (underground stems) of these grasses. The roots and rhizomes act to anchor plants into the ground and replenish the organic material (humus) in the soil when they die and decay.
Figure 8. The American bison (Bison bison), more commonly called the buffalo, is a grazing mammal that once populated American prairies in huge numbers. (credit: Jack Dykinga, USDA ARS)
Fires, which are a natural disturbance in temperate grasslands, can be ignited by lightning strikes. It also appears that the lightning-caused fire regime in North American grasslands was enhanced by intentional burning by humans. When fire is suppressed in temperate grasslands, the vegetation eventually converts to scrub and dense forests. Often, the restoration or management of temperate grasslands requires the use of controlled burns to suppress the growth of trees and maintain the grasses.
Temperate forests are the most common biome in eastern North America, Western Europe, Eastern Asia, Chile, and New Zealand (Figure 9 below). This biome is found throughout mid-latitude regions. Temperatures range between –30 o C and 30 o C (–22 o F to 86 o F) and drop to below freezing on an annual basis. These temperatures mean that temperate forests have defined growing seasons during the spring, summer, and early fall. Precipitation is relatively constant throughout the year and ranges between 75 cm and 150 cm (29.5–59 in).
Deciduous trees are the dominant plant in this biome with fewer evergreen conifers. Deciduous trees lose their leaves each fall and remain leafless in the winter. Thus, little photosynthesis occurs during the dormant winter period. Each spring, new leaves appear as temperature increases. Because of the dormant period, the net primary productivity of temperate forests is less than that of tropical rainforests. In addition, temperate forests show far less diversity of tree species than tropical rainforest biomes.
The trees of the temperate forests leaf out and shade much of the ground. However, more sunlight reaches the ground in this biome than in tropical rainforests because trees in temperate forests do not grow as tall as the trees in tropical rainforests. The soils of the temperate forests are rich in inorganic and organic nutrients compared to tropical rainforests. This is because of the thick layer of leaf litter on forest floors and reduced leaching of nutrients by rainfall. As this leaf litter decays, nutrients are returned to the soil. The leaf litter also protects soil from erosion, insulates the ground, and provides habitats for invertebrates and their predators.
Figure 9. Deciduous trees are the dominant plant in the temperate forest. (credit: Oliver Herold)
The boreal forest, also known as taiga or coniferous forest, is found roughly between 50 o and 60 o north latitude across most of Canada, Alaska, Russia, and northern Europe (Figure 10 below). Boreal forests are also found above a certain elevation (and below high elevations where trees cannot grow) in mountain ranges throughout the Northern Hemisphere. This biome has cold, dry winters and short, cool, wet summers. The annual precipitation is from 40 cm to 100 cm (15.7–39 in) and usually takes the form of snow relatively little evaporation occurs because of the cool temperatures.
The long and cold winters in the boreal forest have led to the predominance of cold-tolerant cone-bearing plants. These are evergreen coniferous trees like pines, spruce, and fir, which retain their needle-shaped leaves year-round. Evergreen trees can photosynthesize earlier in the spring than deciduous trees because less energy from the Sun is required to warm a needle-like leaf than a broad leaf. Evergreen trees grow faster than deciduous trees in the boreal forest. In addition, soils in boreal forest regions tend to be acidic with little available nitrogen. Leaves are a nitrogen-rich structure and deciduous trees must produce a new set of these nitrogen-rich structures each year. Therefore, coniferous trees that retain nitrogen-rich needles in a nitrogen limiting environment may have had a competitive advantage over the broad-leafed deciduous trees.
The net primary productivity of boreal forests is lower than that of temperate forests and tropical wet forests. The aboveground biomass of boreal forests is high because these slow-growing tree species are long-lived and accumulate standing biomass over time. Species diversity is less than that seen in temperate forests and tropical rainforests. Boreal forests lack the layered forest structure seen in tropical rainforests or, to a lesser degree, temperate forests. The structure of a boreal forest is often only a tree layer and a ground layer. When conifer needles are dropped, they decompose more slowly than broad leaves therefore, fewer nutrients are returned to the soil to fuel plant growth.
Figure 10. The boreal forest (taiga) has low lying plants and conifer trees. (credit: L.B. Brubaker, NOAA)
The Arctic tundra lies north of the subarctic boreal forests and is located throughout the Arctic regions of the Northern Hemisphere. Tundra also exists at elevations above the tree line on mountains. The average winter temperature is –34°C (–29.2°F) and the average summer temperature is 3°C–12°C (37°F –52°F). Plants in the Arctic tundra have a short growing season of approximately 50–60 days. However, during this time, there are almost 24 hours of daylight and plant growth is rapid. The annual precipitation of the Arctic tundra is low (15–25 cm or 6–10 in) with little annual variation in precipitation. And, as in the boreal forests, there is little evaporation because of the cold temperatures.
Plants in the Arctic tundra are generally low to the ground and include low shrubs, grasses, lichens, and small flowering plants (Figure 11 below). There is little species diversity, low net primary productivity, and low above-ground biomass. The soils of the Arctic tundra may remain in a perennially frozen state referred to as permafrost. The permafrost makes it impossible for roots to penetrate far into the soil and slows the decay of organic matter, which inhibits the release of nutrients from organic matter. The melting of the permafrost in the brief summer provides water for a burst of productivity while temperatures and long days permit it. During the growing season, the ground of the Arctic tundra can be completely covered with plants or lichens.
Figure 11. Low-growing plants such lichen and grasses are common in tundra. Credit: Nunavut tundra by Flickr: My Nunavut is licensed under CC BY 2.0