Key Concepts

After completing this chapter, you will be able to:

1. Explain the differences between renewable and non-renewable natural resources.
2. Outline the ways that appropriate management practices can increase the harvest of biological resources.
3. Describe at least two case studies of the degradation of potentially renewable resources and explain why those damages occurred.
4. Distinguish between economic growth and economic development and outline the nature of a sustainable economy.

Introduction

For about five decades now, we have been able to examine photographs of Earth as viewed from space. Images from that perspective show that Earth is a spherical mass, with a blue oceanic surface, brownish-green landmasses, and a clear atmosphere except where visibility is obscured by whitish clouds. Such images also reveal that beyond Earth and its atmosphere is the immense, black void of space – an extremely dilute, universal matrix. If we divert our attention from this compelling image of spaceship Earth and focus instead on the unimaginably larger abyss of space, we cannot fail to be stirred by the utter isolation of our lonely planet, the only place in the cosmos that is known to sustain life and ecosystems.

With such a lucid image of Earth in mind, it is not difficult to understand that the resources necessary to sustain life are limited to those already contained on the planet. That is, with one critical exception – the electromagnetic radiation that is continuously emitted by the Sun. A tiny fraction of that solar energy irradiates Earth, warms the planet, and drives photosynthesis. With the exception of sunlight, however, Earth’s resources are entirely self-contained and finite.

It is an undeniable reality that all organisms must have continuous access to resources obtained from their environment. Plants and algae, for example, require sunlight and inorganic nutrients, while animals and heterotrophic microbes must feed on the living or dead biomass of other organisms. Because their organisms must be nourished by environmental capital, the concept can also be extended to ecosystems in their totality. The necessary resources must be available in at least the minimal amounts needed to sustain life, and in larger quantities in ecosystems that are increasing in biomass and complexity, as occurs during succession.

The same reality holds for individual humans, our societies, and our economic systems. All people and their enterprises are subsidized by the harvesting of resources from the environment (including those taken from ecosystems). These necessities must be available in the minimal amounts needed to sustain human life, and in much larger quantities in economic systems that are growing over time. An obvious conclusion is that economic and ecological systems are inextricably linked. Indeed, this is an undeniable fact.

The main connections between economic systems and the natural world involve flows of resources from the environment (including ecosystems) into the human economy, and offsetting flows of disused materials, by-products, and heat (these are sometimes referred to as wastes) from the economy back to the environment. Associated with these interchanges of materials and energy are many kinds of damage caused to natural and managed ecosystems. The damages may be caused by disturbances associated with harvesting natural resources, by emissions of pollutants, and by other stressors related to anthropogenic activities, especially those occurring in heavily industrialized economies.

An ultimate goal of environmental studies is to understand how the use of natural resources and changes in environmental conditions are related to a sustainable economic system and to the quality of human life. Ultimately, a sustainable economy is one that runs forever, and that operates without a net consumption of natural capital – the rates of resource use are equal to or smaller than the rates at which the resources are regenerated or recycled. This definition focuses on the resource-related aspects of sustainability. Also important, however, are environmental damages that may be caused by the extraction and management of natural resources. The social context must also be considered, particularly the ways that wealth is shared among the people who are participating in an economy.

In this chapter, we examine the broader issues related to the use of natural resources in economic systems. Initially, we examine the characteristics of non-renewable and renewable resources. Non-renewable resources are finite, do not regenerate, and therefore are diminished by use. In contrast, renewable resources can regenerate and may be managed to maintain or increase their productivity, and we describe practices that foster those goals. This is followed by an investigation of the reasons for a catastrophic but remarkably common phenomenon – the depletion of potentially renewable resources through excessive use. Finally, we consider the notion of sustainability, a topic that is critically important to the long-term health of both economic and ecological systems. This chapter deals with natural resources in a conceptual manner.

Natural Resources

All natural resources (also known as natural capital) can be divided into two categories: non-renewable and renewable.

Non-Renewables

Non-renewable resources are present in a finite quantity and do not regenerate after they are harvested and used. Consequently, as non-renewable resources are used, their remaining stocks in the environment are depleted. This means that non-renewable resources can never be used in a sustainable fashion – they can only be “mined.” Examples of non-renewable resources include metal ores, petroleum, coal, and natural gas (Image 24.1).

Although continuing exploration may discover additional stocks of non-renewable resources that can be exploited, this does not change the fact that there is a finite quantity of these resources present on Earth. For example, the discovery of a large amount of metal ore in a remote place may substantially increase the known, exploitable reserves of those non-renewable materials. That discovery does not, however, affect the amounts of the metal present on Earth.

Metals are often used to manufacture parts of buildings and machinery. To some degree, the metals can be recovered after these uses and recycled back into the economy, effectively extending the lifespan of their reserves. However, due to the growth and increasing industrialization of the economy, the demand for metals is accelerating. Because recycling cannot keep up with the increasing demands for metals, large additional quantities must be mined from their known reserves in the environment. For valuable metals, such as gold and platinum, there is a high efficiency of recycling, but it is much less so for iron and other less-costly metals.

Fossil fuels are the other major category of non-renewable resources. They are mostly combusted to provide energy for transportation and heating, which converts their organic compounds into carbon dioxide and water, which are released into the environment. Some of that CO₂ and H₂O may be absorbed by plants and other photosynthetic organisms and be converted back into organic materials, a process that might be interpreted as being a kind of recycling. However, given that the rate at which this happens is insignificantly small compared with the release of the CO₂ and H₂O by the combustion of fossil fuels, and that fossil fuels take millions of years to form, these materials should be viewed as being as non-renewable as metals are.

A more minor use of fossil fuels is to manufacture various kinds of plastics. These synthetic materials can be recycled after initial uses, which does help to extend the lifespan of the reserves of fossil fuels (though the recycling process itself often relies on fossil fuel energy). Nevertheless, because the dominant use of fossil fuels is as sources of energy, they essentially flow through an industrial economy, with little new recycling.

Renewables

Renewable resources are capable of regenerating after harvesting, so potentially their stocks can be utilized forever. Most renewable resources are biological, although some are non-biological.

Biological renewable resources are renewable resources that are biological in nature (bio-resources) include the following:

• Wild animals that are hunted as food or for bio-materials, such as deer, moose, hare, ducks, fish, lobster, and seals
• Forest biomass that is harvested for lumber, fiber, or energy
• Wild plants that are gathered as sources of food
• Plants cultivated as sources of food, medicine, materials, or energy (Image 24.2)
• The organic-based capability of soil to sustain the productivity of agricultural crops

Image 24.2. Ethanol Production. Renewable resources, such as many plants and fish, are capable of regenerating after they are harvested. Provided they are not over-harvested or managed inappropriately, renewable resources can be harvested in a sustainable fashion. However, a critique of renewable biofuel is that burning for energy still releases carbon emissions and does not offset existing emissions contributing to climate change. This photo shows a combine harvesting corn for ethanol production, the primary feedstock used in the U.S. in the biofuel industry. While theoretically renewable, it comes under scrutiny for the unsustainable agricultural practices often used in corn monoculture production. Source: “Combine-harvesting-corn” via Wikimedia Commons originally from USDA.

Non-biological renewable resources refer to the following:

• Sunlight, of which there is a continuous input to Earth
• Surface water and groundwater, which are renewed through the hydrologic cycle
• Winds, which are renewed through the heat-distribution system of the atmosphere
• Water currents and waves, which are renewed through the heat-distribution system of the oceans, as well as the tidal influence of the Moon

Many renewable resources can be managed to increase their rates of recruitment and productivity and to decrease mortality. In the following section we explain how management practices can be used to increase the productivity of biological resources.

Although a renewable resource can regenerate after harvesting, it can also be badly degraded by excessive use or by inappropriate management. These practices can damage the ability to regenerate and may ultimately cause a collapse of the stock. If this happens, the renewable resource is being “mined,” or used as if it were a non-renewable resource. As such, it becomes depleted by excessive use. For this reason, ecologists commonly use the qualified term: potentially renewable resources.

Management Of Renewables

Potentially at least, populations of animals and plants, and their assemblages known as communities and ecosystems (such as a tract of forest), can be harvested in a sustainable manner – that is, without depleting the size of the resource or its capability for renewal. Essentially, this is due to the fact that, within limits, bio-resources are able to regenerate after some of their biomass is harvested. As long as the rate of harvesting does not exceed that of regeneration, a bio-resource can be used in a sustainable way.

Ultimately, the upper limits of the productivity of an individual organism are limited by genetically determined factors that influence its fecundity, longevity, and growth rate. To reach that potential limit of productivity, an organism must experience optimal environmental conditions. In a collective sense, genetic factors also set a ceiling on the potential productivity of populations or organisms, as well as communities and larger ecosystems. However, in the real world it is typical that environmental conditions are not optimal, and so the actual (or realized) recruitment, growth, and maturation of individuals and biomass are less than their potential amounts. As a result, it is possible to increase the size of a harvest by the use of management practices that enhance the productivity of bio-resources. When these practices are used in a coordinated way, they are called a management system.

In general, the various management practices are designed to alleviate environmental constraints on productivity. This is done by mitigating factors that may be preventing some recruitment, or are causing mortality, or are constraining the rate of productivity. In addition, the selective breeding of individuals with desirable traits may be used to alleviate genetically based constraints to productivity – ultimately, such genetic “improvements” may result in domesticated varieties of crops.

In any case, however, the expression of many genetic factors is influenced by environmental conditions, various of which restrict productivity (Figure 24.1). Therefore, in the real world of ecosystems, the actual productivity of bio-resources is less than their potential.

Figure 24.1. Factors Affecting the Yield of a Biological Resource. The biomass and productivity of a bio-resource are determined by the recruitment of individuals into the population, their growth rates, and their mortality through either harvesting or natural means. These factors are affected by both genetically determined and environmental influences. Often, environmental and biological factors can be managed to increase the productivity and size of the stock of a bio-resource. Source: Modified from Begon et al. (2005).

If resource managers understand the nature of constraints on the productivity of bio-resources, and can devise ways to reduce those influences, then the yield of harvested products can be increased. In any truly sustainable system of resource management, those increases in yield must be obtained without degrading the capability of the resource for renewal (they cannot be obtained by over-harvesting the resource or by degrading environmental conditions). The most important practices that are used to increase the productivity of bio-resources are described below. (Note, however, that while these are commonly used methods of increasing the productivity of bio-resources, all management practices cause some degree of ecological damage.)

Selective Breeding

In all species, there is some degree of genetically based influence on biological attributes of individuals such as fecundity, longevity, and productivity. Plant and animal breeders deliberately select individuals that display traits that are considered desirable and use them in breeding programs intended to develop “improved” varieties of crops. This is the basis by which all domesticated species used in agriculture were developed, and cultural selection is still an important way in which crop varieties are produced. In addition, since the 1980s, new methods have been developed for transferring genetic information from one species to another – these have been used to create so-called transgenic crops.

Enhancement of Recruitment

The rate of recruitment of new individuals into an exploited population can be increased in various ways. Some commonly used methods are described below.

• Planting: In intensively managed agricultural, aquacultural, and forestry systems, managers may try to achieve an optimally spaced monoculture of the crop. This is done so that the productivity will not be limited by competition with non-crop species or by individuals of the crop growing too closely together. The recruitment of plant crops is often managed by sowing seeds under conditions that favor their germination and establishment, while optimizing density to minimize competition. Sometimes young plants are grown elsewhere and then out-planted, a practice that is used to cultivate paddy rice, develop fruit-tree orchards, and establish plantations in forestry.
• Regeneration of Perennial Crops: Some management systems encourage perennial crops to regenerate by re-sprouting from surviving rhizomes or stumps after the above-ground biomass is harvested. This regeneration system is used with sugar cane and with stands of ash, aspen, maple, and poplar in forestry. In some cases, the regenerating population may have to be thinned to optimize its density.
• Stock Enhancement: Recruitment of many fishes, particularly salmon and trout, is often enhanced by stripping wild animals or hatchery stock of their eggs and milt (sperm). The eggs are then fertilized under controlled conditions and incubated until they hatch. The larval fish (called fry) are cultivated until they reach a fingerling size, when they are released to suitable habitat to supplement the natural recruitment of wild fish.
• Site Preparation: Certain practices favor the recruitment of economically preferred tree species in forestry. For instance, some pines recruit well onto clear-cuts that have been site-prepared by burning, as long as a supply of seeds is available. Seedlings of other tree species establish readily onto exposed mineral soil and are favored by mechanical scarification that exposes that substrate by disrupting the organic surface mat.
• Managing the Sex Ratio: Recruitment of some hunted animals can be maintained by allowing only adult males to be harvested. For example, most species of deer are polygynous (males breed with more than one female). Consequently, a hunt can be restricted to males, on the assumption that the surviving bucks will still be able to impregnate all of the females in the local population.
• Harvest Season: Recruitment of some animals can be managed by limiting the hunting season to a particular time of the year. For example, restricting the hunt of waterfowl to the autumn allows ducks and geese to breed during the spring and summer so that recruitment can occur. Hunting in the springtime interferes with that reproduction.

Enhancement of Growth Rate

As noted previously, the productivity of all plants and animals is constrained by environmental influences, which include inorganic factors such as nutrient availability and temperature and biological ones such as competition and disease. Often, management practices can be used to manipulate environmental conditions to reduce their limitation on growth rate, allowing an increased harvestable yield. Sometimes a management system is used, involving a variety of practices applied in a coordinated manner. Some examples follow.

• Agricultural Systems: In intensive agricultural systems, high-yield varieties of crops are grown and managed to optimize their productivity. The management practices typically combine some or all of the following: fertilizer addition to enhance nutrient availability, irrigation to reduce the effects of drought, tillage (ploughing) or herbicide use to decrease competition from weeds, fungicide use and other practices to control diseases, and insecticide use and other practices to lessen damage caused by insects and other pests.
• Forestry: The intensity of management used in forestry varies greatly, but crop-tree productivity can be increased through silvicultural practices such as thinning young stands to reduce competition among crop trees, using herbicide to control weeds, and using insecticide to cope with infestations of insects.
• Aquaculture: High-yield varieties of fish, crustaceans, or mollusks may be grown at high density in ponds or pens, where they are well fed and protected from diseases and parasites through the use of antibiotics and other chemicals.

Management of Mortality Rate

Mortality of juveniles and adults can seriously affect the sizes of plant and animal stocks. However, by thinning out the stock, mortality also influences the intensity of competition and that can increase the growth rate of survivors. Natural mortality can be caused by predation, disease, or disturbance, while harvesting mortality is associated with use by humans. Resource depletion occurs when the total rate of mortality (natural plus harvesting) exceeds the regenerative capability of the stock.

• Natural mortality associated with predators, parasites, diseases, and accidents; can be decreased in various ways:
  • Diseases, Parasites, and Herbivores: Mortality of crop plants caused by herbivorous insects may be managed by using insecticide or by changing the growth conditions to develop a habitat that is less favorable to the pest. Livestock are commonly affected by parasites, a problem that may also be reduced by using a pesticide. For example, sheep infested with ticks are dipped in chemical baths that kill the pests. Similarly, mortality caused by disease may be reduced by using medicines that treat the symptoms, by administering antibiotics to deal with bacterial infections, or by changing cultivation methods to decrease vulnerability. All such practices allow diseases, parasites, and herbivores to be controlled over the short term, but none are long-term solutions to these causes of productivity loss and mortality.
  • Natural Predators: It is uncommon for coyote, wolf, cougar, or bears to be important predators of livestock, but many farmers still consider any losses to these species to be unacceptable. Some hunters feel the same way about mortality that natural predators cause to hunted wildlife, such as deer, moose, and caribou. Consequently, in many regions these large predators have been relentlessly persecuted by shooting, trapping, and poisoning. An alternative to killing the predators is to restrict their access to livestock using fences or guard animals such as dogs and donkeys.
• Harvesting mortality must also be closely managed to ensure that the total mortality (natural plus anthropogenic) stays below the threshold for depleting the resource. For an ideal population, the maximum sustainable yield (MSY) is the largest amount of harvesting mortality that can occur without degrading the stock (see below for detailed discussion on MSY). Theoretically, a harvest rate less than MSY would leave a “surplus” of the stock to natural sources of mortality, while any greater than MSY would impair regeneration. Note that any harvest rate equal to or less than MSY would theoretically sustain the resource. Harvest-related mortality is influenced by many factors, including the amount and kinds of harvesting equipment and personnel, and the time the units spend harvesting. Resource managers can adjust the mortality by controlling the total harvesting effort, which is a function of both the means (such as the kinds of fishing boats and their gear) and the intensity (the number of boats and the amount of time each spends fishing) of harvesting.
  • Technology: Equipment has a great influence on harvesting rate. Consider, for example, the various methods of catching fish, summarized in Figure 24.2. These technologies vary greatly in efficiency, which might be indicated by the amount of fish caught per-person fishing, per-unit of energy expended, or per-unit of investment in equipment. In general, much greater harvesting mortality is associated with the more intensive technologies, such as drift nets, trawls, and seines, compared with simpler methods such as hand-lines. The more efficient methods may also have a much greater by-catch of species that are not the target of the fishery and are often thrown away. Similarly, a hunter armed with a rifle is more efficient than one using a bow-and-arrow, and trees can be harvested more quickly using a feller-buncher than a chainsaw or an axe. (A feller-buncher is a large machine that cuts and de-limbs trees and then stacks the logs into piles.).

  • Selection of Species and Sizes: The great variation in selectivity of harvesting methods, with regards to both species and size, can be an important consideration in resource management. In a fishery, for example, a change in the net-mesh diameter influences the sizes of animals that are caught. Usually, it is advantageous to not harvest smaller individuals, which may not yet have bred and often have a smaller value-per-unit-weight than bigger animals. In forestry, size- or species-selective cutting might be used in preference to clear-cutting, perhaps to encourage regeneration of the most desirable tree species. Those methods also reduce environmental damage, by keeping the physical structure of the forest relatively intact.

  • Number of Harvesting Units: An obvious way to manage mortality associated with harvesting is to limit the number of units that are participating in a harvest. In a fishery, for example, the government could limit the number of fishers by issuing only a certain number of licenses. Usually, the kind of technology that the harvesters can use is also specified, such as the number of boats using a particular fishing gear.

  • Time Spent Harvesting: The harvesting effort is also influenced by the amount of time that each unit works. Often there is strong pressure on regulators to allow harvesting to occur for as long as possible, because of the great economic value of investments made in machinery and personnel. Even so, in some cases, the harvesting time is closely regulated. For example, certain herring fisheries in coastal waters of western North America are only allowed to operate for as little as several hours per year.

    

Figure 24.2. Fishing Technologies. Methods of catching fish vary enormously in their efficiency and in the associated harvesting mortality. (a-b) A trawl is an open, broad-mouthed net that is dragged along the bottom or through the water column. (c) A gill-net can be set on the bottom or attached to drifting buoys and can range up to tens of kilometers in length, catching fish and other animals as they try to swim through the mesh. (d) A purse seine is positioned around a school of fish near the surface and then pulled shut with a bottom draw-line. (e-f) Long lines consist of baited hooks set out behind the vessel for a period of time, either in the water column or on the bottom sediment. Source: NOAA.

• Regulatory tools are legal and administrative procedures that managers use to achieve a measure of control over the harvesting effort, and therefore over the mortality associated with exploitation. Relatively direct controls include licenses that regulate the numbers of participants, the technology they may use, their resource quotas, and the times and places they may harvest. Indirect tools can be used to influence the profitability of different harvesting strategies, such as the following:
  • Fines for non-compliance, which decrease profit by raising costs
  • Taxes on more harmful harvesting methods, or subsidies on less harmful ones, which influence profit by increasing or reducing costs, respectively
  • Buyouts of inappropriate or excess harvesting capacity (either equipment or licenses), which increase profit for the remaining harvesters by improving their relative allocation

Maximum Sustainable Yield

Potentially, all management options (including selective breeding, enhancement of growth and recruitment rates, and management of mortality rate) can result in larger yields of bio-resources. However, the factors that influence the size and productivity of stocks of renewable resources are imperfectly understood. Consequently, the management systems advocated by resource scientists are also imperfect. Despite this caveat about uncertainty, enough is usually known about ecological factors affecting bio-resources to design harvesting and management systems that will not degrade the capability for renewal.

At the very least, precautionary levels of harvesting that are small enough to avoid over-exploiting the resource can be predicted, even though the harvest might be smaller than the potential maximum sustainable yield. It is not necessary that harvests of natural resources are as large as are potentially attainable. If resource managers cannot predict an accurate MSY, then it is ecologically prudent to harvest at a rate known to be smaller than the MSY, but that is clearly sustainable. Of course, such strategies result in smaller harvests and less short-term profit. These are, however, more than offset by the longer-term economic and ecological benefits of adopting prudent strategies of resource use.

Moreover, the regional economic benefits of smaller (but sustainable) harvests can be enhanced by taking steps to ensure that the manufactured outputs of resource-dependent industries focus more so on “value-added” products. In forestry, for example, the export of raw logs might be prohibited, while local manufacturing of value-added products such as lumber, furniture, and violins would be encouraged. Similarly, a regional fishing industry might focus on the production and export of higher-valued products, such as prepared foods, rather than unprocessed fish. These kinds of integrations of resource harvesting and manufacturing can optimize the regional economic benefits of resource-based industries, while allowing smaller, sustainable harvests of the resource to take place.

Regrettably, non-sustainable rates of harvesting have been common in the real world of open, poorly regulated, bio-resource exploitation. This has happened even where so-called “scientific” management was being used. These facts become clear from the examples of resource degradation described in this chapter.

Non-Sustainable Use

Many potentially renewable resources have been used by humans in an unsustainable manner. Either these resources were excessively harvested (a condition known as over-harvesting or over-exploitation), or their post-harvest regeneration was inappropriately managed. Either of these can result in depletion or exhaustion of the resource by so-called mining (a term more usually applied to a non-renewable resource).

There are many examples of the non-sustainable use of potentially renewable resources. A few species have even been made extinct by excessive hunting, such as the dodo, passenger pigeon, and great auk (the latter two occurred in North America. In other cases, seemingly abundant species were rendered endangered by over-harvesting, including American ginseng, Eskimo curlew, northern fur seal, plains bison, right whale, trumpeter swan, and other once-common species. In fact, there are remarkably few examples of economically valuable, potentially renewable resources that have not been severely depleted at one time or another through excessive use or inappropriate management.

Additional examples of the mining of potentially renewable resources include the following:

• Extensive deforestation of many parts of the world, which has resulted in losses of timber and fuelwood resources as well as environmental damages such as erosion and regional changes in climate
• Extensive degradation of the quality of agricultural soil, resulting in declining crop yields and sometimes the abandonment of previously arable land
• Widespread depletions of groundwater by over-use for irrigated agriculture, which is rapidly drawing down local and even regional aquifers
• Exhaustion of fisheries, such as those of cod, salmon, and herring off the coasts of North America
• Depletion of many hunted resources – various species of fish, antelope, deer, furbearers, waterfowl, whales, and others

Not all cases of the mining of potentially renewable resources have occurred in modern times. Examples that are prehistoric are described in Global Focus 24.1 and 24.2. These well-known cases demonstrate that even relatively unsophisticated human societies with primitive technologies have caused enormous damage to their crucial resource base.

In some cases, an early depletion of potentially renewable resources was followed by efforts of conservation or improved management, which subsequently restored the depleted stocks (but not ones that had been made extinct). (In the sense used here, conservation refers to the “wise use” of natural resources, including recycling and other means of efficient utilization, as well as ensuring that the harvesting of renewable resources does not exceed their regeneration.) For example, regulating the hunting of white-tailed and mule deer has allowed those species to remain abundant in regions where habitat is suitable. Comparable successes have been achieved with other once-depleted animals, such as certain sportfish, ducks, and geese.

Overall, however, there is more bad news than good about future stocks and regeneration of many potentially renewable resources. Although some renewables are being used in a manner that is supportive of their future availability, many are not. If this situation does not change for the better in the near future, there will be grim consequences for the human economy, and also for biodiversity and natural ecosystems.

GLOBAL FOCUS 24.2. PREHISTORIC EXTINCTIONS

Paleontologists have found clear evidence of prehistoric mass extinctions of animal species, apparently caused by over-hunting by stone-age humans (Martin, 1967, 1984; Diamond, 1982, 2004). Although the extinctions occurred at different times and places, all of them coincided with the discovery and colonization of a landmass that was previously uninhabited by people. The extinct animals were seemingly naïve to predation by efficient groups of hunters and were unable to adapt to the onslaught. These mass extinctions represent cases of non-sustainable harvesting of wild-animal populations, which were potentially renewable bio-resources for the Neolithic hunters.

In North America, a wave of extinctions began about 11-thousand years ago, soon after people colonized the continent by migrating across a land bridge from Siberia. (The land bridge existed because sea level was much lower than today, as a result of so much water being tied up in glacial ice.) Within a relatively short time, at least 56 species of large mammals (weighing more than 44 kg), 21 smaller mammals, and several large birds had become extinct. The extinctions included 10 species of horses (genus Equus), the giant ground sloth (Gryptotherium listai), four kinds of camels (family Camelidae), two buffalo (genus Bison), a cow (genus Bos), the saiga antelope (Saiga tatarica), and four kinds of elephants including the mastodon (Mammut americanum) and mammoth (Mammuthus primigenius; Image 24.3). Predators and scavengers that depended on these large herbivores also became extinct, including the sabre-toothed tiger (Smilodon fatalis), the American lion (Panthera leo atrox), and a huge scavenging bird (Terratornis merriami). The best collection of fossil bones of many of the extinct animals has been excavated from the La Brea tar pits in southern California. However, bones of many species are widespread, and some have been found in various places in North America, including Canada. As colonizing people spread from North America into Central America, and then into South America, extinctions of many other vulnerable species also occurred there.

Image 24.3. Wooly Mammoth and American Mastodon. An artistic impression of a wooly mammoth (left) and an American mastodon (right). Source: “MammothVsMasodon” by Dantheman9758 is licensed under CC BY-SA 3.0.

Similar events of mass extinction have occurred elsewhere, also coinciding with the colonization of places by stone-age hunters. In New Guinea and Australia, waves of extinction occurred about 50-thousand years ago, following the discoveries of those islands by Melanesians migrating south from Asia. These extinctions involved the losses of many large marsupials, flightless birds, and tortoises.

In New Zealand, an extinction wave occurred less than 1,000 years ago, following the discovery of those islands by Polynesians. This swept away numerous large, flightless birds, including a 250-kg, 3-m giant moa (Dinornis maximus), 26 other species of moa, a goose (Cnemiornis calcitrans), a swan (Cygnus sumnerensis), a giant coot (Fulica chathamensis), a pelican (Pelecanus novaezealandiae), an eagle (Harpagornis moorei), and fur seals and various large lizards and frogs. The extinctions of the moas progressed as a wave from North Island to South Island over a two-century period following the Polynesian colonization. Great quantities of bones have been discovered at places where the moas were herded and butchered. Some of the bone deposits were mined by European colonists and used as phosphate fertilizer.

The human colonization of Madagascar occurred about 1,500 years ago. This also resulted in many extinctions, including the loss of 6-12 species of huge elephant birds, 14 lemurs, 2 giant tortoises, and other large animals. Prehistoric mass extinctions also occurred in Hawaii, New Caledonia, Fiji, the West Indies, and other island groups. All are believed to have resulted from over-hunting by newly colonizing people.

Clearly, the unsustainable use of bio-resources, resulting in irretrievable losses of species important to people, is not only a modern phenomenon. Prehistoric humans could also be rapacious, given appropriate opportunities in the form of naïve and edible species.

Patterns Of Over-Exploitation

In cases where only a particular species is being harvested, over-exploitation generally involves an excessive harvesting rate, occurring without sufficient attention to regeneration. Under such conditions, the stocks are quickly mined, and they collapse to economic or biological extinction.

Sometimes, a “virgin” (or previously unexploited) resource is dominated by large, old-growth individuals, which are harvested selectively during the initial stages of resource “development.” This changes the structure of the resource to one that is dominated by smaller, younger individuals. Because younger individuals are often relatively fast growing, the productivity of the resource is not necessarily smaller than that of the initial old-growth stock, although the total biomass may be less. However, if this kind of resource degradation is taken too far, the population may collapse in both productivity and biomass. The collapse may be caused by inadequate recruitment into the harvested population because the fecundity of younger individuals is not sufficient to offset the harvesting mortality.

Patterns of resource degradation are more complicated in the case of mixed-species resources, which are often over-exploited in a sequential manner. At first, only certain species in the virgin mixed-species resource may be considered desirable from the economic perspective. In addition, some individual organisms may be very large, especially in the case of old-growth resources. For instance, old-growth forests of the American Pacific Northwest are typically dominated by large individuals of valuable tree species, which are coexisting with many smaller individuals. Many pre-exploitation communities of fish, whales, and other species are also typically dominated by large individuals of desirable species.

The exploitation of a mixed-species resource usually involves a sequential harvest of commodities with progressively smaller economic value (measured as value per individual, as well as per unit of biomass and of harvested area). Initially, the largest individuals of the most valuable species are harvested selectively and are rapidly depleted. In an old-growth forest, for example, the largest logs of the most desirable species have the highest value per unit of biomass – they can be used to manufacture large-dimension lumber of precious species or costly veneer products. The intention of post-harvesting management is not to recreate another old-growth forest, because this would take too much time and would also involve an extended period of relatively low productivity. Instead, the site is typically converted into a second-growth forest.

Next, smaller individuals of the most desired species might be harvested selectively, along with the largest individuals of secondarily desired species. In a forest, the economic products might be smaller-dimension lumber. If management of the regenerating stand is intended to produce timber for manufacturing into lumber, the subsequent harvests would be on a relatively long rotation, say 60-100 years, depending on the growing conditions.

However, area-harvesting methods might then be used to harvest all individuals of all species for manufacturing into bulk commodities. In the case of forestry, trees might be clear-cut for the production of small lumber, pulp, industrial fuel, charcoal, or domestic fuelwood. Subsequent harvests for such purposes would be on a short rotation, perhaps 30-50 years. Sometimes the area-harvesting system is followed by management that regenerates a productive resource, although it has a different character from the original, natural ecosystem. In forestry, for example, natural mixed-species forest might be converted into a single-species plantation or perhaps into an agricultural ecosystem.

Intensive harvesting, sequential or otherwise, can also lead to a collapse of biological productivity and therefore to a huge loss of resource value. For instance, clear-cut forests sometimes regenerate into shrub-dominated ecosystems that resist the establishment of tree seedlings. This severe resource degradation may require expensive management to restore another economically useful forest (Image 24.4).

Image 24.4. Clear-cut Forest. Ecologically rich old-growth forest in tropical countries is being rapidly cleared to provide agricultural land. The conversion results in destruction of the forest (the mining of a potentially renewable resource), often to develop agricultural land that may not be productive for very long. In this case, the rice paddies may be cultivated for many years, but the hillsides have been badly degraded by temporary agricultural use. This scene is from East Kalimantan, Indonesia. Source: “Cleared land” by CIFOR is licensed under CC BY-NC-ND 2.0.

Reasons For The Abuse Of Natural Resources

To function over the longer term, an economy depends on a sustained input of natural resources. Given this vital context, it would appear to be economically self-destructive to degrade renewable resources by over-harvesting them or by inappropriate management. Nevertheless, this maladaptive behavior has occurred frequently throughout human history. In fact, most uses of potentially renewable resources have been decidedly non-sustainable, and have caused stocks to become depleted. The most important reasons for this foolish behavior are outlined below.

• The world’s dominant cultures have developed an ethic that presumes that humans have the “right” to take whatever they want from nature for subsistence or economic benefits. This is an expression of the anthropocentric world view.
• Particularly noteworthy is the so-called Judeo-Christian ethic (White, 1967), which is based on the Biblical story of creation. In that story, God directed humans to “be fruitful, and multiply, and replenish the earth, and subdue it,” and to “have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth and over every creeping thing that creepeth upon the earth” (Genesis 1:28). From a purely ecological perspective, this is an arrogant attitude, but it is typical of the world’s dominant cultures and religions. Modern technological ethics have developed from this commanding world view and are now used to legitimize the rapid mining of natural resources and the collateral ecological damage.
• Individual people and their societies are intrinsically self-interested. This attitude is responsible for many cases of over-exploitation of resources in order to optimize short-term profit. At the same time, ecological damage associated with resource depletion is discounted as being unimportant. This is easily done because, in most economic systems, the consequences of ecological damage are usually shared broadly across society (by degradation of the common environment, or by tax monies being used to fix the problem), rather than being considered the responsibility of the persons or corporations who cause the damage.
• Natural resources are perceived as being boundless. Many people believe that nature and its resources are unlimited in their extent, quantity, and productivity. This is referred to as the cornucopian world view (Chapter 16; a cornucopia is the mythical horn of plenty that yields food in boundless amounts). In actual fact, Earth has limited stocks of resources available for use by humans, and most of these are being rapidly depleted.
• Investments of money in some sectors of the economy may accumulate profit faster than the growth rate of renewable resources. Consequently, apparent profit can be increased over the shorter term by liquidating natural resources and then investing the money earned in a faster-growing sector of the economy (see In Detail 24.1). Following this line of reasoning, the growth of many regional and national economies has been jump-started by economic “capital” gained through the non-sustainable mining of natural “capital.”
• Not all of the true costs of over-exploitation are taken into account. The economic strategies suggested above only work if the ecological costs of over-exploitation are not paid for. In fact, some kinds of environmental damage can be interpreted as being “good” for the economy because they add to the gross national product (GNP). For example, the wreck of the Exxon Valdez in Alaska and the cleanup of the spilled petroleum were responsible for billions of dollars of “growth” in the GNP of the U.S. over several years. In actual fact, however, the environmental damages represent a depletion of natural capital and contribute to a “natural debt.” When conventional economics (meaning economics as it is usually practiced) calculates the apparent profit gained through over-exploitation, it does not properly account for costs associated with resource depletion and other environmental damages. In theory, at least, ecological economics (a type of economics advocated by many enlightened economists and environmentalists) would fully cost those damages. An economic system that tallies all costs, including those of environmental damage, is known as a full-cost accounting system.

Within an economic context that involves free access to common-property resources (these are owned by all of society), the above factors inevitably lead to the over-harvesting of potentially renewable resources. In a highly influential essay, Garrett Hardin (1968) called this frequently observed phenomenon “the tragedy of the commons.” He explained this economic misadventure using the analogy of a publicly owned pasture (the “commons”) to which all local farmers had open access for grazing their sheep. Individual, self-interested farmers believed that they would gain additional economic benefits by having as many of their own sheep as possible grazing the pasture. This led to an excessive aggregate use of the pasture, which damaged the forage resource. Hardin’s major conclusion was that “freedom in a commons brings ruin to all,” and this has generally been true of the ways in which many renewable resources have been abused.

Many nations are experiencing crises because of diminishing stocks of natural resources and the associated ecological damage caused by disturbances, pollution, and loss of biodiversity. Remarkably, many of these countries have not yet attempted to deal effectively with resource depletion. With few exceptions, the design and implementation of intelligent strategies for using natural resources has so far proven to be beyond the capability of modern political and economic systems.

Nevertheless, people are definitely capable of designing and implementing systems that would conserve natural resources and the healthy ecosystems that are required to sustain economies. The solutions to resource-related predicaments require an integration of scientific knowledge and social change, along with the adoption of ecologically based economic policies that pursue true sustainability. Such solutions are far preferable to unfettered economic growth based on the depletion of natural resources.

Growth, Development, And Sustainability

To an economist, growth and development are different phenomena. Economic growth is a feature of an economy that is increasing in size over time. It is associated with increases in both the numbers of people and their per-capita use of resources. Particularly in developed countries, economic growth is typically achieved by a rapid consumption of natural resources. Non-renewable resources, such as metals and fossil fuels, are consumed in especially large quantities in a growing economy. Potentially renewable resources are also frequently mined, rather than being harvested on a sustainable basis.

In recent times, almost all national economies have been growing quite rapidly. Moreover, most economic planners, politicians, and businesspeople hope for additional increases in economic activity into the foreseeable future. They feel this way because economic growth is viewed as a means of generating more wealth for countries and companies, while providing a better life for citizens.

Unfortunately, there are well-known limits to growth – these constraints are related to the finite resources on planet Earth plus the laws of thermodynamics. Consequently, economic growth can never be sustained over the longer term. In the perspectives of ecologists and environmentally minded economists, growth is not necessarily desirable: “Economic growth as it now goes on is more a disease of civilization than a cure for its woes” (Ehrlich, 1989).

Economic development is different from growth. It implies an improving efficiency in the use of materials and energy, and it thereby represents progress being made toward a sustainable economic system. In this sense, sustainable economic development involves the following actions:

• Increasing the efficiency of use of non-renewable resources – for example, by recycling and reusing metals and other materials; by minimizing the use of energy for industrial, transportation, and space-heating purposes; and by improving the designs of other materials and products
• Increasing the use of renewable materials in the economy, such as products manufactured from trees or agricultural biomass
• Rapidly increasing the use of renewable sources of energy, such as electricity generated using hydro, solar, wind, or biomass technologies
• Improving social equity, ultimately to such a degree that all citizens (and not just a minority of wealthy people) have access to the necessities and amenities of life

Sustainable Development

Sustainable development refers to making progress toward an economic system that uses natural resources in ways that do not deplete their capital or otherwise compromise their availability to future generations of people. In this sense, the present human economy is obviously non-sustainable because it involves rapid economic growth that is achieved by vigorously mining both non-renewable and potentially renewable resources.

Many politicians, economists, resource managers, and corporate spokespersons have publicly stated that they are in favor of sustainable development. However, most of these people are confusing genuine sustainable development with “sustained economic growth,” which by definition is not possible.

The term “sustainable development” was first popularized in the widely acclaimed report Our Common Future, by the World Commission (1987) on Environment and Development, an agency of the United Nations. (This report, published in 1987, is often referred to as the Brundtland Report, after Gro Harlem Brundtland, the chair of the Commission at the time.) However, even this report obscured some important differences between economic growth and development. In fact, the Brundtland Report advocated a large expansion of the global economy: “It is . . . essential that the stagnant or declining growth trends of this decade [the 1980s] be reversed.” The report suggested that economic growth, coupled with a more equitable distribution of wealth, was required to improve the living standards of poorer peoples of the world. It further presumed that real progress toward a no-growth, equilibrium economy could not be made until society had achieved the equitable socio-economic conditions that are required for stopping both population growth and the rampant depletion of natural resources.

One of the recommendations of the Brundtland Report was that the global average per-capita income should grow by 3% per year (if maintained, this would double per-capita income every 23 years). However, because the global population was increasing at about 2% per year at the time, the economic growth rate would have to compensate, requiring a 5% per year increase in the total economy (3% plus 2%, resulting in a 14-year doubling time). Of course, in those regions where population growth is more rapid, such as much of Africa, South Asia, Latin America, and most cities (see Chapter 10), economic growth rates might have to be even higher in order to achieve a 3% per year increase in real per-capita income. Ultimately, the Brundtland Report estimated that a 5- to 10-fold expansion of the global human economy was needed in order to set the stage for attaining a condition of sustainable development.

The authors of the Brundtland Report believed that this growth would best be achieved through “policies that sustain and expand the environmental resource base.” Such policies would include the advancement of efficient technologies that could help to achieve economic growth while consuming fewer material and energy resources. In addition, a redistribution of some wealth from richer people and regions to poorer ones would be central to achieving the growth of average per-capita income that is championed in the Brundtland Report. It is important to understand that the Brundtland Report was developed through a consensus-building process that involved wide-ranging consultations among diverse interested parties. Therefore, representatives of many nations and cultures had to agree on its content. Considering the diversity of the interests involved, it is not surprising that the report advocated substantial economic growth as a component of its “development” strategy. The growth-related aspects of the report made it easier for politicians and business to support its recommendations.

From the ecological perspective, however, it is doubtful that a 5- to 10-fold increase in the human economy could be sustained. Many have argued that it would be much more sensible to pursue solutions that aggressively attack both economic growth (as it is currently achieved) and population growth. These solutions would include vigorous actions toward population control, a more equitable distribution of wealth, reduced use of resources by wealthier peoples of the world, more equitable access of women to education and social empowerment, development and use of more efficient technologies, and rigorous conservation of natural resources. These sustainable solutions are more difficult and unpopular than the policies advocated by most mainstream politicians and economists, but they appear to be necessary if a sustainable human economy is to be achieved.

Sustainable Economies

The proper definition of a sustainable economy is one that can be maintained over time without causing a depletion of its capital of natural resources. Ultimately, a sustainable economy can be supported only by the “wise use” of renewable resources, which would be harvested at rates equal to or less than their productivity. Therefore, “economic development” should refer only to progress made toward a sustainable economic system. Unfortunately, there have been few substantial gains in this direction. This is because most actions undertaken by politicians, economists, planners, and businesspeople have supported rapid economic growth rather than sustainable economic development. In large part, they do this because they believe they are following the wishes of the public to have greater access to wealth and employment.

Because non-renewable resources are always depleted by their use, they cannot provide an ultimate foundation of a sustainable economic system. However, non-renewable resources do have an important role to play in a sustainable economy. Their use should, however, be tied to improving the stocks of comparable renewable resources so that a net depletion of capital does not occur. For example, if people want to use non-renewable coal, they might act to provide a compensating increase in forest area and biomass. This could result in no net consumption of potential energy (because tree biomass and coal are both fuels), and no net increase in the concentration of atmospheric carbon dioxide or other pollutants (because trees absorb CO₂ as they grow, and mature forest can store carbon for a long time if not disturbed). Of course, any non-renewable materials already in use in the economy should continue to be used and be recycled as efficiently as possible.

Symptoms Of Non-Sustainability

As was previously mentioned, the dominant trends of local, national, and global economies are mostly toward vigorous economic growth, rather than toward sustainable development. The key indicators of these trends of non-sustainable growth are summarized below.

• Rapid Growth of Economies: Because of increases in both population and per-capita use of materials and energy, almost all economies are growing. This well-known fact is reflected by trends in many economic indicators, such as stock markets, growth indexes, and the gross domestic product (GDP). GDP is the value of all goods and services produced by a country in a year; it is equal to gross national product (GNP) minus net investment from foreign nations. The U.S. GDP grew from $2.8 trillion in 1980 to $20.9 trillion in 2020 (Bureau of Economic Analysis, 2021). During the same time span, the U.S. population grew from 229 million in 1980 to 329 million in 2019. Furthermore, there was an increase of per-capita GDP during the period, which reflects an improvement of individual wealth in terms of access to goods and services in the economy despite a growing population (Figure 24.3).

Figure 24.3. GDP per capita vs Population Density, 1990 to 2017. GDP per capita depicts average wealth per individual and population density refers to the number of individuals in a given area of land (per km²). With 2011 constant dollars, the U.S. GDP per capita grew from $37,062 in 1990 to $54,225 in 2017 while population density increased from 27 people/km² to 36 people/km². The data show a positive correlation between economic growth and density of the American population. This pattern can also be observed in other countries, perhaps due in part to increased urbanization of countries as they become more developed and require jobs of higher skill. Source: visualization provided by OurWorldinData.org is licensed under CC BY 4.0. Data from World Bank.

• Depletion of Non-Renewable Resources: All stocks of metal ores, petroleum, natural gas, coal, and other non-renewable resources are finite, being limited to what is present on Earth. These resources are being rapidly consumed, and their exploitable reserves will eventually become depleted. However, discoveries of additional exploitable stocks will extend the economic lifetimes of non-renewable resources, as will efficient recycling. Nevertheless, global stocks of non-renewable resources are being rapidly depleted.
• Depletion of (Potentially) Renewable Resources: Around the globe there are crises of depletion of renewable resources. In many regions, for example, once enormous fish stocks are collapsing, deforestation is proceeding rapidly, the fertility of agricultural soil is declining, supplies of surface water and groundwater are being depleted and polluted, and hunted animals are becoming scarcer. Not all stocks of renewable resources are being severely depleted, but the declines are becoming more common and widespread.
• Depletion of Non-Economic, Environmental Resources: Some resources that are necessary for the health of economic systems are not assigned value in the marketplace—that is, they are not valuated in dollars, and are not actively traded. Nevertheless, these resources are important to the health of the ecosystems that sustain the human economy. Examples of such non-valuated environmental resources include: (1) the ability of ecosystems to cleanse the environment of toxic pollutants such as sulfur dioxide and ozone, (2) ecological services such as the production of atmospheric O₂ and consumption of CO₂ (the latter being an important anthropogenic pollutant), and (3) ecosystem functions that support the productivity of conventional resources, such as the plant and algal productivity that ultimately allows the growth of stocks of hunted deer, fish, and other animals.
• Depletion of Other Ecological Values: Some ecological values are not directly or indirectly important in the human economy, but they still have intrinsic (or existence) worth. This makes these values significant, regardless of any perceived importance to human welfare (see Chapter 1). The most important examples of these ecological values are associated with biodiversity, especially the many species and natural ecosystems that are indigenous to particular regions. These biodiversity values are increasingly being threatened and lost in all regions of the globe. Such losses would never be tolerated in an ecologically sustainable economy (that is, one in which resources are used in ways that do not compromise their future availability and do not endanger species or natural ecosystems).

Modern economies deliver great benefits to people who are wealthy enough to purchase a happy and healthy lifestyle, replete with sufficient food, shelter, material goods, and recreational opportunities. For less-wealthy people, however, current economic systems may allow only minimal access to the most fundamental basics of subsistence. If a fairer, more equitable delivery of economic benefits to the poorer people of the world is to be achieved, then either non-sustainable economic growth or a substantial redistribution of some of the existing wealth will be required.

Ultimately, the global scale and long-term sustainability of the human economy will be limited by the ability of the biosphere to deliver renewable resources and ecosystem services. However, the limits of many potentially renewable resources have already been exceeded, resulting in stock declines or collapses. These well-documented damages should be regarded as warnings of the likely future of the human enterprise, unless there are marked improvements in resource-use systems. If critical resources are no longer available to support economic activity, the non-sustainable economy will be forced to contract in size, and may collapse.

It must be recognized that an ecologically sustainable economy might not be very popular with much of the public, or with politicians, government administrators, and business interests. These stakeholders would experience short-term pain (likely felt over decades) to achieve long-term, sustainable, societal gains. The pain would be associated with a less-intensive use of natural resources, the abandonment of the paradigm of economic growth, and the rapid stabilization – and perhaps downsizing – of the human population. The gains would be associated with an ecologically sustainable economic system that could support human society, and the rest of the biosphere, for a long time.

As we have repeatedly observed, people rely on natural resources to sustain their enterprise. Throughout history, resources that are essential to economies have been consumed to exhaustion (assuming there was a technological capability to do so). It is clear that better, more sustainable systems must be found that will allow us to use natural capital without depleting its stocks and without degrading ecosystems in unacceptable ways. Human societies desperately require these sustainable systems, but it remains to be seen whether we will design and implement them.

Conclusions

The human economy can function only if it has continuous access to an input of natural resources, of which there are two kinds: non-renewable and renewable. Non-renewable resources cannot regenerate, so they always become depleted as they are used. In contrast, renewable resources are capable of regeneration, so they can potentially be available forever. Nevertheless, excessive harvesting or inappropriate management can degrade potentially renewable resources, causing them to become diminished or even disappear. The human economy has been growing rapidly, and this has been achieved by the vigorous consumption and depletion of both non-renewable and potentially renewable resources. However, this process is clearly unsustainable because it has relied on a rapid depletion of natural resources, while also causing other kinds of environmental damage, for example, to biodiversity. Ultimately, a sustainable human economy must be based on the wise use of renewable resources – meaning use that does not compromise their availability in the future. In addition, an ecologically sustainable economy would not cause unacceptable damage to other parts of the biosphere, such as putting other species and natural ecosystems at risk of extinction.

Questions For Review

1. What are the differences between non-renewable and (potentially) renewable natural resources? Give examples of each.
2. How can the productivity of biological resources be increased through management?
3. Describe three cases of the “mining” of (potentially) renewable natural resources. Why did the overexploitation occur?
4. What are the key differences between conventional economics and ecological (environmental) economics?

Questions For Discussion

1. What are the differences between economic growth and development? Relate economic growth and development to the notion of sustainable development. Do you believe that the American economy is making much progress toward sustainable development? Explain your answer.
2. Can you think of any examples of economically valuable, potentially renewable resources that have not been severely depleted through excessive use or inappropriate management? Explain your answer.
3. List three environmental values that do not directly contribute to the human economy, but are nevertheless important to the healthy functioning of ecosystems. Could these services be valuated (measured in dollars) in order to allow their degradation to be considered a true “cost” of doing business? What would be the benefits of such an ecological cost-accounting?
4. In this chapter, we defined sustainable development as “progress toward an economic system based on the use of natural resources in a manner that does not deplete their stocks nor compromise their availability for use by future generations of humans.” We also defined ecologically sustainable development as “considering the human need for resources within an ecological context, and including the need to sustain all species and all components of Earth’s life-support system.” Discuss the key similarities and differences in these two kinds of economic sustainability.

Exploring Issues

1. Show how natural resources are important in your life by making a list of resources that you use daily for energy, food, or as materials in manufactured products.
2. You have been asked to help develop a plan for sustainable forest management on a large tract of land. What practices would you recommend to ensure that the timber harvesting does not deplete the resource? What about other economic resources, such as fish, hunted animals, and opportunities for outdoor recreation? How would your plan also accommodate the need to sustain native species and natural ecosystems?

References Cited And Further Reading

Begon, M., R.W. Howorth, and C.R. Townsend. 2014. Essentials of Ecology. 4th ed. Wiley, Cambridge, UK.

Brown, L.R. 2001. Eco-Economy: Building an Economy for the Earth. W.W. Norton and Company, New York, NY.

Brown, L.R. 2003. Plan C: Rescuing a Planet Under Stress and a Civilization in Trouble. W.W. Norton and Company, New York, NY.

Bureau of Economic Analysis. 2021. National Income and Product Accounts Table 1.1.5. Gross Domestic Product. https://apps.bea.gov/iTable/iTable.cfm?reqid=19&step=2#reqid=19&step=2&isuri=1&1921=survey. Accessed June 9, 2021.

Chambers, N., C. Simmons, and N. Wackernagel. 2001. Sharing Nature’s Interest: Ecological Footprints as an Indicator of Sustainability. Earthscan Publications, London, UK.

Chiras, D.D. and J.P. Reganold. 2009. Natural Resource Conservation: Management for a Sustainable Future. 10th ed. Prentice Hall, Upper Saddle River, NJ.

Clark, W.C. 1989. Clear-cut economies: Should we harvest everything now? The Sciences, Jan./Feb.: 16-19.

Clark, W.C. and R.E. Munn (eds.). 1986. Sustainable Development of the Biosphere. Cambridge University Press, New York, NY.

Costanza, R. 1991. Ecological Economics: The Science and Management of Sustainability. Columbia University Press, New York, NY.

Costanza, R. and H.E. Daly. 1992. Natural capital and sustainable development. Conservation Biology, 6: 37-46.

Daly, H.E. 1997. Beyond Growth: The Economics of Sustainable Development. Beacon Press, Boston, MA.

Diamond, J.M. 1982. Man the exterminator. Nature, 298: 787-789.

Diamond, J. 2004. Collapse: How Societies Choose to Fail or Succeed. Viking, East Rutherford, NJ.

Ehrlich, P.R. 1989. Facing the habitat crisis. BioScience, 39: 480-482.

Faber, M.M., R. Manstetter, and J. Proops. 1996. Ecological Economics: Concepts and Methods. Edward Elgar Publishers, New York, NY.

Farley, J. and H.E. Daly. 2003. Ecological Economics: Principles and Applications. Island Press, Washington, DC.

Freedman, B. 1995. Environmental Ecology. 2nd ed. Academic Press, San Diego, CA.

Freedman, B. 2010. Environmental Science. A Canadian Perspective. 5th ed. Pearson Education Canada, Toronto, ON.

Hardin, G. 1968. The tragedy of the commons. Science, 162: 1243-1248.

Holechek, J.L., R.A. Cole, J.T. Fisher, and R. Valdez. 2003. Natural Resources: Ecology, Economics, and Policy. 2nd ed. Prentice Hall, East Rutherford, NJ.

Martin, P.S. 1984. Catastrophic extinctions and late Pleistocene blitzkrieg: two radiocarbon tests. pp. 153-589 In: Extinctions (M.H. Nitecki, ed.). University of Chicago Press, Chicago, IL.

Martin, P.S. and H.E. Wright (eds.). 1967. Pleistocene Extinctions: The Search for a Cause. Yale University Press, New Haven, CT.

Meadows, D.H., D.L. Meadows, and J. Randers. 1992. Beyond the Limits: Global Collapse or a Sustainable Future. Earthscan, London, UK.

Meadows, D.H., J. Randers, and D.L. Meadows. 2003. Limits to Growth: The 30-Year Update. Chelsea Green Publishing Co., White River Junction, VT.

Mowat, F. 1984. Sea of Slaughter. McClelland & Stewart, Toronto, ON.

Ponting, C. 1991. A Green History of the World. Penguin, Middlesex, UK.

Rees, W.E. 1990. The ecology of sustainable development. Ecologist, 20(1): 18-23.

Thirgood, J.V. 1981. Man and the Mediterranean Forest: A History of Resource Depletion. Academic Press, New York, NY.

Tietenberg, T. 2005. Environmental and Natural Resource Economics. 9th ed. Addison Wesley Boston, MA.

White, L. 1967. The historical roots of our ecological crisis. Science, 155: 1203-1207.

World Commission on Environment and Development (WCED). 1987. Our Common Future. Oxford University Press, Oxford, UK.