Conservation Biology 5
In the 1968 book The Population Bomb, biologist Dr. Paul R. Ehrlich wrote,
"The battle to feed all of humanity is over. In the 1970s hundreds of millions of people will starve to death in spite of any crash programs embarked upon now. At this late date nothing can prevent a substantial increase in the world death rate." [8] While many critics view this statement as an exaggeration, the laws of exponential population growth are still in effect, and unchecked human population growth cannot continue indefinitely.
Many dire predictions have been made about the world's population leading to a major crisis called the
"population explosion."
In 1983 the United Nations General Assembly passed resolution 38/161 entitled Process of Preparation of the Environmental Perspective to the Year 2000 and Beyond3 , establishing a special commission whose charge was:
(a) To propose long-term environmental strategies for achieving sustainable development to the year 2000 and beyond; (b) To recommend ways in which concern for the environment may be translated into greater co-operation among developing countries and between countries at dierent stages of economic and social development and lead to the achievement of common and mutually supportive objectives which take account of the interrelationships between people, resources, environment and development; (c) To consider ways and means by which the international community can deal more eectively with environmental concerns, in the light of the other recommendations in its report; (d) To help to dene shared perceptions of long-term environmental issues and of the appropriate eorts needed to deal successfully with the problems of protecting and enhancing the environment, a long-term agenda for action during the coming decades, and aspirational goals for the world community, taking into account the relevant resolutions of the session of a special character of the Governing Council in 1982.
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.
Organizations such as the World Commission on Environment and Development, the Millennium Ecosystem Assessment, and several others including the Intergovernmental Panel on Climate Change21 , the Organization for
Economic Cooperation and Development,22 and the National Academy Report to Congress23 have all issued reports on various aspects of the state of society and the environment. The members of these groups are among the best experts available to assess the complex problems facing human society in the 21st century, and all have reached a similar conclusion: absent the enactment of new policies and practices that confront the global issues of economic disparities, environmental degradation, and social inequality, the future needs of humanity and the attainment of our aspirations and goals are not assured.
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, and then population growth decreases as resources become depleted. This accelerating pattern of increasing population size is called exponential growth.
The commission later adopted the formal name World Commission on Environment and Development (WCED) but became widely known by the name of its chair
Gro Harlem Brundtland4 , a medical doctor and public health advocate who had served as Norway's Minister for Environmental Aairs and subsequently held the post of Prime Minister during three periods. The commission had twenty-one members5 drawn from across the globe, half representing developing nations. In addition to its fact-nding activities on the state of the global environment, the commission held fteen meetings in various cities around the world seeking rsthand experiences on the how humans interact with the environment. The Brundtland Commission issued its nal report Our Common Future6 in 1987.
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.
Thus if we wish to maintain environmental impacts (I) at their current levels (i.e. I2050 = I2010), then
P2010 × A2010 × T2010 = P2050 × A2050 × T2050 (1.2) or T2050 T2010 = P2010 P2050 × A2010 A2050 = 1 1.35 × 1 4 = 1 5.4 (1.3) This means that just to maintain current environmental quality in the face of growing population and levels of auence, our technological decoupling will need to reduce impacts by about a factor of ve. So, for instance, many recently adopted climate action plans for local regions and municipalities, such as the Chicago Climate Action Plan12 , typically call for a reduction in greenhouse gas emissions (admittedly just one impact measure) of eighty percent by mid-century. The means to achieve such reductions, or even whether or not they are necessary, are matters of intense debate; where one group sees expensive remedies with little demonstrable return, another sees opportunities for investment in new technologies, businesses, and employment sectors, with collateral improvements in global and national well-being.
Yeast, a microscopic fungus used to make bread and alcoholic beverages, exhibits the classical
S-shaped curve when grown in a test tube (Figure 45.10a). Its growth levels off as the population depletes the nutrients that are necessary for its growth.
In 1865 William Jevons14 (1835-1882), a British economist, wrote a book entitled
The Coal Question15 , in which he presented data on the depletion of coal reserves yet, seemingly paradoxically, an increase in the consumption of coal in England throughout most of the 19th century. He theorized that signicant improvements in the eciency of the steam engine had increased the utility of energy from coal and, in eect, lowered the price of energy, thereby increasing consumption. This is known as the Jevons paradox,
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.
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 45.9).
Oak trees grow very slowly and take, on average, 20 years to produce their first seeds, known as
acorns. As many as 50,000 acorns can be produced by an individual tree, but the germination rate is low as many of these rot or are eaten by animals such as squirrels. In some years, oaks may produce an exceptionally large number of acorns, and these years may be on a two- or three-year cycle depending on the species of oak (r-selection).
Countries with rapid growth have a pyramidal shape in their
age structure diagrams, showing a preponderance of younger individuals, many of whom are of reproductive age or will be soon (Figure 45.16). This pattern is most often observed in underdeveloped countries where individuals do not live to old age because of less-than-optimal living conditions.
However, these treaties have not been ratified by every country, and many underdeveloped countries trying to improve their economic condition may be less likely to
agree with such provisions if it means slower economic development. Furthermore, the role of human activity in causing climate change has become a hotly debated socio-political issue in some developed countries, including the United States. Thus, we enter the future with considerable uncertainty about our ability to curb human population growth and protect our environment.
Concepts of animal population dynamics can be applied to human population growth. Humans are not unique in their ability to
alter their environment. For example, beaver dams alter the stream environment where they are built. Humans, however, have the ability to alter their environment to increase its carrying capacity sometimes to the detriment of other species (e.g., via artificial selection for crops that have a higher yield).
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.
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.
This inevitably leads to the problem of metrics: what should be measured and how should the values obtained be interpreted, in light of the broad goals of the sustainability paradigm? The Chapter ProblemSolving, Metrics, and Tools for Sustainability (Section 9.1) addresses this in detail, but presented here is a brief summary of the ndings of the Millennium Ecosystem Assessment19 (MEA), a project undertaken by over a thousand internationally recognized experts, from 2001-2005, who
assessed the state of the world's major ecosystems and the consequences for humans as a result of human-induced changes. In its simplest form, a system20 is a collection of parts that function together. The MEA presents ndings as assessments of ecosystems and ecosystem services: provisioning services such as food and water; regulating services such as ood control, drought, and disease; supporting services such as soil formation and nutrient cycling; and cultural services such as recreational, spiritual, religious and other nonmaterial benets. MEA presents three overarching conclusions:
classical conditioning
association of a specific stimulus and response through conditioning
Exponential growth is possible only when infinite natural resources are
available; this is not the case in the real world.
The best example of exponential growth is seen in
bacteria.
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").
Populations of Kselected species tend to exist close to their
carrying capacity (hence the term K-selected) where intraspecific competition is high.
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 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).
In real-life situations, population regulation is very
complicated and density-dependent and independent factors can interact. A dense population that is reduced in a density-independent manner by some environmental factor(s) will be able to recover differently than a sparse population. For example, a population of deer affected by a harsh winter will recover faster if there are more deer remaining to reproduce.
Efforts to control population growth led to the one-child policy in China, which used to include more severe
consequences, but now imposes fines on urban couples who have more than one child. Due to the fact that some couples wish to have a male heir, many Chinese couples continue to have more than one child.
The intersection of social and economic elements can form the basis of social equity. In the sense of enlightened management, "viability" is formed through
consideration of economic and environmental interests. Between environment and social elements lies bearability, the recognition that the functioning of societies is dependent on environmental resources and services. At the intersection of all three of these lies sustainability.
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.
Thus there are three dimensions that sustainability seeks to integrate: economic, environmental, and social (including sociopolitical). Economic interests dene the framework for making
decisions, the ow of nancial capital, and the facilitation of commerce, including the knowledge, skills, competences and other attributes embodied in individuals that are relevant to economic activity. Environmental aspects recognize the diversity and interdependence within living systems, the goods and services produced by the world's ecosystems, and the impacts of human wastes. Socio-political refers to interactions between institutions/rms and people, functions expressive of human values, aspirations and well-being, ethical issues, and decisionmaking that depends upon collective action. The report sees these three elements as part of a highly integrated and cohesively interacting, if perhaps poorly understood, system.
Whether or not the paradox is correct, the fact that it has been postulated gives us pause to examine in somewhat greater
depth consumption patterns of society. If we let Q be the quantity of goods and services delivered (within a given time period) to people, and R be the quantity of resources consumed in order to deliver those goods and services, then the IPAT equation can be rewritten in a slightly dierent way as: I = P × GDP P × Q GDP × R Q × I R (1.4) where h R Q i represents the resource intensity, and I R is the impact created per unit of resources consumed. Rearranging this version of the equation gives: R = Q × R Q (1.5) which says simply that resources consumed are equal to the quantity of goods and services delivered times the resource intensity. The inverse of resource intensity h Q R i is called the resource use eciency, also known as resource productivity or eco-eciency, an approach that seeks to minimize environmental impacts by maximizing material and energy eciencies of production. Thus we can say: R = Q × 1 Eco − eciency (1.6) that is, resources consumed are equal to goods and services delivered divided by eco-eciency. Whether or not gains in eco-eciency yield genuine savings in resources and lower environmental impacts depends on how much, over time, society consumes of a given product or service (i.e. the relative eciency gain, ∆e e ) must outpace the quantity of goods and services delivered ∆Q Q . In the terms of Jevons paradox, if ∆Q Q ≥ ∆e e then the system is experiencing backre.
Humans can construct shelter to protect them from the elements and have
developed agriculture and domesticated animals to increase their food supplies. In addition, humans use language to communicate this technology to new generations, allowing them to improve upon previous accomplishments.
As attractive as the concept of sustainability may be as a means of framing our thoughts and goals, its denition is rather broad and
dicult to work with when confronted with choices among specic courses of action. The Chapter Problem-Solving, Metrics, and Tools for Sustainability (Section 9.1) is devoted to various ways of measuring progress toward achieving sustainable goals, but here we introduce one general way to begin to apply sustainability concepts: the IPAT equation.
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: dN dT = rmax N
Earth's human population is growing rapidly, to the extent that some worry about the ability of the
earth's environment to sustain this population, as long-term exponential growth carries the potential risks of famine, disease, and large-scale death.
The three main elements of the sustainability paradigm are usually thought of as equally important, and within which tradeos are possible as courses of action are charted. For example, in some instances it may be deemed necessary to degrade a particular
ecosystem in order to facilitate commerce, or food production, or housing. In reality, however, the extent to which tradeos can be made before irreversible damage results is not always known, and in any case there are denite limits on how much substitution among the three elements is wise (to date, humans have treated economic development as the dominant one of the three). This has led to the notion of strong sustainability, where tradeos among natural, human, and social capital are not allowed or are very restricted, and weak sustainability, where tradeos are unrestricted or have few limits. Whether or not one follows the strong or weak form of sustainability, it is important to understand that while economic and social systems are human creations, the environment is not. Rather, a functioning environment underpins both society and the economy.
Many countries have attempted to reduce the human impact on climate change by reducing their
emission of the greenhouse gas carbon dioxide.
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.
By the second half of the twentieth century, the concept of K- and r-selected species was used
extensively and successfully to study populations. The concept relates not only reproductive strategies, but also to a species' habitat and behavior, especially in the way that they obtain resources and care for their young. It includes length of life and survivorship factors as well. For this analysis, population biologists have grouped species into the two large categories—K-selected and r-selected—although they are really two ends of a continuum.
Over the years, several studies attempted to confirm the theory, but these attempts have largely
failed. Many species were identified that did not follow the theory's predictions. Furthermore, the theory ignored the age-specific mortality of the populations which scientists now know is very important. New demographic-based models of life history evolution have been developed which incorporate many ecological concepts included in r- and K-selection theory as well as population age structure and mortality factors.
The world's human population is currently experiencing exponential growth even though human reproduction is
far below its biotic potential (Figure 45.14). To reach its biotic potential, all females would have to become pregnant every nine months or so during their reproductive years. Also, resources would have to be such that the environment would support such growth. Neither of these two conditions exists. In spite of this fact, human population is still growing exponentially
An example of density-dependent regulation is shown in Figure 45.11 with results from a study focusing on the
giant intestinal roundworm (Ascaris lumbricoides), a parasite of humans and other mammals. [3] Denser populations of the parasite exhibited lower fecundity: they contained fewer eggs.
The policy itself, its social impacts, and the effectiveness of limiting overall population growth are controversial. In spite of population control policies, the human population continues to
grow. At some point the food supply may run out because of the subsequent need to produce more and more food to feed our population. The United Nations estimates that future world population growth may vary from 6 billion (a decrease) to 16 billion people by the year 2100. There is no way to know whether human population growth will moderate to the point where the crisis described by Dr. Ehrlich will be averted.
Nature regulates population growth in a variety of ways. These are grouped into density-dependent factors, in which the density of the population at a given time affects
growth rate and mortality, and density-independent factors, which influence mortality in a population regardless of population density. Note that in the former, the effect of the factor on the population depends on the density of the population at onset. Conservation biologists want to understand both types because this helps them manage populations and prevent extinction or overpopulation.
K selected species have few, large offspring, a long gestation period, and often give long-term care to their offspring (Table B45_04_01). While larger in size when born, the offspring are relatively
helpless and immature at birth. When they reach adulthood, they must develop skills to compete for natural resources. In plants, scientists think of parental care more broadly: how long fruit takes to develop or how long it remains on the plant are determining factors in the time to the next reproductive event. Examples of K-selected species are primates including humans), elephants, and plants such as oak trees (Figure 45.13a).
The concept of sustainability has engendered broad support from almost all quarters. In a relatively succinct way it expresses the basis upon which human existence and the quality of
human life depend: responsible behavior directed toward the wise and ecient use of natural and human resources. Such a broad concept invites a complex set of meanings that can be used to support divergent courses of action. Even within the Brundtland Report a dichotomy exists: alarm over environmental degradation that typically results from economic growth, yet seeing economic growth as the main pathway for alleviating wealth disparities.
A consequence of exponential human population growth is the time that it takes to add a particular number of
humans to the Earth is becoming shorter. Figure 45.15 shows that 123 years were necessary to add 1 billion humans in 1930, but it only took 24 years to add two billion people between 1975 and 1999. As already discussed, at some point it would appear that our ability to increase our carrying capacity indefinitely on a finite world is uncertain.
As is the case for any equation, IPAT expresses a balance among interacting factors. It can be stated as I = P × A × T (1.1) where I represents the
impacts of a given course of action on the environment, P is the relevant human population for the problem at hand, A is the level of consumption per person, and T is impact per unit of consumption. Impact per unit of consumption is a general term for technology, interpreted in its broadest sense as any human-created invention, system, or organization that serves to either worsen or uncouple consumption from impact. The equation is not meant to be mathematically rigorous; rather it provides a way of organizing information for a rst-order analysis
While reproductive strategies play a key role in life histories, they do not account for
important factors like limited resources and competition. The regulation of population growth by these factors can be used to introduce a classical concept in population biology, that of K-selected versus r-selected species.
Humans are unique in their ability to alter their environment with the conscious purpose of
increasing its carrying capacity. This ability is a major factor responsible for human population growth and a way of overcoming density-dependent growth regulation. Much of this ability is related to human intelligence, society, and communication.
Other factors in human population growth are migration and public health. Humans originated in Africa, but have since migrated to nearly all
inhabitable land on the Earth. Public health, sanitation, and the use of antibiotics and vaccines have decreased the ability of infectious disease to limit human population growth. In the past, diseases such as the bubonic plaque of the fourteenth century killed between 30 and 60 percent of Europe's population and reduced the overall world population by as many as 100 million people.
Today, the threat of infectious disease, while not gone, is certainly
less severe. According to the World Health Organization, global death from infectious disease declined from 16.4 million in 1993 to 14.7 million in 1992. To compare to some of the epidemics of the past, the percentage of the world's population killed between 1993 and 2002 decreased from 0.30 percent of the world's population to 0.24 percent. Thus, it appears that the influence of infectious disease on human population growth is becoming less significant.
Animals that are r-selected do not give
long-term parental care and the offspring are relatively mature and self-sufficient at birth. Examples of r-selected species are marine invertebrates, such as jellyfish, and plants, such as the dandelion (Figure 45.13b).
One possible explanation for this is that females would be s
maller in more dense populations (due to limited resources) and that smaller females would have fewer eggs. This hypothesis was tested and disproved in a 2009 study which showed that female weight had no influence. [4] The actual cause of the density-dependence of fecundity in this organism is still unclear and awaiting further investigation.
biotic potential (rmax)
maximal potential growth rate of a species
Although the Brundtland Report did not technically invent the term sustainability, it was the rst credible and widely-disseminated study that probed its
meaning in the context of the global impacts of humans on the environment. Its main and often quoted denition refers to sustainable development as . . .development that meets the needs of the present without compromising the ability of future generations to meet their own needs. The report uses the terms sustainable development, sustainable, and sustainability interchangeably, emphasizing the connections among social equity, economic productivity, and environmental quality. The pathways for integration of these may dier nation by nation; still these pathways must share certain common traits: the essential needs of the world's poor, to which overriding priority should be given, and the idea of limitations imposed by the state of technology and social organization on the environment's ability to meet present and future needs.
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:
The logistic model of population growth, while valid in many
natural populations and a useful model, is a simplification of real-world population dynamics. Implicit in the model is that the carrying capacity of the environment does not change, which is not the case.
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: ΔN (change in number) ΔT (change in time) = B (birth rate) - D (death rate)
birth rate (B)
number of births within a population at a specific point in time
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").
Eventually, the growth rate will
plateau or level off (Figure 45.9). This population size, which represents the maximum population size that a particular environment can support, is called the carrying capacity, or K.
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. dN dT = bN − dN = (b - d)N 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: dN dT = rN 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.
Age structure allows better prediction of
population growth, plus the ability to associate this growth with the level of economic development in the region.
Many factors, typically physical or chemical in nature (abiotic), influence the mortality of a
population regardless of its density, including weather, natural disasters, and pollution. An individual deer may be killed in a forest fire regardless of how many deer happen to be in that area. Its chances of survival are the same whether the population density is high or low. The same holds true for cold winter weather.
The age structure of a population is an important factor in
population dynamics. Age structure is the proportion of a population at different age ranges.
Most density-dependent factors are biological in nature (biotic), and include
predation, inter- and intraspecific competition, accumulation of waste, and diseases such as those caused by parasites. Usually, the denser a population is, the greater its mortality rate. For example, during intra- and interspecific competition, the reproductive rates of the individuals will usually be lower, reducing their population's rate of growth. In addition, low prey density increases the mortality of its predator because it has more difficulty locating its food source.
Suppose we wish to project future needs for maintaining global environmental quality at
present day levels for the mid-twenty-rst century. For this we need to have some projection of human population (P) and an idea of rates of growth in consumption (A).
As oak trees grow to a large size and for many years before they begin to
produce acorns, they devote a large percentage of their energy budget to growth and maintenance. The tree's height and size allow it to dominate other plants in the competition for sunlight, the oak's primary energy resource. Furthermore, when it does reproduce, the oak produces large, energy-rich seeds that use their energy reserve to become quickly established (K-selection).
Bacteria are prokaryotes that reproduce by
prokaryotic fission.
Age structures of areas with slow growth, including developed countries such as the United States, still have a
pyramidal structure, but with many fewer young and reproductive-aged individuals and a greater proportion of older individuals. Other developed countries, such as Italy, have zero population growth. The age structure of these populations is more conical, with an even greater percentage of middle-aged and older individuals. The actual growth rates in different countries are shown in Figure 45.17, with the highest rates tending to be in the less economically developed countries of Africa and Asia.
A graph of this equation yields an S-shaped curve (Figure 45.9), 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.
The r- and K-selection theory, although accepted for decades and used for much groundbreaking research, has now been
reconsidered, and many population biologists have abandoned or modified it.
The Brundtland Report makes it clear that while sustainable development is enabled by technological advances and economic viability, it is
rst and foremost a social construct that seeks to improve the quality of life for the world's peoples: physically, through the equitable supply of human and ecological goods and services; aspirationally, through making available the widespread means for advancement through access to education, systems of justice, and healthcare; and strategically, through safeguarding the interests of generations to come. In this sense sustainability sits among a series of human social movements that have occurred throughout history: human rights, racial equality, gender equity, labor relations, and conservation, to name a few
In the real world, however, there are variations to this idealized curve. Examples in wild populations include
sheep and harbor seals (Figure 45.10b). 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.
Without new technological advances, the human growth rate has been predicted to
slow in the coming decades. However, the population will still be increasing and the threat of overpopulation remains.
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.
Part of the problem in analyzing data pertaining to whether or not such overconsumption is happening depends on the
specic good or service in question, the degree to which the data truly represent that good or service, and the level of detail that the data measure. Table Historical Eciency and Consumption Trends in the United States (Table 1.1) summarizes some recent ndings from the literature on the comparative eciency and consumption for several activities over extended periods of observation. Taken collectively these activities capture several basic enabling aspects of modern society: major materials, transportation, energy generation, and food production. In all cases the data show that over the long term, consumption outpaces gains in eciency by wide margins, (i.e., ∆Q Q ≥ ∆e e ). It should also be noted that in all cases, the increases in consumption are signicantly greater than increases in population. The data of Table Historical Eciency and Consumption Trends in the United States (Table 1.1) do not verify Jevons paradox; we would need to know something about the prices of these goods and services over time, and examine the degree to which substitution might have occurred (for instance aluminum for iron, air travel for automobile travel). To see if such large increases in consumption have translated into comparable decreases in environmental quality, or declines in social equity, other information must be examined. Despite this, the information presented does show a series of patterns that broadly reect human consumption of goods and services that we consider essential for modern living and for which eciency gains have not kept pace; in a world of nite resources such consumption patterns cannot continue indenitely.
K-selected species are species selected by
stable, predictable environments.
behavioral biology
study of the biology and evolution of behavior
The US Environmental Protection Agency8 (US EPA) takes the extra step of drawing a distinction between sustainability and
sustainable development, the former encompassing ideas, aspirations and values that inspire public and private organizations to become better stewards of the environment and that promote positive economic growth and social objectives, the latter implying that environmental protection does not preclude economic development and that economic development must be ecologically viable now and in the long run.
Dandelions have small seeds that are wind dispersed long distances. Many seeds are produced simultaneously to ensure
that at least some of them reach a hospitable environment. Seeds that land in inhospitable environments have little chance for survival since their seeds are low in energy content. Note that survival is not necessarily a function of energy stored in the seed itself.
To model the reality of limited resources, population ecologists developed
the logistic growth model.
Another result of population growth is the endangerment of
the natural environment.
The Jevons paradox is what?
the principle that as technological progress increases the eciency of resource utilization, consumption of that resource will increase. Increased consumption that negates part of the eciency gains is referred to as rebound, while overconsumption is called backre. Such a counter-intuitive theory has not been met with universal acceptance, even among economists (see, for example, The Eciency Dilemma16 ). Many environmentalists, who see improvements in eciency as a cornerstone of sustainability, openly question the validity of this theory. After all, is it sensible to suggest that we not improve technological eciency?
The Chapter The Evolution of Environmental Policy in the United States (Section 2.1) presents information on how the
three components that comprise sustainability have inuenced the evolution of environmental public policy. The Chapter Sustainability: Ethics, Culture, and History (Section 10.1) explores in greater detail the ethical basis for sustainability and its cultural and historical signicance.
Our consumption of goods and services creates a viable economy, and also reects our social needs. For example, most of us consider it a social good that we can
travel large distances rather quickly, safely, and more or less whenever we feel the need. Similarly, we realize social value in having aluminum (lightweight, strong, and ductile) available, in spite of its energy costs, because it makes so many conveniences, from air travel to beverage cans, possible. This is at the center of the sustainability paradigm: human behavior is a social and ethical phenomenon, not a technological one. Whether or not we must overconsume to realize social benets is at the core of sustainable solutions to problems.
Batesian mimicry
type of mimicry where a non-harmful species takes on the warning colorations of a harmful one
Although humans have increased the carrying capacity of their environment, the technologies used to achieve this transformation have caused
unprecedented changes to Earth's environment, altering ecosystems to the point where some may be in danger of collapse. The depletion of the ozone layer, erosion due to acid rain, and damage from global climate change are caused by human activities. The ultimate effect of these changes on our carrying capacity is unknown. As some point out, it is likely that the negative effects of increasing carrying capacity will outweigh the positive ones—the carrying capacity of the world for human beings might actually decrease.
In contrast, r-selected species have a large number of small offspring (hence their r designation (Table 45.2). This strategy is often employed in
unpredictable or changing environments.
Aggressive display
visual display by a species member to discourage other members of the same species or different species
aposematic coloration
warning coloration used as a defensive mechanism against predation
The carrying capacity varies annually: for example, some summers are hot and dry whereas others are cold and wet. In many areas, the carrying capacity during the
winter is much lower than it is during the summer. Also, natural events such as earthquakes, volcanoes, and fires can alter an environment and hence its carrying capacity. Additionally, populations do not usually exist in isolation. They engage in interspecific competition: that is, they share the environment with other species, competing with them for the same resources. These factors are also important to understanding how a specific population will grow.
