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Chapter 26 Population Growth and Regulation Lecture Outlines by Gregory Ahearn, University of North Florida Copyright © 2011 Pearson Education Inc. Chapter 26 At a Glance 26.1 How Does Population Size Change? 26.2 How Is Population Growth Regulated? 26.3 How Are Populations Distributed in Space and Time? 26.4 How Is the Human Population Changing? Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? A population consists of all the members of a particular species that live within an ecosystem A community is group of interacting populations Communities exist within ecosystems, which include all the living an nonliving components of a defined geographical area The biosphere is the enormous ecosystem that encompasses all of Earth’s habitable surface Ecology is the study of the interrelationships of organisms with each other and with the nonliving environment Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? Population size is the outcome of opposing forces – Four factors determine whether and how much the size of a population changes –Births –Deaths –Migration of individuals into the population (immigration) –Migration of individuals out of the population (emigration) Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? Population size is the outcome of opposing forces (continued) – Birth and immigration add individuals to a population – Death and emigration remove individuals from the population Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? Population size is the outcome of opposing forces (continued) – A simple equation describes the change in population size within a given time period: –Change in population size = (births – deaths) + (immigrants – emigrants) Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Population Change births deaths immigration emigration (births deaths) (immigrants emigrants) change in population size Biology: Life on Earth, 9e Fig. 26-1 Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? Population size is the outcome of opposing forces (continued) – Two opposing forces that determine birth and death rates are biotic potential and environmental resistance – Biotic potential is the theoretical maximum rate at which a population could increase, assuming a maximum birth rate and minimal death rate – Environmental resistance refers to the curbs on population growth that are set by the living and nonliving environment Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? Population size is the outcome of opposing forces (continued) – Examples of environmental resistance include: – Interactions among species, such as competition, predation, and parasitism – The always-limited availability of nutrients, energy, and space – Natural events, such as storms, fires, freezing weather, floods, and droughts In nature, the interaction between biotic potential and environmental resistance usually results in a balance between the size of a population and the resources available to support it Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? Biotic potential can produce exponential growth – Evolutionarily successful organisms possess traits that make them well adapted to their environment – These organisms pass these inherited traits on to as many healthy offspring as possible – If environmental resistance is reduced, populations can grow extremely rapidly Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? Population growth is a function of the birth rate, the death rate, and population size – The growth rate (r) of a population, also called the rate of natural increase, is the change in the population size per individual per unit of time – Growth rate is expressed by the equation: –r (growth rate) = b (birth rate) – d (death rate) Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? The growth rate of a population – Birth rate (b) is the number of births per individual during a specific unit of time, such as a month or a year – For example, if there are 150 births among 1,000 individuals in a year, b = 0.15 – Death rate (d) is the number of deaths per individuals during a specific unit of time – For example, if there are 50 deaths among 1,000 individuals in a year, d = 0.05 – If the birth rate exceeds the death rate, the population will grow – If the death rate exceeds the birth rate, the population will decline Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? The growth rate of a population (continued) – The growth rate of this population is therefore: –r (growth rate) = 0.15 (birth rate) – 0.05 (death rate) = 0.1 = 10% per year Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? The growth rate of a population (continued) – Population growth (G), which is the number of individuals added to a population in a given time period, can be calculated by multiplying growth rate (r) by the original population size (N) –Population growth (G) = r (growth rate) x N (population size) – In the previous example, population growth (rN) = 0.1 x 1,000 = 100, so the population has grown by one hundred individuals in the first year Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? If births exceed deaths by a constant percentage, population growth produces a J-curve – A patter of continuously accelerating increase in population size is called exponential growth – Exponential growth occurs when, over a given period of time, a population grows by a fixed percentage of its size at the beginning of that time period – Thus, an increasing number of individuals is added to the population during each succeeding time period, causing the population to grow at an accelerating pace – When graphed against time, a shape called a J-curve is produced Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? If births exceed deaths by a constant percentage, population growth produces a Jcurve (continued) – This high biotic potential evolved because it helps ensure that, in a world filled with forces of environmental resistance, some offspring survive to reproduce Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? Several factors influence biotic potential – The age at which the organism first reproduces – The frequency at which reproduction occurs – The average number of offspring produced each time – The length of the organism’s reproductive life span – The death rate of individuals – Increased death rates can slow the rate of population growth significantly Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? The age at which an organism first reproduces affects the size of the future population – For example, consider two populations of golden eagles that are followed for 30 years – Individuals in one population begin reproducing at the age of 4 years – Individuals in the other population begin reproducing at 6 years – Both populations will follow a J-shaped population growth curve, but more individuals will be added to the earlier reproducing population, resulting in a steeper increase in population numbers – At 30 years, the earlier reproducing population would be 10 times the size of the other population Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Exponential Growth Curves are J-Shaped reproduce at 4 years (pop. 1) reproduce at 6 years (pop. 2) At 24 years, this population has 2,504 eagles Number Number of of Time eagles eagles (years) (pop. 1) (pop. 2) 0 2 2 6 8 4 12 52 18 18 362 86 24 2,504 392 30 17,314 1,764 At 24 years, this population has 392 eagles Fig. 26-2 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? The death rate affects population size – As long as birth rate exceeds death rate, population size will follow a J-shaped rate of increase – However, the time for each population to reach a specific number of individuals will depend upon the magnitude of the death rate Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.1 How Does Population Size Change? The death rate affects population size (continued) – An example would be to examine three hypothetical bacteria populations with different death rates –In the bacterial colony with no deaths, it takes 3.5 hours to produce 1,500 bacteria –In the colony with a 10% death rate, it takes 4 hours to generate 1,500 bacteria –In the colony with a 25% death rate, it takes 5.5 hours to generate 1,500 bacteria Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Effect of Death Rates on Population Growth no deaths 10% death rate 25% death rate It takes about 4 hours to produce 1,500 bacteria It takes about 5.5 hours to produce 1,500 bacteria It takes about 3.5 hours to produce 1,500 bacteria Fig. 26-3 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Exponential growth only occurs under special conditions – Exponential growth cannot continue indefinitely – All populations that exhibit exponential growth must eventually stabilize or crash – Exponential growth can be observed in populations that undergo boom-and-bust cycles, in which periods of rapid population growth are followed by a sudden, massive die-off Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Exponential growth only occurs under special conditions (continued) – Boom-and-bust cycles can be seen in short-lived, rapidly reproducing species, such as microbes and insects – Seasonal populations are linked to changes in rainfall, temperature, or nutrient availability – Ideal conditions encourage rapid growth; deteriorating conditions encourage massive die-off – For example, each year, photosynthetic bacteria in a lake may exhibit exponential growth when conditions are ideal, but crash when they have depleted their nutrient supply Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Boom-and-Bust Population Cycles Nutrients are depleted, and water temperature falls Favorable growth conditions occur “boom” “bust” (a) A boom-and-bust cycle in photosynthetic bacteria Fig. 26-4a Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Boom-and-bust cycles can be seen in short lived, rapidly reproducing species (continued) – Complex factors produce four-year cycles for small rodents, such as lemmings –Lemming populations may grow until lack of food, large migrations, and predators and starvation cause sudden high mortality Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Boom-and-Bust Population Cycles (b) Boom-and-bust cycles in a lemming population in Alaska Fig. 26-4b Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Exponential growth occurs when environmental resistance is reduced – In populations that do not experience boom-and-bust cycles, exponential growth may occur temporarily under special circumstances such as: – When food supply is increased – When population-controlling factors, such as predators, are reduced – For example, the whooping crane population has grown exponentially since they were first protected from hunting and human disturbance in 1940 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Exponential Growth of Wild Whooping Cranes Fig. 26-5 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Exponential growth can occur when individuals invade a new habitat – Invasive species are organisms with a high biotic potential that are introduced into ecosystems where they did not evolve and where they encounter little environmental resistance – When they are introduced into a new ecosystem, population numbers may explode due to a lack of natural predators Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth – Many populations that exhibit exponential growth eventually stabilize –As resources become depleted, reproduction slows and the growth rate would eventually drop to zero, causing the population size to remain constant Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – This growth pattern, where populations increase to the maximum number sustainable by their environment and then stabilize, is called logistic population growth – The maximum population size that can be sustained by an ecosystem for an extended time without damage to the ecosystem is called its carrying capacity (K) – When logistic growth is plotted, it results in an S-shaped growth curve, or S-curve Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Population Growth and Regulation Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Logistic Growth Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The S-Curve of Logistic Population Growth Growth rate slows Growth stops and the population stabilizes close to the carrying capacity Population grows rapidly (a) An S-shaped growth curve stabilizes at carrying capacity Fig. 26-6a Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – If a population far exceeds the carrying capacity of its environment, excess demands placed on the ecosystem are likely to destroy crucial resources – This can permanently and severely reduce carrying capacity, causing the population to decline to a fraction of its former size or disappear entirely Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The S-Curve of Logistic Population Growth The population overshoots the carrying capacity; the environment is damaged Low damage; resources recover, and the population fluctuates Extreme damage; the population dies out High damage; the carrying capacity is permanently lowered (b) Consequences of exceeding carrying capacity Fig. 26-6b Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – For example, when reindeer were introduced onto an island with no large predators, their population increased rapidly, seriously overgrazing the vegetation they relied on for food – As a result, the reindeer population plummeted Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Effects of Exceeding Carrying Capacity exponential growth population crash Fig. 26-7 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Logistic population growth can occur in nature when a species moves into a new habitat –For example, new barnacle settlers along a rocky coast may find ideal conditions that allow their population to grow exponentially –As population density increases, however, individuals begin to compete for space, energy, and nutrients Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. number of barnacles (per cm2) A Logistic Curve in Nature time (weeks) Biology: Life on Earth, 9e Fig. 26-8 Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – These forms of environmental resistance can reduce the reproductive rate and increase the death rate, as has been demonstrated in laboratory populations of fruit flies –In response to crowding, the fruit flies show a decrease in both reproductive rate and life span Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Density-Dependent Environmental Resistance days offspring per day life span population density Biology: Life on Earth, 9e Fig. 26-9 Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – As environmental resistance increases, population growth slows and eventually stops – In nature, conditions are never completely stable, so both carrying capacity and the population size will vary somewhat from year to year – However, environmental resistance ideally maintains populations at or below the carrying capacity of their environment Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Environmental resistance can be classified into two broad categories – Density-independent factors, which limit population size regardless of the population density – Density-dependent factors, which increase in effectiveness as the population density increases –Nutrients, energy, and space are all densitydependent regulators of population size Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Density-independent factors limit populations regardless of their density – climate and weather – For example, most insects and annual plant populations are limited in size by the number of individuals that can be produced before the first hard freeze – Hurricanes, droughts, floods, and fire can have profound effects on local populations, particularly small, short-lived species Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? – Human activities can also limit the growth of natural populations –Pesticides and pollutants can cause drastic declines in natural populations –Overhunting has driven some species to extinction Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Density-dependent factors become more effective as population density increases – Populations of organisms with a life span of more than a year have evolved adaptations that allow them survive seasonal changes, such as the onset of winter – Many mammals develop thick coats and store fat; some hibernate – Many birds migrate long distances to find food and a hospitable climate – Tree and bushes enter dormancy, dropping leaves and slowing their metabolic activities Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Important density-dependent factors limiting population growth are: – Predation – Parasitism – Competition Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Predators exert density-dependent controls on populations – Predators are organisms that eat other organisms, called their prey – Predation becomes important as prey populations grow because predators eat a variety of prey, depending upon what is most abundant and easiest to find – In this way, predators exert density-dependent population control over more than one prey population Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Predators Help Control Prey Populations Fig. 26-10a Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Predator populations often grow as their prey become more abundant – The number offspring produced is determined by the abundance of prey –For example, snowy owls hatch up to 12 chicks when lemmings (their prey) are abundant, but may not reproduce at all in years when the lemming population has crashed Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Predators Help Control Prey Populations Fig. 26-10b Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Some predator-prey population cycles are outof-phase when predators cause a dramatic decline in prey populations, which in turn results in a decline in the predator population at a future date Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Parasites spread more rapidly among dense populations – Parasitism involves a parasite living on or in a host organism, harming it but not generally killing it because many parasites benefit by having their host remain alive – Most parasites cannot travel long distances, so they spread more readily among hosts in dense populations – Parasites influence population size by weakening their hosts and making them more susceptible to death from other causes, such as harsh weather or predators – Examples of parasites include the bacterium that causes Lyme disease, some fungi, intestinal worms, ticks, and some protists Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Competition for resources helps control populations – Competition is defined as the interaction among individuals who attempt to use the same limited resource, and this interaction limits population size in a density-dependent manner Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? There are two major forms of competition: – Interspecific competition, between individuals of different species – Intraspecific competition, between individuals of the same species –Because the needs of members of the same species for resources are almost identical, intraspecific competition is an important density-dependent mechanism of population control Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Organisms have evolved ways to deal with intraspecific competition – Most plants and many insects engage in scramble competition—a free-for-all scramble as individuals try to beat others to a limited pool of resources – For example, gypsy moth females each lay a mass of up to 1,000 eggs on tree trunks in eastern North America – As the eggs hatch, armies of caterpillars crawl up the tree – Huge outbreaks of this invasive species can completely strip large trees of their leaves in a few days – Competition for food may be so great that most of the caterpillars die before they can metamorphose into egg-laying moths Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Scramble Competition Fig. 26-12 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Organisms have evolved ways to deal with intraspecific competition (continued) – Many animals have evolved contest competitions, where social or chemical interactions determine access to important resources – Territorial species—such as wolves, fish, rabbits, and songbirds—defend areas that contain important resources – Only the best adapted individuals are able to defend their territories, whereas those without territories may not reproduce or may become easy prey Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Organisms have evolved ways to deal with intraspecific competition (continued) – As population densities increase and competition becomes more intense, some animals react by emigrating – Large numbers leave their homes to colonize new areas; many die in the quest –For example, locusts emigrate, consuming vegetation in their path Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Emigration Fig. 26-13 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.2 How Is Population Growth Regulated? Density-independent and density-dependent factors interact to regulate population size – The size of a population at any given time is the result of complex interactions between densityindependent and density-dependent forms of environmental resistance –For example, a caribou weakened by hunger (density-dependent) and attacked by parasites (density-dependent) is more likely to be killed by an exceptionally cold winter (densityindependent) Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.3 How Are Populations Distributed in Space and Time? Populations exhibit different spatial distributions – There are three major types of spatial distributions –Clumped –Uniform –Random Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.3 How Are Populations Distributed in Space and Time? Populations who live in groups exhibit clumped distribution – Examples include elephant herds, wolf packs, prides of lions, flocks of birds, and schools of fish – Advantages of clumped distributions include: – Many eyes that can search for localized food sources – Movement of the group (e.g., schools of fish or flocks of birds) can confuse predators by their sheer numbers – Predators, in turn, may hunt in groups, cooperating to bring down larger prey Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Clumped Distribution Fig. 26-14a Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.3 How Are Populations Distributed in Space and Time? Organisms with a uniform distribution maintain a relatively constant distance between individuals – This is common among territorial animals defending scarce resources or breeding territories – An example among plants is desert creosote bushes, which are spaced evenly resulting from competition among their root systems for water and nutrients – An advantage of uniform distribution is that it helps ensure adequate resources for each individual Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Uniform Distribution Fig. 26-14b Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.3 How Are Populations Distributed in Space and Time? Organisms with a random distribution are rare, exhibited by individuals that do not form social groups – Random distribution occurs when resources are not scarce enough to require territorial spacing –Examples include trees and other plants in rain forests – There are probably no vertebrate species that maintain a random distribution throughout the year; most interact socially, at least during the breeding season Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Random Distribution Fig. 26-14c Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.3 How Are Populations Distributed in Space and Time? Survivorship in populations follows three basic patterns – Late-loss populations – Constant-loss populations – Early-loss populations Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.3 How Are Populations Distributed in Space and Time? Survivorship in populations follows three basic patterns (continued) – Survivorship describes the pattern of survival in a population – Survivorship tables track groups of organisms born at the same time throughout their life span, and record how many continue to survive in each succeeding year Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.3 How Are Populations Distributed in Space and Time? Survivorship in populations follows three basic patterns (continued) – A survivorship curve for a population can be produced by graphing survivorship table data –The Y-axis logs the number of individuals surviving to a particular age out of an initial population size born at a specific time –The X-axis plots increasing age categories after a specific birth date Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.3 How Are Populations Distributed in Space and Time? Late-loss populations produce convex survivorship curves – These populations have relatively low juvenile death rates; many or most individuals survive to old age – Late-loss curves are seen in many long-lived animals, who produce few offspring that receive substantial parental care during early life –Examples include humans and many large mammals, such as elephants and mountain sheep Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.3 How Are Populations Distributed in Space and Time? Constant-loss populations produce relatively straight lines – In these populations, individuals have an equal chance of dying at any time during their life span –Examples include some bird species, such as gulls and the American robin, as well as organisms that reproduce asexually, such as Hydra and bacteria Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.3 How Are Populations Distributed in Space and Time? Early-loss populations produce concave survivorship curves – These curves are characteristic of organisms that produce large numbers of offspring that receive little or no parental care – Many of these species engage in scramble competition early in life – The death rate is high among the young, but those that reach adulthood have a reasonable chance to survive to old age – Examples include most invertebrates, most plants, and many fish Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Survivorship Tables and Survivorship Curves Age Number of survivors 0 (birth) 100,000 10 99,124 20 98,713 30 97,754 40 96,489 50 93,698 60 87,967 70 76,241 80 54,117 90 22,312 100 2,523 (a) A survivorship table late loss (human) constant loss (American robin) early loss (dandelion) (b) Survivorship curves Fig. 26-15 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? Demographers track changes in human populations – Demography is the study of the changing human population – Demographers measure human populations, track population changes in different countries and regions, and make comparisons between developed and developing countries – Demographic data are used to formulate policies in public health, housing, education, employment, immigration, and environmental protection Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? The human population continues to grow rapidly – In the last few centuries, the human population has grown at nearly an exponential rate following a J-shaped growth curve – Over the last decade, however, the human population has been growing at a relatively constant rate, suggesting that it may not longer be growing exponentially – However, Earth’s human population grows by 75 million people each year – Are we entering the final bend of the S-shaped growth curve? Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Human Population Growth Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Human Population Growth Billions Time to add each billion (years) 1804 1 All of human history 1927 1960 1975 1987 1999 2012 2 3 4 5 6 7* 123 33 14 13 12 13 1999 1987 1975 year Date 2012* 2008 1960 *projected 1927 Bubonic plague Technical advances Agricultural advances 1804 Industrial and medical advances Fig. 26-16 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? A series of advances have increased Earth’s carrying capacity to support people – Humans have manipulated the environment to increase the Earth’s carrying capacity – Several technological advances have greatly influenced the human ability to make resources available –Technical advances –Agricultural advances –Industrial and medical advances Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? Technical advances – Early humans discovered fire, invented tools and weapons, built shelters, and designed protective clothing –Tools and weapons allowed them to hunt more effectively to increase their food supply –Shelter and clothing increased the habitable areas of the globe Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? Agricultural advances – Around 8000 B.C., animals and plants were domesticated, providing a larger and more stable food supply – This resulted in a longer life span and more childbearing years, although disease continued to restrain population growth Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.5 How Is the Human Population Changing? Industrial and medical advances – Beginning in England in the mid-eighteenth century, medical and industrial advances permitted a population explosion –Industrial advances allowed fewer people to produce more food –Medical advances decreased the death rate from infectious disease Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? The demographic transition explains trends in population size – In developed countries, people benefit from a relatively high standard of living, with access to modern technology and medical care, including contraception – Examples include Australia, New Zealand, Japan, and countries in North America and Europe Most of Earth’s people, however, live in developing countries, which lack these advantages – Examples include countries in Central and South America, Africa, and much of Asia Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? The rate of population growth in countries that are now developed has changed over time in predictable stages, in a pattern called demographic transition – Pre-industrial stage: The population was relatively small and stable, with high birth rates and high death rates – Transitional stage: Food production increased and health care improved, which caused death rates to fall; because birth rates remained high, there was an explosive population increase – Industrial stage: Birth rates fell as contraceptives were more available, and as people moved from farms to cities, where children were less important as a source of labor – Post-industrial stage: Populations are relatively stable, with low birth and death rates Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Demographic Transition Pre-industrial Stage birth rate death rate population size Birth and death rates are high Population grows rapidly Transitional Stage Industrial Stage Post-industrial Stage Population stabilizes Population growth slows Birth rate remains high natural rate of population increase Birth rate declines Birth and death rates are low Population remains low Death rate declines Fig. 26-17 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? The demographic transition explains trends in population size (continued) – A population will eventually stabilize if parents have just the number of children to replace themselves, called replacement-level fertility (RLF) –Because not all children survive to maturity, RLF is 2.1 per woman Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? World population growth is unevenly distributed – Many developing countries still have rapidly growing populations, as birth rates vastly exceed death rates –This results from low incomes and the need for many children to raise family income or produce food –In these countries, knowledge of, and access to, contraception are limited Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? World population growth is unevenly distributed (continued) – In spite of the population reduction of some developing countries, zero population growth will not be achieved globally –The U.N. predicts a global human population of over 9 billion, and growing, by the year 2050 –7.9 billion of those people will live in developing countries Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Historical and Projected World Population 2009: 6.8 billion developing countries developed countries Fig. 26-18 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? The current age structure of a population predicts its future growth – Age structure diagrams show the distribution of human populations by age and gender – Age structure can be shown graphically –Age is shown on the vertical axis –The number of individuals in each age group is shown on the horizontal axis, with males and females placed on opposite sides Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? The current age structure of a population predicts its future growth (continued) – All age-structure diagrams peak at the maximum life span, but the shape below the peak reveals if the population follows one of three basic agestructure patterns –A growing population –A stable population –A shrinking population Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? A growing population – If adults of reproductive age (15 to 44 years) are having more children than are needed to replace themselves, the population is above RLF and is expanding –The age-structure diagram will be roughly triangular –An example of a country with a growing population is Mexico Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Age-Structure Diagram Mexico 2009 male (a) Population pyramid for Mexico Biology: Life on Earth, 9e female Fig. 26-19a Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? A stable population – If adults of reproductive age have just the number of children needed to replace themselves, the population is at RLF and is stable –The age-structure diagram will have relatively straight sides –An example of a country with a stable population is Sweden Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Age-Structure Diagram Sweden 2009 male (b) Population pyramid for Sweden Biology: Life on Earth, 9e female Fig. 26-19b Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? A shrinking population – If adults of reproductive age have fewer children than are needed to replace themselves, the population is below RLF and is shrinking –The age-structure diagram will be narrow a the base –An example of a country with a shrinking population is Italy Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Age-Structure Diagram Italy 2009 male (c) Population pyramid for Italy Biology: Life on Earth, 9e female Fig. 26-19c Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? Average-age structure diagrams have been plotted for developed and developing countries for 2009, with predictions for 2050 – These diagrams reveal that even if developing countries were to achieve RLF immediately, their population increases would continue for decades –A large population of children today create a momentum for future growth as they enter their reproductive years Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Age-Structure Diagrams of Developed Countries 2009 male 2050 female 75 60–74 postreproductive (45–79 yr) 45–59 30–44 reproductive (15–44 yr) 15–29 0–14 prereproductive (0–14 yr) (a) Developed countries Fig. 26-20a Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Age-Structure Diagrams of Developing Countries 2009 male 2050 female 75 60–74 45–59 30–44 15–29 0–14 (b) Developing countries Fig. 26-20b Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? Fertility in Europe is below replacement level – A comparison of growth rates for various world regions shows Europe as the only one with an average rate of change in population that is negative –The average fertility rate is 1.5, which is substantially below RLF –Concerns about the availability of future workers and taxpayers have prompted several countries to offer incentives for couples to have children at an earlier age Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Population Change by World Regions World average: 1.2% Developing countries average: 1.5% Africa: 2.4% Latin America/Caribbean: 1.5% Asia (excluding China): 1.5% China: 0.5% Developed countries average: 0.2% N. America: 0.6% Europe: 0.0% Fig. 26-21 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? The U.S. population is growing rapidly – With a population of over 307 million and a growth rate of about 1% per year, the U.S. population is the fastest growing of all developed countries –The U.S. fertility rate is about 2.0—actually below RLF –However, immigration is adding rapidly to the population, because the fertility rate of new immigrants is above RLF Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. U.S. Population Growth 325 300 275 U.S. population (in millions) 250 225 200 175 150 125 100 75 50 25 0 1800 Biology: Life on Earth, 9e 1850 1900 year 1950 2000 Fig. 26-22 Copyright © 2011 Pearson Education Inc. 26.4 How Is the Human Population Changing? Rapid population growth in the U.S. may have serious implications for the environment of the U.S. and the Earth – Americans consume far more resources and produce far more pollution than the global average – The spread of housing, commercial establishments, and energy-extracting enterprises degrades and destroys natural habitats, reducing the carrying capacity for nonhuman life of ecosystems in the United states Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc.