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BIOLOGY Life on Earth WITH PHYSIOLOGY Tenth Edition Audesirk Audesirk Byers 26 Population Growth and Regulation Lecture Presentations by Carol R. Anderson Westwood College, River Oaks Campus © 2014 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 Age? 26.4 How Is the Human Population Changing? © 2014 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 defined geographical area Each population forms an integral part of a larger community, defined as a group of interacting populations © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? 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 © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? Changes in population size result from natural increases and net migration – Population size changes through – Births – Deaths – Net migration © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? Changes in population size result from natural increases and net migration (continued) – The natural increase of a population is the difference between births and deaths – Natural “increase” can be negative (decrease) if deaths exceed births – The net migration of a population is the difference between immigration (migration into the population) and emigration (migration out) © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? Changes in population size result from natural increases and net migration (continued) – A population thus grows when the sum of natural increase and net migration is positive and declines when this sum is negative – A simple equation for the change is – Change in population size natural increase net migration (births deaths) (immigration emigration) © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? Populations grow based on the birth rate, the death rate, and the population size – The size of most natural populations of organisms fluctuates over the course of a year because reproduction tends to be seasonal – Growing populations add individuals in proportion to the population’s size, much like a bank account accumulates compound interest – If conditions are the same, a population will grow at a constant percentage of its size over a given interval © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? Populations grow based on the birth rate, the death rate, and the population size (continued) – The growth rate (r) of a population is the percentage change in the population size per unit time – The population growth rate is the birth rate (b) minus its death rate (d) – r (growth rate) b (birth rate) d (death rate) © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? Populations grow based on the birth rate, the death rate, and the population size (continued) – If the birth rate exceeds the death rate, the population growth rate will be positive and population size will increase – If the death rate exceeds the birth rate, the growth rate will be negative and the population size will decrease © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? Populations grow based on the birth rate, the death rate, and the population size (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) × N (population size) © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? Populations grow based on the birth rate, the death rate, and the population size (continued) – If births exceed deaths, exponential growth occurs – A constant growth rate (r) produces exponential growth – During exponential growth, an ever-larger number is added to the population during each succeeding time period © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? Populations grow based on the birth rate, the death rate, and the population size (continued) – If births exceed deaths, exponential growth occurs – This pattern of growth will occur in any population in which each individual, on average over the course of its life span, produces more than one offspring that survives to reproduce – If the size of an exponentially growing population is graphed against time, a characteristic shape called a Jcurve will be produced © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? Populations grow based on the birth rate, the death rate, and the population size (continued) – If births exceed deaths, exponential growth occurs (continued) – The age at which an organism first reproduces affects the size of the future population © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? Populations grow based on the birth rate, the death rate, and the population size (continued) – If births exceed deaths, exponential growth occurs (continued) – 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 age 6 years © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? Populations grow based on the birth rate, the death rate, and the population size (continued) – If births exceed deaths, exponential growth occurs (continued) – 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 © 2014 Pearson Education, Inc. Figure 26-1 Exponential growth curves are J-shaped 2,600 reproduce at 4 years (pop. 1) reproduce at 6 years (pop. 2) 2,400 2,200 At 24 years, this population has 2,504 eagles 2,000 number of eagles Number Number of of Time eagles eagles (years) (pop. 1) (pop. 2) 0 2 2 6 8 4 1,400 12 52 18 1,200 18 362 86 1,000 24 2,504 392 800 30 17,314 1,764 1,800 1,600 600 At 24 years, this population has 392 eagles 400 200 0 5 © 2014 Pearson Education, Inc. 10 15 20 time (years) 25 30 26.1 How Does Population Size Change? Populations grow based on the birth rate, the death rate, and the population size (continued) – If births exceed deaths, exponential growth occurs (continued) – 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 on the magnitude of the death rate © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? Populations grow based on the birth rate, the death rate, and the population size (continued) – If births exceed deaths, exponential growth occurs (continued) – The death rate has a major impact on population size – As long as births exceed deaths, the population eventually becomes enormous – As the death rate increases, it takes longer to reach any given population size © 2014 Pearson Education, Inc. Figure 26-2 The effect of death rates on population growth 2,500 no deaths 10% death rate 25% death rate number of bacteria 2,000 It takes about 4 hours to produce 1,500 bacteria It takes about 5.5 hours to produce 1,500 bacteria 1,500 It takes about 3.5 hours to produce 1,500 bacteria 1,000 500 0 1 © 2014 Pearson Education, Inc. 2 3 4 time (hours) 5 6 26.1 How Does Population Size Change? Biotic potential determines the maximum rate at which a population can grow – The ability to produce many offspring is an inherited attribute – Natural selection favors organisms whose attributes adapt them to their environments and who pass these adaptations on to as many healthy offspring as possible – Biotic potential refers to the maximum rate at which a particular population could increase © 2014 Pearson Education, Inc. 26.1 How Does Population Size Change? Biotic potential determines the maximum rate at which a population can grow (continued) – Several factors influence biotic potential – The age at which the organism first reproduces – The frequency of reproduction – The average number of offspring produced each time – The length of the organism’s reproductive life span – The death rate of individuals © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? In 1859, Charles Darwin wrote: “There is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the Earth would soon be covered by the progeny of a pair.” © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Population size results from the interaction between biotic potential and environmental resistance, or all the curbs on population growth imposed by the living and nonliving environment – Examples include interactions among organisms such as predation and competition for limited resources – Environmental resistance also includes natural events such as freezing weather, storms, fires, floods, and droughts © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Exponential growth only occurs under unusual conditions – Under unusual and temporary circumstances, natural populations exhibit exponential growth, producing Jshaped growth curves © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Exponential growth only occurs under unusual conditions (continued) – Exponential growth occurs in populations with boomand-bust cycles – 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 © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Exponential growth only occurs under unusual conditions (continued) – Exponential growth occurs in populations with boomand-bust cycles (continued) – Seasonal populations are linked to changes in rainfall, temperature, or nutrient availability – Ideal conditions encourage rapid growth; deteriorating conditions encourage massive die-off © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Exponential growth only occurs under unusual conditions (continued) – Exponential growth occurs in populations with boomand-bust cycles (continued) – 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 © 2014 Pearson Education, Inc. population density Figure 26-3a A boom-and-bust cycle in photosynthetic bacteria Nutrients are depleted, and water temperature falls Favorable growth “boom” conditions occur Jan Mar May Jul month “bust” Sep Nov A boom-and-bust cycle in photosynthetic bacteria © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Exponential growth only occurs under unusual conditions (continued) – Exponential growth occurs in populations with boomand-bust cycles (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 © 2014 Pearson Education, Inc. number per 100 trap nights Figure 26-3b Boom-and-bust cycles in a lemming population in the Canadian Arctic 14 12 10 8 6 4 2 0 1985 1990 1995 2000 year Boom-and-bust cycles in a lemming population in the Canadian Arctic. © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Exponential growth only occurs under unusual conditions (continued) – Exponential growth occurs temporarily 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 – An increase of food supply or habitat – A reduction in predation © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Exponential growth only occurs under unusual conditions (continued) – Exponential growth occurs temporarily when environmental resistance is reduced (continued) – For example, the whooping crane population has grown exponentially since they were first protected from hunting and human disturbance in 1940 – The whooping crane remains among the world’s rarest birds, so continued population growth will be necessary for its survival © 2014 Pearson Education, Inc. number of cranes Figure 26-4 Exponential growth of wild whooping cranes © 2014 Pearson Education, Inc. 450 425 400 375 350 325 300 275 250 225 200 175 150 125 100 75 50 25 0 1940 1950 1960 1970 1980 1990 2000 2010 year 26.2 How Is Population Growth Regulated? Exponential growth only occurs under unusual conditions (continued) – Exponential growth occurs temporarily when environmental resistance is reduced (continued) – Exponential growth can occur when individuals invade a new habitat with little competition – 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 © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Exponential growth only occurs under unusual conditions (continued) – Exponential growth occurs temporarily when environmental resistance is reduced (continued) – When they are introduced into a new ecosystem, population numbers may explode due to a lack of natural predators © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth – Many populations that exhibit exponential growth eventually stabilize to match the resources available to support them – As resources become depleted, reproduction slows and the growth rate eventually drops to zero, causing the population size to remain constant © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Logistic growth occurs when new populations stabilize as a result of environmental resistance – 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) © 2014 Pearson Education, Inc. Figure 26-5a An S-shaped growth curve stabilizes at carrying capacity number of individuals carrying capacity Growth rate slows Growth stops and the population stabilizes close to the carrying capacity Population grows rapidly 0 time An S-shaped growth curve stabilizes at carrying capacity © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Logistic growth occurs when new populations stabilize as a result of environmental resistance (continued) – When logistic growth is plotted, it results in an Sshaped growth curve, or S-curve – In nature, an increase in population size (N) above carrying capacity (K) can be sustained for a short time © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Animation: Population Growth and Regulation 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Logistic growth occurs when new populations stabilize as a result of environmental resistance (continued) – If a population far exceeds the carrying capacity of its environment, excess demands placed on the ecosystem are likely to destroy crucial resources © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Logistic growth occurs when new populations stabilize as a result of environmental resistance (continued) – This can permanently and severely reduce carrying capacity, causing the population to decline to a fraction of its former size or disappear entirely © 2014 Pearson Education, Inc. Figure 26-5b Consequences of exceeding carrying capacity The population overshoots its carrying capacity; the environment is damaged carrying capacity (original) Low damage; resources recover, and the population fluctuates carrying capacity (reduced) Extreme damage; the population dies out High damage; the carrying capacity is permanently lowered 0 time Consequences of exceeding carrying capacity © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Logistic growth occurs when new populations stabilize as a result of environmental resistance (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 © 2014 Pearson Education, Inc. Figure 26-6 The effects of exceeding carrying capacity 2,000 number of reindeer 1,600 1,200 exponential growth population crash 800 400 0 1910 © 2014 Pearson Education, Inc. 1920 1930 year *Data not taken for 1943–1946 1940 1950 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Logistic growth occurs when new populations stabilize as a result of environmental resistance (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 © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Logistic growth occurs when new populations stabilize as a result of environmental resistance (continued) – Logistic population growth can occur in nature when a species moves into a new habitat (continued) – As population density increases, however, individuals begin to compete for space, energy, and nutrients © 2014 Pearson Education, Inc. number of barnacles (per cm2) Figure 26-7 A logistic curve in nature © 2014 Pearson Education, Inc. 80 60 40 20 0 1 4 2 3 time (weeks) 5 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Logistic growth occurs when new populations stabilize as a result of environmental resistance (continued) – Two forms of environmental resistance usually maintain populations at or below the carrying capacity of their environment – Density-independent – Density-dependent © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Logistic growth occurs when new populations stabilize as a result of environmental resistance (continued) – Density-independent factors limit population size regardless of the population density – Density-dependent factors increase in effectiveness as the population density increases © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-independent factors limit populations regardless of their density – The most important natural density-independent factors are climate and weather, which are responsible for most boom-and-bust population cycles – Many insects and annual plant populations are limited in size by the number of individuals that can be produced before the first hard freeze © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-independent factors limit populations regardless of their density (continued) – The most important natural density-independent factors are climate and weather, which are responsible for most boom-and-bust population cycles (continued) – Hurricanes, droughts, floods, and fire can have profound effects on local population, regardless of density © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-independent factors limit populations regardless of their density (continued) – 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 © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-independent factors limit populations regardless of their density (continued) – Habitat destruction by humans, a density-independent factor, is the single greatest threat to wildlife worldwide – The ivory-billed woodpecker has been driven to extinction in the United States © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Populations of organisms with a life span of more than a year have evolved adaptations that allow them to survive density-independent controls imposed by seasonal changes, such as cold and lack of food during the winter © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Many mammals develop thick coats and store fat for the winter – Some mammals hibernate – Migration is another coping mechanism – Many birds migrate long distances to find food and a hospitable climate © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Most trees and bushes survive the rigors of winter by entering a period of dormancy – For long-lived species in undisturbed habitats, the most important elements of environmental resistance are density dependent © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Density-dependent factors exert a negative feedback effect on population size, because they become increasingly effective as the population density increases © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Predators exert density-dependent controls on populations – Predators are organisms that eat other organisms, called their prey – Prey are killed directly and eaten, but not always © 2014 Pearson Education, Inc. Figure 26-8a Predators often kill weakened prey Predators often kill weakened prey © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Predators exert density-dependent controls on populations (continued) – Predation becomes important as prey populations grow because predators eat a variety of prey, depending on what is most abundant and easiest to find © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Predators exert density-dependent controls on populations (continued) – Predator populations often grow as their prey becomes more abundant, which makes them even more effective as control agents © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Predators exert density-dependent controls on populations (continued) – 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 © 2014 Pearson Education, Inc. Figure 26-8b Predator populations often increase when prey are abundant Predator populations often increase when prey are abundant © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Predators exert density-dependent controls on populations (continued) – Some predator-prey population cycles are out-of-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 © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Predators exert density-dependent controls on populations (continued) – Predation may maintain healthy prey populations near a density that can be sustained by the resources of the ecosystem © 2014 Pearson Education, Inc. Figure 26-9 Experimental predator–prey cycles adult population 1,600 1,200 bean weevils (prey) braconid wasp (predator) A high predator population reduces the prey population The prey population peaks when the predator population is low 800 400 0 5 © 2014 Pearson Education, Inc. 10 15 generation 20 25 30 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Parasites spread more rapidly among dense populations – A parasite feeds on a larger organism, its host, harming it – Parasites include tapeworms that live in the intestines of mammals, ticks that cling to the host’s skin, and diseasecausing microorganisms © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Parasites spread more rapidly among dense populations (continued) – Parasites influence population size by weakening their hosts and making them more susceptible to death from other causes, such as harsh weather or predators – Organisms weakened by parasites are less likely to reproduce © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Parasites spread more rapidly among dense populations (continued) – Parasites, like predators, more often contribute to the death of less-fit individuals, producing a balance in which the host population is regulated but not eliminated © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Competition for resources helps control populations – Competition is the interaction among individuals who attempt to use the same limited resource, which limits population size in a density-dependent manner © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Competition for resources helps control populations (continued) – There are two major forms of competition – Interspecific competition, between individuals of different species – Intraspecific competition, between individuals of the same species © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Competition for resources helps control populations (continued) – 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 © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Competition for resources helps control populations (continued) – 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 © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Competition for resources helps control populations (continued) – For example, gypsy moth females each lay a mass of up to 1,000 eggs on tree trunks in eastern North America © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Competition for resources helps control populations (continued) – 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 © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Competition for resources helps control populations (continued) – Competition for food may be so great that most of the caterpillars die before they can metamorphose into egglaying moths © 2014 Pearson Education, Inc. Figure 26-10 Scramble competition Gypsy moths laying eggs © 2014 Pearson Education, Inc. Gypsy moths caterpillars 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Competition for resources helps control populations (continued) – Many animals have evolved contest competitions, where social or chemical interactions determine access to important resources © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Competition for resources helps control populations (continued) – 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 that supply adequate food and shelter © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-dependent factors become more effective as population density increases (continued) – Competition for resources helps control populations (continued) – As population densities increase and competition becomes more intense, some animals react by emigrating © 2014 Pearson Education, Inc. Figure 26-11 Emigration © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – 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 density-independent and density-dependent forms of environmental resistance © 2014 Pearson Education, Inc. 26.2 How Is Population Growth Regulated? Environmental resistance limits population growth (continued) – Density-independent and density-dependent factors interact to regulate population size (continued) – For example, a caribou weakened by hunger (densitydependent) and attacked by parasites (densitydependent) is more likely to be killed by an exceptionally cold winter (density-independent) © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations of different types of organisms show characteristic spacing of their members, determined by their behavioral characteristics and their environments Each population exhibits patterns of reproduction and survival that are characteristic of its species © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations exhibit different spatial distributions – Spatial distribution describes how individuals within a population are distributed within a given area – Spatial distribution may vary with time, changing with the breeding seasons – Ecologists recognize three major types of spatial distribution: – Clumped – Uniform – Random © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations exhibit different spatial distributions (continued) – Populations whose members live in groups exhibit clumped distribution – Examples include elephant herds, wolf packs, prides of lions, flocks of birds, and schools of fish © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations exhibit different spatial distributions (continued) – 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 © 2014 Pearson Education, Inc. Figure 26-12a Clumped distribution clumped Clumped distribution © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations exhibit different spatial distributions (continued) – 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 © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations exhibit different spatial distributions (continued) – Territorial behavior is more common among animals during their breeding seasons – Seabirds may space their nests evenly along the shore, just out of reach of one another – Mature desert creosote bushes are often spaced very evenly – This spacing comes from competition among their root systems, which occupy a circular area around each plant © 2014 Pearson Education, Inc. Figure 26-12b Uniform distribution uniform Uniform distribution © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations exhibit different spatial distributions (continued) – Organisms with a random distribution are relatively rare – Such individuals do not form social groups – The resources needed are more or less equally available throughout the area they inhabit – Resources are not scarce enough to require territorial spacing – Examples include trees and other plants in rain forests © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations exhibit different spatial distributions (continued) – There are probably no vertebrate species that maintain a random distribution throughout the year – Most interact socially, at least during the breeding season © 2014 Pearson Education, Inc. Figure 26-12c Random distribution random Random distribution © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations exhibit different age distributions – Animals of different species differ considerably in their chances of dying at any given phase of their life cycle – Some species produce many offspring that are provided with very few resources; most die before they can reproduce – Others produce few offspring, which are each given far more resources and often survive to reproduce © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations exhibit different age distributions (continued) – Three types of survivorship curves are described according to the part of the life cycle during which most deaths occur – Late-loss populations – Constant-loss populations – Early-loss populations © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations exhibit different age distributions (continued) – To determine the pattern of survivorship, researchers construct survivorship tables, which track groups of organisms (born at the same time) throughout their lives, recording how many survive in each succeeding year – If these numbers are graphed, they reveal the survivorship curves characteristic of the species in the environment where the data were collected © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations exhibit different age distributions (continued) – 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 characteristic of humans and other large and long-lived animals such as elephants and mountain sheep – Relatively few offspring are produced by these species © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations exhibit different age distributions (continued) – Constant-loss populations produce straight-line survivorship curves – In these populations, individuals have an equal chance of dying at any time during their life span – This pattern is seen in some birds such as gulls and the American robin, in some species of turtles, and in laboratory populations of organisms that reproduce asexually, such as hydra and bacteria © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations exhibit different age distributions (continued) – 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 © 2014 Pearson Education, Inc. 26.3 How Are Populations Distributed in Space and Age? Populations exhibit different age distributions (continued) – Early-loss populations produce concave survivorship curves (continued) – The death rate is high among the young, but those that reach adulthood have a reasonable chance to survive to old age – Most invertebrates, many fish and amphibians, and most plants exhibit early loss survivorship curves © 2014 Pearson Education, Inc. Figure 26-13 Survivorship tables and survivorship curves 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 survivorship table © 2014 Pearson Education, Inc. number of survivors 1,000 Age 100 late loss (human) 10 constant loss (American robin) early loss (dandelion) 0 percent of maximum life span Survivorship curves 26.4 How Is the Human Population Changing? No force on Earth rivals that exerted by humans – Humans possess enormous brainpower – We possess dexterous hands that can shape the environment by our demands Natural selection favored those with the ability and the drive to bear and nurture offspring, which helped make sure that few would survive – This characteristic now threatens us and the biosphere on which we depend © 2014 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 Jshaped growth curve – Over the last decade, however, the human population has been growing at a relatively constant rate, suggesting that it may no longer be growing exponentially © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? The human population continues to grow rapidly (continued) – Are humans starting to enter the final bend of the Sshaped logistic growth curve that will eventually lead to a stable population? – Despite the fact that our annual growth rate has declined from 1.8% in 1960 to 1.2% in 2011, Earth’s human population is adding people faster than ever – Having reached 7 billion in 2011, our numbers now grow by about 83 million each year © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Animation: Human Population Growth Figure 26-14 Human population growth 1927 1960 1975 1987 1999 2011 2025 1 All of human history 2 3 4 5 6 7 8* 123 33 14 13 12 12 14 6 1999 5 1987 4 3 *projected 1975 year 1804 2011 billions of people Date Billions Time to add each billion (years) 7 1960 2 1927 1 1804 bubonic plague 12,000 11,000 10,000 9,000 B.C. B.C. B.C. B.C. Technical advances © 2014 Pearson Education, Inc. 8000 7000 6000 5000 4000 3000 2000 1000 B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. Agricultural advances B.C./A.D. 1000 0 2000 A.D. A.D. Industrial and medical advances 26.4 How Is the Human Population Changing? A series of advances has increased Earth’s carrying capacity to support people – Human population growth has been spurred by a series of advances, each of which circumvented some type of environmental resistance, increasing Earth’s carrying capacity for people © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? A series of advances has increased Earth’s carrying capacity to support people (continued) – Early humans – Discovered fire – Invented tools and weapons – Built shelters – Designed protective clothing © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? A series of advances has increased Earth’s carrying capacity to support people (continued) – A series of technical advances increased carrying capacity – Tools and weapons allowed humans to hunt more effectively and obtain additional high-quality food, while shelter and clothing expanded the habitable areas of the globe © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? A series of advances has increased Earth’s carrying capacity to support people (continued) – Domesticated crops and animals had supplanted hunting and gathering in many parts of the world by 8000 B.C. – These agricultural advances provided a larger and more stable food supply for people, further increasing Earth’s carrying capacity for humans © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? A series of advances has increased Earth’s carrying capacity to support people (continued) – Human population growth continued slowly for thousands of years until major industrial and medical advances permitted a population explosion – These advances began in England in the mideighteenth century – Medical progress dramatically decreased the death rate by reducing environmental resistance caused by disease © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? A series of advances has increased Earth’s carrying capacity to support people (continued) – The discovery of bacteria and their role in infection resulted in better control of bacterial diseases through improved sanitation and antibiotics – Vaccines for diseases such as smallpox reduced deaths from viral infections © 2014 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 readily available contraception – Developed countries include Australia, New Zealand, Japan, and countries in North America and Europe © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? The demographic transition explains trends in population size (continued) – Average income in developed countries is relatively high – Education and employment opportunities are available to both sexes – Death rates from infectious diseases are low – Less than 20% of the world’s population lives in developed countries © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? The demographic transition explains trends in population size (continued) – In the developing countries of Central and South America, Africa, and much of Asia—home to more than 80% of humanity—the average person lacks these advantages – The historical rate of population growth in developed countries has changed over time in reasonably predictable stages, producing a pattern called demographic transition © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? The demographic transition explains trends in population size (continued) – 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 © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? The demographic transition explains trends in population size (continued) – 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 © 2014 Pearson Education, Inc. Figure 26-15 The demographic transition increase in rates or size Pre-industrial Stage birth rate death rate population size Birth and death rates are high Population grows rapidly Transitional Stage Industrial Stage Population growth slows Birth rate remains high Population stabilizes natural rate of population increase Birth rate declines Birth and death rates are low Population remains low Death rate declines time © 2014 Pearson Education, Inc. Post-industrial Stage 26.4 How Is the Human Population Changing? The demographic transition explains trends in population size (continued) – A population’s fertility rate reflects the average number of children that each woman bears – If immigration and emigration rates are balanced, a population will eventually stabilize if parents have just the number of children to replace themselves – This is called replacement-level fertility (RLF) – RLF is 2.1 children per woman because not all children survive to maturity © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? World population growth is unevenly distributed – In developing countries, medical advances have decreased death rates and increased life span, but birth rates remain relatively high – Although China is a developing country, its population has approached one billion © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? World population growth is unevenly distributed (continued) – Most other developing countries are within the latetransitional or the industrial stage of the demographic transition – Adult children provide financial security for aging parents – Young children may also contribute significantly to the family income by working on farms or factories © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? World population growth is unevenly distributed (continued) – Most other developing countries are within the latetransitional or the industrial stage of the demographic transition (continued) – Social factors drive population growth in countries where children confer prestige because religious beliefs promote large families – Many individuals who would like to limit their family size lack access to contraceptives © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? World population growth is unevenly distributed (continued) – A lack of education and a lack of access to contraceptives then contributes to continued high birth rates – Of the 7 billion people on Earth in 2011, about 5.8 billion resided in developing countries – The prospect for world population stabilization in the near future is nonexistent © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? The age structure of a population predicts its future growth – Age structure diagrams show age groups on the vertical axis and the numbers (or percentages) of individuals in each age group on the horizontal axis, with males and females shown on opposite sides – Age structure diagrams all rise to a peak that reflects the maximum human life span – The shape of the rest of the diagram reveals whether the population is expanding, stable, or shrinking © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? The age structure of a population predicts its future growth (continued) – If adults of reproductive age (15 to 44 years) are having more children (the 0- to 14-year age group) than are needed to replace themselves, the population is above RLF and is expanding – The age-structure diagram will be roughly triangular © 2014 Pearson Education, Inc. Figure 26-16a Africa: A rapidly growing population 100 90 Africa 2010 female male 80 age 70 60 50 40 30 20 10 0 6 4 2 4 2 0 percent of population Africa: A rapidly growing population © 2014 Pearson Education, Inc. 6 26.4 How Is the Human Population Changing? The age structure of a population predicts its future growth (continued) – If adults of reproductive age have just the number of children needed to replace themselves, the population is at RLF – A population that has been at RLF for many years will have an age structure diagram with relatively straight sides © 2014 Pearson Education, Inc. Figure 26-16b North America: A slowly growing population 100 90 North America 2010 female male 80 age 70 60 50 40 30 20 10 0 6 4 2 4 2 0 percent of population 6 North America: A slowly growing population © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? The age structure of a population predicts its future growth (continued) – In a shrinking population, the reproducing adults have fewer children than are required to replace themselves – The age-structure diagram will be narrow at the base – The median age depends on the age structure – The lower the median age, the more rapidly the population will expand © 2014 Pearson Education, Inc. Figure 26-16c Europe: A slowly declining population 100 90 Europe 2010 female male 80 age 70 60 50 40 30 20 10 0 6 4 2 4 2 0 percent of population Europe: A slowly declining population © 2014 Pearson Education, Inc. 6 26.4 How Is the Human Population Changing? The age structure of a population predicts its future growth (continued) – Average-age structure diagrams have been plotted for developed and developing countries for 2012, with predictions for 2050 © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? The age structure of a population predicts its future growth (continued) – 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 creates a momentum for future growth as they enter their reproductive years © 2014 Pearson Education, Inc. Figure 26-17a Developed countries 2010 2050 100 90 female male 80 postreproductive (45100 years) 70 age 60 50 40 reproductive (1544 years) 30 20 prereproductive (014 years) 10 0 300 275 250 225 200 175 150 125 100 75 50 25 0 25 50 75 100 125 150 175 200 225 250 275 300 millions of people Developed countries © 2014 Pearson Education, Inc. Figure 26-17b Developing countries 2010 2050 100 90 male female 80 70 age 60 50 40 30 20 10 0 300 275 250 225 200 175 150 125 100 75 50 25 0 25 50 75 100 125 150 175 200 225 250 275 300 millions of people Developing countries © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? The age structure of a population predicts its future growth (continued) – The United Nations has developed high, medium, and low projections for future growth based on assumptions about fertility rates – For the year 2050, the medium projection is that Earth’s population will have increased by about 33% to over 9.3 billion – Eight billion people live in developing nations © 2014 Pearson Education, Inc. world population (billions) Figure 26-18 United Nations world population projections 12 11 high 10.6 10 9.3 9 medium 8 low 8.1 7 6 2000 © 2014 Pearson Education, Inc. 2010 2020 2030 2040 2050 26.4 How Is the Human Population Changing? Fertility in some nations 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 1.6, 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 © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? Fertility in some nations is below replacement level (continued) – Japan’s government is concerned about the country’s low fertility rate (1.4) and provides subsidies to encourage larger families – Japan is about the size of the state of Montana in the United States and home to 128 million people (equivalent to 41% of the entire U.S. population) © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? Fertility in some nations is below replacement level (continued) – The current economic structures in countries throughout the world are based on growing populations – The difficult adjustments necessary as populations decline—or even merely stabilize—motivate governments to adopt policies that encourage more childbearing and continued growth © 2014 Pearson Education, Inc. Table 26-1 © 2014 Pearson Education, Inc. 26.4 How Is the Human Population Changing? The U.S. population is growing rapidly – The United States has a population of more than 313 million and a growth rate of about 0.7% (adding one person every 15 seconds) and is the fastest-growing developed country in the world – Continued immigration, which accounts for about 30% of the population increase, will ensure growth for the indefinite future © 2014 Pearson Education, Inc. Figure 26-19 United States population growth 325 300 275 U.S. population (in millions) 250 225 200 175 150 125 100 75 50 25 0 © 2014 Pearson Education, Inc. 1800 1850 1900 year 1950 2000 26.4 How Is the Human Population Changing? The U.S. population is growing rapidly (continued) – The rapid growth of the U.S. population has major environmental implications for local ecosystems and for the planet – The average U.S. resident uses nearly four times as much energy as the average person worldwide © 2014 Pearson Education, Inc.