Population Growth and Regulation

Population Growth and Regulation
26 Population Growth and Regulation 1

How Does Population Size Change?
Population  community  ecosystem  biosphere Ecology is the study of the interrelationships of organisms with each other and with the nonliving environment A population consists of all the members of a particular species that live within an ecosystem, a defined geographical area (nonliving and living) Each population forms an integral part of a larger community, defined as a group of interacting populations The biosphere is the enormous ecosystem that encompasses all of Earth’s habitable surface

Changes in population size result from natural increases and net migration Population size changes through Births Deaths Net migration

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) Immigration = migration into a population Emigration = migration out (young animals leaving the population) For simplicity, we will just focus on natural increase not migration.

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

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) 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

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 If the size of an exponentially growing population is graphed against time, a characteristic shape called a J-curve will be produced The age at which an organism first reproduces affects the size of the future population 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

Figure 26-1 Exponential growth curves are J-shaped
2,600 reproduce at 4 years (pop. 1) Number of eagles (pop. 1) Number of eagles (pop. 2) 2,400 reproduce at 6 years (pop. 2) Time (years) 2,200 At 24 years, this population has 2,504 eagles 2,000 2 2 1,800 6 8 4 1,600 12 52 18 1,400 number of eagles 1,200 18 362 86 1,000 24 2,504 392 800 30 17,314 1,764 600 At 24 years, this population has 392 eagles We assume that golden eagles live to be 30 and each pair produces two offspring annually after sexual maturity. Two populations – each founded by a single breeding pair of eagles over their lifespan. 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 (red) Individuals in the other population begin reproducing at age 6 years (blue) 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 400 200 5 10 15 20 25 30 time (years) 8

Populations grow based on the birth rate, the death rate, and the population size (continued) If births exceed deaths, exponential growth occurs 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 2,500 2,000 1,500 1,000 500 1 2 3 4 5 6 time (hours) number of bacteria no deaths 10% death rate 25% death rate It takes about 4 hours to produce 1,500 bacteria It takes about 3.5 hours to produce 1,500 bacteria It takes about 5.5 hours to produce 1,500 bacteria

Biotic potential determines the maximum rate at which a population could increase 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 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

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: interactions among organisms such as predation and competition for limited resources natural events such as freezing weather, storms, fires, floods, and droughts

Exponential growth only occurs under unusual conditions Under unusual and temporary circumstances, natural populations exhibit exponential growth, producing J-shaped growth curves

Exponential growth only occurs under unusual conditions (continued) Exponential growth occurs in populations with boom-and-bust cycles periods of rapid population growth are followed by a sudden, massive die-off Seasonal populations are linked to changes in rainfall, temperature, or nutrient availability Ideal conditions encourage rapid growth; deteriorating conditions encourage massive die-off Short-lived, rapidly reproducing species (insects) have seasonal cycles.

Nutrients are depleted, and water temperature falls
Favorable growth conditions occur “boom” “bust” Jan Mar May month A boom-and-bust cycle in photosynthetic bacteria Jul Sep Nov population density 14 12 10 8 6 4 2 1985 number per 100 trap nights 1990 1995 2000 year Boom-and-bust cycles in a lemming population in the Canadian Arctic. 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 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 14

Exponential growth only occurs under unusual conditions (continued) Exponential growth occurs temporarily when environmental resistance is reduced Populations that do not experience boom-and-bust cycles Special circumstances: A reduction in predation An increase of food supply or habitat 450 425 400 375 350 325 300 275 250 225 200 175 150 125 100 75 50 25 1940 1950 1960 1970 1980 1990 2000 2010 year number of cranes For example, the whooping crane population has grown exponentially since they were first protected from hunting and human disturbance in (15 birds remained) The whooping crane remains among the world’s rarest birds, so continued population growth will be necessary for its survival

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 Introduced accidentally or deliberately to an ecosystem. When they are introduced into a new ecosystem, population numbers may explode due to a lack of natural predators

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

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)

Figure 26-5a An S-shaped growth curve stabilizes at carrying capacity
Growth rate slows Growth stops and the population stabilizes close to the carrying capacity Population grows rapidly number of individuals When logistic growth is plotted, it results in an S-shaped growth curve, or S-curve time An S-shaped growth curve stabilizes at carrying capacity 19

Environmental resistance limits population growth (continued) Logistic growth (continued) In nature, an increase in population size (N) above carrying capacity (K) can be sustained for a short time 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

Logistic growth (continued) Occurs 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 80 60 40 20 3 4 5 time (weeks) number of barnacles (per cm2) 1 2

Environmental resistance limits population growth (continued) Two forms of environmental resistance usually maintain populations at or below the carrying capacity of their environment Density-independent factors limit population size regardless of the population density Density-dependent factors increase in effectiveness as the population density increases

Two forms of environmental resistance 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 Hurricanes, droughts, floods, and fire Insects - are limited in size by the number of individuals that can be produced before the first hard freeze Fire - can have profound effects on local population, regardless of density

Two forms of environmental resistance Density-independent factors (continued) Human activities can also limit the growth of natural populations Pesticides and pollutants can cause drastic declines Overhunting has driven some species to extinction Habitat destruction by humans is the single greatest threat to wildlife worldwide

Two forms of environmental resistance Density-dependent factors (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 Many mammals develop thick coats and store fat for the winter Some mammals hibernate Most trees and bushes survive the rigors of winter by entering a period of dormancy Migration is another coping mechanism Many birds migrate long distances to find food and a hospitable climate

Two forms of environmental resistance Density-dependent factors (continued) Density-dependent factors exert a negative feedback effect on population size, because they become increasingly effective as the population density increases Predators exert density-dependent controls on populations Prey are killed directly and eaten, but not always Not always eaten: deer eat some buds on a maple tree; gypsy moth larvae feed on the leaves of oaks – the trees are harmed but not killed.

Two forms of environmental resistance Density-dependent factors (continued) Predators exert density-dependent controls Predation becomes important as prey populations grow because predators eat a variety of prey, depending on what is most abundant and easiest to find Predator populations often grow as their prey becomes more abundant, which makes them even more effective as control agents Predator populations often increase when prey are abundant Coyotes eat more field mice when the mouse population is high then switch to ground squirrels as the mouse population declines. 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

Two forms of environmental resistance Density-dependent factors (continued) Some predator-prey population cycles are out-of-phase 1,600 1,200 800 400 5 10 15 20 25 30 generation adult population bean weevils (prey) braconid wasp (predator) A high predator population reduces the prey population The prey population peaks when the predator population is low Predation may maintain healthy prey populations near a density that can be sustained by the resources of the ecosystem weeding out poorly adapted individuals and weak or old animals.

Two forms of environmental resistance Density-dependent factors (continued) Parasites spread more rapidly among dense populations Cannot travel long distances 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 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 disease-causing microorganisms 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

Two forms of environmental resistance Density-dependent factors (continued) Competition for resources helps control populations Competition is the interaction among individuals who attempt to use the same limited resource 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

Two forms of environmental resistance 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 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 (density-independent)

Review Describe exponential growth.
When would exponential growth occur? Describe a logistic growth curve. What are the two major forms of environmental resistance?

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 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

Populations exhibit different spatial distributions 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 Clumped distribution clumped Temporary clusters for mating and parenting. Localized resources.

Populations exhibit different spatial distributions 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

Populations exhibit different spatial distributions Uniform distribution maintain a relatively constant distance between individuals This is common among territorial animals defending scarce resources or breeding territories uniform Uniform distribution 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

Populations exhibit different spatial distributions Random distribution is 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 random Random distribution There are probably no vertebrate species that maintain a random distribution throughout the year Most interact socially, at least during the breeding season

Figure 26-13 Survivorship tables and survivorship curves
1,000 Age Number of survivors 0 (birth) 100,000 10 99,124 100 20 98,713 late loss (human) 30 97,754 40 96,489 number of survivors constant loss (American robin) 50 93,698 10 60 87,967 70 76,241 early loss (dandelion) 80 54,117 Late loss – low infant mortality, most survive to old age. Produce few offspring Constant loss – equal chance of dying any time during life Early loss – produce large numbers of offspring with little parental care  high infant mortality, few survive to old age 90 22,312 100 2,523 percent of maximum life span A survivorship table Survivorship curves 38

How Is the Human Population Changing?
The human population continues to grow rapidly Date Billions Time to add each billion (years) 1804 1927 1960 1975 1987 1999 2011 2025 *projected 1 2 3 4 5 6 7 8* All of human history 123 33 13 12 14 12,000 11,000 10,000 9,000 8000 7000 6000 5000 4000 3000 2000 1000 bubonic plague billions of people year Technical advances Agricultural advances B.C. B.C./A.D. A.D. Industrial and medical advances 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 no longer be growing exponentially 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

A series of advances has increased Earth’s carrying capacity to support people Early humans Discovered fire Invented tools and weapons Built shelters Designed protective clothing – small stable populations Agricultural advances – longer life span but high death rate Medical advances – decreased birth and death rate

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

Figure 26-16a Africa: A rapidly growing population
male 100 90 80 70 60 50 40 30 20 10 6 age 4 2 percent of population female Africa: A rapidly growing population 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 (replacement level fertility) and is expanding The age-structure diagram will be roughly triangular 42

Figure 26-16b North America: A slowly growing population
100 North America 2010 male female female 90 80 70 60 age 50 40 30 20 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 10 6 4 2 2 4 6 percent of population North America: A slowly growing population 43

Figure 26-16c Europe: A slowly declining population
100 male female 90 80 70 60 age 50 40 30 20 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 10 6 4 2 2 4 6 percent of population Europe: A slowly declining population 44

The age structure of a population predicts its future growth Median age – age at which half the population is younger and half is older The median age depends on the age structure The lower the median age, the more rapidly the population will expand In a stable human population, fewer than 20% of individuals are younger than age 15

Figure 26-18 United Nations world population projections
12 11 high 10.6 10 9.3 9 medium world population (billions) 8.1 8 low 7 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 6 2000 2010 2020 2030 2040 2050 46

Review Explain the three basic types of distribution within populations. Describe the three general types of survivorship curves. How does the shape of an age structure diagram predict future changes in population sizes?

Population Growth and Regulation

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