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Chapter 27

Chapter 27. Population Growth. 27.1 How Are Populations Distributed In Space And Time?. Organisms arrange themselves in space in many different ways. Ecologists recognize three major types of spatial distribution: Clumped Uniform Random.

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Chapter 27

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  1. Chapter 27 Population Growth

  2. 27.1 How Are Populations Distributed In Space And Time? • Organisms arrange themselves in space in many different ways. • Ecologists recognize three major types of spatial distribution: • Clumped • Uniform • Random

  3. 27.1 How Are Populations Distributed In Space And Time? • Individuals in many populations clump together in groups, and include social groups such as elephant herds, wolf packs, lion prides, flocks of birds, and schools of fish. clumped (a) Clumped distribution Fig. 27-1a

  4. 27.1 How Are Populations Distributed In Space And Time? • What are the advantages of clumping? • Many eyes can search for localized food. • Part of a large group may reduce the odds that one individual will be killed by a predator. • A group may increase the individual’s chance of finding a mate.

  5. 27.1 How Are Populations Distributed In Space And Time? • Some individuals disperse themselves evenly. • Populations with uniform distributions maintain relatively even spacing between individuals. • This type of distribution occurs among animals that defend territories. • Male Galapagos iguanas establish regularly spaced breeding territories.

  6. 27.1 How Are Populations Distributed In Space And Time? • Creosote bushes release chemicals into the soil around them that inhibit germination of seeds from other plants. uniform (b) Uniform distribution Fig. 27-1b

  7. 27.1 How Are Populations Distributed In Space And Time? • In a few populations, individuals are distributed at random. • In a population with random distribution, the distance between individuals varies unpredictably. • Individuals in such populations do not form social groups. • The resources they need are not in short supply and are available throughout the year.

  8. 27.1 How Are Populations Distributed In Space And Time? • Trees and other plants in rain forests may be randomly distributed. random (c) Random distribution Fig. 27-1c

  9. 27.2 How Do Populations Grow? • Births, deaths, and migration determine population growth. • A population’s size remains stable if, on average, as many individuals join as leave. • Individuals enter a population by birth or immigration, and leave it by death or emigration. • A population grows when the number of births plus immigrants exceeds the number of deaths plus emigrants. • Populations shrink when the opposite occurs.

  10. 27.2 How Do Populations Grow? • A simple equation for the change in population size within a given period is as follows: • (births – deaths) + (immigrants – emigrants) = change in population size

  11. 27.2 How Do Populations Grow? • Population growth can be expressed as a rate. • The per capita growth rate (r) of a population is measure of how fast a population grows, expressed as a change in population size per individual per unit of time (that is, as a percentage). • This value is determined by subtracting the per capita death rate (d) from the per capita birth rate (b). • b (births) – d (deaths) = r (growth rate)

  12. 27.2 How Do Populations Grow? • Population growth can be expressed as a rate (continued). • For example, to calculate the annual growth rate of a human population of 10,000 in which there are 1,500 births and 500 deaths each year, we first calculate the annual per capita birth rate. • b = 1,500 births/10,000 people • = 0.15 births per person per year • Next, the annual per capita death rate. • d = 500 deaths/10,000 people • = 0.05 deaths per person per year

  13. 27.2 How Do Populations Grow? • Population growth can be expressed as a rate (continued). • Finally, we calculate the per capita growth rate by subtracting the death rate from the birth rate. • r = 0.15 births per person per year – 0.05 deaths per person per year • = 0.10 (that is, 10% increase per year) • If the death rate exceeds the birth rate, the growth rate will be negative and the population will shrink.

  14. 27.2 How Do Populations Grow? • A constant growth rate increases population size rapidly. • To calculate the number of individuals added to a population in a year, multiply the annual per capita growth rate (r) by the original population size (N). • Population growth = rN • In our example, population growth in the first year (rN) is 0.10 x 10,000 = 1,000 people; at the end of year 1, the population has 11,000 people.

  15. 27.2 How Do Populations Grow? • A constant growth rate increases population size rapidly (continued). • If the per capita growth rate r remains constant, the number of people added to the population increases each year. • This pattern of continuously accelerating increase in population size is exponential growth.

  16. 27.2 How Do Populations Grow? • A population’s growth rate depends on patterns of reproduction. • A population grows during periods when births exceed deaths. • Growth will persist if, on average, each individual produces more than one surviving offspring during its lifetime. • Each individual, of course, has the potential to replace itself many times during its lifetime.

  17. 27.2 How Do Populations Grow? • A population’s growth rate depends on patterns of reproduction. • Every species has a built-in capacity for population growth, but the speed of this potential growth varies among species, dependent upon the following factors. • The age of first reproduction • The frequency of reproduction • The average number of offspring produced each time • The length of an organism’s reproductive life span • The organism death rate

  18. 27.2 How Do Populations Grow? • Some species produce large numbers of offspring quickly. • The harmless bacterium Staphylococcus is found on the human body. • Each bacterial cell can divide every 20 minutes, doubling the population three times each hour. • The larger the population growth, the more cells there are to divide.

  19. 27.2 How Do Populations Grow? • Some species produce large numbers of offspring quickly (continued). • The growth rate is so great that, unchecked, the offspring of one bacterium could produce a layer around the Earth 7 feet deep in 48 hours. • Because this does not happen, many bacteria must die.

  20. 27.2 How Do Populations Grow? • Other species produce fewer but longer-lived offspring. • The golden eagle is a long-lived, rather slowly reproducing species. • Figure 27-2 compares the potential population growth of eagles with that of bacteria, assuming no deaths. • All three curves on this figure are J-shaped, indicating exponential growth.

  21. 27.2 How Do Populations Grow? • J-shaped exponential growth curves in bacteria time(minutes) number ofbacteria bacteria 1,200 1,100 1,000 900 800 700 600 500 400 300 200 100 0 020406080 100120140160180 200220 12481632641282565121,0242,048 number of individuals Exponentialgrowth curvesare J-shaped 120 180 240 60 0 (a) Bacteria time (minutes) Fig. 27-2a

  22. 27.2 How Do Populations Grow? • Delayed onset of reproduction slows population growth. • Figure 27-2b shows what happens if there is a difference in the age at which individuals in a population begin reproduction. • Growth in both populations is exponential, but the population whose members begin reproduction later takes longer to reach a particular size. • For humans, delayed childbearing slows population growth.

  23. 27.2 How Do Populations Grow? • J-shaped exponential growth curve in eagles numberofeagles (i) numberofeagles (ii) time(years) eagles 2,000 1,800 1,600 1,400 1,200 1,000 800 600 400 200 0 024 6810121416182022242628 30 22481428521001903626301,3142,5044,7709,08817,314 2224 8 1218 325486142 2383926441,0661,764 (i) Reproductionbegins at4 years (ii) number of individuals Reproductionbegins at6 years (b) Eagles 5 10 15 20 25 30 0 time (years) Fig. 27-2b

  24. 27.2 How Do Populations Grow? • Death rate also influences growth rate. • Figure 27-3 compares three bacterial populations with different death rates. • The J shapes of the curves are the same. • However, the time required to reach any given population size is longer at higher death rates.

  25. 27.2 How Do Populations Grow? • The effects of death rates on population growth 2,500 bacteria Nodeaths 10% diebetweendoublings 2,000 25% diebetweendoublings 1,500 number of individuals 1,000 500 0 0 1 5 6 2 3 4 Fig. 27-3 time (hours)

  26. 27.3 How Is Population Growth Regulated? • A population’s growth is influenced by its biotic potential—the maximum rate at which a population can increase, assuming ideal conditions that allow the highest possible birth rate and the lowest possible death rate. • The ultimate size of a population is also affected by limits that oppose this potential for growth. • These limits, which are set by the living and nonliving environments, are collectively known as environmental resistance.

  27. 27.3 How Is Population Growth Regulated? • Environmental resistance is imposed by several things. • Availability of food and space • Interactions with competitors • Predators • Disease-causing organisms • Natural catastrophes (e.g., storms, fires, freezing weather, floods, and droughts)

  28. 27.3 How Is Population Growth Regulated? • Environmental resistance limits population size by the following factors: • Increasing death rates • Decreasing birth rates • Both factors together • For example, a drought might restrict growth of an animal population both by increasing the number of deaths from starvation, and by causing malnutrition that reduces the number of births.

  29. 27.3 How Is Population Growth Regulated? • Rapid growth cannot continue indefinitely. • Populations grow rapidly only under certain circumstances and only for a limited time before environmental resistance brings their expansion to a halt.

  30. 27.3 How Is Population Growth Regulated? • Some populations fluctuate cyclically. • Short-term explosive growth occurs in populations that undergo regular cycles of rapid population growth followed by a sudden, massive die-off—a boom-and-bust cycle. • Many such animals have seasonal population cycles that are linked to predictable changes in rainfall, temperature, or nutrient availability.

  31. 27.3 How Is Population Growth Regulated? • A boom-and-bust population cycle Nutrients are depleted population density Favorable growthconditions occur “boom” “bust” 0 May Nov Jan Jul Mar Sep month Fig. 27-4

  32. 27.3 How Is Population Growth Regulated? • Some populations fluctuate cyclically (continued). • A lemming population may grow until the animals overgraze their fragile arctic tundra ecosystem. • Then, lack of food, increasing populations of predators, and social stress caused by overpopulation all contribute to a suddenly high death rate. • Eventually, the lemming population shrinks so much that number of predators declines.

  33. 27.3 How Is Population Growth Regulated? • Lemming population cycles Fig. 27-5

  34. 27.3 How Is Population Growth Regulated? • Ecosystem changes may allow temporary rapid growth. • A population may grow rapidly if population- controlling factors, such as predators, are eliminated or if the food supply is increased. • Growth can also be explosive when individuals invade a new habitat that has favorable conditions and few competitors. • For example, a species’ population may grow explosively when people introduce the species into an ecosystem.

  35. 27.3 How Is Population Growth Regulated? • Environmental resistance limits population growth. • Populations that grow rapidly must eventually stabilize or crash. • They tend to stabilize at or below their ecosystem’s carrying capacity, the maximum population of a particular species that an ecosystem can support indefinitely.

  36. 27.3 How Is Population Growth Regulated? • Environmental resistance limits population growth (continued). • As a growing population approaches the carrying capacity of the environment, its growth rate gradually declines and finally stops when the population reaches a state of equilibrium. • In this equilibrium, the birth rate is balanced by the death rate, and population size is stable. • This type of population growth is represented graphically by an S-shaped growth curve.

  37. 27.3 How Is Population Growth Regulated? • An S-shaped logistic growth curve stabilizes at carrying capacity. carryingcapacity Growthrate slows Growth stops and thepopulation stabilizes closeto the carrying capacity number of individuals Populationgrows rapidly 0 time (a) An S-shaped growth curve stabilizes at carrying capacity Fig. 27-6a

  38. 27.3 How Is Population Growth Regulated? • Environmental resistance limits population growth (continued). • A population may grow to a size larger than its ecosystem’s carrying capacity. • This overshooting of carrying capacity is necessarily temporary. • A small overshoot of carrying capacity is likely to be followed by a decrease in population size until the resources recover and the original carrying capacity is restored.

  39. 27.3 How Is Population Growth Regulated? • Consequences of exceeding carrying capacity Populationovershootsthe carryingcapacity; theenvironmentis damaged carryingcapacity(original) Low damage; resourcesrecover, and thepopulation fluctuates carryingcapacity(reduced) Extremedamage; thepopulationdies out High damage; thecarrying capacity ispermanently lowered 0 time (b) Consequences of exceeding carrying capacity Fig. 27-6b

  40. 27.3 How Is Population Growth Regulated? • Environmental resistance limits population growth (continued). • The factors that usually maintain populations at or below the carrying capacity of their environment can be classified into two broad categories. • Density-independent factors limit population size regardless of the population density (number of individuals per given area). • Density-dependent factors increase in effectiveness as the population density increases.

  41. 27.3 How Is Population Growth Regulated? • Density-independent factors limit population size. • Natural events—including hurricanes, droughts, floods, and fire—can reduce the size of populations. • The effectiveness of such events in limiting the size of a population does not generally depend on the population’s density.

  42. 27.3 How Is Population Growth Regulated? • Density-independent factors limit population size (continued). • The most important natural density-independent factor is weather. • Human activities, such as the use of pesticides and pollutants, can also limit population growth in ways independent of population density.

  43. 27.3 How Is Population Growth Regulated? • Density-dependent factors have a greater effect as population density increases. • Density-dependent factors become increasingly effective as population density increases, thus exerting negative feedback that limits the size of populations. • Conversely, density-dependent factors become less effective as population density decreases, allowing population size to stabilize or grow.

  44. 27.3 How Is Population Growth Regulated? • Density-dependent factors have greater effect as population density increases (continued). • The most important density-dependent factors are predation and competition. • In predation, one organism feeds on another, harming it in the process. • Often, one organism (the predator) kills another (its prey) in order to eat it, but the prey is not always killed.

  45. 27.3 How Is Population Growth Regulated? • Predators help control prey populations. Fig. 27-7

  46. 27.3 How Is Population Growth Regulated? • Density-dependent factors have greater effect as population density increases (continued). • Prey also often survive the special form of predation known as parasitism. • In parasitism, the predator (the parasite) lives on or inside another organism (its host) and feeds on the host’s body without killing it—at least not immediately.

  47. 27.3 How Is Population Growth Regulated? • Predators exert density-dependent controls on populations. • Predation plays an increasingly important role in population control as prey populations increase. • The increased frequency of encounters with predators increases the prey populations’ death rate, because predators tend to eat larger numbers of whichever prey species is most abundant and easiest to find. • Increasing numbers of prey organisms also increase the numbers of predators, and this tends to control prey numbers.

  48. 27.3 How Is Population Growth Regulated? • Parasites spread faster when the host population density is high. • Like other forms of predation, parasitism is density-dependent. • Most parasites have limited ability to move, so they spread more readily from host to host at high host-population densities. • Parasites can affect the death rate of a host population because the damage inflicted by the parasite on its host’s body may kill the host.

  49. 27.3 How Is Population Growth Regulated? • Populations can soar or crash when predator-prey relationships are disrupted. • The population balance in ecosystems can be disrupted when predators are introduced to regions in which they were not previously present and where prey species have had no opportunity to evolve defenses against them. • Rats, snakes, and mongooses were introduced in Hawaii and many other Pacific islands, and have drastically reduced or exterminated many native bird populations.

  50. 27.3 How Is Population Growth Regulated? • Populations can soar or crash when predator-prey relationships are disrupted (continued). • Prey populations can grow out of control when introduced to areas where they have no predators. • The prickly pear cactus was introduced into Australia from Latin America, and, lacking predators, it spread uncontrollably. • In the 1920s, a cactus moth was imported from Argentina to feed on the cacti. • Within a few years, the cacti were almost eliminated, and today, the moth keeps the population density of its prey very low.

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