Lesson Overview

1
Lesson Overview

• 3.1 What Is Ecology?

2
Studying Our Living Planet

• The biosphere consists of all life on Earth and
  all parts of the Earth in which life exists,
  including land, water, and the atmosphere.
• The biosphere extends from about 8 km above
  Earths surface to as far as 11 km below the
  surface of the ocean.

3
The Science of Ecology

• Ecology is the scientific study of interactions
  among and between organisms and their physical
  environment.
• Interactions within the biosphere produce a web
  of interdependence between organisms and the
  environments in which they live.
• Organisms respond to their environments and can
  change their environments, producing an
  ever-changing biosphere.

4
Ecology and Economics

• Economics is concerned with interactions based
  on money.
• Economics and ecology share the same word root.
  Indeed, human economics and ecology are linked.
  Humans live within the biosphere and depend on
  ecological processes to provide such essentials
  as food and drinkable water that can be bought
  and sold for money.

5
Levels of Organization

• Ecological studies may focus on levels of
  organization that include the following
• Individual organism
• Populationa group of individuals that belong to
  the same species and live in the same area
• Communityan assemblage of different populations
  that live together in a defined area
• Ecosystemall the organisms that live in a
  place, together with their physical environment
• Biomea group of ecosystems that share similar
  climates and typical organisms
• Biosphereour entire planet, with all its
  organisms and physical environments

6
Biotic Factors

• A biotic factor is any living part of the
  environment with which an organism might
  interact, including animals, plants, mushrooms
  and bacteria.
• Biotic factors relating to a bullfrog might
  include algae it eats as a tadpole, the herons
  that eat bullfrogs, and other species competing
  for food or space.

7
Abiotic Factors

• An abiotic factor is any nonliving part of the
  environment, such as sunlight, heat,
  precipitation, humidity, wind or water currents,
  soil type, etc.
• For example, a bullfrog could be affected by
  abiotic factors such as water availability,
  temperature, and humidity.

8
Biotic and Abiotic Factors Together

• The difference between abiotic and biotic
  factors is not always clear. Abiotic factors can
  be influenced by the activities of organisms and
  vice versa.
• For example, pond muck contains nonliving
  particles, and also contains mold and decomposing
  plant material that serve as food for bacteria
  and fungi.

9
Biotic and Abiotic Factors Together

• In addition, trees and shrubs affect the amount
  of sunlight the shoreline receives, the range of
  temperatures it experiences, the humidity of the
  air, and even the chemical conditions of the
  soil.
• A dynamic mix of biotic and abiotic factors
  shapes every environment.

10
Ecological Methods

• What methods are used in ecological studies?
• Regardless of their tools, modern ecologists use
  three methods in their work observation,
  experimentation, and modeling. Each of these
  approaches relies on scientific methodology to
  guide inquiry.

11
Observation

• Observation is often the first step in asking
  ecological questions.
• Questions may form the first step in designing
  experiments and models.

12
Experimentation

• Experiments can be used to test hypotheses.
• An ecologist may set up an artificial
  environment in a laboratory or greenhouse, or
  carefully alter conditions in selected parts of
  natural ecosystems.

13
Modeling

• Many ecological events occur over such long
  periods of time or over such large distances that
  they are difficult to study directly.
• Ecologists make models to help them understand
  these phenomena.

14
Lesson Overview

• 3.2 Energy, Producers, and Consumers

15
Primary Producers

• Organisms need energy for growth, reproduction,
  and metabolic processes.
• No organism can create energyorganisms can only
  use energy from other sources.
• For most life on Earth, sunlight is the ultimate
  energy source.
• For some organisms, however, chemical energy
  stored in inorganic chemical compounds serves as
  the ultimate energy source for life processes.
• Plants, algae, and certain bacteria can capture
  energy from sunlight or chemicals and convert it
  into forms that living cells can use. These
  organisms are called autotrophs.
• Autotrophs are also called primary producers.

16
Primary Producers

• Primary producers store energy in forms that
  make it available to other organisms that eat
  them, and are therefore essential to the flow of
  energy through the biosphere.
• For example, plants obtain energy from sunlight
  and turn it into nutrients that can be eaten and
  used for energy by animals such as a caterpillar.

17
Energy From the Sun

• The best-known and most common primary producers
  harness solar energy through the process of
  photosynthesis.
• Photosynthesis captures light energy and uses it
  to power chemical reactions that convert carbon
  dioxide and water into oxygen and energy-rich
  carbohydrates. This process adds oxygen to the
  atmosphere and removes carbon dioxide.
• Plants are the main photosynthetic producers on
  land. Algae fill that role in freshwater
  ecosystems and the sunlit upper ocean.
• Photosynthetic bacteria, most commonly
  cyanobacteria, are important primary producers in
  tidal flats and salt marshes.

18
Life Without Light

• Biologists have discovered thriving ecosystems
  around volcanic vents in total darkness on the
  deep ocean floor.
• Deep-sea ecosystems depend on primary producers
  that harness chemical energy from inorganic
  molecules such as hydrogen sulfide.
• The use of chemical energy to produce
  carbohydrates is called chemosynthesis.

19
Consumers

• Organisms that must acquire energy from other
  organisms by ingesting in some way are known as
  heterotrophs.
• Heterotrophs are also called consumers.

20
Types of Consumers

• Consumers are classified by the ways in which
  they acquire energy and nutrients.
• Carnivores kill and eat other animals, and
  include snakes, dogs, cats, and this giant river
  otter.
• Catching and killing prey can be difficult and
  requires energy, but meat is rich in nutrients
  and energy and is easy to digest.
• Scavengers, like a king vulture, are animals
  that consume the carcasses of other animals that
  have been killed by predators or have died of
  other causes.
• Decomposers, such as bacteria and fungi, feed by
  chemically breaking down organic matter. The
  decay caused by decomposers is part of the
  process that produces detritussmall pieces of
  dead and decaying plant and animal remains.

21
Types of Consumers

• Herbivores, such as a military macaw, obtain
  energy and nutrients by eating plant leaves,
  roots, seeds, or fruits. Common herbivores
  include cows, caterpillars, and deer.
• Omnivores are animals whose diets naturally
  include a variety of different foods that usually
  include both plants and animals. Humans, bears,
  and pigs are omnivores.
• Detritivores, like giant earthworms, feed on
  detritus particles, often chewing or grinding
  them into smaller pieces. Detritivores commonly
  digest decomposers that live on, and in, detritus
  particles.

22
Beyond Consumer Categories

• Categorizing consumers is important, but these
  simple categories often dont express the real
  complexity of nature.
• For example, herbivores that eat different plant
  parts often differ greatly in the ways they
  obtain and digest their food.
• In addition, organisms in nature often do not
  stay inside the categories we put them in.
• For example, some carnivores will scavenge if
  they get the chance. Many aquatic animals eat a
  mixture of algae, bits of animal carcasses, and
  detritus particles.
• It is important to expand upon consumer
  categories by discussing the way that energy and
  nutrients move through ecosystems.

23
Lesson Overview

• 3.3 Energy Flow in Ecosystems

24
Food Chains

• A food chain is a series of steps in which
  organisms transfer energy by eating and being
  eaten.
• Food chains can vary in length. An example from
  the Everglades is shown.

25
Food Chains

• In some aquatic food chains, such as the example
  shown, primary producers are a mixture of
  floating algae called phytoplankton and attached
  algae. These producers are eaten by small fishes,
  such as flagfish.
• Larger fishes, like the largemouth bass, eat the
  small fishes.
• The bass are preyed upon by large wading birds,
  such as the anhinga, which may ultimately be
  eaten by an alligator.

26
Food Chains

• There are four steps in this food chain.
• The top carnivore is four steps removed from the
  primary producer.

27
Food Webs

• In most ecosystems, feeding relationships are
  much more complicated than the relationships
  described in a single, simple chain because many
  animals eat more than one kind of food.
• Ecologists call this network of feeding
  interactions a food web. An example of a food web
  in the Everglades is shown.

28
Food Chains Within Food Webs

• Each path through a food web is a food chain.
• A food web, like the one shown, links all of the
  food chains in an ecosystem together.
• The orange highlighted food chain, presented
  earlier, is one of many that make up this web.

29
Decomposers and Detritivores in Food Webs

• Most producers die without being eaten. In the
  detritus pathway, decomposers convert that dead
  material to detritus, which is eaten by
  detritivores, such as crayfish, grass shrimp, and
  worms.
• Pig frogs, killifish, and other fishes eat the
  detritivores.

30
Decomposers and Detritivores in Food Webs

• At the same time, the decomposition process
  releases nutrients that can be used by primary
  producers. They break down dead and decaying
  matter into forms that can be reused by
  organisms, similar to the way a recycling center
  works.
• Without decomposers, nutrients would remain
  locked in dead organisms.

31
Food Webs and Disturbance

• When disturbances to food webs happen, their
  effects can be dramatic.
• For example, all of the animals in this food web
  depend directly or indirectly on shrimplike
  animals called krill.
• Krill are one example of small, swimming animals
  called zooplankton.

32
Food Webs and Disturbance

• In recent years, krill populations have dropped
  substantially. Given the structure of this food
  web, a drop in the krill population can cause
  drops in the populations of all other members of
  the food web shown.

33
Trophic Levels and Ecological Pyramids

• Each step in a food chain or food web is called
  a trophic level.
• Primary producers always make up the first
  trophic level.
• Various consumers occupy every other level. Some
  examples are shown.

34
Trophic Levels and Ecological Pyramids

• Ecological pyramids show the relative amount of
  energy or matter contained within each trophic
  level in a given food chain or food web.
• There are three different types of ecological
  pyramids pyramids of energy, pyramids of
  biomass, and pyramids of numbers.

35
Pyramids of Energy

• There is theoretically no limit to the number of
  trophic levels in a food web or the number of
  organisms that live on each level.
• However, only a small portion of the energy that
  passes through any given trophic level is
  ultimately stored in the bodies of organisms at
  the next level.

36
Pyramids of Energy

• Organisms expend much of the energy they acquire
  on life processes, such as respiration, movement,
  growth, and reproduction.
• Most of the remaining energy is released into
  the environment as heata byproduct of these
  activities.

37
Pyramids of Energy

• Pyramids of energy show the relative amount of
  energy available at each trophic level.
• On average, about 10 percent of the energy
  available within one trophic level is transferred
  to the next trophic level.
• The more levels that exist between a producer
  and a consumer, the smaller the percentage of the
  original energy from producers that is available
  to that consumer.

38
Pyramids of Biomass and Numbers

• The total amount of living tissue within a given
  trophic level is called its biomass.
• The amount of biomass a given trophic level can
  support is determined, in part, by the amount of
  energy available.
• A pyramid of biomass illustrates the relative
  amount of living organic matter at each trophic
  level.
• Typically, the greatest biomass is at the base
  of the pyramid, as is seen in the field ecosystem
  modeled here.

39
Pyramids of Biomass and Numbers

• A pyramid of numbers shows the relative number
  of individual organisms at each trophic level in
  an ecosystem.
• In most ecosystems, the shape of the pyramid of
  numbers is similar to the shape of the pyramid of
  biomass for the same ecosystem, with the numbers
  of individuals on each level decreasing from the
  level below it.

40
Pyramids of Biomass and Numbers

• In some cases, however, consumers are much
  smaller than organisms they feed upon.
• Thousands of insects may graze on a single tree,
  for example. The tree has a lot of biomass, but
  represents only one organism.
• In such cases, the pyramid of numbers may be
  turned upside down, but the pyramid of biomass
  usually still has the normal orientation.

41
Lesson Overview

• 3.4 Cycles of Matter

42
Recycling in the Biosphere

• Unlike the one-way flow of energy, matter is
  recycled within and between ecosystems.
• Elements pass from one organism to another and
  among parts of the biosphere through closed loops
  called biogeochemical cycles, which are powered
  by the flow of energy.

Biogeochemical cycles of matter involve
biological processes, geological processes, and
chemical processes. As matter moves through
these cycles, it is never created or
destroyedjust changed.
43
Biological Processes

• Biological processes consist of any and all
  activities performed by living organisms.
• These processes include eating, breathing,
  burning food, and eliminating waste products.
• Geological processes include volcanic eruptions,
  the formation and breakdown of rock, and major
  movements of matter within and below the surface
  of the earth.

44
Chemical and Physical Processes

• Chemical and physical processes include the
  formation of clouds and precipitation, the flow
  of running water, and the action of lightning.

45
Human Activity

• Human activities that affect cycles of matter on
  a global scale include the mining and burning of
  fossil fuels, the clearing of land for building
  and farming, the burning of forests, and the
  manufacture and use of fertilizers.

46
Recycling in the Biosphere

• Biogeochemical cycles of matter pass the same
  atoms and molecules around again and again.

47
The Water Cycle

• How does water cycle through the biosphere?
• Water continuously moves between the oceans, the
  atmosphere, and landsometimes outside living
  organisms and sometimes inside them.

48
The Water Cycle

• Water molecules typically enter the atmosphere
  as water vapor when they evaporate from the ocean
  or other bodies of water.
• Water can also enter the atmosphere by
  evaporating from the leaves of plants in the
  process of transpiration.

49
The Water Cycle

• If the air carrying it cools, water vapor
  condenses into tiny droplets that form clouds.
• When the droplets become large enough, they fall
  to Earths surface as precipitation in the form
  of rain, snow, sleet, or hail.

50
The Water Cycle

• On land, some precipitation flows along the
  surface in what scientists call runoff, until it
  enters a river or stream that carries it to an
  ocean or lake.
• Precipitation can also be absorbed into the
  soil, and is then called groundwater.

51
The Water Cycle

• Groundwater can enter plants through their
  roots, or flow into rivers, streams, lakes, or
  oceans.
• Some groundwater penetrates deeply enough into
  the ground to become part of underground
  reservoirs.

52
Nutrient Cycles

• What is the importance of the main nutrient
  cycles?
• Every organism needs nutrients to build tissues
  and carry out life functions. Like water,
  nutrients pass through organisms and the
  environment through biogeochemical cycles.
• The three pathways, or cycles, that move carbon,
  nitrogen, and phosphorus through the biosphere
  are especially critical for life

53
Nutrient Cycles

• The chemical substances that an organism needs
  to sustain life are called nutrients.
• Every organism needs nutrients to build tissues
  and carry out life functions.
• Nutrients pass through organisms and the
  environment through biogeochemical cycles.

54
Nutrient Cycles

• Oxygen participates in parts of the carbon,
  nitrogen, and phosphorus cucles by combining with
  these elements and cycling with them through
  parts of their journeys.
• Oxygen gas in the atmosphere is released by one
  of the most important of all biological
  activities photosynthesis.
• Oxygen is used in respiration by all
  multicellular forms of life, and many
  single-celled organisms as well.

55
The Carbon Cycle

• Carbon is a major component of all organic
  compounds, including carbohydrates, lipids,
  proteins, and nucleic acids.

56
The Carbon Cycle

• Carbon dioxide is continually exchanged through
  chemical and physical processes between the
  atmosphere and oceans.

57
The Carbon Cycle

• Plants take in carbon dioxide during
  photosynthesis and use the carbon to build
  carbohydrates.
• Carbohydrates then pass through food webs to
  consumers.

58
The Carbon Cycle

• Organisms release carbon in the form of carbon
  dioxide gas by respiration.

59
The Carbon Cycle

• When organisms die, decomposers break down the
  bodies, releasing carbon to the environment.

60
The Carbon Cycle

• Geologic forces can turn accumulated carbon into
  carbon-containing rocks or fossil fuels.

61
The Carbon Cycle

• Carbon dioxide is released into the atmosphere
  by volcanic activity or by human activities, such
  as the burning of fossil fuels and the clearing
  and burning of forests.

62
The Carbon Cycle

• Important questions remain about the carbon
  cycle.
• How much carbon moves through each pathway?
• How do ecosystems respond to changes in
  atmospheric carbon dioxide concentration?

63
The Nitrogen Cycle

• All organisms require nitrogen to make amino
  acids, which are used to build proteins and
  nucleic acids, which combine to form DNA and RNA.

64
The Nitrogen Cycle

• Nitrogen gas (N2) makes up 78 percent of Earths
  atmosphere.

65
The Nitrogen Cycle

• Nitrogen-containing substances such as ammonia
  (NH3), nitrate ions (NO3), and nitrite ions (NO2)
  are found in soil, in the wastes produced by many
  organisms, and in dead and decaying organic
  matter.

66
The Nitrogen Cycle

• Dissolved nitrogen exists in several forms in
  the ocean and other large water bodies.

67
The Nitrogen Cycle

• Although nitrogen gas is the most abundant form
  of nitrogen on Earth, only certain types of
  bacteria that live in the soil and on the roots
  of legumes can use this form directly.
• The bacteria convert nitrogen gas into ammonia,
  in a process known as nitrogen fixation.

68
The Nitrogen Cycle

• Other soil bacteria convert fixed nitrogen into
  nitrates and nitrites that primary producers can
  use to make proteins and nucleic acids.

69
The Nitrogen Cycle

• Consumers eat the producers and reuse nitrogen
  to make their own nitrogen-containing compounds.

70
The Nitrogen Cycle

• Decomposers release nitrogen from waste and dead
  organisms as ammonia, nitrates, and nitrites that
  producers may take up again.

71
The Nitrogen Cycle

• Other soil bacteria obtain energy by converting
  nitrates into nitrogen gas, which is released
  into the atmosphere in a process called
  denitrification.

72
The Nitrogen Cycle

• A small amount of nitrogen gas is converted to
  usable forms by lightning in a process called
  atmospheric nitrogen fixation.

73
The Nitrogen Cycle

• Humans add nitrogen to the biosphere through the
  manufacture and use of fertilizers. Excess
  fertilizer is often carried into surface water or
  groundwater by precipitation.

74
The Phosphorus Cycle

• Phosphorus forms a part of vital molecules such
  as DNA and RNA.
• Although phosphorus is of great biological
  importance, it is not abundant in the biosphere.

75
The Phosphorus Cycle

• Phosphorus in the form of inorganic phosphate
  remains mostly on land, in the form of phosphate
  rock and soil minerals, and in the ocean, as
  dissolved phosphate and phosphate sediments.

76
The Phosphorus Cycle

• As rocks and sediments wear down, phosphate is
  released.
• Some phosphate stays on land and cycles between
  organisms and soil.

77
The Phosphorus Cycle

• Plants bind phosphate into organic compounds
  when they absorb it from soil or water.

78
The Phosphorus Cycle

• Organic phosphate moves through the food web,
  from producers to consumers, and to the rest of
  the ecosystem.

79
The Phosphorus Cycle

• Other phosphate washes into rivers and streams,
  where it dissolves. This phosphate eventually
  makes its way to the ocean, where marine
  organisms process and incorporate it into
  biological compounds.

80
Nutrient Limitation

• How does nutrient availability relate to the
  primary productivity of an
• ecosystem?

81
Nutrient Limitation

• How does nutrient availability relate to the
  primary productivity of an ecosystem?
• If ample sunlight and water are available, the
  primary productivity of an ecosystem may be
  limited by the availability of nutrients.

82
Nutrient Limitation

• Ecologists are often interested in an
  ecosystems primary productivitythe rate at
  which primary producers create organic material.
• If an essential nutrient is in short supply,
  primary productivity will be limited.
• The nutrient whose supply limits productivity is
  called the limiting nutrient.

83
Nutrient Limitation in Soil

• The growth of crop plants is typically limited
  by one or more nutrients that must be taken up by
  plants through their roots.
• Most fertilizers contain large amounts of
  nitrogen, phosphorus, and potassium, which help
  plants grow better in poor soil. Carbon is not
  included in chemical fertilizers because plants
  acquire carbon dioxide from the atmosphere.
• Micronutrients such as calcium, magnesium,
  sulfur, iron, and manganese are necessary in
  relatively small amounts, and are sometimes
  included in specialty fertilizers.

84
Nutrient Limitation in Soil

• All nutrient cycles work together like the gears
  shown.
• If any nutrient is in short supplyif any wheel
  sticksthe whole system slows down or stops
  altogether.

85
Nutrient Limitation in Aquatic Ecosystems

• Oceans are nutrient-poor compared to many land
  areas.
• In the ocean and other saltwater environments,
  nitrogen is often the limiting nutrient.
• In streams, lakes, and freshwater environments,
  phosphorus is typically the limiting nutrient.

86
Nutrient Limitation in Aquatic Ecosystems

• Sometimes an aquatic ecosystem receives a large
  input of a limiting nutrientfor example, runoff
  from heavily fertilized fields.

87
Nutrient Limitation in Aquatic Ecosystems

• The result of this runoff can be an algal
  blooma dramatic increase in the amount of algae
  and other primary producers due to the increase
  in nutrients.
• If there are not enough consumers to eat the
  algae, an algal bloom can cover the waters
  surface and disrupt the functioning of an
  ecosystem.

88
Lesson Overview

• 5.1 How Populations Grow

89
THINK ABOUT IT

• In the 1950s, a fish farmer in Florida tossed a
  few plants called hydrilla into a canal. Hydrilla
  was imported from Asia for use in home aquariums
  because it is hardy and adaptable. The few plants
  he tossed in reproduced quickly and kept on
  reproducing. Today, their tangled stems snag
  boats in rivers and overtake habitats native
  water plants and animals are disappearing. Why
  did these plants get so out of control? Is there
  any way to get rid of them?

90
THINK ABOUT IT

• Meanwhile, people in New England who fish for a
  living face a different problem. Their catch has
  dropped dramatically, despite hard work and new
  equipment. The cod catch in one recent year was
  3,048 metric tons. Back in 1982, it was 57,200
  metric tonsalmost 19 times higher! Where did all
  the fish go? Can anything be done to increase
  their numbers?

91
Describing Populations

• How do ecologists study populations?

92
Describing Populations

• How do ecologists study populations?
• Researchers study populations geographic range,
  density and distribution,
• growth rate, and age structure.

93
Describing Populations

• The stories of hydrilla and cod both involve
  dramatic changes in the sizes of populations.
• A population is a group of organisms of a single
  species that lives in a given area, such as the
  hydrilla population represented on this map.
• Researchers study populations geographic range,
  density and distribution, growth rate, and age
  structure.

94
Geographic Range

• The area inhabited by a population is called its
  geographic range.
• A populations range can vary enormously in
  size, depending on the species.

95
Geographic Range

• A bacterial population in a rotting pumpkin may
  have a range smaller than a cubic meter, whereas
  the population of cod in the western Atlantic
  covers a range that stretches from Greenland down
  to North Carolina.
• Humans have carried hydrilla to so many places
  that its range now includes every continent
  except Antarctica, and it is found in many places
  in the United States.

96
Density and Distribution

• Population density refers to the number of
  individuals per unit area.
• Populations of different species often have very
  different densities, even in the same
  environment.
• A population of ducks in a pond may have a low
  density, while fish and other animals in the same
  pond community may have higher densities.

97
Density and Distribution

• Distribution refers to how individuals in a
  population are spaced out across the range of the
  populationrandomly, uniformly, or mostly
  concentrated in clumps.

98
Density and Distribution

• An example of a population that shows random
  distribution is the purple lupine. These wild
  flowers grow randomly in a field among other
  wildflowers. The dots in the illustration
  represent individual members of a population with
  random distribution.

99
Density and Distribution

• An example of a population that shows uniform
  distribution is the king penguin. The dots in the
  illustration represent individual members of a
  population with uniform distribution.

100
Density and Distribution

• An example of a population that shows clumped
  distribution is the striped catfish. These fish
  organize into tight groups. The dots in the
  illustration represent individual members of a
  population with clumped distribution.

101
Growth Rate

• A populations growth rate determines whether
  the population size increases, decreases, or
  stays the same.
• Hydrilla populations in their native habitats
  tend to stay more or less the same size over
  time. These populations have a growth rate of
  around zero they neither increase nor decrease
  in size.
• The hydrilla population in Florida, by contrast,
  has a high growth ratewhich means that it
  increases in size.
• Populations can also decrease in size, as cod
  populations have been doing. The cod population
  has a negative growth rate.

102
Age Structure

• To fully understand a plant or animal
  population, researchers need to know the
  populations age structurethe number of males
  and females of each age a population contains.
• Most plants and animals cannot reproduce until
  they reach a certain age.
• Also, among animals, only females can produce
  offspring.

103
Population Growth

• What factors affect population growth?

104
Population Growth

• What factors affect population growth?
• The factors that can affect population size are
  the birthrate, death rate, and
• the rate at which individuals enter or leave the
  population.

105
Population Growth

• A population will increase or decrease in size
  depending on how many individuals are added to it
  or removed from it.
• The factors that can affect population size are
  the birthrate, death rate, and the rate at which
  individuals enter or leave the population.

106
Birthrate and Death Rate

• A population can grow when its birthrate is
  higher than its death rate.
• If the birthrate equals the death rate, the
  population may stay the same size.
• If the death rate is greater than the birthrate,
  the population is likely to shrink.

107
Immigration and Emigration

• A population may grow if individuals move into
  its range from elsewhere, a process called
  immigration.
• A population may decrease in size if individuals
  move out of the populations range, a process
  called emigration.

108
Exponential Growth

• What happens during exponential growth?

109
Exponential Growth

• What happens during exponential growth?
• Under ideal conditions with unlimited resources,
  a population will grow
• exponentially.

110
Exponential Growth

• If you provide a population with all the food
  and space it needs, protect it from predators and
  disease, and remove its waste products, the
  population will grow.
• The population will increase because members of
  the population will be able to produce offspring,
  and after a time, those offspring will produce
  their own offspring.
• Under ideal conditions with unlimited resources,
  a population will grow exponentially.
• In exponential growth, the larger a population
  gets, the faster it grows. The size of each
  generation of offspring will be larger than the
  generation before it.

111
Organisms That Reproduce Rapidly

• In a hypothetical experiment, a single bacterium
  divides to produce two cells every 20 minutes.
• After 20 minutes, under ideal conditions, the
  bacterium divides to produce two bacteria. After
  another 20 minutes, those two bacteria divide to
  produce four cells. After three 20-minute
  periods, we have 222, or 8 cells.

112
Organisms That Reproduce Rapidly

• Another way to describe the size of the bacteria
  population is to use an exponent 23 cells (three
  20-minute periods).
• In another hour (six 20-minute periods), there
  will be 26, or 64 bacteria.
• In one day, this bacterial population will grow
  to 4,720,000,000,000,000,000,000 individuals.
• If this growth continued without slowing down,
  this bacterial population would cover the planet
  within a few days!

113
Organisms That Reproduce Rapidly

• If you plot the size of this population on a
  graph over time, you get a J-shaped curve that
  rises slowly at first, and then rises faster and
  faster.
• If nothing were to stop this kind of growth, the
  population would become larger and larger, faster
  and faster, until it approached an infinitely
  large size.

114
Organisms That Reproduce Slowly

• Many organisms grow and reproduce much more
  slowly than bacteria.
• For example, a female elephant can produce a
  single offspring only every 2 to 4 years. Newborn
  elephants take about 10 years to mature.
• If exponential growth continued and all
  descendants of a single elephant pair survived
  and reproduced, after 750 years there would be
  nearly 20 million elephants!

115
Organisms in New Environments

• Sometimes, when an organism is moved to a new
  environment, its population grows exponentially
  for a time.
• When a few European gypsy moths were
  accidentally released from a laboratory near
  Boston, these plant-eating pests spread across
  the northeastern United States within a few
  years.
• In peak years, they devoured the leaves of
  thousands of acres of forest. In some places,
  they formed a living blanket that covered the
  ground, sidewalks, and cars.

116
Logistic Growth

• What is logistic growth?

117
Logistic Growth

• What is logistic growth?
• Logistic growth occurs when a populations growth
  slows and then stops,
• following a period of exponential growth.

118
Logistic Growth

• Natural populations dont grow exponentially for
  long.
• Sooner or later, something stops exponential
  growth. What happens?

119
Phases of Growth

• Suppose that a few individuals are introduced
  into a real-world environment.
• This graph traces the phases of growth that the
  population goes through.

120
Phase 1 Exponential Growth

• After a short time, the population begins to grow
  exponentially.
• During this phase, resources are unlimited, so
  individuals grow and
• reproduce rapidly.
• Few individuals die, and many offspring are
  produced, so both the
• population size and the rate of growth increase
  more and more rapidly.

121
Phase 2 Growth Slows Down.

• In real-world populations, exponential growth
  does not continue for long. At some point, the
  rate of population growth begins to slow down.
• The population still grows, but the rate of
  growth slows down, so the population size
  increases more slowly.

122
Phase 3 Growth Stops.

• At some point, the rate of population growth
  drops to zero and the size of the population
  levels off.
• Under some conditions, the population will
  remain at or near this size indefinitely.

123
The Logistic Growth Curve

• This curve has an S-shape that represents what
  is called logistic growth.
• Logistic growth occurs when a populations
  growth slows and then stops, following a period
  of exponential growth.
• Many familiar plant and animal populations
  follow a logistic growth curve.

124
The Logistic Growth Curve

• Population growth may slow for several reasons.
• Growth may slow if the populations birthrate
  decreases or the death rate increasesor if
  births fall and deaths rise together.
• In addition, population growth may slow if the
  rate of immigration decreases, the rate of
  emigration increases, or both.

125
Carrying Capacity

• When the birthrate and the death rate are the
  same, and when immigration equals emigration,
  population growth stops.
• There is a dotted, horizontal line through the
  region of this graph where population growth
  levels off. The point at which this dotted line
  intersects the y-axis represents the carrying
  capacity.

126
Carrying Capacity

• Carrying capacity is the maximum number of
  individuals of a particular species that a
  particular environment can support.
• Once a population reaches the carrying capacity
  of its environment, a variety of factors act to
  stabilize it at that size.

127
Lesson Overview

• 5.2 Limits to Growth

128
THINK ABOUT IT

• What determines the carrying capacity of an
  environment for a particular species?
• In its native Asia, populations of hydrilla
  increase in size until they reach carrying
  capacity, and then population growth stops. But
  here in the United States, hydrilla grows out of
  control.
• Why does a species that is well-behaved in one
  environment grow out of control in another?

129
Limiting Factors

• What factors determine carrying capacity?

130
Limiting Factors

• What factors determine carrying capacity?
• Acting separately or together, limiting factors
  determine the carrying
• capacity of an environment for a species.

131
Limiting Factors

• A limiting factor is a factor that controls the
  growth of a population.
• There are several kinds of limiting factors.
• Somesuch as competition, predation, parasitism,
  and diseasedepend on population density.
• Othersincluding natural disasters and unusual
  weatherdo not depend on population density.

132
Density-Dependent Limiting Factors

• What limiting factors depend on population
  density?

133
Density-Dependent Limiting Factors

• What limiting factors depend on population
  density?
• Density-dependent limiting factors include
  competition, predation,
• herbivory, parasitism, disease, and stress from
  overcrowding.

134
Density-Dependent Limiting Factors

• Density-dependent limiting factors operate
  strongly only when population densitythe number
  of organisms per unit areareaches a certain
  level. These factors do not affect small,
  scattered populations as much.
• Density-dependent limiting factors include
  competition, predation, herbivory, parasitism,
  disease, and stress from overcrowding.

135
Competition

• When populations become crowded, individuals
  compete for food, water, space, sunlight, and
  other essentials.
• Some individuals obtain enough to survive and
  reproduce.
• Others may obtain just enough to live but not
  enough to enable them to raise offspring.
• Still others may starve to death or die from
  lack of shelter.
• Competition can lower birthrates, increase death
  rates, or both.

136
Competition

• Competition is a density-dependent limiting
  factor. The more individuals living in an area,
  the sooner they use up the available resources.
• Often, space and food are related to one
  another. Many grazing animals compete for
  territories in which to breed and raise
  offspring. Individuals that do not succeed in
  establishing a territory find no mates and cannot
  breed.
• For example, male wolves may fight each other
  for territory or access to mates.

137
Competition

• Competition can also occur between members of
  different species that attempt to use similar or
  overlapping resources.
• This type of competition is a major force behind
  evolutionary change.

138
Predation and Herbivory

• The effects of predators on prey and the effects
  of herbivores on plants are two very important
  density-dependent population controls.

139
Predator-Prey Relationships

• This graph shows the fluctuations in wolf and
  moose populations on Isle Royale over the years.
• Sometimes, the moose population on Isle Royale
  grows large enough that moose become easy prey
  for wolves. When wolves have plenty to eat, their
  population grows.

140
Predator-Prey Relationships

• As wolf populations grow, they begin to kill
  more moose than are born. This causes the moose
  death rate to rise higher than its birthrate, so
  the moose population falls.

141
Predator-Prey Relationships

• As the moose population drops, wolves begin to
  starve. Starvation raises wolves death rate and
  lowers their birthrate, so the wolf population
  also falls.
• When only a few predators are left, the moose
  death rate drops, and the cycle repeats.

142
Herbivore Effects

• Herbivory can also contribute to changes in
  population numbers. From a plants perspective,
  herbivores are predators.
• On parts of Isle Royale, large, dense moose
  populations can eat so much balsam fir that the
  population of these favorite food plants drops.
  When this happens, moose may suffer from lack of
  food.

143
Humans as Predators

• In some situations, human activity limits
  populations.
• For example, fishing fleets, by catching more
  and more fish every year, have raised cod death
  rates so high that birthrates cannot keep up. As
  a result, cod populations have been dropping.
• These populations can recover if we scale back
  fishing to lower the death rate sufficiently.
• Biologists are studying birthrates and the age
  structure of the cod population to determine how
  many fish can be taken without threatening the
  survival of this population.

144
Parasitism and Disease

• Parasites and disease-causing organisms feed at
  the expense of their hosts, weakening them and
  often causing disease or death.
• For example, ticks feeding on the blood of a
  hedgehog can transmit bacteria that cause
  disease.
• Parasitism and disease are density-dependent
  effects, because the denser the host population,
  the more easily parasites can spread from one
  host to another.

145
Parasitism and Disease

• This graph shows a sudden and dramatic drop in
  the wolf population of Isle Royale around 1980.
  At this time, a viral disease of wolves, canine
  parvovirus (CPV), was accidentally introduced to
  the island.
• This virus killed all but 13 wolves on the
  islandand only three of the survivors were
  females.

146
Parasitism and Disease

• The removal of wolves caused the moose
  population to skyrocket to 2400.
• The densely packed moose then became infested
  with winter ticks that caused hair loss and
  weakness.

147
Stress From Overcrowding

• Some species fight amongst themselves if
  overcrowded.
• Too much fighting can cause high levels of
  stress, which can weaken the bodys ability to
  resist disease.
• In some species, stress from overcrowding can
  cause females to neglect, kill, or even eat their
  own offspring.
• Stress from overcrowding can lower birthrates,
  raise death rates, or both, and can also increase
  rates of emigration.

148
Density-Independent Limiting Factors

• What limiting factors do not typically depend on
  population density?

149
Density-Independent Limiting Factors

• What limiting factors do not typically depend on
  population density?
• Unusual weather such as hurricanes, droughts, or
  floods, and natural
• disasters such as wildfires, can act as
  density-independent limiting factors.

150
Density-Independent Limiting Factors

• Density-independent limiting factors affect all
  populations in similar ways, regardless of
  population size and density.
• Unusual weather such as hurricanes, droughts, or
  floods, and natural disasters such as wildfires,
  can act as density-independent limiting factors.

151
Density-Independent Limiting Factors

• A severe drought, for example, can kill off
  great numbers of fish in a river.
• In response to such factors, a population may
  crash. After the crash, the population may
  build up again quickly, or it may stay low for
  some time.

152
True Density Independence?

• Sometimes the effects of so-called
  density-independent factors can actually vary
  with population density.
• It is sometimes difficult to say that a limiting
  factor acts only in a density-independent way.

153
True Density Independence?

• On Isle Royale, for example, the moose
  population grew exponentially for a time after
  the wolf population crashed. Then, a bitterly
  cold winter with very heavy snowfall covered the
  plants that moose feed on, making it difficult
  for moose to move around to find food.

154
True Density Independence?

• Because this was an island population,
  emigration was not possible. Moose weakened and
  many died.

155
True Density Independence?

• In this case, the effects of bad weather on the
  large, dense population were greater than they
  would have been on a small population. In a
  smaller population, the moose would have had more
  food available because there would have been less
  competition.

156
Controlling Introduced Species

• In hydrillas natural environment,
  density-dependent population limiting factors
  keep it under control.
• Perhaps plant-eating insects or fishes devour
  it, or perhaps pests or diseases weaken it. Those
  limiting factors are not found in the United
  States, and the result is runaway population
  growth!
• Efforts at artificial density-independent
  control measuressuch as herbicides and
  mechanical removaloffer only temporary solutions
  and are expensive.

157
Controlling Introduced Species

• Researchers have spent decades looking for
  natural predators and pests of hydrilla.
• The best means of control so far seems to be an
  imported fish called grass carp, which views
  hydrilla as an especially tasty treat.
• Grass carp are not native to the United States.
  Only sterilized grass carp can be used to control
  hydrilla. Can you understand why?

158
Lesson Overview

• 5.3 Human Population Growth

159
THINK ABOUT IT

• How quickly is the global human population
  growing?
• In the United States and other developed
  countries, the population growth rate is low. In
  some developing countries, the population is
  growing very rapidly. Worldwide, there are more
  than four human births every second.
• What does the present and future of human
  population growth mean for our species and its
  interactions with the rest of the biosphere?

160
Historical Overview

• How has human population size changed over time?

161
Historical Overview

• How has human population size changed over time?
• The human population, like populations of other
  organisms, tends to
• increase. The rate of that increase has changed
  dramatically over time.

162
Historical Overview

• For most of human existence, the population grew
  slowly because life was harsh. Food was hard to
  find. Predators and diseases were common and
  life-threatening.

163
Historical Overview

• These limiting factors kept human death rates
  very high. Until fairly recently, only half the
  children in the world survived to adulthood.
• Because death rates were so high, families had
  many children, just to make sure that some would
  survive.

164
Exponential Human Population Growth

• As civilization advanced, life became easier,
  and the human population began to grow more
  rapidly. That trend continued through the
  Industrial Revolution in the 1800s.

165
Exponential Human Population Growth

• Several factors, including improved nutrition,
  sanitation, medicine, and healthcare,
  dramatically reduced death rates. Yet, birthrates
  in most parts of the world remained high.
• The combination of lower death rates and high
  birthrates led to exponential growth.

166
The Predictions of Malthus

• This kind of exponential growth could not
  continue forever.
• Two centuries ago, English economist Thomas
  Malthus suggested that only war, famine, and
  disease could limit human population growth.
• Malthus thought that human populations would be
  regulated by competition (war), limiting
  resources (famine), parasitism (disease), and
  other density-dependent factors.
• Malthuss work was vitally important to the
  thinking of Charles Darwin.

167
World Population Growth Slows

• Exponential growth continued up to the second
  half of the twentieth century, reaching a peak
  around 19621963, and then it began to drop.
• The size of the global human population is still
  growing rapidly, but the rate of growth is
  slowing down.

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