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Lesson Overview How Populations Grow 5.1 How Populations Grow BENCHMARK SC.912.L.17.5 Analyze how population size is determined by births, deaths, immigration, emigration, and limiting factors (biotic and abiotic) that determine carrying capacity. Lesson Overview How Populations Grow 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? Lesson Overview How Populations Grow 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 tons—almost 19 times higher! Where did all the fish go? Can anything be done to increase their numbers? Lesson Overview How Populations Grow Describing Populations: How do ecologists study populations? • population: group of organisms of a single species that lives in a given area e.g. hydrilla population (see map) • Researchers study populations’ geographic range, density and distribution, growth rate, and age structure Lesson Overview How Populations Grow Geographic Range • geographic range: area inhabited by a population • varies enormously in size, depending on the species Lesson Overview How Populations Grow Geographic Range : EXAMPLES 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. Lesson Overview How Populations Grow Density and Distribution • Population density: number of individuals per unit area • Populations of different species often have very different densities, even in the same environment • EXAMPLE: 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. Density and Distribution RANDOM 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. UNIFORM 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. CLUMPED 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. distribution: how individuals in a population are spaced out across the range of the population Lesson Overview How Populations Grow Growth Rate • How populations change over time • determines whether the population size increases, decreases, or stays the same EXAMPLE: Hydrilla populations • in their native habitats tend to stay more or less the same size over time • have a growth rate of around zero; (neither increase nor decrease in size) • in Florida, by contrast, has a high growth rate—which means that it increases in size EXAMPLE: Cod • decrease in size • has a negative growth rate Lesson Overview How Populations Grow Age Structure • age structure—the number of males and females of each age a population contains • Most plants and animals cannot reproduce until they reach a certain age • among animals, only females can produce offspring Lesson Overview How Populations Grow Population Growth Factors Affecting Population Growth • increase or decrease in size depending on how many individuals are added to or removed from it • ADDED: birthrate, immigration • REMOVED: death rate, emigration Lesson Overview How Populations Grow Population Trends: 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. Lesson Overview How Populations Grow Immigration and Emigration • immigration: population may grow if individuals move into its range from elsewhere • emigration: population may decrease in size if individuals move out of the population’s range Lesson Overview How Populations Grow Exponential Growth: What Happens? What does a population need to grow? • food and space it • protection from predators and disease • remove its waste products Why does the population grow? • members of the population will be able to produce offspring • those offspring will produce their own offspring What is exponential growth? • under ideal conditions with unlimited resources, a population will grow exponentially • the larger a population gets, the faster it grows • size of each generation of offspring will be larger than the generation before it Lesson Overview How Populations Grow Exponential Growth: Organisms That Reproduce Rapidly • • • • plot the size of this population J-shaped curve rises slowly at first, and then rises faster and faster if no limits leads to an infinitely large size population • 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 2×2×2, or 8 cells. Lesson Overview How Populations Grow Organisms That Reproduce Slowly NOT ALL ORGANISMS REPRODUCE FAST EX: a female elephant • produce a single offspring only every 2 to 4 years • newborn elephants take about 10 years to mature After 750 years there would be nearly 20 million elephants! Lesson Overview How Populations Grow NON-NATIVE SPECIES/INVASIVE SPECIES: Organisms in New Environments • an organism is moved to a new environment, its population grows exponentially for a time EXAMPLE”: European gypsy moths (plant-eating pest) • accidentally released from a laboratory near Boston • spread across the northeastern United States within a few years. • Ate the leaves of thousands of acres of forest • formed a living blanket that covered the ground, sidewalks, and cars. Lesson Overview How Populations Grow Logistic Growth • occurs when a population’s growth slows and then stops • follows a period of exponential growth Natural populations don’t grow exponentially for long. Sooner or later, something stops exponential growth. What happens? Lesson Overview How Populations Grow Phases of Growth This graph traces the phases of growth that the population goes through. Lesson Overview How Populations Grow Phase 1: Exponential Growth • • • • • resources are unlimited individuals grow and reproduce rapidly few individuals die many offspring are produced both the population size and the rate of growth increase rapidly Lesson Overview How Populations Grow Phase 2: Growth Slows Down. • rate of population growth begins to slow down • limiting factors (space, food, competition) don’t allow population to continue to grow • population still grows, but the rate of growth slows down • population size increases more slowly. Lesson Overview How Populations Grow Phase 3: Growth Stops • rate of population growth drops to zero • size of the population levels off • under some conditions, the population will remain at or near this size indefinitely Lesson Overview How Populations Grow The Logistic Growth Curve • S-shape that represents what is called logistic growth • occurs when a population’s growth slows and then stops, following a period of exponential growth Lesson Overview How Populations Grow Carrying Capacity When the birthrate and the death rate are the same, and when immigration equals emigration, population growth stops. • dotted, horizontal line through the region of this graph where population growth levels off Lesson Overview How Populations Grow Carrying Capacity • 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 (limiting factors) act to stabilize it at that size Lesson Overview How Populations Grow Factors That Affect Population Growth Density-Dependent Factors Density-Independent Factors Competition Natural Disasters Predation Environmental Changes Parasitism/Diseases Human Activities (Pollution, Pesticides and Competition, Removal of Predators, Introduction of New Species) Food Supply 26 Lesson Overview How Populations Grow Density-Dependent Factors Predator-Prey Competitive Exclusion 27 Lesson Overview How Populations Grow Predator-Prey Relationships Lesson Overview How Populations Grow Food Webs • A network of food chains • Graphic representation of relationships between species Lesson Overview How Populations Grow Self Check What determines how large a population can grow? Limiting factors – Living space, competition, predation, diseases. How does a density-dependent limiting factor affect carrying capacity? Space availability, food availability How do density-independent limiting factors affect how a population grows? Natural disasters 30 Lesson Overview How Populations Grow HOT Question! In the survivorship curve below, the probability of an individual in this population surviving to 8 years old is about: a. 100% b. 80% c. 60% d. 50% e. 20% 31 Lesson Overview How Populations Grow HOT Question! 2. As the population gets larger, density-dependent factors ________ and density-independent factors _____. a. are unaffected; intensify b. intensify; diminish c. intensify; are unaffected d. diminish; are unaffected e. are unaffected; diminish 32 Lesson Overview How Populations Grow HOT Question! 3. What is the value of the carrying capacity, for the population described by the growth curve below? a. 5,000 b. 4,000 c. 3,000 d. 2,000 e. 1,000 33 Lesson Overview How Populations Grow HOT Question! 4. Which of the following interactions are best classified as predator-prey relationships? I. A lion eats an antelope. II. A virus enters a human cell. III. A flea bites a dog. IV. A dog gets tapeworms from the flea bite. V. An antelope eats grass. a. I , II and IV b. II, III, and IV c. I and V d. I, IV, and V e. II and III 34 Lesson Overview How Populations Grow HOT Question! 5. What is the maximum number of individuals of a given species that a particular environment can support for a prolonged period of time? A. B. C. D. population density maximum growth rate carrying capacity biotic potential 35 Lesson Overview How Populations Grow HOT Question! 6. Which of the following is a density-dependent factor that regulates population size? A. flood C. freezing B. fire D. disease 36