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Communities and Populations Succession (Revisited) • As an ecosystem changes, older inhabitants gradually die out and new organisms move in, causing further changes in the community = ecological succession – – – – Primary succession = occurs on bare rock Pioneer species = first species to populate an area (lichens) Secondary succession = a disturbance changes an existing community Climax community = mature, stable community after specific, predictable stages of succession • Biotic factors develop on abiotic factors Succession Do Now • What may cause the size of a population to grow? • What may cause the size of a population to decrease? How populations grow? • Since organisms are constantly interacting, growth or decline of one population will effect another population! • Remember: Relationships in food chains show effects of population growth or decline. Characteristics of Populations • Important characteristics of a population: – Geographic distribution = “range” of area inhabited by a population – Density = number of individuals per unit area – Growth rate = change in the number of individuals over time Population Growth • Populations may stay the same size from year to year, or may grow/decline rapidly due to the removal/introduction of a predator or the introduction/removal of prey. • Population size is effected by: – Number of births – Number of deaths – Number of individuals that enter or leave the population • Immigration = movement of individuals into area (growth) • Emigration = movement of individuals out of population (decline) Exponential Growth • If a population has abundant space and food, and is protected from predators and disease, then organisms in that population will multiply and the population size will increase. • Exponential growth occurs when the individuals in a population reproduce at a constant rate – Think of asexually reproduction: population doubles over and over again approaching an infinitely large size – Results in “J”-shaped curve • Under ideal conditions with unlimited resources, a population will grow exponentially. Logistic Growth • Exponential growth does not continue in natural populations for very long. • As resources become less available, the growth of a population slows or stops. • Logistic growth occurs when a population’s growth slows or stops following a period of exponential growth – Results in “S” shaped curve – The largest number of individuals that a given community can support = carrying capacity Quick Activity • Analyze data Interest Grabber continued Section 5-2 Do Now 1. Imagine a small island that has a population of five rabbits. How might each of the following factors affect the rabbit population? a. climate b. food supply c. predation 2. Now imagine another small island that has a population of 500 rabbits. How would the same factors affect this population? 3. Which of the factors depend on population size? Which factors do not depend on population size? Go to Section: Limits to growth • An environmental interaction can change the growth rate of a population by changing the relationship between birthrate and death rate, or between rates of immigration and emigration. • A limiting factor = a factor that causes population growth to decrease • Some limiting factors include: – – – – – Competition stronger effect as Predation population grows Parasitism and disease regardless of Drought and other climate extremes population size Human disturbances Figure 5-7 Wolf and Moose Populations on Isle Royale Section 5-2 60 2400 50 2000 40 1600 30 1200 20 800 10 400 0 1955 1960 0 1965 1970 1975 Moose Go to Section: 1980 Wolves 1985 1990 1995 Density-Dependent Factors • A limiting factor that depends on population size including: – Competition – organisms struggle for resources • When organisms of different species compete, it forces evolutionary change to decrease competition (evolve to occupy separate niches) – Predation – predator-prey relationship controls population – Parasitism – control population by weakening/killing host – Disease - control population by weakening/killing host • Density dependent factors become limiting only when the population density (number of organisms per unit area) reaches a certain level • These factors operate most strongly when a population is large and dense Density-Independent Factors • Affect all populations in similar ways, regardless of the population size including: – – – – Unusual weather Natural disasters Seasonal cycles Human activities (damming rivers/clearing forests) • Many species will show a characteristic “crash,” then the population will build back up right away, or stay low for some time • Environments are always changing, and most populations can adapt to a certain amount of change by growing or shrinking in size. • Major upsets in the ecosystem can lead to long-term declines in certain populations (human activities) Quick Activity • Gray Wolf Population Reading Human Population Growth • Like other populations, the size of the human population tends to increase with time. – Long ago, the human population was limited by food, disease, and survival to adulthood rates were low – About 500 years ago, the human population began growing rapidly with agriculture and industry making resources readily available, improved sanitation, medicine and health care. Human Population Growth Section 5-3 Industrial Revolution begins Agriculture begins Plowing and irrigation Go to Section: Bubonic plague Patterns of Population Growth • The human population cannot grow exponentially forever, because resources are limited. • In 1800s, Malthus predicted war, famine and disease would limit human population growth • Today, scientists study human population including birthrates, death rates, and age structure to predict growth rates = demography Demographic Transition • Demographers have developed a hypothesis to explain a slowing of population growth using changing in birth and death rates = demographic transition – Initially, birthrate and death rate are equally high – Then, advances decrease death rate, while birth rate remains high (more births than deaths and population increases) – Then, modernization decreases birthrate, and population growth slows – Eventually, birthrate and death rates are equally low and population growth stops Age Structure • Population growth also depends on how many people of different ages make up a given population • Demographers can predict future growth using models that graph the numbers of people in different age groups in a population = age-structure diagrams or population profiles Figure 5-13 Age Distribution Section 5-3 U.S. Population Males Go to Section: Females Rwandan Population Males Females Future Population Growth • Demographers can predict worldwide human population growth in future by analyzing age structures of every country – By 2025, 7.8 billion – By 2050, 9 billion – However, by 2050, the growth rate may begin to slow in a demographic transition • Many ecologists suggest that if human population growth doesn’t slow down, there could be serious and lasting damage to the environment and global economy • Other scientists believe that science, technology, and positive changes in society will help control negative impacts of humans Quick Activity • Analyze human population growth