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Chapter 5 Populations Location of the ecosystem – Aleutian Islands, Alaska Estimated Population Size Change in sea otter populations around the Aleutian Islands 100000 80000 60000 40000 20000 0 1900 1920 1940 1960 Year 1980 2000 2020 WHY? With your group, discuss as many reasons that you can think of that might be causing the sea otter population decline Change in sea otter populations around the Aleutian Islands Killer whale prey in the Aleutians If you were a killer whale… • With your partners, rank the order of the three prey species that you would prefer to eat. Give a reasoning for your ranking. Great Whales Harbor Seals Steller Sea Lions Sea Otters 1.0 Proportion of maximum 0.8 0.6 0.4 0.2 0.0 1950 1960 1970 1980 Year 1990 2000 So who cares if the otters go extinct? • With your partners, make a prediction about what will happen to the kelp forests of the Aleutian Islands habitat if the sea otters become extinct What’s next? • With your partners, sum everything up. What will eventually happen to all the members of this ecosystem if the sea otters become extinct? 5 – 1 How Populations Grow • What characteristics are used to describe a population? • What factors affect population size? • What are exponential growth and logistic growth? Characteristics of Populations • Three important characteristics of a population are: 1. geographic distribution – The area inhabited by a population 2. density - # of individuals per unit area 3. growth rate – how fast the population grows Population Growth • Three factors can affect population size: 1. The # of births 2. The # of deaths 3. How fast the population numbers grow Exponential Growth • Under ideal conditions with unlimited resources, a population will grow exponentially • When the individuals in a population reproduce at a constant rate Exponential Growth Logistic Growth • Obviously, the conditions for exponential growth are rarely ever met • 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 Q: Why does this happen? A: Lack of resources, lack of space Carrying Capacity • The largest number of individuals that a population can support Q: What is the growth rate when a population is at carrying A: Zero 5 – 2 Limits to Growth • What factors limit population growth? Limiting Factor • A factor that causes population growth to decrease Ex.) Parasitism, disease, drought, extreme weather • A resource base that is limited can also affect the long term survival of a species Density – dependent Factors • Limiting factors that depend on population size • Density dependent factors become limiting only when the population density reaches a certain level • These factors operate most strongly when a population is large and dense • Density dependent factors include: Competition Predation Parasitism Disease Density – Independent Factors • Affect all populations in similar ways regardless of the population • Ex.) unusual weather, natural disasters, seasonal cycles, human activities Unusual Weather Natural Disasters Seasonal Cycles Human Activities Human Activities Density – Independent Factors • In response to such factors, many species show a characteristic crash in population size • Environments are always changing, and most populations can adapt to a certain amount of change • Populations often grow and shrink in response to such changes • Major upsets in an ecosystem can lead to long term declines in certain populations 5 -3 Human Population Growth • How has the size of the human population changed over time? • Why do population growth rates differ in countries throughout the world? Historical Overview • Like the populations of many other living organisms, the size of the human population tends to increase with time • In the U.S. and other developed countries, the current growth is low • In some developing countries, the human population is growing at a rate of 3 people per. second • The human population is well on its way to reaching 9 billion in your lifetime Exponential Human Growth • The following factors have caused a rapid increase in population growth: Agriculture Industry Reliable food source Shipping Improved sanitation Medicine Technology Patterns of Population Growth Q: Why can’t the human population keep growing exponentially forever? A: The resources on earth are limited Demography • The scientific study of human population • Birthrates, death rates, and age structure of a population help to project why some countries have a high growth rate while others grow more slowly The Demographic Transition Age Structure • Population growth depends, in part, on how many people of different ages make up a given population Age-structure diagrams • (population profile) – show the population of a country broken down by gender and age group World Population: 1950 - 2050 Year Average Annual Growth Rate (%) Population 1950 1.47 2,555,360,972 1960 1.33 3,039,669,330 1962 2.19 3,136,556,092 1963 2.19 1970 2.07 3,708,067,105 1980 1.69 4,454,607,332 1990 1.58 5,275,407,789 2000 1.23 6,078,684,329 2010 1.06 6,812,009,338 2020 .87 7,515,218,898 2030 .68 8,127,227,506 2040 .54 8,646,671,023 2050 .43 9,078,850,714 3,206,072,286