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POPULATION BIOLOGY CHAPTER 5 What affects population size? • Remember, a population is a group of organisms belonging to a single species that lives in a given area. • Answer the following questions about what factors can affect population size. 1. Think of a population in your area. How might this population grow in size? 2. How might this same population decrease in size? How is population size estimated? • For stationary organisms: – Quadrant method • For moving organisms: – Mark and Recapture – Great Turtle Race • For people: – Census DEFINITIONS • Species – groups of organisms that are able to interbreed and produce fertile offspring • Population – a group of organisms belonging to the same species Characteristics of Populations • Geographic distribution • Population Density - the number of individuals of a population within a given area • Growth rate POPULATION GROWTH • Three factors that affect population size: – Number of births (birthrate) – Number of deaths (death rate) – Number of individuals that leave (emigration) or enter (immigration) a population EXPONENTIAL GROWTH • Exponential Growth – highest rate of reproduction under ideal conditions # organisms Exponential Growth 160 140 120 100 80 60 40 20 0 0 2 4 6 8 10 12 14 16 18 AGE OF POPULATION (HOURS) LOGISTIC GROWTH • Logistic growth – occurs when a population’s growth slows or stops following a period of exponential growth • Carrying capacity – maximum number of individuals in a population that the environment can support LOGISTIC GROWTH # organisms Carrying capacity 700 600 500 400 300 200 100 0 0 2 4 6 8 10 12 14 16 18 AGE OF POPULATION (HOURS) QUESTIONS 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? 4. Which factors do not depend on population size? LIMITING FACTORS • Limiting factors –causes population growth to decrease – List examples DENSITY-DEPENDENT LIMITING FACTORS • Limiting factors that become limiting only when the population reaches a certain density – Examples: • Competition • Disease (contagious) • Predation • Parasitism • Competition – competing for resources such as food, space, water, sunlight, mates etc. – Can occur between the same species (intraspecific) or different species (interspecific) – What are the general outcomes of competition? • Extinction • Adaptation • Emigration – How do organisms avoid intraspecific (same species) competition? • Life cycles (frogs and tadpoles do not compete) • Life span (adults die shortly after young are produced) • Parental care of young • Dominance and Social Hierarchy (chicken’s pecking order) •Role separation (bees and ants) •Behavioral and Physiological Changes (killing young, no mating) •Emigration •Territoriality (birds’ songs, a buck’s scent) • Disease (contagious) – The more crowded an area, the quicker disease spreads • Predation – hunting, killing, and eating an animal – Example: eagles (predators) and mice (prey) – Predation may be a limiting factor for the prey population while food availability may be a limiting factor for the predator – Predator-prey relationships show cycles of increases and decreases in both populations in response to one another – Predators often kill the old, weak, and young – Predation may keep prey population very near carrying capacity • Example: Moose vs. wolf on an island in Lake Superior • Crocs 60 2400 30 1200 0 0 1955 1975 Moose 1995 Wolves • Parasitism – when organisms live on or in a host; always harmful to host •Example: tics and lice (parasites) live on humans (host) •The greater the population density, the greater number of parasites are passed from one organism to another DENSITY-INDEPENDENT LIMITING FACTORS • Limiting factors that limit a population regardless of its density – Examples: • Temperature (as with ladybugs) • Climate and natural disasters NUMBER OF LADYBUGS POPULATION GROWTH DENSITY-INDEPENDENT LIMITING FACTOR 80 60 40 20 0 SPRING SUMMER AUTUMN How fast are we growing? Until about 500 years ago, the world’s human population remained fairly stable. Then, as advances in medicine, agriculture, and technology occurred, the human population began growing very rapidly. Today, the world’s human population is greater than 6.5 billion people, and it continues to grow, but at a slower rate. 1. The human population is increasing by about 1.4 percent each year. Assume that the population is 6 billion (6,000,000,000). How large will the population be in one year? 2. If the human population continues to grow at a rate of 1.4 percent per year, the population would double in size (to 12 billion people) in only 51 years! What effect might this increase in population have on the environment and on other people? HUMAN POPULATION • World Population as of 9/15/08 (Estimate) 6,723,708,602 • World Population as of 2050 (projection) 9,404,251,020 • U.S. Population as of 9/15/08 (Estimate) 305,158,990 One birth every.................................. 7 seconds One death every.................................. 13 second One international migrant (net) every............ 29 seconds Net gain of one person every..................... 9 seconds • U.S. Population for 2050 (projection) 419,854,000 • A World Population Clock • World Clock with extra data • Source for USA and World Pop data: http://www.census.gov/ipc/www/idb/worldpopinfo.html and www.census.gov/population/www/popclockus.html Industrial Revolution begins Agriculture begins Plowing and irrigation Bubonic plague • • • • • • • • World Population reached: 1 billion in 1804 2 billion in 1927, (123 years later) 3 billion in 1960, (33 years later) 4 billion in 1974, (14 years later) 5 billion in 1987, (13 years later) 6 billion in 1999, (12 years later) October 12, 1999 was Y6B day (the day the world population reached 6 billion) PATTERNS OF POPULATION GROWTH • Demography – study of human populations • Since the 1970’s the worldwide population growth rate is decreasing • However, the population size is still increasing (just at a slightly slower rate) • Growth rates higher in developing countries – Birthrates higher – More people at reproductive age • Zero population growth is when birthrate = death rate • Birthrate, death rate, and age structure affect population growth rate • Demographic transition – as countries modernize, death rates decrease at first without birthrates decreasing. This causes populations to increase rapidly (USA 1790-1910) Later, birthrates also decrease slowing population growth. • Age-structure diagrams – show numbers of people in different age groups Figure 5-13: Age Distribution U.S. Population Rwandan Population RAPID GROWTH 2.1% ANNUALLY STABLE GROWTH 1.7% ANNUALLY NEGATIVE GROWTH -0.1% ANNUALLY Male Female Population of the USA by Age and Sex from 1950-2050 (in millions)