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How Populations Grow Chapter 5 Section 1 Let’s Review! What is a population? A group of similar organisms that can breed and produce fertile offspring Species or not a species? Characteristics of Populations 1. Populations often change due to a variety of influential factors There are three important characteristics of a population. Geographic distribution – the range of area inhabited by a population Characteristics of Populations 2. Population density – the number of individuals per unit area Population Density Can vary tremendously depending on the species and its ecosystem A field of sunflowers A flock of birds A few saguro cactus Turtles grouped to sun Population Density What are some benefits of living close together or in a clumped population? 1. 2. 3. Help finding food Protection from predators or wind Easier to find species PopulationDensity What are some drawbacks (negatives) of living in a clumped population? 1. 2. 3. Crowded living areas Battles over food/shelter Easier for disease to spread Characteristics of Populations 3. Growth rate- the amount by which a population’s size changes in a given time Population Growth 1. 2. 3. Three factors affect population size Number of births Number of deaths Number of individuals that enter or leave Population Growth Migration- periodic movements of populations away from and back to their place of origin (not what we are talking about here) Irregular movements of some individuals out of an area, into a new area, with no return - affects population size Immigration – movement of individuals into an area Population increases in size Emigration – movement of individuals out of an area Population decreases in size Population Growth Why would an animal immigrate to or emigrate from a population? Competitive pressures on some individuals To find food, better living condition, or a mate Exponential Growth If a population has plenty of food and space and is protected from predators and disease, what will happen to the size of the population? It will increase! Exponential Growth Bacteria reproduce in ~20 minutes by splitting in half Example: 1 bacteria after 20 min = 2 bacteria 2 bacteria after 40 min = 4 bacteria 4 bacteria after 60 min = 8 bacteria In 180 min (3 hours) = 512 bacteria In one day = 4,720,000,000,000,000,000,000 bacteria Exponential Growth Exponential growth occurs when the individuals in a population reproduce at a constant rate This is what a graph Creates a “J” shaped curve Under ideal conditions with unlimited resources and no predation, a population will grow exponentially of bacterial growth would look like if the growth went unchecked Logistic Growth Do bacteria cover the planet? Exponential growth does not continue in populations very long. As resources become less available, the growth of a population slows or stops What might cause population growth to stop or slow down? Predators, less food, less space, less resources, loss of habitat, disease, etc. Logistic Growth Logistic growth occurs when a population’s growth slows or stops following a period of exponential growth Creates an “S” shaped curve Population growth may slow or stop for a number of reasons: Birthrate ↓ death rate ↑ immigration ↓ emigration ↑ At some point, the growth of a population will level off Number Logistic Growth Time (hours) Logistic Growth Occurs after a period of exponential growth When a population reaches carrying capacity Logistic Growth capacity – the largest number of individuals an environment can support Carrying Most plant and animal populations follow this type of Carrying capacity growth Number of Yeast Cells Time (hours) Comparing Exponential and Logistic Growth Limits to Growth Limiting Factor: any factor that causes population growth to stop Limits the size of a population Almost anything can be a limiting factor… Examples: competing for sunlight, space, water, food, etc. Predation, parasitism, disease, natural disasters, etc. Limits to Growth As a population increases, so does competition Not all individuals will get what they need to survive and will die The number of births decreases and the number of deaths increases The population gets smaller Limits to Growth Plant roots compete for space, nutrients, and water with the roots of other plants Taller plants block out sun for shorter plants