# How Do Populations Change in Size?

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How Do Populations Change in Size?

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Populations A population is all the members of a species living in the same place at the same time.
A population is a reproductive group because organisms breed with members of their own population. Ex: Daisies in a field in Mt. Holly will breed with each other, not with daisies in Kentucky.

Properties of Populations Properties are used to describe populations and to predict changes within them 1) Population Size 2) Population Density – number of individuals per unit area or volume (ex: number of trout per cubic meter of water in a lake) 3) Population Dispersion – the relative distribution/arrangement of its individuals within a given amount of space (ex: even, clumped, or random) Flamingos live in clumped flocks Rattlesnakes are randomly dispersed

How Does a Population Grow?
A population gains individuals with each birth and loses them with each death. Growth Rate = Birth Rate – Death Rate THINK – PAIR - SHARE 1) What happens to the growth rate if there are more births than deaths? 2) What happens to the growth rate if there are more deaths than births?

How Do Populations Change in Size? THINK ABOUT IT…
Wild female rabbits can have up to 12 babies per litter (average 5/litter). Female rabbits can have between 1-7 litters per year (average 3-4/year). Theoretically, how many babies could 1 female rabbit birth in one year? If half of the babies from #1 are female, how many more baby rabbits could be born the next season? What would happen to the population if these birth rates continued? What happens to keep the population from climbing indefinitely? 84 babies 3528 babies The population would increase The environment limits their survival

Population Growth Activity
1) Start out with 100 g of beans. 2) Count out 5 beans. These represent the starting population of a species. 3) Assume that each year 20% of the beans each have 2 offspring. 4) Assume that 20% of the beans die each year. 5) Calculate the number of beans to add or subtract for 1 year. Round to the nearest whole number. 6) Continue for 10 years. Graph your results. Year 1 2 3 4 5 6 7 8 9 10 Starting Births Deaths Ending

How Fast Can a Population Grow?
Reproductive Potential – the maximum number of offspring that an organism can produce. Different organisms have different generation times (the average time it takes a member to reach reproduction age) Ex: human vs. bacteria RP increases with organisms that produce more offspring at a time, reproduce more often, and reproduce earlier in life.

Exponential Growth Exponential Growth – increased growth that gets faster and faster over time Occurs in nature only when a population has: Plenty of food and space Little to no competition or predators Occurs regularly in bacteria and mold growth

Think – Pair - Share How does the curve of this graph show exponential growth? What would the graph look like if it had linear growth? How does exponential growth relate to reproductive potential?

Think – Pair - Share How does the curve of this graph show exponential growth? The curve rises more and more steeply, meaning the population increases by greater amounts during each time period. What would the graph look like if it had linear growth? It would show a straight line, increasing by the same amount during each time period. How does exponential growth relate to reproductive potential? Most organisms have the potential to reproduce multiples of themselves, thus creating exponential growth rates. Exponential growth shows nearly unlimited population growth to a point.

Bacteria Colony Growth
Observe the growth of bacteria. Graph the growth. 1) Is this growth exponential? Explain. 2) Will the bacteria continue to grow at this rate indefinitely? Why or why not?

What Limits Population Growth?
Carrying Capacity – the maximum population that the ecosystem can support indefinitely.

Think About It 1) How was the reindeer population able to increase so quickly? 2)Why did the reindeer population decline? 3) How is this related to carrying capacity?

How does a population reach its carrying capacity?
When resources become limited Limiting resource – when a resource is consumed at the same rate as it is being produced by the ecosystem. Ex: plant growth is limited by water supplies, mineral nutrients, and sunlight. Competition within a population As the population approaches the carrying capacity, members of the population begin to compete for limited resources Territory – an area defended by one or more individuals against other individuals because of the space, shelter, food , or breeding sites it contains.

Population Regulation
Population growth is regulated by deaths Density Dependent – death that happens when individuals of a population are densely packed together. Ex: Limited resources, predation, and disease Density Independent – deaths that affect all members of a population in a general way. Ex: Severe weather and natural disasters Diseased trees in a forest Winter storm froze plants

Favorite Populations Choose a favorite plant or animal. Write it down.
Answer the following questions: Where in the world can you find populations of this organism? What kinds of resources are limiting to its growth? How are the individuals dispersed within their habitat? How do the organisms find each other to mate? How many offspring do they produce on average?

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