Bellwork: Describe the conditions under which logistic growth occurs

Chapter 5 - populations 5.1 How populations grow

How could one harmless plants cause widespread problems

In Florida the hydrilla population has got out of control Why?

Fish stocks in New England have plummeted I recent years Why are fish populations in decline?

What do both of these stories have in common? Both involve dramatic changes in the size of a population Population – A group of organisms of a single species that live in a given area Researchers study a population’s Geographic range Density Distribution Growth Rate Age Structure

Geographic range The area inhabited by a population Can vary from less than a cubic meter to continents

Density and distribution Population density = Number of individuals per unit area Different species have different densities – even in the same area

Patterns of distribution How individuals are spaced out

Growth rate High growth rate – population size is increasing Neutral growth rate – constant population size Negative growth rate – population is in decline

Age Structure Why is it important to know the age structure of a population Why is it important to know the distribution between male and females?

What factors affect population growth? Birth rate Death rate Immigration Emigration

Birth and Death rate If more people are born than die, a population will grow If birthrate is higher than it’s death rate Birth can mean different things to different species Mammals give birth Fish hatch from eggs Some countries have a higher death rate than birth rate

Immigration and Emigration Immigration – influx of organisms moving into an area Emigration – mass exodus of organisms due to a food shortage or, overcrowding or another reason….

Exponential growth Under ideal condition, with unlimited resources, a population will grow exponentially Why is this unrealistic?

Bacterial growth If one bacteria can divide every 30 minutes, how many bacteria could you have at the end of 24 hr period? Answer = 281,474,976,710,656

Organisms that reproduce slowly Example - elephants that were protected from hunting in the kruger national park (South Africa) Single offspring every 2 – 4 years

Organism in new environment Organisms can sometimes flourish in a new environment - why? Example – Burmese pythons in florida

Why don’t we see many examples of exponential growth? Many populations start with exponential growth At some point growth rate will slow down Increased competition for food, land etc… At some point growth rate will stop

Logistic growth When a population’s growth slows and then stops following exponential growth Growth can slow due to Decreased birth rate Increased death rate Immigration decrease Emigration increase

Carrying capacity When birth rate = death rate, and immigration = emigration, population growth stops Some small variation, but on the whole stable Where this line intersects y axis = carrying capacity Definition = The maximum number of individuals of a particular species that a particular environment can support A number of factors will stabilize population at this size

Writing exercise How is the carrying capacity of a cities roads similar to the carrying capacity of an ecosystem?