 Populations

What is a population?  Population: a group of organisms all of the same species, which interbreed and live in the same place at the same time  Ex: Humans in Negaunee  Grey squirrels in Marquette  White-tailed deer in Ishpeming

Population Growth  There are three main factors that determine how fast a population will grow  1. Birth Rate  2. Death Rate  3. Migration Rate  Immigration: movement into a population  Emigration: movement out of a population

Types of growth  There are two main types of population growth  1. Exponential  2. Logistic

Exponential Growth  As a population gets larger, it grows at a faster rate  Ex: The population doubles with each generation  Occurs when resources are unlimited and conditions are optimal  Results in unchecked growth  Cannot continue indefinitely

Exponential Growth: “J” Shaped Curve  Starts off slow  Eventually grows faster and faster  Unchecked growth  Does NOT continue indefinitely

Logistic Growth  A population cannot grow indefinitely. Eventually, certain factors will slow population growth.  Limiting factors such as food, disease, predators, and lack of space will cause population growth to slow down and stabilize.

Logistic Growth: “S” Shaped Curve  Growth starts fast but limiting factors cause growth to slow and reach a stable size  Populations will slow down due to either an increased death rate or a decreased birth rate  Will reach a stable level called carrying capacity

Carrying Capacity  Carrying capacity is the number of organisms of one species that an environment can support  Once the carrying capacity is reached, certain factors work to keep population in check (including lack of food, overcrowding, predations, accumulation of waste)

Carrying Capacity  If a natural population overshoots the carrying capacity, three things can happen:  1. It will die back to the original carrying capacity  2. It will die back but the damage to the environment will lower carrying capacity  3. It will become extinct

Limiting Factors  Factors that limit the growth and size of a population  Two types:  1. Density Dependent  2. Density Independent

Bill Nye Video  1:20-3:07, 12:05 – 13:05 

Density Dependent Factors  Depend on the size of a population  Increase in effect as the population increases  Factors such as disease, competition, predation, parasitism, crowding stress  Usually not enough to eliminate a population

Density Independent Factors  Affect populations regardless of density  Usually abiotic factors  Ex: pollution, habitat destruction, natural disasters, weather, temperature, chemical pesticides  Usually just brings populations below carrying capacity but these factors have the potential to cause a population to become extinct

Predator Prey Relationships  Predator and prey populations follow similar patterns  If prey populations are high, the predator population will also be high  If prey populations are low, predators will have less food and their population size will decrease

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Dynamics in a Population: Age Structure  1. Very young and very old are more susceptive to disease  2. If there are huge numbers of young adults, the population will grow; if there are mostly elderly, the population will decline.

Age Structure

Sex Ratios  In a monogamous species, the ratio of males to females should be about equal.  Ex: Humans  In deer and lion groups, this is not as important, because one male often fertilizes many females.

Behavior 1. Territory- a defended area which insured the occupants will have enough resources for themselves and their offspring. 2. Social hierarchy – wolves, chickens, social status determines which individuals eat or breed.

The End!  Any questions?