Populations

Remember a population is… A group of the same species in the same area at the same time. A group of the same species in the same area at the same time. The boundary of a population can be physical or arbitrary for the purpose of study The boundary of a population can be physical or arbitrary for the purpose of study Populations have 4 important properties that interact to define the population: Populations have 4 important properties that interact to define the population: 1. Size 2. Density 3. Dispersion 4. Growth rate

Population Size and Density: 1. Population size The number of individuals it contains The number of individuals it contains Can be difficult to measure so, many times sizes are estimates, usually done by sampling Can be difficult to measure so, many times sizes are estimates, usually done by sampling 2. Population density How crowded the population is (on average) How crowded the population is (on average) Expressed as the number of individuals per unit of area or volume Expressed as the number of individuals per unit of area or volume

3. Population Dispersion The spatial distribution of individuals in the population The spatial distribution of individuals in the population Can be dependent on the area being measured, resources available, or social interactions or the individuals Can be dependent on the area being measured, resources available, or social interactions or the individuals Three types: Three types: Clumped – Individuals are clustered Clumped – Individuals are clustered Even – Individuals are consistently spaced Even – Individuals are consistently spaced Random – Individual location is independent of other individuals Random – Individual location is independent of other individuals

Population Dispersions

4. Growth Rate The amount a populations size changes in time The amount a populations size changes in time Dependent on 4 processes Dependent on 4 processes Birth Birth Death Death Immigration Immigration Emigration Emigration When calculating growth rate, scientists assume immigration and emigration cancel out so: When calculating growth rate, scientists assume immigration and emigration cancel out so: Growth rate = birth rate – death rate There are 2 models of population growth There are 2 models of population growth Exponential growth model Exponential growth model Logistic growth model Logistic growth model

The changes in populations size and composition The changes in populations size and composition Dependent on: Dependent on: Birth rate - # of individuals born in a time period Birth rate - # of individuals born in a time period Death rate - # of individuals dying in a time period Death rate - # of individuals dying in a time period These can affect the make up of a population including the age structure and mortality curves These can affect the make up of a population including the age structure and mortality curves

Exponential Growth Model Defined as a population that grows rapidly after the first few generations Defined as a population that grows rapidly after the first few generations Assumes the birth and death rates are constant, no matter the population size Assumes the birth and death rates are constant, no matter the population size J-shaped curve J-shaped curve

Exponential growth model (cont.) Predictions Predictions The larger the population gets the faster it grows The larger the population gets the faster it grows Limitations Limitations Occurs only rarely, under specific conditions, and for short periods of time Occurs only rarely, under specific conditions, and for short periods of time All populations have limiting factors that prevent exponential growth, for example resources of food, water, and space. All populations have limiting factors that prevent exponential growth, for example resources of food, water, and space.

Logistic Growth Model Expands exponential growth model to include the limiting factors Expands exponential growth model to include the limiting factors Identifies a carrying capacity (K), the number of individuals the environment can support over time Identifies a carrying capacity (K), the number of individuals the environment can support over time S-shaped curve S-shaped curve

Logistic growth model (cont.) Predictions Predictions When populations are small birth rates are high and death rates are low When populations are small birth rates are high and death rates are low When populations approach carrying capacity, birth rate falls and death rate rises When populations approach carrying capacity, birth rate falls and death rate rises At the carrying capacity birth rates and death rates are equal and growth stops At the carrying capacity birth rates and death rates are equal and growth stops Limitations Limitations Carrying capacity is not constant, it fluctuates with the environment Carrying capacity is not constant, it fluctuates with the environment

Population Regulation There are 2 types of limiting factors There are 2 types of limiting factors Density Independent – reduce the population by the same amount, no matter the size. Density Independent – reduce the population by the same amount, no matter the size. Ex: weather, floods, fires Ex: weather, floods, fires Density Dependent – reduce the population more the higher the population. Density Dependent – reduce the population more the higher the population. Ex: food, shelter Ex: food, shelter Limiting factors can cause population fluctuations Limiting factors can cause population fluctuations Population size fluctuations are natural. Some are environmentally connected others are not Population size fluctuations are natural. Some are environmentally connected others are not