Populations CHAPTER 5. 5.1 Describing Populations ▪ Geographic Range ▪ Density and Distribution ▪ Growth Rate ▪ Age Structure.

PopulationsCHAPTER 5

5.1 Describing Populations

▪ Geographic Range

▪ Density and Distribution

▪ Growth Rate

▪ Age Structure

Describing Populations

▪ Geographic Range: The area inhabited by a population

▪ For some, the range is very small (bacteria) while others have a large geographic range (fish)

Describing Populations

▪ Density: The number of individuals per unit area

-Ex: Small density of ducks versus large density of fish in a pond

▪ Distribution: how individuals in a population are spaced out across the range of the population

- Randomly, Uniformly, or Clumped

Describing Populations

▪ Growth Rate: determines whether the populations increases (positive) , decreases (negative) , or stays the same.

Describing Populations

▪ Age Structure: the number of males and females of each age in the population

- Why? Most animals cannot reproduce until a certain age, and only females can produce offspring.

Population Growth

▪ Factors that affect population size are ▪ birth rate▪ death rate▪ rate at which

individuals enter (immigration) or leave (emigration) the population

Population Growth

Birth Rate

▪ Increases the population size if it’s greater than the death rate

Death Rate

▪ Decreases the population size if it’s greater than the birth rateIf birth rate = death rate, population stays about the same size

Population Growth

Immigration

▪ Individuals moving into the area (grows the population)

Emigration

▪ Individuals moving out of the area (decreases the population)

Exponential Growth

▪ The larger a population gets, the faster its population grows

▪ Under ideal conditions with unlimited resources, a population will grow exponentially

▪ Usually occurs with organisms that reproduce rapidly, mature at a young age, and produce many offspring at once.

Exponential Growth

Logistic Growth

▪ Organisms can’t grow exponentially… otherwise we’d have world domination! So what DOES happen?

Logistic Growth

▪ Phases of Growth:▪ Exponential growth: resources

are unlimited, few individuals die, more offspring are produced and reproduction is rapid

▪ Growth slows down: population is STILL growing, just at a much slower rate due to outside influences

▪ Growth Stops: rate of population growth drops to 0, population will usually remain stable at this size

Logistic Growth

▪ Graph has an S shape as it goes through the different phases

▪ Most populations experience this kind of growth, especially when first introduced into the area

▪ What are some reasons that the population growth may slow down?▪ Death rate increases▪ Birth rate decreases▪ Immigration decreases▪ Emigration increases

Carrying Capacity

▪ The period at the end of the graph where it’s not growing is because it has reached its carrying capacity: the maximum # of individuals of a particular species that a particular environment can support.

5.2 Limits to Growth

▪ Limiting factor: a factor that controls the growth of a population.▪ Competition▪ Predation▪ Parasitism and disease▪ Natural disasters▪ Unusual weather▪ The first three depend on population density…

the last two do not.

Limits to Growth

▪ Acting separately or together, limiting factors determine the carrying capacity of an environment for a species.

▪ They keep populations from becoming extinct or overrunning the planet.

Density-Dependent Limiting Factors

▪ Affect large, clumped populations – too many individuals

▪ These factors include▪ Competition▪ Predation▪ Herbivory▪ Parasitism▪ Disease▪ Stress from overcrowding

Density-Dependent Interactions

▪ Competition: Too much crowding leads to competition for food, water, space, sunlight, and other essentials▪ Can lower birthrates, increase death rates, or both.▪ Is density-dependent because the more individuals in an

area, the sooner they use up all the resources.▪ Can be between the same species or different species

Density-Dependent Interactions

▪ Predation and Herbivory: Too many predators can affect the prey population, which can in turn affect the plant population. ▪ Continuous cycle of up and down:

Wolf population will increase when Moose population increases, but eventually the wolf will eat more Moose than are being produced and the Moose population will drop, and so will the Wolf population.

▪ Same with Moose and their plant food – cycle of up and down

▪ Can happen with Humans as predators – our fishing cycles

Density-Dependent Interactions

▪ Parasitism and Disease: Density dependent because the more host population there is, the easier the parasites and infection can spread.

Density-Dependent Interactions

▪ Stress from Overcrowding: Too many species causes fighting.▪ Some females will be so stressed that they neglect, kill or

even eat their own offspring▪ Causes lower birthrates, higher death rates, and can

increase the rate of emigration (leaving of individuals).

Density-Independent Interactions

▪ Affects all populations in similar ways, regardless of population size and density▪ Examples: hurricanes, droughts, floods, wildfires, tornadoes,

etc. ▪ Populations may or may not bounce back

5.3 Human Population Growth

▪ The human population, like populations of other organisms, tends to increase. The rate of that increase has changed dramatically over time.

▪ We are expected to reach over 10 billion by 2030!

Human Population Growth

▪ Was slow at first.. Half the children born didn’t make it to adulthood due to disease and very little food.

▪ Families had many children to make sure some survived to continue the family line.

Human Population Growth

▪ Civilization advanced, leading to improved:▪ Nutrition▪ Sanitation▪ Medicine▪ Healthcare

▪ Death rate lowered, but birth rate dramatically grew… leads to what kind of growth??

Human Population Growth

▪ Thomas Malthus knew this exponential growth could not continue, and so he predicted that the limiting factors for the human population would be:▪ Famine (limited resources)▪ War (competition)▪ Disease (disease)

Patters of Human Population Growth

▪ We have been growing exponentially since the 1900’s… yet we are starting to slow down (although increasing!)

▪ We may be entering the second phase of logistic growth! Why???

Patterns of Human Population Growth

▪ Demography: the study of human populations▪ looks at birth rates, death rates, and age structure to help

predict why some countries have a higher growth rate than others.

Patterns of Human Population Growth

▪ Demographic Transition: has 3 stages

1. Birth rate and Death rate are equally high

2. Death rate begins to fall, but birth rate remains high for a time

3. The birth rate falls to meet the death rate

▪ The U.S., Japan, and Europe have already completed this transition. Developing countries like Guatemala are still in stage 2.

Patterns of Human Population Growth

▪ Scientists look to age structures to help determine whether a country is growing or not.

United States is stable, age structure is balanced Madagascar is growing, looks like a pyramid