Population Growth

Chapter 5

How do ecologists study populations?

• Geographic range: where are they distributed?

• Density and distribution: in what manner are they distributed?

• Growth rate: how is the population increasing or declining?

• Age structure: males and females? number at reproductive age?

Geographic range: manatees and dugongs

Patterns of distribution: why???

Clumped distribution

• Food – clumped around food source

• Protection or shelter

• Small differences in habitat: shade, soil, wind, water

• Patchy resources!

Uniform distribution

• Can be caused by competition for resources

• Some organisms have specialized mechanisms…

• Creosote bushes release terpines, chemicals which inhibit growth of other plants around

• Tree planting

• Farming

Random distribution• Happens when none of

these factors exist• Least common in nature

because biotic factors usually cause other type

• Homogeneous habitats (same all over)

• Wind dispersal of seeds• Water dispersal of larvae

Population growth

• Birth rate, death rate, immigration, emigration

• Resources are often the driver: food, water abundance or shortage, overcrowding

Exponential population growth

What caused human population expansion?

Logistic population growth• Stages: exponential growth, growth slows

down, growth stops

• Carrying capacity: number of organisms the area can support

Limiting factors

• Any factor that slows or stops the growth of a population

• Determine the carrying capacity

• Can be biotic or abiotic: availability of food or water, parasitism or disease, competition and predation, natural disasters such as floods or droughts

Density dependent limiting factors

• Act strongly when population density reaches a certain level

• Competition, predation, parasitism and disease, hunting, overcrowding stress

• Tend to be the BIOTIC limiting factors

Density independent limiting factors

• Affect all populations equally, regardless of population size or density

• Drought, floods, unusual weather such as hurricanes, wildfires

• Tend to be ABIOTIC limiting factors

Invasive species

• Any non-native species that is introduced to an ecosystem

• Accidental in intentional introductions• Don’t have the usual limiting factors

because the species didn’t evolve there:– No natural predators– No biological controls– Often short life-history species (rapid growth,

lots of reproduction

Examples:

• Brazilian pepper trees outcompete our mangroves

• Brown hoplo “armored catfish” in St. Johns River are outcompeting native fish we eat

• Jellyfish brought in ballast water of ships

• Invertebrates attached to oil rigs moving from ocean to ocean

Demography: study of human populations

• When you refer to “demographics” it usually means: ages, genders, ethnicities, socioeconomics

• Age structure very important: how many members of the population are reproducing?

The Demographic Transition

• Stage 1: high birth rate and high death rate

• Stage 2: high birth rate, but death rate begins to fall

• Stage 3: birth rate falls to meet the death rate

Factors that influence human birth and death rates: nutrition, sanitation, medical treatment, education, higher standard of living

Countries by population

Top 400 urban areas in 2006:1 million +

• Humans have a clumped distribution

• Notice that Florida has 3 dots!

Fertility rate by country: number of children (0-8)

More people = less resources

• Food competition: shortages, prices

• Other resources: fuels, water, space, trees

• Impact on the environment:– Deforestation– Air pollution– Water pollution– Overcrowding

What can we do? What should we do?

• Write a 1 page essay about human population growth

• Should we address population as a society, or leave it up to individuals?

• What are some of the things that we can do?