Chapter 4Population Ecology

Lesson 4.1Lesson 4.1

Studying Ecology

Levels of Ecological Organization

–The study of how organisms interact with each other and with their environments

– Scientists study ecology at various levels of organization.

Biotic and Abiotic Factors

• Biotic factors: Parts of an ecosystem that are living or used to be living

• Abiotic factors: Parts of an ecosystem that have never been living

Did You Know? Decaying organisms are biotic factors as long as their structure remains cellular.

Brainstorm Examples

Biotic Factors1. ___________________

2. ___________________

3. ___________________

4. ___________________

5. ___________________

Abiotic Factors1. ___________________

2. ___________________

3. ___________________

4. ___________________

5. ___________________

Ecological Levels of Organization

BBECPO

Levels of Organization

– Species: group of similar organisms that can breed and produce offspring

– Population: group of individuals of the same species living in the same area at the same time– Must be able to interbreed

– Each pond has a separate population

– Community: group of populations living and interacting with each other in a common habitat– Only biotic factors

Levels of Organization

Adding in ABIOTIC factors

– Ecosystem: all the organisms that live in a place, together with their physical environment

– Biome: collection of ecosystems sharing common climatic conditions

– Biosphere: part of the Earth inhabited by organisms 

Component Local Example Global Example

Species

Population

Community

Ecosystem

Biome

Habitat

–The specific environment in which an organism lives

–Habitats provide an organism with resources—anything an organism needs to survive and reproduce, including food, shelter, and mates.

Lesson 4.2Lesson 4.2

Describing Populations

Population Size

• The number of individuals in a population at a given time

• Sudden and dramatic decreases in population size can indicate an unhealthy population headed toward extinction. 

• Ecologists often use samplingtechniques to estimate population size.

Did You Know? The passenger pigeon was once North America’s most abundant bird. Hunting drove them to extinction in less than 100 years.

Counting Laysan Albatross Nests

Population Density

• Measure of how crowded a population is

• Larger organisms generally have lower population densities.  Why?

• Low population density: More space, resources; finding mates can be difficult (Angler Fish)

• High population density: Finding mates is easier; tends to be morecompetition; more infectious disease; more vulnerability to predators Northern pintail ducks

Learning Check

–Who would have a higher population density?– Polar Bears: Roam large territories alone.  Mothers will care for cubs for a period of time after their birth.  Eventually the cubs will go off on their own, leaving the mother alone again.

– Flying Fish: Live in large schools of 100 or more fish.  They cooperate to find food and avoid predators.

Population Distribution

• How organisms are arranged within an area:

• Random distribution: Organisms arranged in no particular pattern

• Uniform distribution: Organisms evenly spaced

• Clumped distribution: Organisms grouped near resources; most common distribution in nature

Learning Check

–Who has which distribution?– Lions live in groups called prides.  Prides will always live close to a watering hole.

– Dandelion seeds are dispersed by the wind.  The plant grows wherever its seed lands.

– Sage plants secrete toxins from their roots killing other plants near by.  This leads to all of the sage plants being equally spaced in their environment. 

Sex Ratios

• Proportion of males to females

• Age structure diagrams give information about sex ratios.

• For a monogamous species, the ideal sex ratio is 50:50.

Learning Check

– Are their more males or more females?– Bald Eagles are monogamous and mate for life.

– The males of a species of marsupial mouse die after mating season leaving a population of only pregnant females to eventually raise the next generation.

Measuring Populations

– Random Sampling: Works well with stationary organisms– Uses quadrats

– Capture‐Mark‐Recapture: works better with moving organisms– Catch animal

– Tag and release animal

– Recapture and see the percentage of the animals that were tagged from the first group.

Capture-Mark-RecaptureFor this method to work…

– Sample must me truly random– Tag can’t hurt animal– Tag can’t prevent movement– Tag can’t make it easier to be found by predators– Population can’t change between captures– Not too long in between captures

– Tagged animals must be able to mix back into the population– Not too short in between captures

Lesson 4.3Lesson 4.3

Population Growth

Birth and Death Rates

• A population’s relative birth and death rates (mortality and natality) affect how it grows.

• Survivorship curves show how the likelihood of death varies with age.

K and R Strategies

r‐strategist– Short life

– Rapid growth

– Early maturity

– Many small offspring

– Adapted to unstable environment

– Prey

K‐strategist– Long life

– Slower growth

– Late maturity

– Fewer large offspring

– High parental care and protection

– High investment in individual offspring

– Adapted to stable environment

– Predators

Immigration and Emigration

• In addition to births and deaths, population growth is affected by immigration and emigration—individuals moving into and out of a population.

• Migration, seasonal movement into and out of an area, can temporarily affect population size.

• Determined by the following equation:  (birthrate + immigration rate) – (death rate + emigration rate)

• Growing populations have a positive growth rate; shrinking populations have a negative growth rate.

• Expressed in terms of individuals per 1000

Calculating Population Growth

Exponential Growth

• Population increases by a fixed percentage every year.

• Normally occurs only when small populations are introduced to an area with ideal environmental conditions

• Rarely lasts long

Logistic Growth and Limiting Factors

• Growth almost always slows and stops due to limiting factors.

• Limiting factors: Environmental characteristics slow population growth and determine carrying capacity.

• Density‐dependent: change with population density.

• Density‐independent: Does not change with population density.

Limiting Factors

– Exponential Growth– Only occurs when a species lives under perfect conditions (enough food, water, space)

– Limiting factors exist– Density‐Dependent Limiting Factors

– Usually biotic factors

– Density‐Independent Limiting Factors– Usually abiotic factors

Density-Dependent Limiting Factors

– Effects increase with increasing population size

– Act as negative feedback mechanisms to stabilize population

– Internal Factors: act within a species– Food supply, availability of territories, reduced fertility

– External Factors: act between different species– Predation (Predator/Prey Cycles)

– Disease (Black Death)

Density-Independent Limiting Factors

– Abiotic factors not related to population density– Weather: Short‐Term (storms)

– Climate: Long‐Term (Cold Winter)

– Volcanic Eruptions

– Floods

–Not part of the feedback system…

Biotic Potential

• An organism’s maximum ability to produce offspring in ideal conditions

• Many factors influence biotic potential, including gestation time and generation time.

• Organisms with high biotic potential can recover more quickly from population declines than organisms with low biotic potential.

• Would an r or K species be higher?