Lecture 12: PopulationsLecture 12: Populations

Human Population

Humans live on every continent, but are not evenly spread

Concentrated on coasts

Asia, esp. China and India, has 56% of world’s population

Human PopulationsGrowth rate is the difference between the number of individuals born and the number of individuals who die each year

Human growth rate is approximately 1.3% annually

At this rate, the doubling time of the human population is 35 years

Human Population

A doubling of the population means a doubling of the resources needed to sustain those individuals

However, people in different parts of the world do not use resources in the same way

Carrying capacity

Population growth becomes zero when there are not enough resources for the population to continue to grow

Carrying capacity is the number of individuals an environment can sustain

What is Earth’s carrying capacity for humans? We don’t really know.

Different Regions

More developed countries have a high standard of living, and a lower growth rate in general

US has one of the highest growth rates for a more developed country: 0.6% vs. 0.2% for much of Europe

Different RegionsLess developed countries have a lower standard of living and a higher growth rate

Asia has the highest growth rate, due to high childbirth and relatively low adult mortality

Africa has a lower growth rate, even though birth rates are very high, adult mortality is also high due to HIV/AIDS

Replacement Reproduction

This is each couple having two children- replaces themselves

Will eventually result in zero growth rate, but perhaps not initially if many children are entering the population

Environmental Impact

Impact of a population measured by

1. per capita resource consumption

2. pollution per unit of resource used

Overpopulation can be seen in less developed countries, but more developed countries use many many more resources

Ex. more developed countries use 60% of fossil fuels, 80% of metals

Populations

When the environment is stable, populations become adapted to it and tend to increase to carrying capacity

When the environmental conditions change, the population must adapt again- the populations tends to decrease in size until adapted again

ResourcesResources- components of the environment needed to support organisms

light, space, water, food, mates, shelter

Availability of resources influences distribution of organisms

Distribution

Can be clumped, random, or uniform

A species occurs in a range- an area that has appropriate resources for it

Population density- how many individuals occur per unit area

In general, declines with increasing body size

DemographicsAvailability of resources + population characteristics = demographics

Age structure- how many of each age class

Ex. box turtle populations

Survivorship- how the age of death influences population size

Type I- death likely at old age

Type II- death risk same for all ages

Type III- death risk high when young

Demographics

Biotic Potential- highest possible rate of population growth given unlimited resources

Number of offspring, survival until reproductive age, etc.

Patterns of Population GrowthExponential growth- can typically last only as long as resources are unlimited

Patterns of Population GrowthLogistic Growth- Population growth levels off when resources are limited

Patterns of Population Growth

Logistic growth has 4 phases:

Lag phase- growth is slow b/c number of individuals is small

Exponential phase- growth is accelerating due to biotic potential

Deceleration phase- rate slows due to increased competition among individuals

Stable equilibrium phase- births and deaths are about equal

Patterns of Population GrowthStable eq. phase occurs at the carrying capacity for that system

Uses- for resources used by humans, we want to keep things either stable or in exponential phase, because if get too few, will be in lag phase

Ex. fisheries

For pests, best to reduce carrying capacity, not individuals

Human Growth Curve

Factors that Regulate Growth

Density- Independent Factors- many abiotic factors

Weather, natural disasters

Intensity of effect not dependent on population size

Factors that Regulate Growth

Density-Dependent Factors- Usually biotic

Percentage of population affected depends on population size

Typically interactions between individuals

Density-DependentCompetition- individuals try to use a resource that is in limited supply

Not all individuals will get enough of that resource to survive

If more or less of the resource becomes available, the population size may shift

Resource partitioning can occur among different age classes

Density- DependentPredation- One organism eats another

Effect increases as prey increase in density, because they become easier to find (competition for hiding spots)

Cycles often exist between predator and prey densities

Snowshoe Hares & Canada Lynx

Life History

Life history- the mix of characteristics and traits that one species exhibits

Two general patterns:

Opportunistic species

Equilibrium species

Life HistoryOpportunistic species:

exponential growth

small

mature early

short life

many offspring

ex. insects, weeds

Life HistoryEquilibrium species:

logistic growth

best adapted individuals leave most offspring

large

long life

few offspring

slow to mature

Ex. birds, large mammals

ExtinctionWhen every individual of a species disappears

Opportunistic species at less risk for extinction

Three factors influence risk for equilibrium species

Size of geographic range

Habitat tolerance

Size of local populations

Extinction

Most resistant to extinction will have wide range, large population size, and wide habitat tolerance, least resistant will be the opposite

What would the most vulnerable population look like?

Ecology

The study of the interactions between all the living and non-living parts of an environment