Lecture 13: Population Growth

Covers Chapter 26

Some Definitions*

• Population: all of the members of a particular species that live within an ecosystem

• Community: a group of interacting populations• Ecosystem: all of the living and non-living components of

a defined geographic area• Biomes: Large land areas with similar environmental

conditions and characteristic plant communities• Biosphere: all living and non-living components covering

Earth’s surface• Ecology: the study of interrelationships of organisms with

each other and with their non-living environment

Population Change

• *Populations change through – Births– Deaths– Migration: net migration depends on

• Immigration (entering the population) • Emigration (leaving the population)

• The natural increase in population is difference between births and deaths.

• The total change in a population size is births minus deaths plus net migration.

births immigration

deaths

(births deaths) (immigrants emigrants) change in population size

emigration

Population Change

Fig. 26-1

2 opposing factors affect population change*

• Biotic Potential: maximum rate at which a population could increase assuming ideal conditions

• Carrying Capacity: maximum population size that can be sustained– for an extended period of time*

– without damage to the ecosystem*

Biotic Potential stimulates population growth

• Factors influencing biotic potential:– Age at which organism first reproduces*– Frequency of reproduction*– Average number of offspring produced each

time an organism reproduces*– Length of organism’s reproductive lifespan*– Death rate of individuals under ideal

conditions*

Carrying Capacity limits population growth

• If population exceeds carrying capacity, surrounding resources cannot feed all of the members of the population…then – the population will stop growing (and eventually begin

to shrink)

– the carrying capacity will shrink (because resources cannot renew themselves quickly enough)*

Fig. 26-6a

(a) An S-shaped growth curve stabilizes at carrying capacity

Growth stops and thepopulation stabilizes close to the carrying capacity

Populationgrows rapidly

Growthrate slows

Carrying Capacity Adjusts

Fig. 26-6b

(b) Consequences of exceeding carrying capacity

High damage; the carrying capacity ispermanently lowered

Low damage; resourcesrecover, and the population fluctuates

Extremedamage; thepopulationdies out

The populationovershoots thecarrying capacity; the environmentis damaged

Environmental Resistance*

• Degree to which the living and non-living environment limits population growth.

• A way to quantify environmental resistance is to divide factors that limit population growth into – density-dependent factors

– density-independent factors

Density dependent and density independent factors: overview*

• Density Dependent: factors that limit population size more when populations are larger– Competition– Predation– Parasitism– Availability of Nutrients– Availability of Space– Availability of Energy

• Density Independent: factors that limit population size regardless of how large/concentrated the population is– Climate and Weather– Human activities: pesticides, pollution

Density Dependent*• These factors become more effective at

limiting growth as the population grows/becomes more dense:– Competition: interactions among individuals

who attempt to use the same resources-harms both species involved

– Predation: organisms kill/eat other organisms– Parasitism: one organism (parasite) feeds off of

a larger one (host)-harms the host only

Fig. 26-11

bean weevils (prey)

A high predator population reduces the prey population

The prey populationpeaks when thepredator populationis low

braconid wasp (predator)

More next lecture

• Next lecture will be a more in depth look at competition, predation, etc…..community interactions.

• Today we are talking more about how these factors affect population growth.

Density Independent*

• Climate and weather

• Human activity– Some populations evolve adaptations to survive

climate/weather• thick coats for winter that are shed in

spring/summer

• Migration to warmer climate or one with more resources for a period of time

• Plants have dormant period during winter

Population Distribution*

• Populations show characteristic spacing, which may vary with time:

• Clumped: members live in groups (herds, packs, prides, flocks and schools)

– Many eyes to see food

– Protection from predators

– Some species clump near resources (trees near water)

• Uniform: members maintain a relatively constant distance between each other

– Territorial behavior: stay farther apart from others because of limited resources (plants, birds, etc)

• Random: rare

– Resources equally available

– Trees and other plants mostly

Populations Distribution

• Great pacific media (You Tube)

Clumped Distribution

Fig. 26-14a

Uniform Distribution

Fig. 26-14b

Random Distribution

Fig. 26-14c

What about the human population?

• Demography: study of the changing human population– Measure populations in different countries/regions– Track population changes– Make comparisons between developed and third world

countries– Examine birth/death rates among different sexes &

races– Attempt to estimate future population changes– Evaluate the impact of these changes

Humans• Human population growth used to be

exponential: continually accelerating increase. Now it seems we may be leveling off…..at least some think so.

• It took 200,000 years to get to 1 billion (1804), but now in 2012 we are at 7 billion– 1.4 million born each week– What about environmental resistance?– Are we at carrying capacity?

7 billion: Nat Geo Magazine

• You Tube

Humans and Environmental Resistance

• Humans have encountered resistance, but we have developed ways to overcome it:– Discovered fire*– Tools & weapons to kill food*– Shelter and clothing to live in cold areas*– Domesticated crops and animals for food*– Industrial and medical advances*– Scientific advances (discovery of bacteria)*

Population Growth is different in developed & developing countries

• Many factors determine population growth in different countries

Developed countries

• High standard of living• Access to modern technology• Access to medical care, including contraception• Education/employment opportunities• Decreased death rates, increased life span, more

stable birth rates• US is fastest growing developed country

Developing countries

• Higher birth rates (less access to contraception)

• More poverty

• Lack of education

Europe: 0.0%

Latin America/Caribbean: 1.5%

Asia (excluding China): 1.5%

Developing countries average: 1.5%

Africa: 2.4%

N. America: 0.6%

World average: 1.2%

Developedcountries average: 0.2%

China: 0.5%

Fig. 26-21