Chapter 4: Population

Biology

Population

• group of organisms, all of the same species, that live in a specific area

How fast do populations grow?

• Not linear growth – so it is not a straight line

• Graph- starts to increase slowly, then resembles a J-shaped curve

• Initial increase slow, b/c # of organisms that reproduce is small

• Increases b/c the total # of individuals that can reproduce has increased

Is Growth Limited?• J-shaped growth curve illustrates exponential

population growth

• Exponential Growth – means that as a population get larger, it also grows at a faster rate

What Can Limit Growth?

Population growth does have limits

• Limiting Factors – food, disease,

predators, or lack of space will

Cause population growth to SLOW

• Under these conditions the population may stabilize in a S-shape growth curve

Carrying Capacity

• the number of organisms of one species that an environment can support indefinitely

•  Developing population – more births than deaths and the population increases until the carrying capacity is reached or passed

• When a population overshoots the carrying capacity, limiting factors come into affect

• Deaths begin to exceed births and the population falls below the carrying capacity

Reproduction Patterns • Life-History Pattern- organisms

reproduction pattern

• Slow life-history pattern – slow rate of reproduction and produce relatively few offspring–   Ex. Elephant

• Fast life-history pattern – reproduce rapidly and produce many offspring in a short period of time– Ex. Mosquito

Rapid Life- History Patterns• Common among organisms in changeable

or unpredictable environments • Organisms have:

– Small body size– Mature rapidly– Reproduce early– Short life span

• reproduction rises rapidly then decline when environment becomes unsuitable

• population survives and will reproduce again when the environment is favorable

Slow Life- History Patterns

large species that live in more stable environments– Elephants, Bears, Whales, Humans, and plants

(trees)• reproduce and mature slowly• long-lived• maintain population sizes at or near carrying

capacity

Density Factors and Population Growth

 

3 Patterns of Dispersal:1. Random

2. Clumped

3. Uniform

Density-Dependent Factors

• disease

• competition

• predators

• parasites

• food

• increasing affect as the population increases

• Example- corn fields

• Crops grow close together

• Disease can spread rapidly and kill off the whole crop

Density-Independent Factors

can effect most populations regardless of their density• Most are abiotic factors

•   Volcanic eruptions• Temperature• Storms• Floods• Drought• Chemical Pesticides• Major habitat destruction

Usually affect smaller organisms more– Example – Mosquitoes – severe winters kill the adults of most species

Population Size

• limited by abiotic and biotic factors

• controlled by various interactions among organisms that share a community

• Predation – can effect population size in minor and major ways

• Locusts eat acres of lettuce on a farm

• Brown snake introduced into Guam, there were no native predators, and it preyed freely on native birds – drastic effect on the population size

• predator- prey relationships are known to experience cycles or changes in their numbers over periods of time

• prey population increases and there is more food for the predator, so the predator population increases

• predator population increase and predation increase, therefore – the prey population declines.

• With less food, the predator population decrease. • Cycle starts again

Competition within a Population

• density dependent factor

• few individuals compete for resources, usually no problems

• When population increase and the demand exceeds the supply, the population size decreases

Crowding and Stress• populations become

crowded, individuals exhibit stress

• stress symptoms– Aggression– decrease in parental care– decreased fertility– decreased resistance to

disease

• all have negative effects on a population

Human Population•  Demography – study of human population size

– density and distribution– movement– Birth and death rates

Human Population Growth

• Humans change their environment• Developed methods for producing

more food• infant mortality rate decreased• clean water• All enable people to live longer and

produce more offspring• Population Grows

Calculating Growth Rate

• Birthrate – number of live births per 1000 population in a given year

• Death rate – number of deaths per 1000 population in a given year

• Immigration – movement of individuals into a population

• Emigration – movement of individuals out of a population

Calculate a population’s growth rate by

• (Birthrate + Immigration Rate) – (Death rate + Emigration Rate) = Population Growth Rate

• PGR = (B + I) – (D + E)

• Immigration and emigration are not always accurate use:

 • PGR = Birthrate – Death rate

• Positive Growth Rate– unless the growth rate becomes negative the

population continues to grow, just slower

• Doubling Time – time needed for a population to double in size

• Age Structure – refers to the portions of the population that

are in the different age levels