APES Unit 3

Population

Individuals of a particular species that inhabit an area.

Population size

Number of individuals present at a given time

Population density

The number of individuals in a population per unit area

High Density Advantages and disadvantages

• Easier to find mates

• Increased competition and vulnerbility to predation

• Increased transmission of diseases

• Sometimes causes organisms to leave and area if too dense

Population distribution (dispersion)

Spacial arrangement of organisms

• Random - Haphazardly located individuals, with no pattern

• Uniform: Individuals are evenly spaced

• Clumped: Organisms found close to other members of population (most common in nature, cluster around resouces)

Sex ratio

Proportion of males to females in a population

• In monogamous (one partner) species, a 1:1 sex ratio maximizes population growth

• Most species are not monogamous, so ratios vary

Age Distribution (structure)

The relative numbers of organisms of each age in a population

Three age categories

• Pre-Reproductive age

• Reproducing (can contribute to population growth)

• Post-Reproductive age

Natality

Births within the population

Mortality

Deaths within the population

Immigration

arrival of individuals from outside the population

Emigration

departure of individuals from the population

Net migration rate =

immigration - emigration

Crude birth (death) rates

Number of births (or deaths) per 1000 individuals per year

Population Change

(Births + Immigration) - (Deaths + Emigration)

Birth and immigration add individuals, while death and emigration remove individuals

• Positive growth = births > deaths

• Negative growth = birth < deaths

• No growth: births = deaths

Natural rate of population increase =

crude birth rate - crude death rate

Population growth rate

(Crude bith rate + immigration rate) - (crude death rate + emigration rate)

Growth rate as a percentage

Population growth rate x 100

Exponential growth (growth is exponential)

When a population increased by a fixed percentage

• Graphed as a J-curve

• Positive feedback loop (the more you have the more reproduction can occur)

• cannot be sustained indefinitely

Limiting factors

Physical, chemical, and biological attributes of the environment that restrain population growth

Ex: Space, food, water, mates, shelter, suitable breeding sites, temperature, disease, predators

Carrying capacity

The maximum population size of a species that its environment can sustain

limiting factos slow and stop exponential growth

• S-shaped graph, logistic growth

Density-dependent factors

Limiting factors whose influence is affected by population density

• Increased density increases the risk of predation, disease, and competition

• Larger populations have stronger effects of limiting factors

Density-independent factors

limiting factors whose influence is not affected by population density

• Events such as floods, fires, and landslides

Biotic potential

An organism’s capacity to produce offspring

• Basicall growth rate given no restrictions, usually determined by reproduction rate

K-selected species

species with long gestation periods, few offspring, and strong parental care

• low biotic potential

• Experience natural selection at carrying capacity, fight against themselves (compettition), on average 2 children will replace their parents

r-selected species

species that reproduce quickly and offer little or no care for offspring

• Have a high biotic potential

• Populations fluctuate greatly

Survivorship curves

graphs that show that the likelihood of death varies with age

Type I survivorship curve

Higher death rate at older ages, typical of larger animals.

Type II survivorship curve

Same death rate at all ages, typical of medium-sized animals.

Type III survivorship curve

Higher death rate at young ages, typical of small animals and plants.

Rule of 70 (doubling time)

Doubling time of population can be predicted by dividing 70 by % population growth rate

DT (years) = 70/GR (%)

Thomas Malthus

said that humans will outstrip food supplies and War, disease, starvation reduce populations

What causes human population growth

• Education

• More agricultural

• Better medical care

• Better sanitation

• Decline in death rates

What increases birth rates

• Historic need for farm labor

• Restrictions on women’s rights

• prohibitions on birth control

I = P ⋅ A ⋅ T ⋅ S

Total impact (I) on the environment results from:

Population (P)

Affluence (A)

Technology (T)

Sensitivity (S)

Biocapacity

the amount of biologically productive land and sea available to us

Demographic fatigue

occurs when governments face overwhelming challenges related to population growth

Demography

the application of population ecology to the study of change in human populations

Age Strutcture diagrams

Wide base: Many young, High reproduction, rapid population growth

Even age distribution: remains stable as births keep pace with deaths

Narrow base: Denotes fewer young than old Population will likely decline over time

Ecological deficit

ecological footprint > biocapacity

Ecological reserve

ecological footprint < biocapacity

Total fertility rate (TFR)

The average number of children born to each female.

Replacement fertility

The TFR that keeps the size of a population stable, approximately 2.1.

Natural rate of population change

Change due to birth and death rates alone (no migration)

Life expectancy

average number of years that an individual is likely to continue to live

Has increased with reduced rates of infant mortality

Demographic transition

A model explaining declining death and birth rates in industrializing nations

• Pre-industrial stage

• Tansitional stage

• Industrial stage

• Post-industrial stage

Pre-industrial stage

in pre-industrial societies, both birth and death rates are high

• High birth rate to compensate for high infant mortality

• Population growth is slow

Transitional stage

declining death rates due to increased food production and medical care

• Birth rates remain high since people are not used to the low infant mortality rates

• Population grows quickly

Industrial stage

birth rates fall as jobs provide opportunities for women outside the home and children are not needed in the workforce

• Difference between birth and death rates shrinks

• Population growth slows

Post-industrial stage

birth and death rates are low and stable

• Population stabilizes or even shrinks

Life expectancy

Average number of years an individual is likely to continue to live.