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12.2 Demography - SBI4U1 Cheat Sheet by

grade 12 biology - chapter 12.2


Popula­tions are dynamic - they change hourly, daily, season­ally, or annually - and depends on the species and enviro­nmental conditions


- predicts the growth of a population
    - can protect endangered species
- can be determined by natality, morality, immigr­ation, and emigration
retrieving data
- done by routine sampling
- through life tables and surviv­orship curves


the study of the growth rate, age structure, and other charac­ter­istics of popula­tions
the birth rate in a population mortality the death rate in a population
the movement of indivi­duals into a population
the movement of indivi­duals out of a population
Life table
a chart that summarizes the demogr­aphic charac­ter­istics of a population
a group of indivi­duals of similar ages
Age-Sp­ecific Mortality
the proportion of indivi­duals that were alive at the start of an age interval but died during the age interval
Age-Sp­ecific Surviv­orship
the proportion of indivi­duals that were alive at the start of an age interval and survived until the start of the next age interval
Surviv­orship Curve
a graphic display of the rate of survival of indivi­duals over the lifespan of a species
the potential for a species to produce offspring in a lifetime
Generation Time
the average time between the birth of an organism and the birth of its offspring
Sex Ratio
the relative proportion of males and females in a population

Changes in Population Size

- enviro­nmental conditions can increase or decrease population
    - biotic and abiotic factors affect the rates of natality, mortality, immigr­ation, and emigration
population change = (births + immigr­ation) - (deaths + emigra­tion)
- if natality and immigr­ation are equal to mortality and emigration → the popula­tion’s size will remain stable

- when natality and immigr­ation are greater than mortality and emigration → there is population growth

- when mortality and emigration are greater than natality and immigr­ation → population will decrease

Life Tables

- a group of indivi­duals born around the same time are marked
    - monitored until they all die
- the lifespans of the indivi­duals are divided into age intervals
      - short life-span: labelled by days, weeks, or months
      - long life-span: by years or ranges of years
morality and surviv­orship
- the mortality rate is the number of deaths in a population per unit of time
morali­ty/­sur­viv­orship calcul­ations:
age-sp­ecific mortality = # that died during the interval / # alive at the start of the interval

age-sp­ecific surviv­orship = # still alive at the end of the interval / # alive at the start of the interval
survival probab­ility
- the proportion of the cohort that survived at a specific age summarizes the survival probab­ility of a newborn at that age
probab­ility of being alive at a specific age = # alive at the start of the age interval / # alive at the start of the initial age interval


fecundity varies from species to species
- can increase or decrease depending on:
    - enviro­nmental conditions
    - age structure
    - generation time
    - sex ratio
enviro­nmental condit­ions:
- plenty of food and the climate is optimal = species tend to have higher rates of reprod­uction
- little food and precip­itation → reprod­uction rates drop
age structure:
describes the relative number of indivi­duals in each age category
- pre-pr­odu­ctive: younger than the age of sexual maturity
- reprod­uctive: reprod­ucing age
- post-r­epr­odu­ctive: older than the max. age of reprod­uction

this reflects its recent growth history to predict its future growth
generation time:
- i.e. E. coli has a short generation time → they mature very quickly and reproduce quicker

- humans have a much longer generation time in comparison to E. coli
sex ratio:
# of females have a greater impact because they produce offspring
- a male can mate with several females → does not greatly influence reprod­uction

- but; some species have lifelong pairs and the # of males matter as much as females

High Vs. Low Fecundity

high fecundity
= an animal that has many offspring
    - but does little care for them
    - e.x. hawksbill turtle lay as much as 100 eggs, but leave the nest (the offspring fend for themselves at birth
low fecundity
= produces fewer offspring per year
    - has more energy to care for them
    - can be protective
    - e.x. bears stay and teach their cubs how to survive

Surviv­orship Curves

they are like life tables; but a graphical repres­ent­ation
- displays the survival of indivi­duals over the lifespan of the species
- type I
- type II
- type III


- flat at the start
- low death rate in the early and middle years
- death rate increases in older age groups
- has a long gestation period
- common in large animals
    - produce few offspring and provide their offspring
    with a lot of care for a long period of time


- constant rate of mortality in all age groups
- steady declining survival rate
- preyed by type I organisms
- feed on type III organisms
- has a short gestation period


- drops rapidly right at the beginning
- in early life, the death rate is really high and then flattens out later on in the life of the organism IF the organism survives
- organisms that produce a large number of offspring
    - like plants, insects, and fish.

Surviv­orship Curve Examples

Type I example
- humans; when a baby is born, we take great care of them until their young adult years in which they provide for themselves
Type II example
- lizard; they constantly face a number of diseases, predation, and starvation not matter their age
Type III example
- fish; has a high juvenile mortality (when they are babies - constantly surrounded by predators)


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