SBI 4U1 – Population Dynamics 12.2 Demographics SBI 4U1 – Population Dynamics

Learning Goals To learn the factors that affect population demographics and how they do so

Demography Study of population changes Growth Rate is calculated using: Natality Mortality Immigration Emigration

Life tables Used to summarise demography of a species Investigates certain cohorts or age groups (Ex. The double cohort that entered universities when OAC was ditched by the government!)

Survivorship Curves Shows the ability of an individual to survive hardships Types I-III

Survivorship Type I Low death rate in early years High death rate as organism ages Larger animals with few young Ex. Ram, humans, elephants… Long gestation period, high infant investment by parents

Survivorship type II Constant mortality rate in all cohorts Ex. Five lined skink, songbirds, small mammals… Short gestation period

Survivorship Type III Newborns die quickly during critical stage Ex. Plants, insects, invertebrates, fish… Lots of offspring born– few survive

fecundity Ability of an organism to have fertile offspring Ex. Elephants have lower fecundity than horseshoe crabs that lay thousands of eggs Depends on environmental conditions How?

fecundity Larger animals have higher or lower? lower

fecundity Organisms are pre-reproductive, reproductive, or post-reproductive Age structure determines how population will grow Ex. High number of individuals of reproductive age will cause population to increase (Niger has the world’s fastest growing population)

fecundity Affected by generation time (time to complete one generation) Small body size has short generation time Ex. E. coli Large size = long generation time Ex. whales

fecundity Affected by sex ratio, also Females have a larger effect on population growth than males One male can mate with many females Ex. Northern Elephant seals compete for mates—one male may fertilise 100+ females

Some organisms mate for life Male: Female ratio matters more Ex. Swans, geese