Life history

Fecundity

The potential reproductive capacity of an individual organism or population

★ cohort

group of individuals in a population that were all born at around the same time

★ kin selection

a process whereby natural selection favours a trait due to its positive effects on the reproductive success of an organism's relatives

★ clutch size fast facts

clutch size increases with increasing latitude and day length

lay less than expected so that more can survive

★ survivorship curve

plots survivorship against age and characterizes the pattern of mortality

★ age specific fecundity

charcterizes fecundity at a particular age

★ net reproductive rate

average number of female offspring that a female is expected to have in a life time

★ life table parameters

age class, number surviving, number of births, survivorship, fecundity

Fecundity rate

The average number of offspring per reproductive female per year

★ Life history

The sequence, time in, and nature of events in an organism's life, from birth to reproduction to death

★ Life history strategy

Pattern of life history traits that has evolved by natural selection over time in a population in response to particular ecological and environmental conditions

Life history trait

• A heritable trait that determines some aspect of life history of an organism or species

  • Age and size at maturity, number of offspring

★ Key life history questions

• How long and how big to grow

• When sexually mature and how long reproductively active

• How many offspring and how much to invest in each

• How much to invest in growth and survival vs. reproduction

Life history trade offs

• Arise because resources allocated to one trait cannot be used for another

• Every life history strategy comes with both costs and benefits

Constraints

• Limit how an individual can invest in survival and reproduction

• Can be imposed by environment

• Some are imposed by physiology

if allocation to survival and growth increases

• The allocation to reproduction must decrease

• The allocation to each offspring must decrease

Growth rate

• r = b - d

• r > 0 : pop is growing

• r = 0 : pop is stable

• r < 0 : pop is shrinking

Age structure

• Pre-reproductive: 0-20

• Reproductive: 20-50

• Post-reproductive: 50+

★ successful life history

stable or growing populations

Demographic parameters

• Quantitative values that characterize the life history of a species

  • Birth rate 

  • Death rate

  • Fecundity

  • Age of first reproduction

Life table

Summarizes how fecundity and mortality vary with age

Estimate population growth more accurately

★ Survivorship

The fraction of the initial cohort that survives to start an age class

Divide the live individuals by initial cohort size

Type 1

Human, elephant

High survivorship in early and middle ages, quick decline at old age

Type 2

Squirrel, birds

Constant mortality rate throughout life. Same proportion survive each age class

Type 3

Barnacle, trees

Very low survivorship for young individuals, high for those who reach middle and old age

Many offspring

Fecundity schedule

Set of age specific fecundities of a population

Net reproductive rate

• Average number of female offspring that a female has in her lifetime

• Depends on how many offspring she has at each age and her survival to that age

• R₀ = Σl_xm_x

• If R₀ is greater than 1, then the population is growing

Generation time

The average time that elapsed from the birth of a female until the birth of her daughters

Unit of G is in years

Shorter generation time = larger population growth rate

★ Per capita population growth rate

r = b - d

The average number of individuals added to the population per individual per unit time

Depends on how many daughters a typical female produces and how long it takes her to produce daughters

Exponential growth equation

• 

• Estimate the size of a population that has been growing for a time period from an initial size

Demographic transition

Stage one

• High birth

• High death

• Stable population

Stage two

• High birth

• Falling death

• Rapid increase in population size

Stage three

• Falling birth

• Falling death, slower

• Slower increase in population size

Stage four

• Low birth

• Low death

• Falling then stable population

Trade offs with age of first reproduction

• The age at which a fish reaches sexual maturity can affect its fecundity

• Fish that mature early spec into reproduction and spec out of growth and survival

• Fish that delay reproduction can spec into growth

• Fish they delay reproduction are more fecund when they start reproducing

  • Have more gametes

  • Maintain this advantage through life

• They can reproduce early but remain less fecund or they can delay reproduction and remain more fecund for longer

Principle of allocation

Any resource used for one function reduces the amount of energy available for another function

Iteroparity

• Multiple bouts of reproduction during an individual's lifetime

• Iteroparous

• Reserve resources during each reproductive bout to invest in growth, maintenance, and future reproduction

Semelparity

• A single bout of reproduction in a lifetime

• Semelparous

• Produce a huge number of offspring

• Benefit by delaying reproduction until they are grown big, can invest heavily in reproduction

Species are classified as r or K selected based on how they respond to disturbances which reduce population densities and make resources more readily available

r-selected

• Reproduce early and often

• Invest lightly in each offspring

• More successful in environments where disturbances are regular

• Competition isn’t important

K-selected

• Highly competitive

• More successful in stable environments

• Perform well when population size is close to K

• Grow larger, live longer, reproduce later, invest resources into offspring

Ruderal

• r-selected plants stay small, reproduce early, invest heavily in seed production

• Seed survive long periods of dormancy

• Grass, dandelions

Competitive

• K-selected plants have low mortality, efficiently take up water and nutrients, live long, devote more energy to growth and survival and reproduction

• Do best environments what are low stress and low disturbance

• Oak, hickory

Stress tolerant

• Have adaptations for tolerating periods of scarcity and exploiting ephemeral resources

  • Water after a storm

• Slow growth rates, evergreen, long-lived tissues, storage of nutrients and water

• High reproductive rates, low juvenile survivorship, long lifespans

  • Cacti, arctic lichens

Opportunistic

• Early, frequent

• Low Juvenile survivorship

• Low Fecundity

• Guppies, anchovies

Equilibrium

• Late

• High Juvenile survivorship

• Low Fecundity

• Sharks, catfish

Periodic

• Late

• Low Juvenile survivorship

• High Fecundity

• Sturgeon

Large scale variability

Response to strong seasonality or special variability