Life History Strategies

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Life History Strategies

• Chapter 5

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Life History Strategies

• An organisms life history concerns its lifetime
  pattern of growth and reproduction.
• Some organisms produce many, usually smaller
  offspring, while others produce fewer but larger
  offspring.
• Some reproduce only once, others many times.

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Life History Strategies

• Understanding these differences in life history
  strategies is vital to understanding how
  populations grow.

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Reproductive Strategies

• Semelparity
• Organisms that produce all of their offspring in
  a single reproductive event.
• May live several years before reproducing or they
  may have a total lifespan of just one year (e.g.
  annual plants).

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Semelparity

• Examples of semelparous organisms
• Salmon
• Yucca

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Reproductive Strategies

• Iteroparity
• Organisms that reproduce in successive years or
  breeding seasons.
• Variation in the number of clutches and number of
  offspring per clutch.

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Iteroparity

• Some species have distinct breeding seasons.
• Ex. Temperate birds and temperate forest trees.
• Breeding seasons lead to distinct generations.

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Iteroparity

• Some species reproduce repeatedly and at any time
  during the year (continuous iteroparity).
• Ex. Some tropical species, many parasites, and
  humans.

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Reproductive Strategies

• Environmental uncertainty favors iteroparity.
• If survival of juveniles is poor and
  unpredictable, selection favors
• Repeated reproduction.
• Long reproductive life.
• Spread the risk over a longer period (bet
  hedging).

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Reproductive Strategies

• A stable environment favors semelparity.
• An organism can put all of its efforts into
  reproduction (e.g. annual plants producing many
  seeds) instead of self-maintenance.
• If environment becomes stressful, problems arise
  annual plants have to rely on dormant seeds.

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Reproductive Strategies

• Reproductive strategies differ among organisms
  with some breeding continuously, others in
  discrete intervals, and others just once in their
  lifetimes.

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Age Structure

• Reproductive strategy has an effect on the age
  structure of a population.
• Growing populations have a large number of young.

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Age Structure

• Semelparous organisms
• Often produce groups of same-aged young
  cohorts.
• Cohorts grow at similar rates.
• Iteroparous organisms
• Many young at different ages.

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Age Structure

• The age structure of populations can be
  characterized by specific age categories
• Eggs, larvae, or pupae in insects.
• Years in mammals.
• Size classes in plants.

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Age Structure

• Increasing populations large number of young.
• Decreasing populations few young.
• Loss of age classes Influence on population.

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Age Structure

• Ex. Overexploited fish populations older age
  classes removed.
• Reproductive age classes are removed.
• Leads to reproductive failure.
• Results in population collapse.

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Age Structure

• Ex. Removal of younger age classes.
• Deer removing all young trees.
• Population consists of only older trees.
• When they die, there are no replacements.

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Mating Systems

• Sex ratio proportion of males females in the
  population.
• Applied ecology
• Hunters prefer deer populations dominated by
  males.
• Too many males limits population growth.

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Why Is the Sex Ratio Usually 11?

• Arent males superfluous? One male can fertilize
  many females.
• Answer Selfish genes!
• Populations predominately female - Selection
  would favor sons.
• Populations predominately male - Selection
  would favor daughters.
• Over time, sex ratio would be kept at 11.

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Mating Systems

• Exception to 11
• One male dominates in breeding.
• Occurs in species with
• Low powers of dispersal.
• Frequent inbreeding.

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Mating Systems

• Ex. Parasitic Wasps
• Females mate once and store sperm.
• Females control sex ratio
• Use sperm to create females.
• Without sperm to create males.
• Process termed haplodiploid.
• Ex. The mite Acarophenox
• Brood size 20 (1 male).
• Male mates with sisters, dies before birth.

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Mating Systems in Animals

• Monogamy
• Exclusive mating over at least one breeding
  cycle.
• Common among birds (90) of species.
• Advantageous to male to stick around and help
  raise offspring because few would survive if he
  did not.
• Monogamy is rare among other vertebrates.

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Mating Systems

• Polygamy
• Individuals mate with multiple partners.
• Polygyny
• One male mates with multiple females.
• Females mate with only one male.
• Polyandry
• One female mates with multiple males.
• Males mate with only one female.

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Polygyny

• Polygyny one male mates with multiple females.
• In species where females usually care for the
  young, the male may desert.
• Mammals tend to be polygynous.
• Occasionally, females can desert young.
• A fish may deposit eggs in a males territory and
  then leave him to guard the eggs.
• He may guard the eggs of several females.

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Mating Systems

• Mating systems may be influenced by spatial and
  temporal distribution of females.
• Monogamous relationships can result from all
  females becoming sexually receptive at the same
  time.
• Female receptiveness spread over weeks or months
  polygyny can result.

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Mating Systems

• Two contrasting examples
• Common toads lay all their eggs within about one
  week, so a male only has time to mate with one
  female.
• In the bullfrog, the mating season lasts several
  weeks, giving males time to mate with up to six
  females.

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Mating Systems

• Resource-based polygyny
• Critical resource is patchily distributed or in
  short supply.
• Male can dominate resource and breed with more
  than one visiting female.

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Resource-Based Polygyny

• Disadvantages for the female
• Must share resources.
• More females means less success.
• Advantageous for male to have more than one mate.

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Mating Systems

• Non-resource based polygyny
• Harem-based polygyny is common in groups or
  herds.
• Elephant seals
• Protection from predators.
• Harem master does not remain for long.

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Non-Resource Based Polygyny

• Communal courting areas leks
• Females choose a male after he performs an
  elaborate courtship display.
• Many females choose the same male.

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Mating Systems

• Polyandry one female mates with several males.
• Much less common.
• Practiced by a few species of birds.

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Mating Systems

• Ex. Spotted sandpiper in the Arctic tundra
• Reproductive success not limited by food.
• Females lay up to five 20-egg clutches in 40
  days.
• Limited by the number of males needed to incubate
  eggs.
• Females compete for males, defending territories
  where males sit.

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Mating Systems

• Ex. American jacana
• Egg predation is high.
• Females have large territories with several nests
  each attended by a male.
• Males incubate eggs.
• If male tries to abandon nest in order to mate
  with another female, it is likely that he will
  lose his eggs to a predator.

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Life History Strategies

• Success of populations
• Reproductive strategies.
• Survival strategies.
• Habitat usage.
• Competition with other organisms.
• r and K selection represent two extremes of a
  continuum of life history strategies developed
  by McArthur and Wilson (1967)

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Life History Strategies

• r-selected species have
• High rate of population growth (r).
• Poor competitive abilities.
• Example weed that quickly colonizes a vacant
  lot, goes through a few generations and
  disappears as better competitors take over.
• Grows quickly
• Reaches maturity, reproduces dies in 1 year
• Produces lots of small seeds

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Life History Strategies

• K-selected species
• Populations increase slowly toward the carrying
  capacity (K) of the environment.
• Low reproductive allocations.
• Iteroparous.
• High competitive abilities eventually displace
  weedy species.

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K-selected species

• Ex. Mature forest trees that occupy a
  non-disturbed habitat.
• Grow slowly.
• Reach reproductive age late.
• Devote large amounts of energy to growth and
  maintenance.
• Grow to large sizes and shade-out r-selected
  species.
• Long-lived and produce seeds repeatedly every
  year while mature.
• Seeds are bigger than r-selected species
  provide food reserves to help them get started.

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Life History Strategies

• There is a trade-off between the size and number
  of seeds
• A few big seeds or lots of small seeds.
• Animals are r-K selected as well
• Insects often r-selected.
• Many young, short life cycle.
• Large mammals (elephants) K-selected.
• Grow slowly, have few young, reach large sizes.

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Life History Strategies

• K-selected species may be at risk of extinction.
• Bigger need larger areas.
• Fewer offspring populations cant recover as
  fast from disturbance.
• Breed later generation time is long.
• Population size often small greater risk of
  inbreeding.

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Life History Strategies

• The r-K system brings together several life
  history attributes.
• Reproductive strategy.
• Habitat selection.
• Ability to disperse.
• Population size.

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Life History Strategies

• Alternatives to the r and K continuum
• Ruderals, competitors, and stress tolerators
  (Grime 1977 and 1979).
• Ruderals (botanical term for weed)
• Adapted to cope with habitat disturbances.
• Competitors
• Adapted to live in highly competitive but benign
  environments (e.g., tropics).
• Stress tolerators
• Adapted to cope with severe environmental
  conditions (e.g., salt marsh plants).

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Life History Strategies

• Demographic interpretation of this idea
• Species could either survive a long time as
  adults, grow large, or produce a lot of seeds,
  but not all three.
• Trade-off.
• Growth, survival and fecundity triangle.

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Summary

• Life history concerns lifetime patterns in
  reproduction and growth.
• Semelparous
• Iteroparous

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Summary

• Reproductive strategy strongly affects age
  structure.
• Low ratio of young to adults population in
  decline.
• High ratio of young to adults population
  growing.

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Summary

• Sex ratio
• 11 ratio expected in most populations.
• Polygamy is often based on limited resources.
• Polygynous
• Males mate with more than one female.
• Polyandrous
• Females mate with more than one male.
• Monogamous
• Each individual has one mate.

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Summary

• Categorizing life history strategies
• r-K continuum
• r-selected
• Poor competitors
• High population growth rate
• Disperse well
• Colonize new habitats
• Many, smaller offspring
• K-Selected
• Good competitors
• Usually exist in mature habitats, close to the
  carrying capacity.
• Grow slowly
• Produce, few but larger offspring

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Summary

• Alternative life history strategies
• Ruderals, competitors, and stress tolerators
  classification
• Growth-longevity-fecundity triangle