Chap.5 Life History Strategies

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Chap.5 Life History Strategies

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• ????? Ayo
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Life History Strategies
Road Map

• Reproductive strategies
• Species that reproduce throughout their lifetimes
  (iteroparous)
• Species that reproduce just once (semelparous)
• Age structure
• Growing populations
• Declining populations
• Classification of mating systems
• Continuum of life history strategies
• r-selected vs. K-selected
• Carrying capacity

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5.1 Reproductive strategies

• Semelparity
• Organisms that produce all of their offspring in
  a single reproductive event.
• May live several years before reproducing or
  lifespan is one year (ex. Annual plants)
• Ex. Figure 5.1.

The yucca plant, Yucca elata, grows for many
years before it flowers and produces seed.
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Iteroparity

• Organisms that reproduce in successive years or
  breeding seasons
• Variation in the number of clutches and number of
  offspring per clutch.
• Some species have distinct breeding seasons
• Ex. Temperate birds and temperate forest trees
• Lead to distinct generations
• 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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Environmental Uncertainty

• Favors iteroparity
• 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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Environmental Stable

• Favors semelparity
• More energy can be devoted to seed production
  rather than maintenance
• Annuals rely on seed storage during
  environmentally unstable years

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5.2 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

• Increasing populations large number of young
• Decreasing populations few young
• Loss of age classes
• Influence on population
• Ex. Overexploited fish populations older age
  classes
• Reproductive age classes removed
• Reproductive failure
• Results in population collapse
• Ex. Younger age classes, deer removing young
  trees
• Figure 5.2

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(a) Age distribution in an undisturbed forest
40
20
Percent of trees
(b) Age distribution skewed toward adults where
overgrazing has reduced the abundance of young
trees
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40
20
Fig. 5.2
10
20
30
40
50
60
70
Age (years)
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5.3 Mating systems

• Why is the sex ratio usually 11?
• Arent males superfluous?
• 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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Sex ratio

• Exception to 11
• One male dominates in breeding
• Occurs in species with
• Low powers of dispersal
• Inbreeding is frequent
• Ex. The parasitic Hymenoptera
• 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 (Figure 5.3)

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Fig. 5.3 A viviparous mite of the family
Acarophenacidae. Here brothers mate with sisters
while both are still inside the body of the
mother.
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Mating systems in animals

• Monogamy
• Exclusive mating
• Common among birds (90) of species
• Polyandry
• One female mates with multiple males
• Males mate with one female
• Polygyny
• Females must care for the young
• Mammals tend to be polygynous
• Ex. Figure 5.4

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Polygyny

• Influenced by spatial and temporal distribution
  of females
• Monogamous relationships 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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Resource-based polygyny

• Critical resource is patchily distributed or in
  short supply
• Male can dominate resource and breed with more
  than one visiting female
• Disadvantages for the female
• Must share resources
• More females means less success
• Figure 5.5

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Fig. 5.5 yellow-bellied marmots (???)
5
1.25
4
1.0
3
Number of yearlings per male ( )
Number of yearlings per female ( )
0.75
2
0.5
1
1
2
3
4
5
6
Number of females per group
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Non-resource based polygyny

• Harem-based
• Common in groups or herds
• Protection from predators
• Harem master does not remain for long
• Communal courting areas leks
• Figure 5.6

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Fig. 5.6 Male long-tailed manakins at a lek.
Females, shown at lower right, also visit the
leks and choose their prospective mates.
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Polyandry

• Practiced by a few species of birds
• Ex. Spotted sandpiper in the Arctic tundra
• Reproductive success not limited by food
• Limited by the number of males needed to incubate
  eggs.
• Ex. American jacana (Figure 5.7)

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5.4 Life History Strategies

• Success of populations
• Reproductive strategies
• Survival strategies
• Habitat usage
• Competition with other organisms

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• K-Selected
• Populations increase slowly toward the carrying
  capacity
• (K) of the environment
• Low reproductive allocations
• Iteroparous
• High competitive abilities

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Fig.5.8 Life history traits of a dandelion and
an oak tree
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r-K continuum and bet-hedging strategy

• Species can generally be placed somewhere along
  this continuum.
• However, not all species fall neatly onto this
  continuum.
• A bet-hedging strategy combines elements of r and
  K selection.
• If juvenile mortality is variable and
  occasionally high , neither a classic r nor a
  classic K strategy is optimal.
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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)
• Stress, disturbance and competition triangle
• Figure 5.9

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Fig. 5.9 a model in which stress, disturbance,
and competition are the important factors.
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Fig. 5.9b
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Life History Strategies

• Demographic interpretation (Silverton et al.
  1992, 1993)
• Growth-survival and fecundity triangle
• Figure 5.10

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Fig. 5.10 The distribution of species of
perennial plants in the growth-survival-fecundity
triangle.
G
1.0
0.0
0.8
0.2
0.6
0.4
Growth
Survival
0.4
0.6
0.2
0.8
0.0
1.0
0.4
S
F
1.0
0.8
0.6
0.2
0.0
Fecundity
Semelparous herbs
Iteroparous herbs in open habitats
Iteroparous herbs in forests
Woody trees
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C-S selections

• S-selection means specialist selection, which
  favors the present success. Under s-selection,
  the species evolves toward to be a confined and
  endemic species.
• C-selection means colonizer selection, which
  favors the future success. These species are
  high starvation tolerance, and wide distribution,
  a kind of colonizers.

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Life history and the risk of extinction
Applied Ecology

• K-selected species
• All attributes set them at risk to extinction
• Tend to be bigger need bigger habitat
• Fewer offspring populations can not recover as
  fast from disturbance
• Breed later in life generation time is long
• Population size is small high risk of
  inbreeding
• Examples
• Florida panthers
• Giant sequoia tree
• Large terrestrial mammals (elephants, rhinoceros,
  and grizzlies)
• Large marine mammals (blue and sperm whales)

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Japalura_at_hotmail.com Ayo ???
http//mail.nutn.edu.tw/hycheng/