Chap.5
Life History Strategies
鄭先祐
生態主張者 Ayo
Japalura@hotmail.com
Road Map
Life History Strategies
1. Reproductive strategies
– Species that reproduce throughout their lifetimes
(iteroparous)
– Species that reproduce just once (semelparous)
2. Age structure
– Growing populations
– Declining populations
3. Classification of mating systems
4. 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
Percent of trees
40
20
(b) Age distribution skewed toward adults
where overgrazing has reduced the abundance
of young trees
60
40
20
10
20
30
40
50
60
70
Age
(years)
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history strategies
Fig. 5.2
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5.3 Mating systems
• Why is the sex ratio usually 1:1?
– Aren’t 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 1:1
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Sex ratio
• Exception to 1:1
• 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 yellowbellied marmots
(土撥鼠)
1.25
4
1.0
3
0.75
2
0.5
1
1
2
3
4
5
Number of females per group
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Number of yearlings per male (
Number of yearlings per female (
)
)
5
6
16
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.
– 生殖的能量分散投入，以減少完全失敗的風險。
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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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2003 生態學 Chap. 5 Life history strategies
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-survivalfecundity triangle.
G
F 1.0
0.8
0.6
0.4
0.2
0.0 S
Fecundity
Semelparous herbs
Iteroparous herbs in open habitats
Iteroparous herbs in forests
Woody
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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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Environmental
condition
(+)
Constant (+) or
Variable (-)
(-)
Defense against
limiting factor(s)
Body (1) or
Offspring(2)
Body (1) or
Offspring(2)
Energetic
priority
Types of
selection
(1) body growth
C-selection
or K-selection
(2) reproduction
S-selection
(1) body growth
C-selection
(2) reproduction
S-selection
or r-selection
Fig. 11 Evolutionary mechanism of C-S selection.
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Climates and types of selections
Variable climate
r-selection
C-selection
Sub-variable
climate
Constant climate
K-selection
S-selection
Fig. 12. Climates and types of selections
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Applied Ecology
Life history and
the risk of extinction
• 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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 問題與討論！
Japalura@hotmail.com
Ayo 台南站： http://mail.nutn.edu.tw/~hycheng/
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