Chap.5 Life History Strategies 鄭先祐生態主張者 Ayo [email protected]

Chap.5 Life History Strategies

鄭先祐生態主張者 Ayo

[email protected]

2003 Chap. 5 Life history strategies生態學 2

Life History Strategies

1. Reproductive strategies– Species that reproduce throughout their lifetimes (iter

oparous)

– 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

Road Map


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.


Iteroparity• Organisms that reproduce in successive years o

r 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 huma

ns


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”)


Environmental Stable

• Favors semelparity

• More energy can be devoted to seed production rather than maintenance

• Annuals rely on seed storage during environmentally unstable years


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


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


20

40

60

20

40

60

(a) Age distribution in an undisturbed forest

(b) Age distribution skewed toward adults where overgrazing has reduced the abundanceof young trees

10 20 30 40 50 60 70

Age (years)

Per

cent

of

tree

s

Fig. 5.2


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


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)


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.


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


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


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

2003 Chap. 5 Life history strategies生態學 16Number of females per group

1 2 3 4 5 6

1

2

3

4

5

Num

ber

of

yearl

ings

per

male

( )

Num

ber

of

yearl

ings

per

fem

ale

( )

0.5

0.75

1.0

1.25

Fig. 5.5 yellow-bellied marmots

( 土撥鼠 )


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


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


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)


5.4 Life History Strategies

• Success of populations– Reproductive strategies

– Survival strategies

– Habitat usage

– Competition with other organisms


• K-Selected– Populations increase slowly toward the

carrying capacity

– (K) of the environment

– Low reproductive allocations

– Iteroparous

– High competitive abilities


Fig.5.8 Life history traits of a dandelion and an oak tree



r-K continuum and bet-hedging strategy • Species can generally be placed somewhere alon

g 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。


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


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

2003 Chap. 5 Life history strategies生態學 27Fig. 5.9b


Life History Strategies

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

– Figure 5.10


G

Gro

wth

Survival

Fecundity

1.0 0.8 0.6 0.4 0.2 0.0

Semelparous herbs

Iteroparous herbs in open habitats

Iteroparous herbs in forests

Woody trees

0.0

0.2

0.4

0.6

0.8

1.0 0.0

0.2

0.4

0.6

0.8

1.0

SF

Fig. 5.10 The distribution of species of perennial plants in the growth-survival-fecundity triangle.


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.


(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.

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


Variable climate

Sub-variable climate Constant climate

Fig. 12. Climates and types of selections

Climates and types of selections

r-selection C-selection

K-selection S-selection


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)

Applied Ecology


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Documents

Chap.5 Life History Strategies 鄭先祐 生態主張者 Ayo [email protected]

Chap.5 Life History Strategies 鄭先祐生態主張者 Ayo [email protected]