Chap.5 Life History Strategies

1. Chap.5 Life History Strategies 鄭先祐 生態主張者 Ayo Japalura@hotmail.com

2. Road Map Life History Strategies • 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 2003 生態學 Chap. 5 Life history strategies

3. 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. 2003 生態學 Chap. 5 Life history strategies

4. 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 2003 生態學 Chap. 5 Life history strategies

5. 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”) 2003 生態學 Chap. 5 Life history strategies

6. Environmental Stable • Favors semelparity • More energy can be devoted to seed production rather than maintenance • Annuals rely on seed storage during environmentally unstable years 2003 生態學 Chap. 5 Life history strategies

7. 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 2003 生態學 Chap. 5 Life history strategies

8. 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 2003 生態學 Chap. 5 Life history strategies

9. 60 (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 60 40 20 Fig. 5.2 10 20 30 40 50 60 70 Age (years) 2003 生態學 Chap. 5 Life history strategies

10. 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 2003 生態學 Chap. 5 Life history strategies

11. 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) 2003 生態學 Chap. 5 Life history strategies

12. 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. 2003 生態學 Chap. 5 Life history strategies

13. 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 2003 生態學 Chap. 5 Life history strategies

14. 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 2003 生態學 Chap. 5 Life history strategies

15. 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 生態學 Chap. 5 Life history strategies

16. Fig. 5.5 yellow-bellied marmots (土撥鼠) 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 2003 生態學 Chap. 5 Life history strategies

17. 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 2003 生態學 Chap. 5 Life history strategies

18. Fig. 5.6 Male long-tailed manakins at a lek. Females, shown at lower right, also visit the leks and choose their prospective mates. 2003 生態學 Chap. 5 Life history strategies

19. 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) 2003 生態學 Chap. 5 Life history strategies

20. 5.4 Life History Strategies • Success of populations • Reproductive strategies • Survival strategies • Habitat usage • Competition with other organisms 2003 生態學 Chap. 5 Life history strategies

21. K-Selected • Populations increase slowly toward the carrying capacity • (K) of the environment • Low reproductive allocations • Iteroparous • High competitive abilities 2003 生態學 Chap. 5 Life history strategies

22. Fig.5.8 Life history traits of a dandelion and an oak tree 2003 生態學 Chap. 5 Life history strategies

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24. 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。 2003 生態學 Chap. 5 Life history strategies

25. 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 2003 生態學 Chap. 5 Life history strategies

26. Fig. 5.9 a model in which stress, disturbance, and competition are the important factors. 2003 生態學 Chap. 5 Life history strategies

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

28. Life History Strategies • Demographic interpretation (Silverton et al. 1992, 1993) • Growth-survival and fecundity triangle • Figure 5.10 2003 生態學 Chap. 5 Life history strategies

29. 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 F S 1.0 0.8 0.6 0.2 0.0 Fecundity Semelparous herbs Iteroparous herbs in open habitats Iteroparous herbs in forests Woody trees 2003 生態學 Chap. 5 Life history strategies

30. 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. 2003 生態學 Chap. 5 Life history strategies

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33. 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) 2003 生態學 Chap. 5 Life history strategies

34. 問題與討論！ Japalura@hotmail.com Ayo 台南站： http://mail.nutn.edu.tw/~hycheng/ 2003 生態學 Chap. 5 Life history strategies