bios 3010: ecology lecture 14: life histories

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Dr. S. Malcolm BIOS 3010: Ecology Lecture 14: slide 1

BIOS 3010: Ecology Lecture 14: Life Histories:

•  Lecture summary: –  Components of life

histories: •  Growth. •  Fecundity. •  Survivorship.

–  Reproductive value. –  Trade-offs. –  r- and K-selection. –  Habitat templates. –  Clutch size.

Dendrobates, M & P. Fogden, Rainforests, A Celebration

Linnet, D. Attenborough (1998), Life of Birds, Princeton.


2. Components of life histories:

•  Growth, fecundity and survivorship – (i) Growth

• Balance between the benefits and costs of large and small sized individuals:

– (Figs. 14.1 & 4.16). • Development

– Development can be varied independently of growth.



•  (ii) Fecundity – Rapid development can lead to early reproduction – Timing important (pre-reproductive period length). – Duration and frequency of reproduction is also

important (semelparity and iteroparity). •  Offspring size

–  variation in parental investment in individual offspring.

•  Number of offspring –  variation in numbers.

•  Parental care –  variation in how much parents look after offspring.

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•  (iii) Survivorship – Variation in mortality, affected by:

• Energy storage: – Valuable for irregularly supplied resources.

– Dispersal in space and time: • Like migration across space and diapause

through time to escape periods of resource shortage.


5. Reproductive effort:

•  “Proportion of the available resource input that is allocated to reproduction over a defined period of time.”

•  E.g., the allocation of energy or dry weight to different parts at various stages of development (Fig. 14.3).

•  "Natural selection favours those individuals that make the greatest proportionate contribution to the future of the population to which they belong.”

–  Begon et al., 2006, p 111. –  Life histories are evolutionary attempts to maximize fecundity

and survival and these can be compared as a single "currency” »  The reproductive value as a measure of fitness »  Such as the intrinsic rate of natural increase r, or the basic

reproductive rate Ro.


6. Reproductive value:

•  (i) Reproductive value at a stage or age is the sum of the current reproductive output and the future reproductive value.

•  (ii) Future reproductive value includes both future survival and expected fecundity.

•  (iii) Future contribution of an individual is determined relative to the contribution of others.

•  (iv) The life history favored by natural selection will be the one with the highest sum of contemporary output plus future output (Fig. 14.4).

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7. Trade-offs:

•  Life histories are balances of costs and benefits: – One characteristic can result in increased

benefits that are associated with decreased benefits from another characteristic (Fig. 4.19 & Fig. 4.23).

– This figure also shows the cost of reproduction (Fig. 4.22).


8. r and K selection:

•  After MacArthur & Wilson (1967) based on parameters of the logistic equation: – r-selected individuals:

•  Favored for the ability to reproduce rapidly (high r - values) in r-selecting habitats (more unstable).

– K-selected individuals: •  Favored for the ability to make a large proportional

contribution to a population which remains at its carrying capacity (K) in K-selecting habitats (more stable).

– These are extremes of a continuum.


9. Characteristics of r and K- selected individuals:

See Figure 14.20 and Table 4.7

r-selected individuals K-selected individualsHabitat: unstable, unpredictable stable, predictable

Individual size: smaller largerTiming of reproduction: earlier later

Parity: semelparous iteroparousReproductive allocation: higher lower

Number of offspring: many fewSize of offspring: small large

Parental care: no yesSurvivorship investment Low (little defense,

poor competitor)High (good defense,highly competitive)

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10. Habitat and life history

•  Habitat is the templet (template) to which individuals fit their life history (Southwood, 1977).

•  For example, guppies in Trinidad streams: – Selection by different fish predators influences the

cost of reproduction in guppies (Table 4.6) and selects for:

•  Shift in offspring sizes. •  Change in reproductive allocation.


11. Clutch size:

•  How many eggs should a bird lay? •  “Lack clutch size”

– Balance between number produced & subsequent survival to maturity (Fig. 4.29).

– Lack’s predictions not supported because: •  (1) Inadequate assessment of offspring fitness:

–  Subsequent survival of added eggs not measured - so Lack may be correct?

•  (2) Cost of reproduction (Fig 4.29) not considered: –  Impact of present reproduction on future reproduction in

iteroparous species.


Figure 14.1 (3rd ed.): Larger female crabs produce more offspring.

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Figure 4.16: Predicted male damselfly size corresponds to population mode.


Figure 14.3 (3rd ed.):

Percentage allocation of calories to parts of annual plants, (a) Senecio, (b) Chrysanthemum (see Fig. 4.17 4th ed.)


Figure 14.4 (3rd ed.): Change in reproductive value with age in (a) Phlox and (b) grey squirrels.

(see fig. 4.18, 4th ed.)

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Figure 4.19:

Life history trade-offs in: (a) migratory fruit flies, (b) douglas fir, (c) male fruit flies with 8 virgin, 1 virgin + 7 mated or 0 virgin + 8 mated females


Figure 4.23: Trade-offs in (a) goldenrod spp., (b) Hawaiian fruit flies, (c) Sceloporus lizards

Predictable pollen

Unpredictable yeast

Leaf bacteria

3 = Solidago canadensis


Figure 4.22: Cost of reproduction in Senecio (deaths above line)

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Figure 14.20 (3rd ed.):

Plants and the r/K continuum: (a) Reproductive allocation (b) Seed weight (c) age of reproduction variation with life span (see fig 4.30, 4th ed.)


Table 4.7:

(North Dakota) (Texas)


Table 4.6:

Large fish��predator Small fish��

predator

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Figure 4.29: “Lack clutch size” (a) prediction based on offspring fitness trade-off, (b) inclusion of cost of reproduction and maximum net benefit

bios 3010: ecology lecture 14: life histories

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