Chapter 7
Life History strategies
鄭先祐
生態主張者：Ayo 工作室
Life History Strategies
 The
effects of body size
 Metamorphosis
 Diapause and resting stages
 Senescence
 Reproductive strategies
 Constrains and ambiguities in the study
of the life history strategies
 Environmental application (Life histories
and conservation)
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Consider the following life histories
 1. A salmon,
having spent five years in
the North pacific, enters the Yukon
River and swims upstream some 2,000
miles to a small tributary. By the time it
reaches the small stream where its
parents mated, it is near death. Finally,
it spawns and dies.
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Consider the following life histories

2. A female red kangaroo in the desert of
Australia cares simultaneously for three young
at different stage of development. The oldest
has left its mother’s pouch and lives
independently, although it remains near its
mother’s side. The second, a newborn, is
attached to a teat in the pouch. It is helpless
and incompletely developed. The third is a
fertilized egg in the uterus, where it will remain,
unattached to the placenta, for 204 days.
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Consider the following life histories

3. A mayfly egg hatches in a small stream in
the Rockies. After feeding under the surface
of the water for a few weeks, the nymph
swims to the surface and hatches into the first
adult stage. This winged form flies off and
conceals itself in the vegetation along the
stream. In a few hours, it sheds its skin and
becomes a sexually mature adult. After
males and females fly over the water and
mate, the females lay eggs on the surface,
and both sexes die.
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Consider the following life histories
 4. A bamboo
plant in Patagonia
reproduces vegetatively for 100 years.
Along with other individuals, it forms a
dense stand of plants. Then, in one
season, all the individuals in the
population flower simultaneously,
reproduce sexually, and die. Another
100 years later, the process is repeated.
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Consider the following life histories
 A dandelion
seed lands in a wellmanicured lawn and germinates that
same day. Within a week, the plant has
a small rosette of leaves and has
produced a flower only a few inches tall.
The flower asexually produces a huge
number of seeds that are scattered by
the wind. A few days later, the plant
flowers again.
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Life history
 The
term life history refers to any aspect
of the developmental pattern and mode
of reproduction of an organism.
 Five fundamentals aspects:
 1.
Body Size. (體型大小)
 2. Metamorphosis. (蛻變)
 3. Diapause. (休眠)
 4. Senescence. (老化)
 5. Reproductive patterns. (繁殖類型)
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The effects of body size
 The
body sizes of organisms span a
tremendous range.
 In linear dimension this scale extends
from bacteria 1 micrometer in length to
redwoods 100 meters tall (not including
the roots), a span of eight orders of
magnitude.
 Even within a taxon, there is a great
range of body sizes (Fig. 7.1).
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Fig. 7.1 (a) and (b)
10
Fig. 7.1 Frequency distributions of number of
species with respect to log body mass for North
American mammals (a), birds(b), and freshwater
fish ©.
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The effects of body size

Body size has an important influence on life.
 These effects can be ecological, physiological, or
both.
 An organism’s total food requirements increase
with increasing size, while per-gram food
requirements decrease.
 Larger organisms have lower risks of predation.
 Vulnerability to physical factors also varies with size.
 Larger organisms generally have longer life spans
and thus longer generation times, which affect the
potential rate of evolution via natural selection.
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Cope’s law

Within a particular taxonomic group, size
tends to increase over evolutionary time, this
notion is referred to as Cope’s law.
 For example, in minnows, those species
closest to the phylogenetic root of the family
are the smallest.
 The common ancestor of equines,
Hyracotherium, the dawn horse, was a small
animal. The fossil record indicated steadily in
size. But other lines of horse-like mammals
did not show this trend.
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100 grams hypothesis

The distribution of mammalian body sizes has
a mode at about 100 grams.
 100 grams represents an optimal body size in
the mammals.
 The optimal body size is a balance between
these two limitations.


(1) the acquisition of energy, which increases with
mass raised to the 0.75 power, and
(2) the rate of conversion of energy to offspring,
which changes as a function of mass to the –0.25
power.
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Reproductive
power is
maximal for
animals of
about 100
grams.
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Allometry

The situation in which different morphological
characters change at different rates in
referred to as allometry.
 The general equation for allometric
relationships is Y = aXb.
 In Fig. 7.3 the relationship between body
weight and brain weight for mammals is
plotted on a log-log scale. The relationship is
described by the equation
Y=0.16 X 0.67
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Y=0.16 X 0.67
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Allometric changes
= aXb
 Log Y = log a + b log x
Y
 假若
b < 1 , y changes more slowly than x.
 When b>1, the reverse is true.
 譬如：圖7.3
brain mass vs. body wt.
 10,000kg
animal the brain/body ratio is
0.0056, For a 100gram animal, the ratio is
0.018. The brain weights change at a
slower rate.
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Kleiber’s law

Surface area varies as the square of the
linear dimension, whereas the volume varies
as its cube.
 In this case, the allometric equation would be
Y = a X 2/3


Where Y is the surface area and x is the volume.
Mammals show a consistent allometric
relationship between metabolic rate and body
mass in which the slope is 0.75, a
phenomenon known as Kleiber’s law.
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Quarter power scaling
 In
many ecological and physiological
allometries, the value of b is 0.25 or
0.75, a phenomenon known as quarter
power scaling.
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0.375 power
 Enquist
et al. (1998) showed that in
trees, the basal stem diameter scales to
mass to the 0.375 power, a fascinating
result because in mammals, the
diameter of the trachea and aorta scale
to the same power.
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Metamorphosis

Organisms that metamorphose undergo
radical changes in morphology, physiology,
and ecology over the course of their life cycle.
 Species that metamorphose must undertake
complex genetic and physiological processes
in the transformation.
 What sorts of ecological advantages could
outweigh these complications?
 One hypothesis is that metamorphic species
specialize so as to exploit habitats with high
but transient productivity – and hence high
potential for growth.
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Transient productivity hypothesis

Part of this strategy is that specializations for
feeding, dispersal, and reproduction are
separated across stages.
 A frog tadpole occupies an aquatic
environment with extremely high potential for
growth.
 But an aquatic larva is not capable of
dispersal to new ponds, the adult frog is.
 The energy that adults obtain from feeding is
dedicated to dispersal and reproduction.
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Transient productivity hypothesis

Many insects benefit from the same strategy.
 譬如：毛毛蟲快速的成長，蛻變為蝴蝶後，不
再成長。蝴蝶將其能量保留用於dispersal and
reproduction.
 In many marine invertebrates, the pattern is
reversed. The larvae are specialized for
dispersal, whereas the adults grow and
reproduce. 譬如：barnacle.
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Neoteny (幼體化)

Under certain ecological conditions, it is
apparently advantageous for reproduction to
occur in the larval stage.
 Neoteny, a life cycle in which the larvae of
some populations or races become sexually
mature and no longer metamorphose into
adults.
 Neoteny is an example of a secondary
adaptation because it could evolve only
secondarily– after metamorphosis evolved.
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Diapause and Resting stages
 If
conditions occasionally or regularly
become harsh, it may be advantageous
for the organism to have a resistant
stage built into the life cycle.
 This genetically determined resting
stage, characterize by cessation of
development and protein synthesis and
by suppression of the metabolic rate, is
called diapause.
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Variety of resistant stages

The variety of resistant stages among living
organisms is huge.
 Bacteria form highly resistant spores in response
to desiccation, heat, and certain chemical
environments.
 spores of fungi, seeds of plants, pupae of
insects are highly resistant.
 Some of these resistant stages can be extremely
long-lived. 譬如：於極地的lupine (pea family),
recovered from ancient lemming burrows in the
Arctic, germinated in three days even though
they were more than 10,000years old.
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Face two important abiotic
problems

The severity of the abiotic regime, including
temperature extremes, insufficient or excess
water, wind, and so forth.
 The unpredictability probably pose greater
difficulties for organisms than harsh,
predictable conditions.
 譬如：many seeds require a period of
stratification, exposure to low temperatures
for some minimum period, before they will
germinate. (避免因為環境的 unpredictability
造成致命的傷害)。
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範例： red kangaroo (Megaleia rufa)

The marsupial inhabits the deserts of central
Australia, where the onset of rains and the
resulting flush of vegetation are extremely
unpredictable.
 It is advantageous for a kangaroo female to
produce young at a time when plant productivity
is sufficient to support her offspring.
 However, gestation is so long that if a female
waited to mate and carry the young until after
the rains came, the favorable period might be
past.
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
Use of embryonic diapause
during gestation.
 At any one time, the female has
three young at various stages of
development: one in diapause,
one in the pouch, and one on the
hoof.
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範例：Couch’s spadefoot toad

The spadefoot toad inhabit some of the most
severe deserts in North America.
 Adults of this species burrow deeply into the
substrate where it is cooler and perhaps more
moist.
 When it rains, the adults emerge and
congregate to mate at temporary ponds.
 Development is greatly accelerated: The eggs
hatch within 48 hours, and the tadpoles
metamorphose at 16-18 days.
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範例：plants in the deserts

In the Mojave and Sonoran Deserts, rainfall is
infrequent, and even though it may occur in a
predictable season, it often falls over a
narrow and unpredictable geographic area in
any one year.
 Many of the plants in these regions have
adopted an annual habit: They complete their
life cycle in a single year (or season) and die.
 The seeds are highly resistant to desiccation
and can remain dormant in the desert for
many years.
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Senescence
 Why
must organism die?
 Is death truly inevitable?
 Could the timing of death evolve?
 Why do some organisms live so much
longer than others?
 Recent findings suggest the possibility
that life span has some genetic basis
and may, in fact, evolve.
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Senescence and life span

Senescence, as the degenerative changes
that result in an increase in expected
mortality with age. Eventually, the probability
of survival reaches zero.
 The range of life spans in plants, from a few
days to more than 5,000 years.
 Sturgeon(鱘魚) 150 years, earthworm 10
years, captive tarantulas(毒蜘蛛) 數十年。
 A clonal species Huge living fungi (Armillaria
bulbosa) 可能有10,000 or more years old.
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Does life span evolve?

Small animals are known to have shorter life
spans than large ones.
 Because the per gram metabolic rate, heart
rate, and so on are much higher in small
mammals, their shorter life spans were believed
to be a consequence of this rapid physiology–
the organism simply wears out sooner.
 However birds generally live longer than
mammals of comparable size, even though
birds have higher metabolic rates, heat
production, and heart rate.
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Two main hypotheses
 1.
The mutation accumulation hypothesis:
The accumulation of damage ultimately
results in a decrease in survivorship with
age. Senescence per se do not evolve,
rather it is the inevitable result of
exposure to the environment.
 2. The evolutionary senescence
hypothesis: the pattern of senescence
evolves in organisms.
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The concept of
reproductive
value provides
a selective
basis for
different life
history
strategies.
We have a way
to compare the
trade-offs
associated with
different life
histories.
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The reproductive value

V =  (lt/lx)(mx)(Nt/Nx)

Where lx is the age-specific survivorship

mx is the age-specific birth rate

N is population size

x is the current age

(lt/lx) represents the probability of surviving
from age x to age t.

A low probability of survival should result in a
low reproductive value.
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Fig. 7.7 Age-specific changes in (a) mean
annual survival of female sparrowhawks.
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Fig. 7.7 Age-specific changes in (b) mean annual
production of young of female sparrowhawks.
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Natural selection will
have less impact on
deleterious mutations
whose effects are
manifest after the peak
in reproductive value.
Consequently,
senescence may result
from the accumulation
of such mutations.
Fig. 7.7 Age-specific changes in (c) reproductive
value of female sparrowhawks.
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Transgenic Drosophila
 Cells
contain certain enzymes, such as
superoxide dismutase, that scavenge
oxygen radicals and thus protect cells
from damage.
 Orr and Sohal (1994) developed
transgenic Drosophila that contained
three copies of one of the genes for
these enzymes, the flies with extra
copies of the genes had 33% longer life
spans than normal flies.
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Artificial selection on Drosophila
 Partridge
and Fowler (1992) used
artificial selection to generate two
genetic lines: an “old line” generated by
allowing only the oldest individuals to
mate, and a “young line” generated by
mating only young individuals.
 Individuals from the “old line” jad longer
life spans (Fig. 7.8)
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Fig. 7.8 Survivorship curves for (a) male and (b)
female Drosophila from old line(dashed lines) and
young line (solid lines) selected strains.
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Fig. 7.8 Survivorship curves for (a) male and (b)
female Drosophila from old line(dashed lines) and
young line (solid lines) selected strains.
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Altitudinal variation in
Melanoplus grasshoppers

The evolutionary hypothesis predicts that
earlier senescence should evolve at high
elevations where late-age reproductive
opportunities are limited by the shorter
season and harsh conditions.
 Indeed, low-elevation populations haf lower
senescence, as depicted by the survivorship
curves in Figure 7.9.
 The differences in senescence between high
and low-altitude populations have a genetic
basis.
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These data were generated from populations in
laboratory cultures under the same conditions.
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The effect of reproduction on
mortality
 Vanvoorhies
(1992) compared the agespecific mortality rates of two mutant
nematodes – one that does not produce
mature sperm and one that does not
mate – with wild-type animals.
 The wild-type strain had higher agespecific mortality and a shorter life span
than both mutants (Fig. 7.10).
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Fig. 7.10 (a) survivorship curves for nematodes.
Open circles are wild-type males; closed circles
are mutant males that do not produce sperm.
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Fig. 7.10 (b) survivorship curves for mated wildtype males (open circles) and mutant males that
do not mate (closed circles).
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Reproductive Strategies

Both sexual and asexual strategies are
successful, for they are found among virtually
all taxa of both plants and animals.
 Asexual reproduction can have advantages in
a uniform or unchanging environment.
 Sexual reproduction can produce different
offspring with different genotypes via shuffling
of genes by sexual recombination.
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Two basic assumptions:
 1.
Because the amount of energy for
reproduction is finite, the species must
make an evolutionary “decision” about
how to apportion that energy in the
reproductive process.
 A relationship exists between the
demography of a species, particularly its
mortality schedule, and its reproductive
pattern.
Energetics of reproduction
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Trade-offs in reproductive strategies
 Number
of offspring per reproductive
event (每次要生幾個)
 Present versus future reproduction(現在
生殖的投入，與未來生殖的投入)
 Age at sexual maturity (成熟年齡)
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Number of offspring per reproductive event
 Females
can product a few large
offspring, or a large number of smaller
individuals.
 There is an upper bound to the optimal
number in a clutch.
 譬如：kittiwake (三趾鷗)從一窩兩個蛋，
增加到三個蛋，no parents were able to
raise the enlarged broods to fledging.
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The weight at
weaning,
probably an
important
factor in
survivorship,
is significantly
lower in large
litters.
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Mean birth weight declines with increasing litter size.
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緯度愈大，
clutch/litter
size 愈大。
(mammals and
birds)
但是
hibernating
mammals and
hole-nesting
birds, which
have litter or
clutch sizes
smaller than
expected.
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Hypotheses to explain this trend

1. Lack proposed that the longer days at
higher latitudes allow parents more time to
gather food to feed more young, enabling
them to support larger clutches.
 2. A shorter breeding season at high latitudes
selects for larger clutches because fewer can
be produced.
 3. The higher mortality associated with the
more severe northern climates selects for
larger clutches or litters.
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Present versus future reproduction
 Vx
= mx +  (lt/lx)(mt)
 Where
mx represents
the current
reproductive value.
 Survivorship
curves,
type I, II, and III
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Iteroparity vs. semelparity

Type I survivorship curves are typical of
mammals. Mammals are likely to have several
reproductive efforts over the course of their life
span. We refer to this strategy as iteroparity.
 Species with a Type III curve are more likely to
have a single, large reproductive event in their
lifetime, a strategy referred to as semelparity.
 Massive semelparous reproduction is
sometimes referred to as big-bang reproduction.
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Semelparity vs. annual plant
 The
terms semelparity and annual are
not strictly synonymous.
 For example, a desert annual plant is an
organism with a semelparous life history
strategy.
 Similarly, perennial (多年生)未必然就是
iteroparity. 譬如：竹子。
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Semelparous reproductive
strategy 的古典範例

Sockeye salmon.
 They are hatched in freshwater, and after a
few weeks there, the juveniles migrate to salt
water.
 There they exploit the tremendous
productivity of the temperate and arctic
marine environment.
 After several years in the open ocean, the
salmon begin a grueling migration back to the
very stream in which their parents reproduced.
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Fig. 7.13 (a) Migrating
sockeye salmon, (b) death
follows a massive
spawning event.
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Anadromous fish
 Anadromous
fish such as salmon tend
to occur in temperate latitudes, where
the oceans are more productive than the
freshwater systems.
 The frequency of anadromous fish
increases abruptly when the oceans
surpass freshwater in productivity (Fig.
7.14)
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Age at sexual maturity
 It
seems reasonable that selection
would push the age at which sexual
maturity occurs to the lowest possible
age.
 However, if reproductive success
depends on age, size, status, or
experience, delayed reproduction may
be advantageous.
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The theory of r- and K- selection

r-selected species
are commonly
found at low
population
densities, where
growth is
exponential.
 K-selected
species face
intense
intraspecific
competition for
scarce resources.
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範例：two vole species

These species are very closely related.
 Microtus breweri is endemic to Muskeget
island and it evolved recently from the
mainland species, M. pennsylvanicus.
 The island form does not experience the
high-amplitude cycles that are characteristic
of M. pennsylvanicus (Fig. 7.16)
 Instead, it remains at relatively high density.
M. breweri has chracteristics associated with
K selection, where M. pennsylvanicus exhibits
a more r-selected strategy (Table 7.3).
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Fig. 7.16 Log population size over time for (a) M.
breweri . M. breweri remains at high density.
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Fig. 7.16 Log population size over time for (b) M.
pennsylvanicus.
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M. breweri has chracteristics associated with K
selection, where M. pennsylvanicus exhibits a more
r-selected strategy
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Intraspecific 範例

Common dandelion in Massachusetts.
 The species was found in three distinct
environments: a well-traveled pathway, a lawn
that was regularly mowed, and a successional
field that was mowed once a year.
 The three habitats represented different
degrees of density-independent disturbance.
 Electrophoretic studies on plants revealed four
distinct genotypes that were associated with
particular environments.
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Genotype D was
characterized by
adaptations that
favored
competition over
reproduction:
fewer flowering
heads per plant
and larger
photosynthetic
surface area.
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Box
Life Histories and Conservation

Some characteristics of life histories are key
determinants of the vulnerability of a species
to extinction.
 Body size: the size of an organism is
correlated with a number of features related
to its probability of extinction, including
generation time (Fig. 7B.1).
 The populations of the species with larger
body size are more vulnerable to accidents or
chance decreases in population size that may
lead to extinction.
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R0 and G
 R0
measures the average number of
female offspring expected over the life
span of a female.
 G is the generation time.
 R0 = NG/N0 = erG
 lnR0 = rG
These equations demonstrate that he
 r = lnR0/G intrinsic rate of increase, r, is negatively
associated with the generation time.
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r and K-selected species
 We
expect K-selected species to be
vulnerable because of a number of
interacting factors.
 Being large, and thus requiring greater
resources, they find themselves near K.
 The optimal strategy for K-selected
species emphasizes quality of offspring
rather than quantity.
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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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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
S-selection
K-selection
Fig. 12. Climates and types of selections
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Constraints and ambiguities in the
study of life history strategies
 The
theories of the evolution of life
history strategies are optimization
theories.
 What do we conclude if the life history
does not match the predictions?
 It is possible that the poor match results
from one or more of the following
constraints:
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Constraints

1. Evolution is an ongoing process.
 2. Fitness is a relative property. Although our
theory might suggest the optimal life history,
selection cannot choose the solution with
absolute fitness, it can only choose the fittest
of the options available.
 3. It may be difficult for us to ascertain the
appropriate time scale over which to consider
an organism’s life history. (Fig. 7.18)
 4. Allometric relationships may confound our
analysis of life history strategies. (Fig. 7.19)
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Fig. 7.18 Demonstration of how the choice of time
scale can lead to ambiguity in life history depicted
by the product of age-specific birth and mortality
rates (lxbx) . (a) The life histories of two intertidal
barnacles graphed in absolute
time.
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Fig. 7.18 (b) The same life histories graphed in
generation time.
The first graph (a) emphasizes the differences
between the two species; the second (b), the
similarities.
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