BIOL 4120: Principles of Ecology
Lecture 8: Life History
Patterns
Dafeng Hui
Office: Harned Hall 320
Phone: 963-5777
Email: dhui@tnstate.edu
Life History
Life history is species lifetime pattern of
growth, development and reproduction.
Measure of organism’s reproductive success
is fitness: Those individuals who leave the
largest number of mature offspring are the
most fit the environments.
Trade-off between growth and reproduction:
mode of reproduction, age at rep.,
allocation to rep. number and size of eggs,
young or seeds, parental care.
Life History Patterns
8.1 Reproduction may be sexual or asexual
8.2 Sexual reproduction takes a variety of forms
8.3 Mating systems
8.4 Mate selection
8.5 Females may acquire mates based on resources
8.6 Organisms budget time and energy to reproduction
8.7 Species differ in the timing of reproduction
8.8 Parental investment
8.9 Fecundity depends on age and size
8.10 Food supply affects the production of young
8.11 Reproductive effort may vary with latitude
8.12 Habitat selection influences reproduction success
8.13 Environmental conditions influence the evolution of life
history characteristics
8.1 Sexual or Asexual Reproduction
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Asexual reproduction (produce offspring without involving
of egg and sperm)
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New individuals are the same as the parent
Many plants (underground stem) such as strawberry;
some animals (hydra, some aphids, parthenogenesis)
If fitness is high, matches organism to environment
If fitness is low, possible extinction (less variation)
Stress can result in use of sexual cycle to give new gene
combinations (hydra, aphid)
Sexual Reproduction
• More common form.
• Can produce new gene combinations able to cope with a
changing environment.
• Greater energy commitment
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Specific organelles
Production of gametes, courtship activities, and mating are
energetically expensive.
Feeding offspring
The expense of reproduction is not shared equally by both sexes
8.2 Types of sexual reproduction
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Dioecious
• Sexes are separate
individuals
• Greatest diversity of
offspring
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Hermaphroditic
• Perfect
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Male and females
organs in same
flower
Can result in
significant
inbreeding
• Monoecious
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Floral structure
Plants
Separate male and
female flowers
Reduces but does
not eliminate
inbreeding
Animals
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Most familiar form involves male and female
individuals
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Hermaphroditic
• Simultaneous hermaphroditic
 Both sets of organs at same time
• Earthworms
• Outbreeding, but maximizes
offspring (twice)
• Sequential hermaphroditic
 First one sex then the other sex
• Mollusks, echinoderms
Sometimes animals (fish)
• Allows all individual to
participate in both sides of
sexual cycle
Plant can undergo sex change: jack-inthe-pulpit (Arisaema triphyllum)
Parrotfish
Largest female will become male
when the male fish is missing
8.3 Mating Systems describe pairing of
males and females
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Different mating strategies have different advantages
and disadvantages
• Monogamy (one to one, form of a lasting pair bond between one
male and one female)
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Most prevalent among birds, rare among mammals
Seasonal or permanent
• Allows sharing of cost of raising offspring
• Increases survival chances of offspring
• Cheating does occur and has specific advantages to fitness
• Polygamy (one to two or more, a pair bond exists between individual
and each mate)
 More than one mate of one sex for a single individual of the other
sex (polygyny and polyandry)
• Free individual to compete for resources and protect territory
• Better food etc for mates
• Some protection of offspring from competition
• Promiscuity (one to one or many and no pair bound formed)
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Greatest number of offspring
Large amount of competition
Female only responsible for offspring in terms of resources
• Poorer survival chance for offspring
8.4 Sexual Selection
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For Monogamy, Polygamy and
Promiscuity
• All involve the selection of a
mate and therefore sexual
selection
• Selection for secondary sexual
characteristics
 Peacock versus Peahen
• Large tail feathers, more
mating
• Smaller tail feathers, less
mating
 Deer
• Characters that aid
competition such as
horns
 Humans
• Faster sports car such as
a Ferrari
Sexual selection
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Intrasexual selection
• male-to-male competition for the opportunity
to mate
• exaggerated secondary sexual characteristics
such as large size, aggressiveness and organs
of threat such as antler and horns
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Intersexual selection
• Differential attractiveness of individuals of one
sex to another
• Characteristics in male such as bright or
elaborate plumage used in sexual display as
well as these in intrasexual selection.
What is the mate really looking for
in sexual selection
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In most cases the sexually selected
characteristic is an indirect measure of
resources or fitness
• Bigger males have captured more resources
(large territories, abundance of food etc)
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Sports car
• Is this just a display
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Big red car that makes a lot of noise
• Or does it measure resources
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Ferraris are expensive
8.5 Organisms budget time and
energy to reproduction
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Reproductive effort: Time and
energy allocated to
reproduction
Trade-off between growth,
maintenance and reproduction.
• There is a negative
relationship between
annual plant growths and
the allocation to
reproduction.
Percentage of annual
production to reproduction:
Perennials: 15-20%
Wild annuals: 15-30%
Crops:
25-30%
Corn and barley: 35-40%
Lizard:
7-9%
Salamander:
48%
Species’ trait
Production of offspring
Costs of care and nourishment
Bird species (dots) illustrate the tradeoff between “fast” organisms
(high fecundity, high mortality) versus “slow” ones (long life, low
annual fecundity)
Species differ in timing of
reproduction
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Semelparity
• One reproductive effort with all resources, then death
• Most insects and other invertebrates, some fish (salmon)
and many plants (bamboo, ragweed)
• Some are small, short lived, grown in disturbed habitats;
• Environmental effect can be disastrous
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Iteroparity
• Produce fewer young at one time and repeat
reproduction throughout their lifetime
• Multiple cycles of reproduction means the organism
must balance growth, maintenance, escaping predators,
defending territory, etc against reproduction
• Most vertebrates, perennial herbaceous plants, shrubs,
and trees.
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Timing production: When – early or late
How many offspring: cost of the fecundity and its own
survival.
Sockeye salmon, for example, swim as far as
6,000 km from Pacific Ocean feeding grounds
to spawning streams, lay thousands of eggs,
then die from the exertion.
8.6 Parental investment depends on the
number and size of offspring
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Given certain resource allocated to rep., one can produce many
small young or few large ones. The number of offspring affects
parental investment.
Produce large number of offspring, less or no parental care
(fish-eggs, plants-seeds)
Produce helpless offspring (produce young, spend less
energy in incubation, but require considerable parental care)
• Altricial
• Mice
 Longer period suckling
• Robin
 Other bird feeds
Produce more mature offspring (longer gestation, born in
advantaged stage of development)
• Precocial
• Chicken, cow, deer, turkey
Humans ?
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Family care (Grandmothers, Grandfathers, Aunts, Uncles, Brothers and Sisters)
African elephants produce one offspring at a time,
once every few years over a long lifetime, and
protect each offspring intensively (much like
humans)
Few Number
• More resources per individual
• More chance of accidental loss
By contrast, many plants and some insects, reproduce
once (annually), producing vast numbers of
seeds/eggs that are poorly protected, if at all
Large Number
Less resources
per individual
More chances of
success
Desert annuals
Extreme
with
released
eggs of
some fish
such as cod
(millions of
eggs) etc
8.7 Fecundity depends on age and
size
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For many species, number of offspring produced
varies with the age and size of parent.
But some species do not have a characteristic adult size
and can continue to grow through their adults lives
(indeterminate growth)
• Many plants and ectothermic (cold-blooded) animals
(fish, reptiles, amphibians, and invertebrates)
Plants
 Perennial: delay flowering until they have
attained a sufficiently large size
 Biennial: delay flowering beyond 2-yr life
span under poor environments
 Annual: small plants produce less seeds
Animals:
Ectothermic (cold-blooded) animals
Production of offspring in fish increases with size, which increases
with age
Gizzard shad: 2-yr, 59,000 eggs
3-yr, 379,000 eggs
Endothermic (warm-blooded):
similar patterns exist for some animals
European red squirrel: body weight and reproduction success;
<300 g, do not reproduce.
8.8 Reproduction effort may vary
with latitude
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Birds in temperate
regions have a larger
clutch size than
tropical birds
• Food supply, with
longer day length in
springtime to forage for
food to support larger
broods
• large climate variation,
decreases popul. below
carrying capacity, need
more young
• Greater mortality in
winter results in more
food for survivors next
spring
Habitat Selection
Neotropical warblers
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Process in which organisms actively choose a specific location is
called habitat selection.
Filling the available niches (enough food, water etc) and keeping
out competitors (find neighbors), may settle in less ideal habitats
Exception humans
• All habitats
• Left Africa and adapted the environment rather than adapted to
the environment
8.9 Environmental conditions influence the
evolution of life history characteristics
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Idea was conceived by Robert
MacArthur and Edward O. Wilson: “rvs. K-selected strategists”
Derivation of the terminology comes
from population models (see future
lecture):
• “r” is population growth rate; rselected species have traits that
increase r
• “K” is population carrying
capacity; K-selected species have
traits that increase carrying
capacity and competitive ability
when populations fill environment
Spotted and redback salamanders
Examples of r- and K-selected organisms
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r-selected organisms—short-lived, e.g.,
dandelion, with rapid population growth
rate, small body size, early maturity, larger
number of offspring, minimal parental care
(animals). Inhabit unstable conditions,
disturbed areas.
K-selected organisms –competitive species,
long-lived, e.g., oak tree with long life,
production of few, large seeds that can
grow readily in shaded environments, but
lack of mean of wide dispersal, poor
colonizers of new or empty habitats.
Summary of life-history traits in r-selected
versus K-selected strategists
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“strategy”
1. Environment
2. Mortality
3. Population size
4. Competition?
5. Favored traits
r-strategist
K-strategist
variable
constant, predictable
density independent. dens.-dep.
variable, below K
constant, at K
variable, lax
keen
rapid development
(opposites)
early reproduction
small body size
semelparity
short generation time
good dispersal, colonizing ability
high allocation to reproduction (small
offspring/seed sizes; many offspring)
The End
Reproduction effort
may vary with latitude
Set nest boxes at two places
Illinois
Panama
Monitor and collect newly
laid eggs over one breeding
season
Eggs are marked by date
and weighted
Hatched in incubators at
37.8~38oC, RH 85-90%
Another interesting thing is that it takes the same amount of time to
hatch in nature and incubators (for current setting)
Conclusions:
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Ecologists have made great progress explaining
many (but by no means all) life history traits, using
arguments based on individual level natural
selection
Life-history traits are often correlated in their
distribution because of the effect of habitats on
multiple traits
Tradeoffs among different traits are also very
common, indicating the inability to evolve one
phenotype that is perfect in all situations:
Organisms have been selected to allocate resources
differentially in different environments
Ruderal
Small and rapid lifecycle to invade
new sites. Large dispersal area
Competitive
Stable environment, slower lifecycle
With more resources to growth
Stress-tolerant
Limited resources. Ability to adapt