Download Life History Strategies: Trade-offs with reproduction and survival

Life History Strategies:
Trade-offs with reproduction and survival
Sources: Case (2000), UCSD website;
Ricklefs and Miller (1999), Stearns (1992),
Reznick et al. (1997, 2002)
Some slides presented here modified from Case’s website
Motivation:
diversity of life history strategies displayed in nature
and their role on the dynamics of populations
Evolutionary and conservation implications
Just as adaptation in functional form occurs,
so does “demographic adaptation”– life history strategies
(will later link these ideas to elasticity analyses of matrix pop. models)
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Types of survival curves
Pearl’s Type I, II, and III idealized survivorship curves
Type I: Low mortality early on– investment for long life span
Type III: High early (e.g., juvenile) mortality
Types of reproductive strategies
•Semelparity (death after reproduction)
•Iteroparous (repeated bouts of reproduction)
Reproductive parameters:
Age at maturity, size/fecundity relationship,
number of eggs/litter size, frequency, egg size, egg physiology, etc.
Variation at all levels:
individuals, populations, species, and higher taxa
Proximate and ultimate factors
Broad range of genetic heritabilities of reproductive effort
(0-90% based on empirical studies)
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Life-History as Adaptations to the Environment
“… it will be regarded as axiomatic that the reproductive
potentials of existing species are related to their requirements
for survival; that any life history features affecting reproductive
potential are subject to natural selection [and thus must be
heritable] and that such features observed in existing species
Should be considered adaptations, just as purely morphological
or behavioral patterns are so considered”
From Lamont Cole (1954): The population consequences
Of life history phenomena. Quart. Rev. Biol. 29:103-137
Background:
terms and concepts useful for understanding readings
• r = exponential growth rate
• r-selection = selection on traits that determine fecundity and
survival to favor exp growth at low density
• k = carrying capacity
• k-selection = selection on traits that determine fecundity and
survival to favor competitive ability at densities near
carrying capacity (k)
Pop. Density
Logistic Equation
k
r
Time
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Background:
terms and concepts useful for understanding readings
Sx= probability of survival from age-class x to x + 1
lx = probability of survival to age x
bx = fecundity, number of offspring
Ro = lifetime (net) fecundity
8 = Population growth (er)
• Fitness = the genetic contribution by an individual’s
descendants to future generations
• measured as 8 or r in Case (Ch. 7), and thus a function
of survival and fecundity
• selecting a proper metric difficult in the case of densitydependence (Ro sometimes used)
• “measured only by a genotype’s rate of increase relative
to other genotypes” (Futuyma 1986)
What is life history?
• The life history is the schedule of an organism’s
life, including:
– age/size at maturity
– number of reproductive events
– allocation of energy to reproduction
– number and size of offspring
– age and size specific pattern of growth
– life span (patterns of survival)
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What is a life history?
A life-history is a set of environment and
condition-dependent decision rules governing an
organisms scheduling of allocation of resources
towards growth, survival and reproduction.
Life Histories
• Consider the following remarkable differences in
life history between two birds of similar size:
– thrushes
• reproduce when 1 year old
• produce several broods of 3-4 young per year
• rarely live beyond 3 or 4 years
– storm petrels
• do not reproduce until they are 4 to 5 years old
• produce at most a single young per year
• may live to be 30 to 40 years old
Ok, a fish example: some flounders mature in first year, other species at 10 yr
• Similarly, intra-specific and individual variation is often vary high
Ok, a fish example: a species of flounders mature in 3 yr, 20 cm in Scotland
but takes 15 yr, 40 cm in Newfoundland
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What influences life histories?
• Life histories are influenced by:
– body plan and life style of the organism
– evolutionary responses to many factors,
including:
• physical conditions
• food supply
• predators
• other biotic factors, such as competition
• temporal variation of these factors
A Classic Study
• David Lack of Oxford University first
placed life histories in an evolutionary
context
Review details in Case (2000)
The “Lack” Clutch: the clutch size that produces the greatest number
of (reproducing) offspring
Reproductive effort model: a “theoretical tradition” that aims to
optimize reproductive effort taking the costs of reproduction into account
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Lack’s Proposal
• Lack made 3 key points, suggesting that life
histories are shaped by natural selection:
– because life history traits (such as number of
eggs per clutch) contribute to reproductive
success they also influence evolutionary fitness
– life histories vary in a consistent way with
respect to factors in the environment
– hypotheses about life histories are subject to
experimental tests
Optimal Reproductive Effort
The Lack Model and Beyond
What is Lack’s Model? What does it predict?
Most birds produce fewer young than predicted by Lack’s model
1. Probably most studies (incorrectly) equate number of fledglings
and the number young breeding in future population
2. Other costs are involved besides juvenile survival
3. Temporal variability of environment
Bet-hedging: reduce reproductive effort to live longer
and have better chance of reproducing in “good” years
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Cost of reproduction in Kestrels.
Predictions on the Effects of Different Life History Strategies
On Population Growth Rate
Empirical support?
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Life History Studies of the Trinidad Guppy
Trinidadian guppies (Poecilia
reticulata)
• D. Reznick et al. 1990. Nature 345:357-9,
1997. Science 275:1934-1937
• Live in streams in Trinidad
• Exposed to different predation pressures in
different streams and parts of streams.
• Waterfalls often separate populations exposed to a
given predator from those that are not.
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Aripo river, Trinidad. Waterfalls like this
often separate populations exposed to
different predators.
Predators and prey
Crenicichla alta
Rivulus hartii
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Predators impose different
mortality pressures.
• Crenicichla alta (a cichlid) feeds mainly on
adult guppies, lowering adult survival.
• Rivulus hartii (a killifish) preys mainly on
juvenile guppies, lowering juvenile
survival.
Life-history predictions:
• Rivulus predation
• Juvenile survival low,
adult survival high
• Increase size of
offspring at the
expense of offspring
number.
• Lower reproductive
effort.
• Crenicichla predation
• Adult survival low,
juvenile survival high.
• Many small offspring.
• High reproductive
effort
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Comparative approach using
common garden experiments
• Find populations exposed to different predators.
• Collect individuals from each population and
bring to lab.
• Rear fish under common environmental
conditions.
• Measure life history traits and their heritabilities.
Fish room (common garden)
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Life history traits for two populations of
guppies with Rivulus and two
populations with Crenicichla.
Life history trait
Age of females at 1st brood (days)
Rivulus
81.9
Crenicichla
71.5
Size of females, 1st brood (mg wet)
270.0
218.0
Size of 1st litter
3.2
5.2
Offspring size (mg dry), litter 1
Reproductive effort (%)
0.99
19.2
0.84
25.1
Comparative approach:
correlation or causation.
• Differences seen are clearly genetic
(common garden experiment).
• Differences seen correlated with identity of
predator.
• Is the predator the cause of differences?
• Need experimental approach.
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Experiment
• Transplant guppies from a habitat with
Crenicichla (preys on adults) to one with
Rivulus (preys on juveniles).
• Predictions: Guppies should evolve toward
fewer, larger offspring, lower reproductive
effort, longer time to first reproduction and
longer interval between broods.
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Results of transplant experiment
(traits measured in common
environment)
Life history trait
Age of females at 1st brood (days)
Size of females, 1st brood (mg wet)
Size of 1st litter
Offspring size (mg dry), litter 1
Rivulus
(introduction)
92.3
185.6
3.3
0.95
Crenicichla
(control)
85.7
161.5
4.5
0.87
Experimental results in guppies
generally confirm comparative findings.
• Exposure to high juvenile vs. adult
mortality caused the evolution of delayed
reproduction, larger size at reproduction,
fewer larger offspring (particularly for first
brood), and reduced reproductive effort.
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Principal Findings and Relation to Life-History Theory
Multiple Factors
Density-dependent?
r- and k-selection theory useful?
“…it became clear that the predictions of r- and K-selection
Were not always upheld….This dose of reality helped the field
develop a more rigorous theory… The predictions of more heavily
derived models are often dependent on…factors hard to measure in
natural populations. It is this disparity between theoretical concepts
and empirical realities that continue to create a “muddle in life-history
thinking”
Reznick et al. 2002
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