Transcript Lect13 LIfe Histories

Lecture 14 Life Histories
Modes of reproduction – sexual vs. asexual
k vs r selected species
Survivorship tables
Life Histories
• An organism’s life history is its lifetime pattern of
growth, development, and reproduction
• Maximal reproductive success or fitness is
constrained by limited resources and an organism
must balance trade-offs
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Modes of reproduction
Age at reproduction
Allocation to reproduction
Time of reproduction
Number and size of eggs, young, or seeds produced
Parental care
• Sexual reproduction is the fusion of
haploid egg and sperm to form a diploid
zygote
– A major source of genetic variation due to the
recombination of chromosomes during egg
and sperm production
• Asexual reproduction produces offspring
without the involvement of egg and sperm
– Individuals are genetically identical to the
parent
Asexual vs. Sexual Reproduction
 Asexual reproduction
– Benefits
– Offspring are well adapted to current conditions
– Potential for high population growth
– Costs
– Low genetic variability in the population
– May be unable to adapt to a change in environmental conditions
• Sexual
– Benefits
– High genetic variability in the population
– Increased probability that some individuals will survive
environmental changes
– Costs
– Parents only contribute one half of its genes
– Specialized reproductive organs required
– Expense of reproduction not equally shared between parents
How many young are produced?
• Limited access to energy/resources results in
trade-off between number and size of
offspring
– ie.- species producing a larger # if offspring
means offspring are smaller, and vice-versa
• Parent provides extended care for young 
fewer young produced but greater survival
rate
– The amount of energy invested in reproduction
varies for different individuals
– Investment in reproduction includes production,
care, and nourishment of offspring
– An individual’s fitness is determined by the
number of offspring that survive to reproduce
Common Murre
Three Survivorship
Patterns
– Type I = K selected
• Mortality rises in
post-reproductive
years
– Type II
• Mortality constant
throughout life
– Type III = r selected
• Many offspring with
high juvenile
mortality
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K selected species
Low number of young produced
Offspring size tends to be large
Low mortality of young
Extended parental care
High rate of survival past reproductive age
Long time to maturity
Relativly long life span
Live near carrying capacity
r selected Species
• High number of young produced
• Low parental input to each individual young
• Short maturation time
• Breed at young age
• Produce many offspring quickly
• High mortality of young
• Nonexistant parental care
• Opportunists – populations quickly develop but
may crash
• Examples:
– Waterfleas, insects, bacteria
Life History Classification
• MacArthur and Wilson
– r selection (per capita rate of increase)
• Characteristic high population growth rate.
– K selection (carrying capacity)
• Characteristic efficient resource use.
• Pianka : r and K are ends of a continuum,
while most organisms are in-between.
– r selection: Unpredictable environments.
– K selection: Predictable environments.
Life Histories – Age Structure and
Survivorship in Populations
• Cohort populations
– Birthrate and survival of young
– Competitive ability vs population size –
survivorship patterns
• Principle of allocation and reproduction
• Dispersal and seed size
• Ecological succession
• Cohort – a group of individuals of the
same age within a population (individuals
born at same time) - see p. 240-241
– Study of cohort provides information about:
• Mortality and survival vs. age
• Used to construct a cohort life table
– Static life table
• Pattern of survival  survivorship curve
Static life table – ‘snapshot’ of
population at a given time
• Data corrected to 1000 – actual
number sampled 608
• Dall sheep – Murie study 1944
• Collect skulls
• Evaluate age of animal at time
of death
• Allows evaluation of
survivorship: percentage of an
original population that
survives to a given age
Plant Succession and Life History Patterns
JPGrime (pages 286-288)
• Ruderals (highly disturbed habitats)
– Grow rapidly and produce seeds quickly.
• Stress-Tolerant (high stress - no disturbance)
– Grow slowly - conserve resources.
• Competitive (low disturbance low stress)
– Grow well, but eventually compete with others for
resources.
Stress: environmental extremes or competition that
limits (or provides excess) light, temperature,
nutrients
Survivorship and Age structure
• Age structure: Proportion of individuals in
various age classes
• Survivorship is the percentage of an
original population that survives to a given
age
– Involves study development of life table
• Cohort
– Example: Cactus finch
• Static
– Example: Dall sheep
• Age Structure Diagrams: Visualization of
future population growth
• What regulates population size?
Ch 18 p 344
• Diffuse predator–prey interactions
– The lynx, coyote, and horned owl are
responsible for the periodic cycles in the
snowshoe hare population
• Diffuse mutualism
– A single plant species may depend on a
variety of animal species for successful
reproduction
• Is regulation topdown or bottomup?
• ie. primary
productivity vs.
limits imposed by
predator
populations
Hare popul crashes
as:
1. Reduced forage
 weakened
hares, high lynx
prdation
2. Forage produced
after heavy
browsing
accumulates plant
defense chemicals
less palatable
Lynx
predates
weakened
hares –
eventually
crashes
Old Field Succession: Dwight Billings
• Early species to invade: ‘weedy’ or rselected species
– Do not compete well for resources, high
reproductive rate
• Shift to k-selected species
– Changes in nature of habitat favor species
which reproduce successfully at or near
carrying capacity
Plant Life Histories