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
– 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

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

• 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

Lynx predates weakened hares – eventually crashes
Hare popul crashes as:
1. Reduced forage
 weakened hares, high lynx prdation
2. Forage produced after heavy browsing accumulates plant defense chemicals
 less palatable

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