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

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 top-down or bottom-up?

• 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 prdation2. Forage produced after heavy browsing accumulates plant defense chemicals less palatable

Old Field Succession: Dwight Billings

• Early species to invade: ‘weedy’ or r-selected 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