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Lecture

BIOL 103 Lecture Notes - Semelparity And Iteroparity, Natural Selection, Life Table


Department
Biology
Course Code
BIOL 103
Professor
Peter T Boag

Page:
of 3
Population Ecology Continued
- Populations can temporarily exceed carrying capacity
- Logistic Population Growth = stability
- Rare example of exponential population growth when four males and 22
female reindeer introduced to Alaskan island, grew to over 2000 in 30 years,
overgrazed habitat and population crashed to 8 animals by 1950
- At the peek, the population exceeded carrying capacity of the island
- Near the end of the graph, there was no food because the environment was
overgrazed
- i-Clicker Question: Some organisms have peculiar population cycles, such as
the various species of Periodic Cicadas, with species that emerge only every 7,
13, 17, or 19 years from below ground. This might be since these are all
prime numbers, it reduces the likelihood a predator population can track
them with an overlapping cycle of it’s own
- Two or more species with interacting populations can give complex patterns;
some populations are stable while other fluctuate and crash
o Food supply and predators may cause a steep decline
- Life history traits are those closely linked to Darwinian fitness (e.g. clutch
size, age at 1st reproduction, longevity, etc.) and evolve in response to biotic
and abiotic factors
o Semelparous reproduction
Common in insects, fish, some plants which use all their energy
in a single, immense reproductive effort, e.g. salmon dying after
spawning or annual weeds
Reproduce one time in their lifetime
Have a short lived life
o Iteroparaous reproduction
Exhibit repeated reproductive cycles throughout their
lifetimes, often invest more in each offspring, e.g. many large
trees, mammals, birds
o More general comparison of ‘r-selected’ versus ‘K-selected’ species
o r-selected usually are semelparous too they maximize r
o k-selected tend to be larger, longer lived, population size is rather
stable, density dependent stable long term pattern
o Natural selection can influence traits like how many eggs they lay at a
time
- Population dynamics or demography
o Need to collect data and create models to estimate vital statistics
such as bitch and death rates of various types of individuals in a
population (males vs. females, old vs. young)
o Information summarized in a ‘Life Table’ used to compare life
histories between species
o Several items are often important:
Average age individuals start reproducing
Hot often individuals reproduce before dying
How many offspring produced at reproduction
Pattern of age-related survival (survivorship)
How individuals reproduce
- Survivorship curves are important life history component, shaped by
selection acting on the most vulnerable stages of an organism’s life cycle.
Curves can assume any shape, but useful to distinguish three common types
o Type 1:
Morality low in juveniles, greatest in old age, usually associated
with iteroparity
o Type 2:
Morality spread evenly across all ages
There is an equal probability of dying for age category
o Type 3:
Morality greatest among the young, promoting large
reproductive effort; adult survivors may be long-lived
The probability for young dying is higher than the probability
of older individuals dying
- Survivorship curves are sometimes easier to visualize as age-specific
morality curves
- Life tables
o Summarize age-specific birth and death rates for a cohort
o Make predictions about future population trends
o Often for females only if males do no impact demography, Ro, (net
reproductive rate) is approximately equal to r
o There is usually a focus in survival in life tables, but some focus on
other things
o Summarizes how population ages and how many die in order to
compare life-history of species
- Demography of human populations and associated environmental impacts
are discussed in the final week of lectures