9-28-09
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Chapter 13: Populations
Part 3: Population Ecology
Population: unit of evolution
a. Organism: unit of natural selection
What is a population?
a. A group of organisms of the same species occupying a particular space at the
same time. The organisms of the population mate with one another and produce
fertile offspring.
b. A population is characterized by density, age structure, birth rate, and death
rate.
c. Life tables
Metapopulation
a. A series of smaller, separate populations that mutually affect one another. These
different groups of populations are united by the movement of individuals
among them
i. Desert bighorn in the southwest US
Unitary and Modular Population
a. Unitary Population –a population consisting of unitary organisms
i. A unitary organism has a predictable and dertimnate life. When a unitary
individual dies, it is dead.
ii. A unitary individual begins life as a unicellular zygote that resulted from
sexual reproduction
1. Bats, caterpillars
b. Modular Population –a population consisting of modular organisms
i. Modular organism has an unpredictable, indeterminate life. A module is
not dead until all of its component modules are dead.
ii. Begins life as a multi cellular outgrowth of another module
(asexual/vegetatively produced)
1. Trembling aspen (populus tremuloides) –most common woody
species of the world, most widespread tree species in North
America
a. KNOW ITS SCIENTIFIC NAME
iii. Roots of parent sent out to other areas to produce more plants
(genetically identical)
1. Spider plant and snake plant –common house plants
a. Snake plant –cut leaf and each leaf will grow into new
plant
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5. A Modular Population:
a. A Population or Organism?
i. Can be argued both ways
b. The worlds largest living organism –Pando
i. A clone of aspen trees west of Salt Lake City
ii. Covers 200 acre area
iii. Weighed about 13.2 million pounds
1. 60 times heavier than the worlds previously known largest living
organism, a gigantic fungus located in Michigan hardwood forest
2. 33 times heavier than a blue whale (largest animal on earth)
3. 3 times heavier than the worlds largest sequoia tree
6. Population Density and Dispersion
a. Population Density –the size of population per unit of space.
b. There are two types of density
i. Crude density –the number of individuals per unit area
ii. Ecological density –density measured in terms of the amount of area
available as living space
iii. Ex: there are 1200 deer in a 1000 km2 forest. 25% of the forest is covered
by water and not habitable to deer
1. Crude density = 1200/1000 = 1.2 deer per sq km
2. Ecological density = 1200/750
c. Patterns of Population Dispersion
i. Random –distribution is considered random if the position of each
individual is independent of the others or the occupation of each spot is
equally likely
1. Ex: intertidal clams
2. Very uncommon
ii. Spaced –individuals are evenly distributed. Also called uniform
1. Ex: crops and lawn grasses, desert grasses
iii. Clumped –individuals are distributed in patches.
1. Ex: humans, aspen trees
2. The most common patter of dispersion
d. Dispersal Movement
i. Emigration –the movement of an individual out of one habitat and into
another
1. Individual is an emigrant
ii. Immigration –movement of an individual into one habitat from another
1. Individual is an immigrant
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7. Age Structure: Age Distribution
a. Age structure of a population determines –
i. Reproductive rate
ii. Death rates
iii. Vigor
iv. Survival rates
b. Plotting the percentages of different age groups, older groups on top, pyramid
graphs.
c. See powerpoint slides
8. Sex Ratios
a. Male vs. female
b. For some populations a sex ratio is irrelevant
i. Earthworms, apple orchard
c. Mostly talking about mammals
d. Sex Ratio –the proportion of males to females (number of males per 100 females
---x:100)
e. Primary Sex Ratio –the ratio of sexes at conception (plants: fertilization)
f. Secondary Sex Ratio –the ratio of sexes at birth
i. Some organisms may not survive birth or conception period
g. Silene latifolia L. –male and female flowers
i. Sex is determined by x and y chromosomes, just like in humans
ii. Male and female pollen grains grow at different speeds –some
populations are male biased, others are female biased.
h. Males to Females ratio in humans (USA 1965)
i. Age 0-4: 104:100
ii. Age 40-44: 100:100
iii. Age 60-64: 88:100
iv. Age 80-84: 54:100
i. Sex Ratio of Populus tremuloides (Colorado)
i. Below 2,450m elevation --- 79:100
ii. Above 2,900m eleveation --- 178:100
iii. Conditions better at lower elevation or higher? LOWER ELEVATION
iv. Why are more females found at lower elevation?
9. Mortality and Natality
a. Natality
i. Physiological natality –maximum possible number of births per female
under ideal conditions
1. 60 births for one woman –record
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ii. Realized natality – number of successful reproduction that actually occurs
over a period of time
1. 2.8
iii. Crude Birth Rate –number of births per 1000
iv. Age-specific birth rate (Age-specific fecundity) –number of offspring
produced per unit of time by females in different age classes
1. Needed for life table variables
2. Insects: per week, humans: per year
b. Mortality
i. Rate or number of deaths
ii. Crude death rate –number of deaths per 1000
iii. Death rate –number of deaths during a given time interval divided by the
average size of population
iv. Ex: population of 1000, after 1 unit of time 990 remain. 10 deaths
1. 1000  800 -- 200 died
2. For death rate, divide number of dead by either initial pop. or
average population
v. Number of individuals is usually normalized to 1000.
10. Life Table
a. Three Types:
i. Horizontal life table (cohort/dynamic) –following a cohort (group) of
individuals until all are dead
ii. Dynamic-composite life table –constructed by pooling several horizontal
life tables
iii. Vertical life table (time-specific) –constructed by sampling individuals of
different ages during a single time period
b. Life Table Construction:
i. Average size of population –the average of individuals at the beginning
and at the end of a period
ii. Death rate –number of deaths during a given time interval divided by the
average size of population
iii. Probability of dying –ratio of deaths to the number alive at the beginning
of a period
iv. Proportion surviving –the number of survivors as a percentage of the
number of individuals at time 0.
v. Life expectancy –the average number of years to be lived in the future by
members of a given age in the population.
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vi. SEE SLIDE 40 OF POWERPOINTS FOR FORMULAS TO CONSTRUCT LIFE
TABLE.
1. Number dying
2. Proportion surviving
3. Mortality rate
4. Average # alive in class
5. Tx –do this function from the bottom up when you do the life
table
6. Life expectancy
vii. Part of a Life Table ---the end of one age group is the beginning of the
next age group.
viii. KNOW INS AND OUTS OF LIFE TABLE!
ix. Net Reproductive Rate
1. The number of breeding individuals that will be produced by each
breeding individual in a population
2. Ro = sum(lxmx)
a. Mx is a provided number (how many new individuals are
produced by the females in that age class), but you must
find lx
b. When you are done doin lxmx, add them all together.
3. Ro > 1: population is growing in size
4. Ro = 1: population is stable
5. Ro < 1: population is decreasing in size
11. Survivorship
a. Three basic types of survivorship
i. Type I –starts high, outward curve
1. Many mammals
2. Most organisms die late in life
ii. Type II –negative slope, linear
1. Many birds, small mammals, lizards, turtles
2. Uniform rate of decline
3. Survivorship doesn’t change much throughout life
4. Doesn’t differ with age
iii. Type III –inward curve (exponential look but starts high)
1. Many invertebrates –butterfly, insects
2. Huge decline in young
3. Many of young become the food of other organisms
b. How is a survivorship curve constructed?
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i. Plotting age or time on x axis
ii. Plotting survivors # on y axis
iii. Very rapid decrease in population –something bad happened like war or
disease, drought
iv. Example curves in powerpoint slides (sept 28-30)
1. These are what they will look like on test.
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