This lecture discusses life tables , survivorship curves, and age distribution.

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Biology 205
Ecology and Adaptation
Population dynamics
Dr. Erik D. Davenport
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Outline
• Estimating Patterns of Survival
• Survivorship Curves
• Age Distribution
• Rates of Population Change
• Dispersal
– In Response to Climate Change
– In Response to Changing Food Supply
– In Rivers and Streams
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Main concepts
• A survivorship curve summarizes the pattern of
survival (and death) in a population.
• The age distribution of a population reflects its
history of survival, reproduction, and potential for
future growth.
• A life table combined with a fecundity schedule can
be used to estimate net reproductive rate (R0) and
per capita rate of increase (r).
• Dispersal can increase or decrease local population
density.
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Pattern of survival
• Pattern of survival vary a great deal from one species
to another.
• Some species could produce young by the millions,
and die at a high rate.
• Some other species produce a few young and invest
heavily in their care, and have high survival rate.
• Biologists have invented the life table, that list both
the survival ship, and the death (or mortality) in the
population to describe the survival pattern
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Estimating Patterns of Survival
• Three main methods of estimation:
– Cohort life table
• Identify individuals born at same time and keep records
from birth.
– Static life table
• Record age at death of individuals.
– Age distribution
• Calculate difference in proportion of individuals in each
age class.
• Assumes differences from mortality.
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High Survival Among the Young
• Murie collected Dall Sheep skulls, Ovis dalli.
– Major Assumption: Proportion of skulls in each
age class represented typical proportion of
individuals dying at that age.
– Reasonable given sample size of 608.
– Constructed survivorship curve.
– Discovered bi-modal mortality.
– <1 yr.
– 9-13 yrs.
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Survival curve for a plant

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Survival curve for rotifer population

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Constant rates of Survival
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Mortality occurs at approximately equal rates throughout life

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High mortality among the young
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Population has a high mortality rates at young stage

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Survivorship Curves
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Survivorship Curves
• Type I: Majority of mortality occurs among
older individuals.
– Dall Sheep
• Type II: Constant rate of survival throughout
lifetime.
– American Robins
• Type III: High mortality among young,
followed by high survivorship.
– Sea Turtles
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Age Distribution
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Age Distribution
• Age distribution of a population reflects its history of
survival, reproduction, and growth potential.
• Miller published data on age distribution of white
oak (Quercus alba).
– Determined relationship between age and trunk diameter.
– Age distribution biased towards young trees.
– Sufficient reproduction for replacement.
– Stable population
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Age Distribution
• Rio Grande Cottonwood populations (Populus
deltoides wislizenii) are declining.
– Old trees not being replaced.
– Reproduction depends on seasonal floods.
• Prepare seed bed.
• Keep nursery areas moist.
– Because floods are absent, there are now fewer
germination areas.
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Dynamic Population in a Variable
Climate
• Grant and Grant studied Galapagos Finches.
– Drought in 1977 resulted in no recruitment.
– Gap in age distribution.
– Additional droughts in 1984 and 1985.
– Reproductive output driven by exceptional year in
1983.
– Responsiveness of population age structure to
environmental variation.
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Rates of Population Change
• Birth Rate: Number of young born per female.
• Fecundity Schedule: Tabulation of birth rates
for females of different ages.
• Life table and fecundity schedule
1.Estimate net reproduction rate (Ro)
2.Geometric rate of increase ()
3.Generation time (T)
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Estimating Rates for an Annual Plant
• P. drummondii
– Ro = Net reproductive rate: Average number of
seeds produced by an individual in a population
during its lifetime.
Ro=∑lxmx
– X= Age interval in days.
– lx = % pop. surviving to each age (x).
– mx= Average number seeds produced by each
individual in each age category.
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Estimating Rates for an Annual Plant
• Because P. drummondii has non-overlapping
generations, can estimate growth rate.
– Geometric Rate of Increase ():
 =N t+1 / Nt
• N t+1 = Size of population at future time.
• Nt = Size of population at some earlier time.
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Estimating Rates when Generations Overlap
• Common Mud Turtle (K. subrubrum)
– About half turtles nest each year.
– Average generation time:
T = ∑ xlxmx / Ro
– X= Age in years
– Per Capita Rate of Increase:
r = ln Ro / T
– ln = Base natural logarithms
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Concept 4
Dispersal can increase or decrease local
population densities
– Dispersal of expanding population
• African Honeybees
• Collard doves
– Range Change in response to climate change
– Dispersal in response to changing food supply
– Dispersal in rivers and streams
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Africanized Honeybees
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Dispersal
• Africanized Honeybees
– Honeybees (Apis melifera) evolved in Africa and
Europe and have since differentiated into many
locally adapted subspecies.
– Africanized honeybees disperse much faster than
European honeybees.
– Within 30 years they occupied most of South
America, Mexico, and all of Central America.
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Collared Doves
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Collared Doves
• Collared Doves, Streptopelia decaocto, spread
from Turkey into Europe after 1900.
– Dispersal began suddenly.
– Not influenced by humans.
– Took place in small jumps.
– 45 km/yr
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Rapid Changes in Response to Climate Change
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Rapid Changes in Response to Climate Change
• Organisms began to spread northward about
16,000 years ago following retreat of glaciers
and warming climate.
– Evidence found in preserved pollen in lake
sediments.
– Movement rate 100 - 400 m/yr.
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Dispersal in Response to Changing Food Supply
• Holling observed numerical responses to
increased prey availability.
– Increased prey density led to increased density of
predators.
– Individuals move into new areas in response to
higher prey densities.
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Dispersal in Rivers and Streams
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Dispersal in Rivers and Streams
• Stream dwellers have mechanisms to allow them to maintain
their stream position.
– Streamlined bodies
– Bottom-dwelling
– Adhesion to surfaces
• Tend to get washed downstream in spates.
– Muller hypothesized populations maintained via dynamic
interplay between downstream and upstream dispersal.
– Colonization cycle is a dynamic view of stream populations
in which upstream and downstream dispersal, as well as
reproduction, have major influence on stream populations.
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Review
• Estimating Patterns of Survival
• Survivorship Curves
• Age Distribution
• Rates of Population Change
– Overlapping Generations
• Dispersal
– In Response to Climate Change
– In Response to Changing Food Supply
– In Rivers and Streams
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