lecture 9: population ecology

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Biology 205Ecology 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

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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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