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Population Ecology: Estimating Population Size
Population: a group of individuals of the same speciesthat potentially interbreed and living in the same area ata particular time.
- convenient level of abundance for describing thenumber of individuals of a species.
- usually a small subset of the species
- has evolutionary significance
Population ecology is concerned with- measuring changes in population size and
composition, and- identifying the factors causing the changes
observed.- relevant to many applied issues in ecology
deme: a local group of interbreeding organisms
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The population will usually function in a narrower
range of environmental conditions than will theindividuals in a population.
Biolog
icalactivity
Environmentalgradient
optimum
aa bb cc
level required to maintain thepopulation
level required to maintain the
individuallevel required to maintaincritical functions
stream insects: adult body size, metabolic efficiency, fecundity, andabundance are maximized at the center of the latitudinal range
- organisms are only as good as they have to be, which is betterthan conspecifics. No "perfectly" adaptedoptimizedorganisms.
Temperature orMoisture Widest Range
Most NarrowRange
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wet drymoisture gradient
Abundanc
e
mangrovecypress
liquidamberoak pine cactus
Unimodal distributions form the basis of many (statistical)
models for communities.
single peak
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Can be asymmetrical: stenothermic vs. eurythermic
Biolog
icalactivity
Environmentalgradient
(temperature)
Environmentalgradient
(temperature)
stenotherm
eurytherm
thermophilic
species
Narrow
wide range ofdifferent bodytemperatures
Temperature
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How do we quantify the number of individuals in a
population?
- expressed as density: number of individuals/unit area
Some of the basic questions asked by populationbiologists include:
i) How many organisms are there?ii) How does population size vary over space?iii) How does population size vary over time?
How many organisms are there?1) Total counts: absoluteestimation of population
size
Or Volume
CENSUS -Counting everyone
3 ways to count
3 ways to see
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How many organisms are there?
2) representative counts using subsamplingi) quadrats:number of individuals in an area of known
size- can differ in shape
ii) capture-recapture:Animals are captured, marked,released and then recaptured at a later date.
N = total population size: unknownTime 1: mark individuals (M) and release
Time 2, etc: capture individuals:C = total # of individuals in sampleR = number of marked individuals in C.
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based on the assumption:
marked individ. in sample marked individ. in total population--------------------------- = ------------------------------------------total # caught in sample total population size
R M MC----- = -----; RN = MC; N = -----
C N R
e.g., M = 150; C = 400; R = 100; N = ?
150(400)
----------- = 600 = N100
proportion of individual marked in second sample C isrepresentative of total population size when samples originallymarked.
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All capture-recapture models make three critical
assumptions:
1) marked and unmarked individuals are
captured randomly
2) marked individuals are subject to the samemortality rate as unmarked individuals
3) marks are not lost or overlooked.
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3) Estimates of relative density are adequate if we
want an approximation of abundance.- An order of magnitude estimate
Examples:- vocalization frequency- pelt records (e.g. Hudson Bay Company)- number of fecal pellets, tracks, etc.- catch per unit effort: number of fish per
100 hours of trawling- % cover
- roadside counts (e.g., number of raptors perstandard distance)
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Aggregated or clumped: the occurrence of an individualin a sample unit increases the probability that another
individual will occur within the unitIndicates: heterogeneity of resources; aggregation
mechanism
Can lead to: enhanced resource exploitation;protection from predators
e.g., filter-feeding stream-dwelling insects; tentcaterpillars; fish such as herring, jacks, etc.
Dispersion: the pattern ofspacing of individuals within thegeographical boundaries of thepopulation
3 Types of Dispersion
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Regular or spaced: the occurrence of an individual in thesampling unit reduces the probability that anotherindividual will occur there.
Indicates: antagonistic interactionsb/w individuals; territoriality(behavior) or other forms of spacingmechanisms (allelochemicals,shading)
e.g, penguins in a rookery; trees inan orchard
3 Types of Dispersion
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Poisson or random: The occurrence of one individual in asampling unit has no effect on the probability that other
individuals will occur in the same sampling unit.Indicates: chance placement; absence of strong
attractions or repulsions.
e.g., forest trees
3 Types of Dispersion