Lecture 10 Estimating Population Size_1

8/9/2019 Lecture 10 Estimating Population Size_1

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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


(temperature)


(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



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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


Lecture 10 Estimating Population Size_1

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