Population Population EcologyEcology

Cyra Mae R. Soreda

Population EcologyPopulation Ecology

Population ecology is the study of populations in relation to the environment. It includes environmental influences on population density and distribution, age structure, and variations in population size.

Characteristics of Characteristics of PopulationPopulation

Population size Population

densityDispersion

patternsDemographics Survivorship

curvesPopulation

growth

Population sizePopulation size“In population genetics and

population ecology, population size (usually denoted N) is the number of individual organisms in a population”.

Factors that Govern Population Size1.Crude Birth Rate (CBR)2.Crude Death Rate (CDR)3.Immigration4.Emigration

Factors That Increase Population Size

1.Natality is recruitment to a population through reproduction.

2.Immigration from external populations e.g. Bird migration.

Factor Reducing Population Size

1.Mortality which is the death rate from any source e.g. predation.

2.Emigration, where individuals leave the population for another habitat.

NatalityNatalityThe production of new individuals by birth, hatching, germination or fission

2 aspects of reproduction must be distinguished: Fecundity fertility

NatalityNatalityFecundity-physiological notion that

refers to an organism’s potential reproductive capacity

Fertility-ecological concept based on the no. of viable offspring produced during a period time

Realized fertility and potential fecundity-we must be able to distinguish between them

NatalityNatalityE.g, realized fertility rate for a human pop may be only 1 birth per 15 years per female in the child-bearing ages

While the potential fecundity rate for humans is 1 birth per 10 to 11 months per female in the childbearing ages

MortalityMortalityBiologists-interested not only in why organisms die but also why they die at a given age

Longevity-the age of death of individuals in a population

2 types:◦Potential longevity◦Realized longevity

MortalityMortalityPotential longevity

◦The maximum life span of an individual of a particular sp is a limit set by the physiology of the organism, such that it simply dies of old age

◦The average longevity of individuals living under optimum conditions

◦However, organisms rarely live under optimum conditions-most die from disease, or eaten by predators or succumb to a number of natural hazards

MortalityMortalityRealized longevity

◦The actual life span of an organism

◦Can be measured in the field, while potential longevity only in labs or zoos

examplesexamples

European robin has an average life expectation of 1 year in the wild, whereas it can live at least 11 year in captivity

Have more births than deaths?◦Population increases

Have more deaths than births?◦Population decreases

Have equal amounts of births and deaths?◦Population remains constant

What happens to the What happens to the population when we….population when we….

ImmigrationImmigration

“im”= inMigrate= to move from one

place to anotherImmigration is the individual

movement into an areaAnimals in search of mates and

food in new areas

EmigrationEmigration“E” means ‘out’Migrate means to move from

one place to anotherEmigrate means individuals

moving out of one place and into another

Young wolves and bears leaving as they mature

Shortage of food

NATALITYNATALITYThe birthrate, which is the ratio of total

live births to total population in a particular area over a specified period of time

MORTALITYMORTALITYThe death rate, which is also the ratio

of the total number of deaths to the total population.

IMMIGRATIONIMMIGRATIONThe number of organisms moving into

area occupied by the population is called immigration.

EMIGRATIONEMIGRATIONThe number of organisms moving out

of the area occupied by the population is called emigration.

How to estimate population How to estimate population density?density?Techniques differ between

organisms such that the technique to estimate deer cannot be applied to bacteria or protozoa or vice versa

There are 2 fundamental attributes that affect and ecologists choice of technique for population estimation

2 attributes

Size-small animals/plants are usually more abundantthan large animals/plants

Mobility-based on movements of these organisms

Why the need to estimate Why the need to estimate population density?population density?

Estimates of population are made for two reasons:◦How to quantify nature – ecologist role◦Estimates are allows for comparisons

between different populations in terms of space and time measure

2 BROAD APPROACHES TO ESTIMATE POP DENSITY

Absolute densityNo of individual per area/ per volume

Important for conservation and management

Relative densityComparative no of organisms

Two areas of equal sizes, which area has more organisme.g, between area x and y

Area x has more organism than area y

ABSOLUTE DENSITYABSOLUTE DENSITYMaking total counts and by using sampling methods

Total counts - direct counting of populations- human pop census, - trees in a given area, - breeding colonies can be photographed then later counted

- in general total counts are possible for few animals

Measurements of Absolute Measurements of Absolute densitydensity Sampling methods

◦ to count only a small proportion of the population - sample

Using the sample to estimate the total population

2 general sampling techniques:

1)Use of quadrats2)Capture-recapture method

Use of Quadrats

Count all individuals on several quadrats of known size, then extrapolate the average count to the whole area

Quadrat- a sampling area of any shape (may be a rectangle, triangle or circle)

3 requirements:• the pop in the quadrat must be determined exactly• area of the quadrant must be known• quadrant/s must be representative of the area • achieved by random sampling

Quadrant sampling in Quadrant sampling in plant populationplant population

Conduct a transect in the upland hardwood forest

3 transect line, 110 meters long, count all trees taller than 25cm within 1meter of each line

By utilizing the quadrant method sampling for old trees and seedlings, we can determine if populations were likely to change over time

Capture Recapture Method

Capture, marking, release, and recapture-important for mobile animalsWhy?-it allows not only an estimate of density but also estimates of birth rate

and death rate for the population being studied

Capture animal, mark (tag) them and then release them

Peterson method:Involves 2 sampling periods

Capture, mark and release at time 1Capture and check for marked animals at time 2

Time intervals between the 2 samples must be short because this method assumes a closed population with no recruitment of new individuals into the

Population between time 1 and 2 and no losses of marked individuals

Formula for capture-Formula for capture-recapture methodrecapture method

Marked animals in 2nd sample = Marked animals in 1st sampleTotal caught in 2nd sample Total population size

e.g of capture recapture e.g of capture recapture methodmethodDahl marked trout in small Norwegian lakes to estimate the size of the population that was subject to fishing. He marked and released 109 trout, and in 2nd sample a few days later caught 177 trout, of which 57 were marked. From the data, what is the estimate population size?

e.g of capture recapture e.g of capture recapture methodmethodBy using the formula

57 = 109177 Total pop size

Total pop size = (109 X 177)57

= 338 trout

RELATIVE DENSITYRELATIVE DENSITYTraps – no caught per day per trap –

animals caught will depend on their density, activity and range of movement, skill in placing traps – rough idea of abundance – night flying insects, pitfall traps for beetles, suction traps for aerial insects

Fecal pellets – rabbits, deer, field mice – provides an index of pop size

Vocalization frequency – bird calls per 10 mins, can be used for frogs, cicadas, crickets

Pelt records – trapper records dates back 300 years – of lynx

Relative densityRelative densityCatch per unit effort – index of fish

abundance – no of fish per cast net or no of fish per 1 hour trawling

Number of artifacts – thing left behind – pupal cases of emerging insects

Questionnaires – to sportsmen (eg fish)and trappers

Cover - % ground surface covered – in botany, invertebrate studies of the rocky intertidal zone

Feeding capacity – bait taken – for rats and mice – index of density

Roadside counts – birds observed while driving standard distances

Population Population dispersion patternsdispersion patterns

3 types

random

uniformclumped

Population dispersion patternsRandom-when the position of each

individuals in a pop is independent of the others

Uniform-it results as a form of some negative interactions

Common among animal pop where individuals defend an area for their own exclusive use (territoriality) or in plant pop where severe competition exist for belowground resources, i.e water or nutrients

Population dispersion Population dispersion patternspatternsClumped-where individuals occur in groups

Reason-suitable habitat or resources may be distributed as patches on a larger landscape

POPULATION POPULATION GROWTHGROWTH

Population growthPopulation growthRefers to how the number of individuals in a population increases or decreases with time (N, t)

Reflects the difference between rates of birth and death

in pop, if new births occur in pop, if death occurs

2 types of pop growth

Exponential population growth

dN = rmaxNdt

Logistic population growth

dN = rmaxN (K-N)dt K

Population

Growth

Mathematicall

y Defined

N=K/2

Exponential GrowthExponential GrowthContinuous population growth in an

unlimited environment can be modeled exponentially.

dN / dt = rmax N

Appropriate for populations with overlapping generations.◦As population size (N) increases, rate of

population increase (dN/dt) gets larger.

Exponential GrowthExponential GrowthFor an exponentially growing

population, size at any time can be calculated as:

Nt = Noert

Nt = number individuals at time t.N0 = initial number of individuals.e = base of natural logarithms.r (= rmax ) = per capita rate of increase.t = number of time intervals.

PracticePracticeIf the human population size in

1993 was 5.4 billion, what was the projected population size in the year 2000? r=0.0139

No = population size in 1993 = 5.4 billion

t = 7 years (year 2000 - 1993)r = 0.0139

Nt = No ertNt = (540,000,000) e(0.0139)(7)Nt /540,000,000 = e 0.0973

Dust off your high school math skills. To get rid of the exponent, simply take the (ln) of both sides of the equation. Remember, when we take the natural log of a quotient we end up taking the ln of one value and subtracting it from the ln of the other value (see below).

ln (Nt /540,000,000) = ln (e 0.0973)[here we're taking the natural log

of the quotient]= ln(Nt) - ln(540,000,000) =

0.0973[rewrite it as natural log of one

value minus natural log of the other value]

Nt = 595,000,000 or 5.95 billion

Logistic Population GrowthLogistic Population Growth

As resources are depleted, population growth rate slows and eventually stops: logistic population growth.◦Sigmoid (S-shaped) population growth curve.◦Carrying capacity (K) is the number of

individuals of a population the environment can support. Finite amount of resources can only support a

finite number of individuals.

Logistic Population GrowthLogistic Population Growth

dN/dt = rmaxN(1-N/K)

rmax = Maximum per capita rate of increase under ideal conditions.

When N nears K, the right side of the equation nears zero.◦As population size increases, logistic growth

rate becomes a small fraction of growth rate. Highest when N=K/2. N/K = Environmental resistance.

Problem Problem

Suppose a population of butterflies is growing according to the logistic equation. If the carrying capacity is 500 butterflies and r = 0.1 individuals/(individual*month), what is the maximum possible growth rate for the population?

To solve this, you must first determine N, population size. From the plot of dN/dt vs. N, we know that the maximum possible growth rate for a population growing according to the logistic model occurs when N = K/2, here N = 250 butterflies. Plugging this into the logistic equation:

DN/dt = rN [1- (N/K)]= 0.1(250)[1-(250/500)]= 12.5 individuals / month

Fig. 11.9Fig. 11.9

Limits to Population GrowthLimits to Population GrowthEnvironment limits population growth by

altering birth and death rates. Density-dependent factors

Disease, Parasites, Resource Competition Populations do not show continuous geometric increase When density increases other organisms reduces the fertility

and longevity of the individuals in the population This reduces the rate of increase of the pop until eventually

the pop ceases to grow The growth curve is defined as the sigmoid curve, S –

shaped K = carrying capacity (upper asymptote or maximum value)

– the maximum number of individuals that environment can support

Density-independent factors Natural disasters Climate

r- and k-speciesr- and k-speciesCharacteristics of r- species high biotic potential Rapid development Early reproduction Single period reproduction per

individual Short lifecycle Small body size Regulated by the density-

independent factor

Characteristics of k- species low biotic potential slow development delayed reproduction multiple period reproduction per

individual long lifecycle large body size Regulated by the density-dependent

factor

Life history strategiesLife history strategies

K and r selection (MacArthur and Wilson 1967)

r-selected species•r refers to the per capita rate of increase•Selection favoring rapid growth•Should be favored in new or disturbed environments•Less competition

K-selected species•K refers to carrying capacity•More prominent in species that are typically at their carrying capacity •Favors more efficient use of resources•Live with competition

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