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Population Biology

Unit 11

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What is a population?

A group of individuals of

the same species living

in the same place at the

same time Individuals interact with

one another and with

other populations

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Populations

Size

Growth

Structure of populations

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POPULATION GROWTH

Nearly all populations will tend to grow

exponentially as long as there are resources

available.

Two of the most basic factors that affect the

rate of population growth are the birth rate,

and the death rate.

r(rate of growth)=birth rate

death rate

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Biotic Potential Exponential growth

population size is growing by a fixed rate of increase

Populations can not exhibit exponential growth for extendedperiods of time

Generatio n Size

1 1

2 2

3 44 8

5 16

6 32

7 64

8 128

9 256

10 5120

100

200

300

400

500

600

1 2 3 4 5 6 7 8 9 10

Generation or time

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POPULATION GROWTH

Exponential growth curve:population growth plottedagainst time.

As a population gets larger, italso grows at a faster rate.

This is the maximum populationgrowth under idealcircumstances.

Maximum population size

under ideal conditions (food,water, space, no predators)

FACT: No population exhibits this type of growth for long.

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Biotic Potential

Constant death rates do not alter the exponential growth of the

population Exponential growth is not realistic, it assumes that every

individual is immortal

Exercise 1: Exponential growth

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Population Growth

Limiting Factors - Any factor which, by its absence or

reduced supply, will limit an ecological process (food,

nesting sites, shelter). An excess of a limiting factor would

promote exponential growth Carrying Capacity (K)- The maximum number of

individuals that can be sustained in a particular

environment

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PREDICTING POPULATION GROWTH

Logistic model: This model accounts for thedeclining resources available to populations as theygrow.

It assumes the birth and death rates are not

constant. As the population grows, births decline and death

rises.

Eventually birth=death so the population stops

growing.

Carrying capacity (K): The number of organisms ofone species that an environment can supportindefinitely.

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Logistic growth-population initially exhibits exponential growth

but levels off when limiting factors encountered-leveling-off occurs at carrying capacity (K)-S-shaped curve

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Exercise 2: Corn seed organisms

GenerationNumber

Number atstart

Number afterreproduction

Loss due toenv resistance

Num at startof next gen

1 6 12 0 12

2 12 24 12 12

3 12 24 12 12

4 10 20 14 6

5 12 24 8 16

6 16 32 8 24

7 24 48 36 12

8 12 24 10 14

9 14 28 14 14

10 14 28 16 12

0

5

10

15

20

25

30

0 1 2 3 4 5 6 7 8 9 10

Number atstart ofgeneration

Generation number

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Graph for exercise 2

Environmental Resistance - occurs when limiting factorsinhibit the increase in population size and prevent apopulation from realizing its biotic potential

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POPULATION GROWTH

Two models of population growth.

The Exponential curve (also known as a J-curve) occurs when there is no limit to

population size.

The Logistic curve (also known as anS-curve) shows the effect of a limiting

factor (in this case the carryingca acit of the environment .

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Survivorship Curves

Survivorship curves tell us how long individuals

survive in a population at any specific age Populations have a characteristic pattern of

survivorship over time

Three basic types of survivorship curves

Demography

the statistical study of populations. It is used topredict how the size of a population will change

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Parental care, e.g., Humans

Lizards, some jellyfish,some birds

No parental care, e.g., insectsand fish

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Age structure of populations The most important demographic characteristic of a population

is its age structure

Population Pyramids are graphs that show the age distributionfound in a population

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Population Age Structure

Differences in environmental conditions and past history may causepopulations to differ in their age distributions.

The future growth of a population depends on its current age

distribution.

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StableGrowing

f bl

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Example: Ground squirrel (Spermophilus armatus)

Age class l b l*b0-1 yrs 0.332 1.29 0.428

1-2 0.142 2.08 0.295

2-3 0.061 2.08 0.127

3-4 0.026 2.08 0.054

4-5 0.011 2.08 0.023

5-6 0 0 0

Life Tables(age-specific summary of the survival pattern of a population)

l = survivorship (%) to next age class

b = fecundity of females in that age class (Fecundity is the average numberof female offspring produced by each female in the population)

l*b = age-specific fecundity (average number of female offspring produced by afemale in her life time)

R = Net Reproductive Rate (R>1 growing, R

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Population growth is

affected by:mortality (ex 1)space/resources (ex 2)

survival (ex 3 & 4)reproduction & age (ex 5)

+ many other factors

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Sample Test Items

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Sample Test Items

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Sample Test Items