Populations

1

UNIT 7

ECOLOGY

OBJECTIVES FOR TODAY

(a) Understand population dynamics, carrying

capacity, & limiting factors for population

growth.

(b) Explain the effects of uncontrolled
population growth on the Earth’s resources.

Take out
your
guided
notes.

Population

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All the members of a particular species in
geographic area.

Examples:

- All the HUMANS in Charlotte
- All the DEER in Blue Ridge Mountains
- All the BACTERIA in a human gut.

Population Characteristics

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1. Geographic Distribution – The area

inhabited by a population

2. Density - the number of individuals per

unit area

3. Growth Rate – how fast the population

grows

Population Size & Growth

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Three factors can affect population size:

1. The number of births (nacimientos)
2. The number of deaths (muertes)
3. How fast the population numbers grow

How Populations Grow

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Populations grow when the birth rate is HIGHER
than the death rate!

A formula to calculate population growth is:

If your outcome is a POSITIVE number, then your
population is GROWING.

(# Births – # Deaths)/Total Population

Other Growth Factors
(Otros factores de crecimiento)

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Immigration🡪 organisms move into an area.

Populations can also grow or decline depending
on how many individuals move to or away from
an area.

Emigration🡪 organisms move out of (or exit)
an area

Types of Population Growth
(Tipos de crecimiento de la población)

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There are two types of population growth:

- Exponential Growth

(Crecimiento exponencial)

- Logistic Growth

(Crecimiento logístico)

Exponential Growth
(Exponencial)

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- Looks like a J Curve
- Under ideal conditions with unlimited

resources, a population will grow
exponentially.

- This occurs when individuals in a

population reproduce at a constant rate.

- Population doubles regularly.

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Exponential Growth

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The IDEAL conditions for

Exponential Growth are rare

!!!

Logistic Growth
(Logístico)

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- Looks like an S curve.
- Occurs when population growth slows or

stops AFTER a period of exponential
growth

Q: Why does this happen?
A: LACK OF RESOURCES and LACK OF
SPACE

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Logistic Growth

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DO YOU THINK THE
HUMAN POPULATION
GROWS
EXPONENTIALLY OR
LOGISTICALLY?
Why or why not?

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Logistic or
exponential?
Why or why

not?

Carrying Capacity
(Capacidad de carga)

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When a population is in logistic growth, we
say it has met its carrying capacity

Carrying Capacity → the largest number of
individuals that a population can support

Q: What is the growth rate when population is
at carrying capacity?→A:ZERO

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Carrying Capacity

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What are some
ABIOTIC & BIOTIC
factors affect
carrying capacity
(i.e., how big a
population can
grow)?

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BIOTIC FACTORS

ABIOTIC FACTORS

- Amount of food
- Amount of

organisms
competing for food

- Space
- Water
- Climate

Limiting Factors

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Any biotic or abiotic factor in an ecosystem that
causes population growth to stop.

Examples:
- Parasitism
- Disease
- Drought
- extreme weather

Two Types of Limiting Factors

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1. Density-Dependent

2. Density-Independent

Density-Dependent Factors

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The effect on the population depends on
population size.

Become limiting only when the population
density reaches a certain level

These factors operate most strongly when a
population is large and dense.

Density-Independent

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Affect all populations in similar ways, no
matter the population size.

Most density-independent factors are
abiotic, or nonliving.