Posted: October 27th, 2022

Environmental Science

  

1. The age structure for the US was a pyramid indicating that our population is still growing.  Give one reason why our growth is slowing and one reason why we are still have a pyramid age structure.

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2. Explain why organisms like trees and sea turtles have a Type III survivorship curve while most mammals have a Type 1 survivorship

curve.

 
 

3.  Give 4 reasons why humans might have either a Type I or Type III survivorship

curve.
 
 

4.  For each distribution pattern in slide #23, list the factors from slide #22 that would account for that pattern for each of the three patterns

 
 

5.  What might account for a developed nation having a pyramid age structure?

POPULATION ECOLOGY

Population ecology is the branch of ecology that studies the structure and dynamics of populations

.

Population biology is the study of population characteristics and the factors that affect their size and distribution. Environmental Science merges both of these, using basic ecology to understand the structure of populations, and population biology to understand how humans affect population size , distribution, and change over time.

Environmental scientists are interested in:

· Demographics (the study of vital statistics such as births, deaths, immigration, and emigration).

· Biotic potential

· Environmental resistance

· Age structure

· Growth curves

· Survivorship

· Patterns of distribution.

They will use these concepts, along with cultural, societal, religious, political, and economic factors, to predict how populations will grow and affect the environment in the future.

I) DEMOGRAPHICS (for our general purposes, we will use the following simplified definitions):

BIRTHS = the number of humans born in a given time period.

DEATHS = the number of humans that die n a given time period.

IMMIGRATION = individuals entering a population from another related one.

EMIGRATION = individuals leaving a population for another related one.

Thus a simple equation to determine change (∆) in a population’s size over time is:

∆ in pop size = (factors causing growth) – (factors causing decline)

time

So plugging in demographic definitions above, the equation becomes:

∆ in pop size = (births + immigration) – (deaths + emigration)

time

Thus populations grow with births and immigration. They shrink with deaths and emigration

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II) BIOTIC POTENTIAL is the maximum number of offspring a female of the species can produce in her lifetime under optimum (ideal) conditions. (Offspring = births, eggs, seeds, spores.)

Biotic potential is NEVER realized in nature due to ENVIRONMENTAL RESISTANCE.

AN EXAMPLE: If we say that a human female becomes capable of reproducing at 13 and finishes menopause by 53 that gives her a 40 year reproductive life. If she has a child every 9 months, then her biotic potential is 53.3 children. How many women do you know that has produced 53.3 children? Can you give reasons why women don’t reach their biotic potential.

A more reliable measure is TOTAL FERTILITY RATE (TFR) which can be defined as the number of offspring a female of the species can produce in her lifetime under normal conditions. Why is this more reliable? How does it differ from the definition of biotic potential?

III) ENVIRONEMTAL RESISTANCE prevents a species from reaching its biotic potential by controlling growth of a population.

Environmental resistance operates through biotic and abiotic factors, called LIMITING FACTORS (LF) which limits a population’s growth.

The two types of LF are:

1) DENSITY-DEPENDENT LIMITING FACTORS which limit population growth (density) after a
critical size
has been exceeded. They tend to increases with increasing population density.

The critical size referred to above is CARRYING CAPACITY. Carrying capacity is the total number of individuals an environment can support indefinitely. Any population that exceeds carrying capacity will be reduced to a size below carrying capacity by density-dependent limiting factors.

Density-dependent LF are
biotic
(biological) factors such as predators, parasites, diseases, competitors, lack of food, etc., and tend to aid in maintaining population size equilibrium.

Any population that exceeds its carrying capacity will be driven back down below the critical size by the action of density-dependent LF.

2) DENSITY-INDEPENDENT LIMITING FACTORS limit the growth of populations regardless of their size (density). These could be physical factors or catastrophic events.

-3-

These are
abiotic
(physical or environmental) factors such as fire, light availability, drought, storms, natural disasters, etc. These are not involved in maintaining population size equilibrium. (NOTE: some ecologists don’t consider these to be limiting factors, and generally only recognize density-dependent LF. We will recognize both types of LF.

IV) AGE STRUCTURE:

AGE STRUCTURE diagrams show how a population is distributed. It divides the population into
pre-reproductive, reproductive and post-reproductive phases
. The shape of the diagram can show you if a country is growing rapidly, slowly, or negatively by comparing the relative sizes of each group. It can also show is there is zero growth.

1) A simple Pyramid age structure diagram:

This population will grow over time due to the larger number of pre-reproductive individuals.

2) A simple inverted pyramid age structure diagram:

This population will decline over time due to the larger number of post-reproductive individuals.

-5-

3) A simple hour-glass age structure diagram:

This population will remain relatively stable over time (maybe even achieved zero population growth) due to the larger number of reproductive individuals.

Actual age structure diagrams are more complex than these. Please see slides 10 and 11 in the Population ecology PowerPoint presentation on the Moodle page.

V) GROWTH CURVES:

These curves describe how populations grow and what type of Environmental Resistance is limiting the population’s growth

There are three types of growth curves:

1) J-GROWTH CURVE:

This type of growth is unregulated. Thus it never
persists
in nature. It is important because it forms a component of the more common growth curve types

(See the next page for a drawing of this type of curve.)

-6-

a = lag phase

b = exponential growth phase

2) INVERTED J-GROWTH CURVE:

This type of curve is regulated by
DENSITY-INDEPENDENT
limiting factors.

(See the next page for a drawing of this type of curve.)

-7-

a = lag phase

b = exponential growth phase

d = decline phase

3) S-GROWTH CURVE:

-8-

This type of curve is regulated by DENSITY-DEPENDENT limiting factors. NOTE CARRYING CAPACITY IN SECOND EXAMPLE.

A drawing of this type of curve:

Note that the dotted line represents a fluctuation around carrying capacity. If it exceeds carrying capacity, density dependent factors will drive it down.

a = lag phase

b = exponential growth phase

c = equilibrium phase

VI) SURVIVORSHIP:

Suvivorship is a measure of the number of individuals belonging to a cohort (individuals born in the same year) that are still alive in the population at the
end
of a given time period. Survivorship is indicated by curves like those below:

The curves are designated as Type I, Type II, Type III and type IV

-9-

Survivorship Curves Vary by Species

.

There are four general patterns:

1) Full physiological life span if organism survives childhood (TYPE I)

Examples: elephants and bears

2) Probability of death unrelated to age (TYPE II)

Examples: gulls and mice

3) Mortality peaks early in life. (TYPE III)

Examples: trees, sea turtles and fish

4) Mortality occurs early and late in life, with maximum survival during reproductive maturity.

Examples: deer, antelope

VII) DISTRIBUTION PATTERNS:

A DISTRIBUTION PATTERN illustrates how the members of a population are arranged through the populations range.

THERE ARE THREE DISTRIBUTION PATTERNS:

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1) CLUMPED

2) UNIFORM (aka REGULAR)

3) RANDOM

For each distribution pattern, list the factors that would account for that pattern (example 1a, 2b, 3a) for each of the three patterns

The pattern is determined by the interactions of several factors: Three of these are:

1) ENVIRONMENTAL CONDITIONS throughout the range

a) uniform

b) non-uniform

2) TENDENCY FOR SOCIALIZATION (=forming groups like herds, flocks, packs)

a) yes

b) no

3) INTRASPECIFIC (=between members of the same species) COMPETITION

a) little to none

b) intense

In a CLUMPED pattern, the individuals occur in distinct, separated groups.

In a UNIFORM pattern, the individuals occur a specific and equal distance apart.

In a RANDOM pattern, any individual can be found anywhere in the range at any given time.

IX) HUMAN POPULATION ISSUES:

Human population growth is influenced by other factors such as societal norms, culture, religion, economics, wars, and education. Also PUSH AND PULL FACTORS also play a role in population increase or decrease. PLEASE SEE SLIDES 25-34 in the Population ecology PowerPoint presentation on the Moodle page for a discussion of these factors.

Mortality occurs early and late in life, with maximum survival during reproductive maturity. Examples: deer, antelope

Type III

POPULATION ECOLOGY

1

BASIC DEFINITIONS
POPULATION is a group of individuals belonging to the SAME SPECIES living and interacting together in a given area.
COMMUNITY is a group of DIFFERENT populations living and interacting together in a given area.
ECOSYSTEM is a group of DIFFERENT communities living and interacting together in a given area.

2

POPULATION ECOLOGISTS STUDY
1. DEMOGRAPHICS (vital statistics
BIOTIC POTENTIAL
3. ENVIRONMENTAL RESISTANCE
AGE STRUCTURE
GROWTH CURVES
SURVIVORSHIP
7. PATTERNS OF DISTRIBUTION

3

DEMOGRAPHICS
Thus populations grow with births and immigration They shrink with deaths and emigration.
Change in population = (Births + Immigration) – (Deaths + Emigration)
Vital statistics studied by demographers include:
Births
Deaths
Immigration (individuals entering a population from a another related one)
Emigration (individuals leaving a population for a another related
one)

4

BIOTIC POTENTIAL
BIOTIC POTENTIAL is the maximum number of offspring a female of the species can produce in her lifetime under optimum (ideal) conditions.
Offspring = births, eggs, seeds, spores
Biotic potential is NEVER realized in nature due to ENVIRONMENTAL RESISTANCE
An example: If we say that a human female becomes capable of reproducing at 13 and finishes menopause by 53, that gives her a 40 year reproductive life. If she has a child every 9 months, then her biotic potential is 53.3 children. How many women do you know that has produced 53.3 children?

5

ENVIRONMENTAL RESISTANCE
ENVIRONMENTAL RESISTANCE prevents a species from reaching its biotic potential by controlling growth of a population
Environmental resistance works through LIMITING FACTORS that can be either biotic or abiotic which limit a population’s increase

6

LIMITING FACTORS
1) DENSITY-DEPENDENT LIMITING FACTORS begin to limit population size (density) after a critical size has been exceeded. They tend to increases with increasing population density.
The critical size referred to above is CARRYING CAPACITY. Carrying capacity is the total number of individuals an environment can support indefinitely. Any population that exceeds carrying capacity will be reduced to a size below carrying capacity by density-dependent limiting factors.
These are biotic (biological) factors such as predators, parasites, diseases, competitors, lack of food, etc., and tend to aid in maintaining population size equilibrium.
2) DENSITY-INDEPENDENT LIMITING FACTORS limit the growth of populations regardless of their size (density).
These are abiotic (physical or environmental) factors such as fire, light availability, drought, storms, natural disasters, etc., parasites, competitors, lack of food, etc., these are not involved in maintaining population equilibrium.

7

TOTAL FERTILITY RATE (TFR)
A more reasonable measure of reproductive ability of females is TOTAL FERTILITY RATE
TOTAL FERTILITY RATE is the number of offspring an AVERAGE female of the species can produce in her lifetime under NORMAL conditions.

8

AGE STRUCTURE
THIS SHOWS THE NUMBERS OF INDIVIDUALS PRESENT IN CERTAIN AGE LEVELS IN A POPULATION AT ANY GIVEN TIME.
AGE STRUCTURE can be used to predict if population size will increase, remain relatively stable, or decrease over time.
These can be relatively complex or they can be simplified by setting up only three age levels: PRE-REPRODUCTIVE, REPRODUCTIVE, and POST-REPRODUCTIVE. These are typically called PYRAMID AGE STRUCTURE (more pre-reproductive individuals), HOURGLASS (more reproductive, and approximately equal pre- and post-reproductive individuals), and INVERTED PYRAMID (more post-reproductive individuals) [SEE NEXT SLIDES]

9

Ages 0-14
Ages 15-44
Ages 45-85+
Rapid Growth
Guatemala
Nigeria
Saudi Arabia
Slow Growth
United States
Australia
Canada
Male
Female

Zero Growth
Spain
Austria
Greece
Negative Growth
Germany
Bulgaria
Sweden
Note the relative sizes of the 3 primary age groups. This indicates future growth.
Pyramid
Hour-
glass
Inverted
pyramid
SIMPLE AGE STRUCTURE DIAGRAMS

10

A COMPLEX AGE STRUCTURE DIAGRAMS
Note that the levels are divided into 4 year increments.

11

GROWTH CURVES
These describe how populations grow and what limits their growth.
There are three types of growth curves:
J-GROWTH CURVE
INVERTED J-GROWTH CURVE
S-GROWTH CURVE
The type of growth curve can also indicate what type of Environmental Resistance is limiting the population’s growth

12

J-GROWTH CURVE
a
b
This type of growth is unregulated. Thus it never persists in nature. It is important because it forms a component of the more common growth curve types
a = lag phase
b = exponential growth phase

13

INVERTED J-GROWTH CURVE
a
b
d
This type of curve is regulated by DENSITY-INDEPENDENT limiting factors.
a = lag phase
b = exponential growth phase
d = decline phase

14

S (SIGMOID) –GROWTH CURVE
This type of curve is regulated by DENSITY-DEPENDENT limiting factors. NOTE CARRYING CAPACITY IN SECOND EXAMPLE
a = lag phase
b = exponential growth phase
c = equilibrium phase
a
b
c

15

SURVIVORSHIP CURVES
These show the number of individuals belonging to a given COHORT (= group of individuals that were all born in the same year) that are still alive at the END of a particular age increment.
There can be 3 or 4 types depending on the species.

16

Survivorship Curves Vary by Species
There are three general patterns:
Full physiological life span if organism survives childhood (TYPE I)
Example : elephants and bears
Probability of death unrelated to age (TYPE II)
Example : gulls and mice
Mortality peaks early in life. (TYPE III)
Examples: trees and fish

17

A FOURTH TYPE OF SURVIVORSHIP CURVE HAS RECENTLY BEEN DISCOVERED
Mortality occurs early and late in life, with maximum survival during reproductive maturity. (TYPE IV)
Examples: deer, antelope

18

Type IV

19

SPECIFIC SURVIVORSHIP CURVES

20

DISTRIBUTION (SPACING) PATTERNS OF POPULATIONS
A DISTRIBUTION PATTERN illustrates how the members of a population are arranged through the populations range.
THERE ARE THREE DISTRIBUTION PATTERNS:
CLUMPED
UNIFORM (aka REGULAR)
RANDOM

21

FACTORS DETERMINING DISTRIBUTION PATTERN
The pattern is determined by the interactions of several factors: Three of these are:
1) ENVIRONMENTAL CONDITIONS throughout the range
uniform
b) non-uniform
TENDENCY FOR SOCIALIZATION (=forming groups like herds, flocks, packs,
etc.)
a) yes
b) no
3) INTRASPECIFIC (=between members of the same species) COMPETITION
little to none
intense

22

THE DISTRIBUTION PATTERN
In a CLUMPED pattern, the individuals occur in distinct, separated groups.
In a UNIFORM pattern, the individuals occur a specific and equal distance apart.
In a RANDOM pattern, any individual can be found anywhere in the range at any given time.

23

HUMAN POPULATION ISSUES
Besides the ENVIRONMENTAL RESISTANCE LIMITING FACTORS discussed in Lecture # 5, human population growth is influenced by other factors such as societal norms, culture, religion, economics, wars, and education.
The following slides will introduce you to some of the more important factors affecting human population growth.
These may help explain why a human female never reaches her biotic potential, and they can be considered limiting factors unique to humans.

24

SOCIETAL NORMS
In some societies, girls are encouraged, or sometimes forced, to marry at a young age (12-15). Populations tend to increase as a result of these young women having children at a younger age than in other societies. Their “reproductive life” may be extended due to their young marital age.
In the U.S., marrying that early is discouraged, if not illegal in some states. However, prevailing societal beliefs or customs can influence when a women decides to have children. For example, many of the television shows in the 50’s, 60’s, and 70’s encouraged childbearing by young women. On TV, many women were depicted as stay-at-home housewives whose duty it was to take care of her husband and raise children. “Leave it to Beaver”, the “Brady Bunch”, “Father knows best” are examples of TV reinforcing societal norms of the time.
In the 50’s and 60’s high school age women were required to take Home Economics classes which also prepared them to be mothers and housewives.
Has this societal belief changed? If so, how and why?
SOMETHING TO CONSIDER:

25

SOCIETAL NORMS continued
Often society, (especially ours) frowns on women over a certain age having children. Think what you might say if a 50 year old female co-worker came in and announced that she was pregnant.
New reproductive technology may also increase the number of children a women can have as well. Often several embryos are implanted in a uterus and thus multiple births occur as a result.

26

CULTURE
In some cultures, the more children a man produces the higher his value or status is in the community group.
Also, some cultures value male children more than female children. Couples tend to continue having children until 1 or more males are born. This may lead to abandoning or killing female children to have male heirs and reduce economic burdens on the family.

27

RELIGION
NOTE: THE FOLLOWING INFORMATION IS NOT A JUDGEMENT ON ANY RELIGIOUS BELIEF OR ESTABLISHED RELIGION, AND IS NOT TO BE TAKEN IN THAT CONTEXT.
Some religions prohibit the use of artificial birth control devices (oral pill, condom, IUD, diaphragm, etc.) and thus families practicing these religions tend to have larger families in general.
Some religions prohibit abortions for any reason. This may also have an affect on family size.
Some religions prohibit the use of reproductive technologies. This may also have an affect on family size by lowering it.

28

ECONOMICS
In some developing nations, family sizes are large to provide extra income for the family since the children can be sent to work. The children can be workers on the family’s farm or they can work in local factories.
Sometimes a couple may have a lot of children to insure that there is someone available to take care of them when they can no longer work. In other words, the children represent a type of social security for their parents.
In countries where health or environmental conditions are poor, couples may have many children to insure survival of at least some children to meet the above situations.
In developed nations, good economic times tend to increase birth rate and family size (e.g., “baby boomers”), whereas in poor economic times the birth rate and family size decreases.

29

WARS (INCLUDING CIVIL WARS)
Armed conflict in general tends to reduce birth rate for a variety of reasons, such as availability of males, economic conditions, environmental disruptions, women working, etc.
In peacetime, birth rates may increase, especially if the economy is strong.
NOTE: it is not as simple as the above statements might indicate. Many of the other factors discussed in this lecture may interact to affect birth rates after a war.

30

EDUCATION
In the 50’s and early 60’s fewer women went to college as compared to today. Their opportunities for education leading to a career other than the standard jobs employing women during those times (secretary, waitress, maid, etc.) were limited. They had more time to have children compared to women from the 70’s on.
Since the 70’s though, educational opportunities for women have increased to where slightly more women than men attend college today. IN GENERAL (there are exceptions), the level of education a women has greatly affects the number of children she may have in her lifetime. For instance, many women today delay having children until after they have finished college and established themselves in a career, decreasing the number of children she might have as she contends with her “biological clock” (a concept that is frankly over stated).

31

BIOLOGY
A lot of biological factors contribute to a women not reaching her biotic potential. The reproductive organs may be damaged by frequent child birth, thus limiting family size.
She may have trouble getting pregnant frequently due to a variety of factors.
She may not be capable of becoming pregnant with out using the newer reproductive technologies for overcoming infertility.

32

FACTORS INFLUENCING POPULATION GROWTH IN URBAN AREAS
OVERCROWDING IN THE COUNTRY
ECONOMIC CONDITIONS
POLITICAL, RELLIGIOUS, OR RACIAL CONFLICTS
LAND TENURE CHANGES
AGRICULTURAL CHANGES
1) PUSH FACTORS: THESE FORCE PEOPLE OUT OF RURAL AREAS, CAN INCLUDE:
2) PULL FACTORS: THESE ATTRACT PEOPLE TO A CITY VOLUNTARLY, CAN INCLUDE:
JOBS
HOUSING
ENTERTAINMENT OPPORTUNITIES
FREEDOM FROM VILLAGE TRADITIONS
UPWARD SOCIAL MOBILITY
POWER
EDUCATIONAL OPPORTUNITIES
ACCESS TO THE ARTS AND CULTURE
SPECIALIZATION IN PROFESSIONS
CLIMATE
HEALTH CARE
LOCATION

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