Environmental Science

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.

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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.

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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.)

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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.)

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–

a = lag phase

b = exponential growth phase

d = decline phase

3) S-GROWTH CURVE:

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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

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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

 

 

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.

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?

 

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