AP Biology
Population Ecology
Part 1
•
Population Ecology
– This field of Biology deals with species
populations and the population’s
environment.
– A Population is the same species, same
time, same place, and showing signs of
reproduction.
– The human population will soon be over
7 billion people on Earth.
•
We reached 7 billion as of October 2011…current
figures are 7, --- --- ---,--- --- ---,--- --- ---.
http://www.worldometers.info/world-population/
Population
Human Population on Earth
• 8 Billion (2024)
– According to the most recent United Nations estimates, the human population of the world is
expected to reach 8 billion people in the spring of 2024.
• 7 Billion (2011)
– According to the United Nations, world population reached 7 Billion on October 31, 2011.
The US Census Bureau made a lower estimate, for which the 7 billion mark was only reached
on March 12, 2012.6 Billion (1999)
• According to the United Nations, the 6 billion figure was reached on October
12, 1999 (celebrated as the Day of 6 Billion). According to the U.S. Census
Bureau instead, the six billion milestone was reached on July 22, 1999, at
about 3:49 AM GMT. Yet, according to the U.S. Census web site, the date and
time of when 6 billion was reached will probably change because the already
uncertain estimates are constantly being updated.
• Previous Milestones
–
–
–
–
–
5 Billion: 1987
4 Billion: 1974
3 Billion: 1960
2 Billion: 1927
1 Billion: 1804
http://www.worldometers.info/world-population/
Population Ecology
•
It mainly focuses on Density (number of
organisms in a given area) and boundaries.
– Man made boundaries or natural boundaries
exist.
•
Dispersion –This term refers to where
within the boundaries are the organisms
located.
Population Density
Measuring Density
Mark-Recapture Method
N= #Captured and marked in first group x total of second group that is caught
# Recaptured from first time
N is the estimated population size for that defined
area.
Patterns of Dispersion
https://www.youtube.com/watch?v=6K128gg6Soc
•
•
•
Clumped – This usually results from a need
for nutrients, mating, or employment.
(humans, elephants, fish, some species of
plants, fungi)
Uniform (evenly) – This usually results from
territoriality or favorable environment.
(snakes, nesting birds, some plants,)
Random – There is no apparent reason seen
in the dispersion pattern. (dandelions,
Dispersal Patterns
•
Demography – The study of population
sizes and distribution.
•
Growth – This occurs by birth or immigration.(to
enter into a new area).
Decline – This occurs by death or emigration (to exit
an area).
Life Tables
•
•
–
–
Provides Age Specific Traits for cohorts (individuals of the
same age or demographic).
These are expensive and time consuming to produce. (Like
the U.S. census.)
Demography
Terminology
Births
Immigration
Population
size
Emigration
Deaths
Life Tables and Cohorts
Survivorship Curves - probability of newborn
individuals surviving to a particular age
Three basic types of curves can exist in nature.
• Type I (Many young numerous middle  few old)(type of
environment?)
– This environment favors the young and usually indicates that the
environment is favorable and these organisms are usually at the top of
food chain and there seems to be extensive parental care and energy
investment.
• Type II (Constant decline) (type of environment?)
– This indicates that the environment is relatively favorable but the organism
may be a food source for another organism. The parental cares is modest.
• Type III (Many young  few middle and old (type of
environment?)
– This indicates a harsh environment because most of the young die at an
early age. This indicates that they are a food source that is low on the food
chain as well as have practically no parental investment. Young are left to
fend for themselves.
Figure 46.4a
(Late loss)
(Constant loss)
(early loss)
Survivorship Curves
E. Reproductive tables
1. These tables are only concerned with females
of reproductive age or possibility.
These are the only real individuals who will be
able to impact a population size since it is the
female who provides the birth. (Low # of
females indicates a threatened or dying
population; High # indicates a thriving
population.)
AP Biology
Population Ecology
Part 2
Important concepts from previous
units:
• Traits within cells or organisms are directly
associated with inherited DNA (genes).
• DNA is inherited from the parents by
fertilization occurring between a sperm and
egg. (sexual reproduction)
• Positive feedback loops enhance a process
that is already in action.
DNA Inherited
Expression of DNA to create traits
•
Life Histories
–
–
–
A life history can tell a lot about a species’
fitness.
Traits needed for survival in a particular
environment can be determined.
Two types of life histories can typically be seen:
• Semelparity (Big Bang) – Reproduce one time with
huge numbers of offspring.
–
–
–
Organism usually dies after reproducing, so it went out with
a Big Bang. Tremendous amounts of time, energy, and
resources invested in making numerous offspring.
This type of history usually indicates a harsh environment
and low survival rates.
Examples: Salmon or Century Plant
Century Plant
• Iteroparity - Repeated Reproduction year after year.
(Applies to most organisms usually.) This history
indicates a favorable environment and good survival
rates. Modest time, energy, resource investment are
required.
– Environmental Conditions and survival rates?
» Environment is directly related to time, energy involved,
resources available.
» Compromises (When, how often, how many)(Natural
selection?)
•
•
Natural selection is very obvious with
Semelparity…strongest survive.
Natural selection with Iteroparity is directly
related to competition.
•
Population Growth Models
–
Exponential Growth Models (Ideal Growth)
•
Involves r-selection species. (r- think rapid
growth)(A.K. A. Density
•
– independent)
There population size is related to
resources not number of organisms.
– Produces a J curve graph.
– Environment has unlimited resources. (Good for
ideal growth)
– Occurs mainly in a new environments and
pioneer species such as bacteria, lichens, and
mosses. (They are the first organisms to colonize
the new environment.)
Population Growth Characteristics
•
Exponential Growth-as a population gets
larger, it also grows faster (J-shaped curve)
(WHY?)
Exponential Growth
Elephant population
8,000
6,000
4,000
2,000
0
1900
1920
1940
Year
1960
1980
• This is in areas that are just formed like Hawaii
was millions of year ago. (Hawaii started as
barren rock, until the pioneers arrived and
began to make soil. The soil enabled plants to
grow. The seeds of the plants arrived in the
bird dropping of birds that stopped while
migrating to feed on the mosses and lichens.
Larger plant roots sped up soil formation to
allow for larger plants.)
Hawaii millions of years ago
Pioneer species begin to colonize
(lichens and mosses)
Then over time, grasses begin to
grow.
Then over time, shrubs will appear
and they will be followed by trees.
Hawaii today
∆N/∆t = B – D (Means change in population is
equal to births – deaths in that time.)
= (bN) – (dN) (birth rate – death rate)
(This is equal to rN.)
= rN ; r= b – d
(if r is positive = growth greater; if r is negative
= death greater)
ZPG (Zero Population Growth) (r= 0)
Intrinsic growth = rmax (Population is growing as fast as
possible/doubling. This is seen as the curve begins to make a
straight up curve.)
• B = bN
– b(per capita birth rate)
– N (population size)
– B expected number of birth expected based on
the population size
• D=dN
– d(per capita death rate)
– D expected number of death expected based on
the population size
• bN-dN=rN
• r=rate of increase =b-d
• rN-rate of increase x number in population
– Current population growth rate
Formulas related to population growth
Growth rate (r) = birth rate (b) - death rate (d)
# of births / total
# of deaths / total
Practice Problem: In research on beetles, you estimate that the populations size is 3000.
Over the course of a month, you record 400 births and 150 deaths.
What is the growth rate (r)?
Population growth = rN
(r = growth rate, N = original population size)
Calculate the population growth of the beetle population.
US Population size in 2013 (N) = approximately 316 million
Number of births in the US in 2013 (B) = approximately 4.1
million
Number of deaths in the US in 2013 (D) = approximately 2.5
million
What is the per capita birth rate ("b") for the US in 2013?
b=4.1 milliin /316 mil=.013=13/1000
What is the per capita death rate ("d") for the US in 2013?
d=2.5million/316 million = .008 = 8/1000
Exponential “Ideal” Growth
Intrinsic Growth
Logistic Growth Model (Realistic growth)
1. Involves K-selection species usually. (K refers to a
population that is hovering around the carrying capacity
“which is represented by “K”) (Density – dependent)
These species numbers are about number because there
are limited resources because the species is near the carrying
capacity for that environment.
2. Produces an S curve graph. (snakes around the
carrying capacity line)
3. Environment has limited resources; that is why
organisms stay around the K.
a. More organisms than K means damage will be done to
the environment.
b. More damage done to environment can cause K to
drop even farther. This can be an example of a positive
feedback loop.
c. Wars, disease, and famine breakout in a population to
bring numbers down below K. (Extinction is possible? 
depends on damage to environment and K.)
• Carrying Capacity-the number of one species
an environment can support over long periods
of time. (S-shaped curve
Logistic “realistic” growth
4. dN/dt = rN (K-N/K)
K-N the number of individuals that environment can still support
K-N/K the fraction of K still available for growth
(As a population “N” approaches K, K-N approaches zero.)
Meaning when K- N is equal to 0 you are at the carrying capacity
for that environment. When K- N becomes 0, the whole
equation becomes 0.
Before this time, a population is experiencing exponential
growth so you have r maxed.
Logistic Growth
5. Lag time (This accounts for the overshoot.) – It takes
time to begin to see the effects. (So the line goes
above K and this is when death, war, disease, and
famine accelerate.)
6. Allee effect –This situation occurs when you have a
small number of organisms. This low number causes
inbreeding to occur and then this results in no
variation for a gene pool and this leads to increased
genetic diseases that can be lethal to accumulate in
the already small population and then the
population enters what is referred to as the
“Extinction Vortex”. It is extremely difficult to break
out of the vortex. (Like a black whole of extinction.)
Allee affect and the
Extinction Vortex
Small
population
Inbreeding
Genetic
drift
Lower
reproduction
Higher
mortality
Loss of
genetic
variability
Reduction in
individual
fitness and
population
adaptability
Smaller
population