Chapter 4-B1
Population Ecology
Population growth is a critical factor in specie’s ability to maintain homeostasis within its environment.
Main themes
~Scientists study population characteristics to better understand growth and distribution of organisms.
~Populations have different distributions and densities depending on the species represented in the population
~Organisms in a population compete for energy sources, such as food and sunlight
~Homeostasis within a population is controlled by density-dependent and density-independent limiting factors
~Human population varies little overall, but can change greatly within small populations.
I. Population dynamics
A. Population characteristics
1. Population density
a. defined-the number of
organisms per unit area
b. eg. deer in Newark
deer in 5 mi. sq. area
2. Spatial distribution
a. defined-organism pattern in
area (eg. more deer on
Londondale than on
Green Wave Drive)
b. dispersion-how organisms are
placed in an area
c. Types p. 93
-UNIFORM dispersion-
relatively even distribution of
a species. Eg. pine forest
-CLUMPED dispersion-
organism are clumped near
food. Eg. schools of fish,
eg. mushrooms on a fallen log
-RANDOM dispersion-
unpredictable & changing.
Eg. deer, large fish, etc.
3. Population ranges-
a. area where organisms CAN/ARE
living
b. Meaning of Distribution
-in Science~it is area where
something lives and
reproduces
-in everyday language ~it
means distribution (passing
out) eg. of report cards.
B. Population-LIMITING FACTORS
1. Limiting factors-biotic or abiotic
factors that keep a population from
increasing forever (indefinitely).
a. amount of food=biotic
b. amount of space=abiotic
c. amount of disease, etc.=biotic
2. Density-independent factors
a. density-# of organisms
b. independent=doesn’t rely on the
number of organisms, would
happen regardless of the
number of organisms
c. it is usually abiotic
(limits population growth)
-flood (eg. no matter how many
deer in an area)
-hurricane
-blizzard
-drought
-fire (Ponderosa pine needs
fire to kill undergrowth which
takes all the soil nutrients
from them. Heat releases
seeds).
d. man-made density-independent
factors
-building dams
-pollution
3. Density-dependent factors
a. dependent= depends on
b. factors which depend on density
of organisms (eg. more deer
means more predation, disease
and parasites)
c. they are often biotic factors
-parasites
-disease
-predation
-competition
TYPES OF DENSITY DEPENDENT FACTORS
d. Predation (Predator/prey
populations fluctuate due to the
amount of food available
-wolves eat moose
-wolves inc. then moose dec./
moose dec./wolves inc.
e. Disease
-disease spreads by contact
with individual organisms
-the higher the density of an
organism, then the higher the
disease, then the higher the
deathrate (limiting factor)
f. Competition
-when population numbers
increase, so does competition
for available resources
(eg. food)
-within same species for a
resource and/or
-between 2 or more species
Eg. cornfield-
mice, raccoon, deer all
eat corn
High food-then population growth increases exponentially (very quickly)
Low food-then population growth decreases due to starvation
g. Parasites
-parasites spread mostly
through contact
-similar to ‘disease’ as a
limiting factor
-eg. worms&dog
~less dogs then less worms
4. Population growth rate PGR
a. defined-how fast a population
grows
b. Natality-number of organisms
born in a given time
period (birthrate)
~eg.how many males
born in Newark in 2010
c. Mortality-number of deaths in a
given time period
(deathrate)
d. Immigration-number of
organisms moving
into an area
~eg.number of people
moving from Columbus to
Newark in 2010
e. Emigration-number of organisms
moving out of an area
f. usually immigration=emigration
g. Exponential growth model
(J-shaped)
-growth with no limiting
factors
-lag phase-at beginning of
reproduction when
few individuals are
mating
Eg. 2 mice---2 yrs=3 million+
-J-shaped curve on a graph
-all populations grow
exponentially until limiting
factors slows growth
(eg. run out of food)
-Resources become limited
with more individuals
h. Logistics growth model
(S-shaped curve)
-occurs when exponential
growth stopped by limiting
factors
-growth levels off at carrying
Capacity
-limiting factors (lack of food, predators, space, etc. causes exponential growth to stop and level off)
i. Carrying capacity
-the maximum number of
species an area can hold for an
extended period of time
-eg. how many deer in the
summer in Newark and how
many deer in the winter in
Newark
-resources become limited (run
out of food, etc) as population
nears carrying capacity
-populations stabilize
C. Reproductive patterns
1. Based on
a. number of births per cycle
(compare mice/elephant)
b. What age reproduction begins
c. Life span of organism
2. R-strategists (rate strategist)
a. short life span
b. abiotic & biotic factors
fluctuate
-food availability
-temp. change (seasons)
c. plan to reproduce as many
organisms in shortest time
d. normally use little energy to
care for their young
e. usually controlled by
density-independent factors
f. carrying capacity rarely met
-eg. mosquitoes
3. K-strategists
a. large organisms
b. long life span
c. produces few offspring
d. population reaches carrying
capacity
e. they care for their young
-use high energy to get genes
to next generation
successfully
f. usually controlled by
density-dependent factors
Review questions
What are the characteristics of populations and how they are distributed?
What are the differences between density-independent and density-dependent limiting factors?
What are the similarities between the different models used to quantify the growth of a population?
How does carrying capacity affect reproductive rates?
II. Human population
Would you say that the total population is growing faster today or that it grew faster 50 years ago?
A. Human population growth fig. 11 p.100
1. Demography
a. defined-the study of human
population size, density,
distribution, etc.
b. Human population size
-1805-1 billion
-1999-6 billion
-2012-7 billion
-2050-9 billion
2. Technological advances
a. Before technology, the
environment kept the human
population size below carrying
capacity (eg. we ran out of food,
space, disease because no
medicine, etc)
b. Agriculture, technology,
medicine reduced the number of
deaths & increased the
population size
Explain why an improvement in shelter increased the survival rate of the human population.
3. Human population growth rate
a. current rate of growth is
slow~1.2%
b. 1940-1950’s-there was a high
rate of growth~2.2%
c. drops in growth rate may be due
to famine, AIDS & birth control
B. Trends in Human Population Growth
1. Trends influenced by
a. war
b. famine
c. disease etc.
2. Growth rates similar in countries
with similar economics.
3. Demographic transition (change)
a. high birthrate, high deathrate
to low birthrate & low deathrate
b. PGR comparison of a developed
country & a developing country.
Birthrate-deathrate + migration rate=PGR%
10
-developed country (USA) = .87%
-Developing country (Hondurus) = 2.02%
c. eg. Niger 2010-2050 will have
the highest PGR at 2.88%
-this means 13 million people to
53 million!
4. Zero population growth
birth + immigration=death + emigration
5. Age structure
a. Number of males & females in
pre-reproductive, reproductive
& post-reproductive years.
b. affects PGR of a
country/county/city
6. Human carrying capacity
a. technology has allowed an
increase in humans carrying
capacity
b. conservation of resources
changes carrying capacity
c. we must work at keeping humans
at or below carrying capacity.
Why?
d. resources used mostly by
developed countries
-burning fossil fuels causes too
much CO2 in atmosphere
-land then unable to absorb all
the Carbon thus,
-Greenhouse effect, thus-
-Global Warming
e. developing countries have high
PGR & will eventually use more
resources (more food, more
energy needs, etc.)
Why will disease and starvation become larger problems if the population continues to increase?
Review questions
What aspects affect human population growth?
What are the trends in human population growth?
What are the age structures of representative nongrowing, slowly growing, and rapidly growing countries?
What might be the consequences of continued population growth?