CHAPTER 2
Populations
Populations are defined in several
ways
• 1) Ecologists define a population as a singlespecies group of individuals that use common
resources and are regulated by the same
environmental factors
– a) Individuals in a population have a high
likelihood of interacting and breeding with
one another
– b) Researchers must define a population by
geographic boundaries appropriate to the
questions being asked
Density and dispersion patterns
• 1) Population density is the number of
individuals in a given area or volume
– a) It is sometimes possible to count all the
individuals in a population
• 1. More often, density is estimated by
sampling
Density and dispersion patterns
– b) One useful sampling technique for
estimating population density is the markrecapture method
Density and dispersion patterns
– c) The dispersion pattern of a population
refers to the way individuals are spaced
within their area
• 1. Clumped
• 2. Uniform
• 3. Random
• 2) Clumped dispersion is a pattern in which
individuals are aggregated in patches
– a) This is the most common dispersion
pattern in nature
– b) It often results
from an unequal
distribution of
resources in the
environment
• 3) A uniform pattern of dispersion often results
from interactions among individuals of a
population
– a) Territorial behavior and competition for
water are examples of such interactions
Density and dispersion patterns
• 4) Random dispersion is characterized by
individuals in a population spaced in a
patternless, unpredictable way
– a) Example: clams living in a mudflat
– b) Environmental conditions and social
interactions make random dispersion rare
Idealized models
• 1) Idealized models describe two kinds of
population growth
– a) exponential growth
– b) logistic growth
Idealized models
• 2) Exponential growth is the accelerating
increase that occurs during a time when
growth is unregulated
– a) A J-shaped growth curve, described by
the equation G = rN, is typical of exponential
growth
• 1. G = the population growth rate
• 2. r = the intrinsic rate of increase, or an
organism's maximum capacity to
reproduce
• 3. N = the population size
G = population growth rate
r = maximum capacity to reproduce
N = population size
• 3) Logistic growth is slowed by populationlimiting factors
– a) It tends to level
off at carrying
capacity
• 1. Carrying capacity
is the maximum
population size
that an environment
can support at a
particular time
with no degradation
to the habitat
• Homeostasis-the tendency of
A population to remain stable
• 3) 2. What Barriers can prevent dispersal?
– Climate
– Physical- mountains
– Behavioral- area selection like birds
– Passive-wind carrying seeds
– So….why no polar bears
in the South Pole.
Idealized models
• 4) The logistic growth model predicts that
– a) a population's growth rate will be low
when the population size is either small or
large
– b) a population’s growth rate will be highest
when the population is at an intermediate
level relative to the carrying capacity
Multiple factors may limit population
growth
• 1) Increasing population density directly
influences density-dependent rates
• a) such as declining birth rate or
increasing death rate
• 2) The regulation of growth in a natural
population is determined by several factors
– a) limited food supply (resources)
– b) the buildup of toxic wastes
– c) increased disease
– d) predation
Multiple factors may limit population
growth
• 3) Field studies of
the song sparrow
have
demonstrated that
birth rates may
decline as a
limited food
supply is divided
among more and
more individuals
4) Abiotic factors may limit many natural populations
– a) Aphids
show
exponential
growth in the
spring and
then rapidly
die off when
the climate
becomes hot
and dry in the
summer
“Boom-and-bust" cycles
• 1) Some populations go through boom-andbust cycles of growth and decline
– a) Example: the population cycles of the
lynx and the snowshoe hare
• 1. The lynx is one of the main predators
of the snowshoe hare in the far northern
forests of Canada and Alaska
2) About every 10 years, both hare and lynx populations
have a rapid increase (a "boom") followed by a sharp
decline (a "bust")
“Boom-and-bust" cycles
– a) Recent studies suggest that the 10-year
cycles of the snowshoe hare are largely
driven by excessive predation
• 1. But they are also influenced by
fluctuations in the hare's food supply
– b) Population cycles may also result from a
time lag in the response of predators to
rising prey numbers
Evolution shapes life histories
• 1) An organism's life history is the series of
events from birth through reproduction to
death
– a) Life history traits include
• 1. the age at which reproduction first
occurs
• 2. the frequency of reproduction
• 3. the number of offspring
• 4. the amount of parental care given
• 5. the energy cost of reproduction
2) In nature, every population has a particular life
history adapted to its environment
– a) The agave
illustrates what
ecologists call
"big-bang
reproduction"
• 1. It is able to
store nutrients
until
environmental
conditions favor
reproductive
success
The human population
• 1) The human population as a whole has
doubled three times in the last three centuries
– a) Now stands at about 6.87 billion and may
reach 9.3 billion by the year 2050
• 1. Most of the increase is due to
improved health and technology
• 2. These have affected death rates
2) The history of human population growth
The Black Death
Birth, death rates, and age structure
• 1) Population stability is achieved when there
is zero population growth
– a) Zero population growth is when birth
rates equal death rates
– b) There are two possible ways to reach
zero population growth (ZPG)
• 1. ZPG = High birth rates - high death
rates
• 2. ZPG = Low birth rates - low death
rates
2) The demographic transition is the shift from
high birth and death rates to low birth and death
rates
• a) During this
transition,
populations
may grow
rapidly until
birth rates
decline
Birth, death rates, and age structure
• 3) The age structure of a population is the
proportion of individuals in different agegroups
– a) Age structure affects population growth
RAPID GROWTH
SLOW GROWTH
ZERO GROWTH/DECREASE
Kenya
United States
Italy
Male
Female
Male
Female
Ages 45+
Ages 45+
Ages 15–44
Ages 15–44
Under
15
Percent of population
Male
Female
Under
15
Percent of population
Percent of population
The human population
Questions to think about? In your group discuss
the following:
1. Is Earth’s carrying capacity limited?
2. What are some limiting factors that we
face?
3. How much food do you need?
4. What is a famine?
5. How have human activities changed the
environment?
6. Is resource management a necessity?
Practical applications
• 1) Principles of population ecology may be
used to
– a) manage wildlife, fisheries, and forests for
sustainable yield
– b) reverse the decline of threatened or
endangered species
– c) reduce pest populations
Practical applications
• 2) Renewable resource management is the
harvesting of crops without damaging the
resource
– a) However, human economic and political
pressures often outweigh ecological
concerns
– b) There is frequently insufficient scientific
information
3) Integrated pest management (IPM) uses a
combination of biological, chemical, and cultural
methods to control agricultural pests
– a) IPM relies on knowledge of
• 1. the population ecology of the pest
• 2. its associated predators and parasites
• 3. crop growth dynamics
– b) One objective of IPM is to minimize
environmental and health risks by relying on
natural biological control when possible