Download 1. populations

populations
• Population
– Group of organisms of the same species
living in a specific area that can mate and
produce fertile offspring
Why measure populations
• They share a common gene pool
– Reflects interaction between a population and
its environment (natural selection)
• Determine their health or stability
– Is there growth or decline
– How is a new species
– What factors affect growth, stability, decline
• 1st step measuring size
Problems measuring size
• Population density – more meaningful than
total number unless close to extinction
– Number of individuals per unit volume or area
– Comparisons over space and time
Problems measuring size
• Distribution
– The characteristic pattern of spacing of
individuals within a population
Types of distribution
– Random/Patchy – scattered suitable areas w/
population boundries
– Clumped/grouped - most common – herds,
flocks- soil conditions or gathering for reprod.
– Uniform/Even – fairly consistent distance
Age sex distribution
• Age sex differences between populations
are significant indicators of the health of a
population
• Studied by ecologists concerned with a
population’s future
Age structure diagram
• X axis = percentage of population
• Y axis = age groups
• Males left - females right
Age Structure
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Population growth depends, in
part, on how many people of
different ages make up a given
population
Demographers can predict
future growth using models
called age-structure diagrams,
or population profiles
Age-structure diagrams show the
population of a country broken
down by gender and age group
Each bar in the age-structure
diagram represents individuals
within a 5-year group
Percentages of males are to the
left of the center line and females
to the right in each group
Age Structure
• Growing population – usually has more
young individuals than adults at beyond
reproductive age.
• Stable population – equal numbers of
young members and adults
• Declining population – more adults and
fewer young
Patterns in Populations
Characteristics of populations interact to
form several strategies for survival
• Birth rate - number of births per individual
within the population per unit time
• Death rate - number of deaths per
individual within the population per unit
time
• Life expectancy -average survival time for
an individual
Plotting patterns
• Survivorship curves – graph the number of
all individuals still living for each age
• 3 types
– Type I
– Type II
– Type III
• Type I
– highest survival rates w/ individuals living to
old age
– Parental care of offspring
– Low infant death rates
– Produce few offspring
• Type II
– Uniform death rates throughout life span
– Constant loss pattern
– Birds and asexual species
• Type III
– High death rates at young age; few live to old
age
– Parents produce high # of young to offset high
death rates – no parental care
– Plants, invertebrates, fish
Exponential Growth
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The figure at right shows a graph
with the size of the bacterial
population plotted against time
As you can see, the pattern of
growth is a J-shaped curve
The J-shaped curve indicates that
the population is undergoing
exponential growth
Exponential growth occurs when
the individuals in a population
reproduce at a constant rate
At first, the number of individuals in
an exponentially growing population
increases slowly
Over time, however, the population
becomes larger and larger until it
approaches an infinitely large size
Under ideal conditions with
unlimited resources, a population
will grow exponentially
Logistic Growth
• Obviously, neither bacteria nor elephants
cover the planet
• This means that exponential growth
does not continue in natural
populations for very long
• What might cause population growth to
stop or to slow down?
Population Growth on a J-shaped
curve
• Shows that population
grows slowly during
the early lag phase
and then very rapidly
during the
exponential phase
Logistic Growth
Logistic Growth
• Logistic Growth This
graph shows the Sshaped curve of logistic
growth
• As resources become
less available, the
population growth rate
slows or stops
• The growth of this
population has leveled
off at its carrying
capacity
Carrying Capacity
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If you look again at the graph to
the right, you will see a
horizontal line through the
region of the graph where the
growth of the yeast population
has leveled off
The point at which that line
intersects the y-axis tells you
the size of the population when
the average growth rate reaches
zero
That number, in turn, represents
the largest number of
individuals—in this case, yeast
cells—that a given environment
can support
Ecologists call this number the
carrying capacity of the
environment for a particular
species
Population Growth on an S-shaped
curve
• Levels off after the
exponential phase
• The top of this curve
indicates the carrying
capacity of an
ecosystem
Limiting Factors
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Recall from Chapter 3 that the
primary productivity of an
ecosystem can be reduced
when there is an insufficient
supply of a particular nutrient
Ecologists call such
substances limiting nutrients
A limiting nutrient is an example of
a more general ecological
concept: a limiting factor
In the context of populations, a
limiting factor is a factor that
causes population growth to
decrease
Some of the limiting factors that
can affect a population are shown
in the figure at right
Limiting Factors
POPULATION GROWTH
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Limits to Growth
– Density-Dependent Factors: factors that affect indifferent ways depending on population
density
• Food
• Space
• Light
• Parasitic infections
• Disease
• Number of predators / prey
• Oxygen
– Density-Independent Factors: factors that affect populations regardless of population
density
• Usually abiotic
– Changes in weather
– Changes in temperature
– Changes in humidity
– Variations in the amount of sunlight
– Amount of available energy
Limiting Factors
• A resource base that is limited can also affect
the long-term survival of a species
• For example, pandas depend for food on
bamboo that grows in certain kinds of temperate
forests in China
• Since the time that these forests have been
cleared for timber and farmland, panda
populations have fallen dramatically and have
become isolated in small pockets of remaining
forest
Density-Dependent Factors
• A limiting factor that depends on population size is
called a density-dependent limiting factor
• Density-dependent factors become limiting only when
the population density—the number of organisms
per unit area—reaches a certain level
• These factors operate most strongly when a
population is large and dense
• They do not affect small, scattered populations as
greatly
• Density-dependent limiting factors include
competition, predation, parasitism, and disease
Competition
• When populations become crowded, organisms
compete with one another for food, water, space,
sunlight, and other essentials
– Example: puffins must compete for limited nesting sites
• Competition among members of the same species is
a density-dependent limiting factor
• The more individuals living in an area, the sooner
they use up the available resources
• Likewise, the fewer the number of individuals, the
more resources are available to them and the less they
must compete with one another
Predation
Parasitism and Disease
• Parasites can also limit the growth of a
population
• Parasitic organisms range in size from
microscopic, disease-causing bacteria to
tapeworms 30 centimeters or more in length
• These organisms are similar to predators in
many ways
• Like predators, parasites take nourishment at
the expense of their hosts, often weakening
them and causing disease or death
Density-Independent Limiting Factors
• Affect all populations in similar ways, regardless of the
population size
• Unusual weather, natural disasters, seasonal cycles,
and certain human activities—such as damming
rivers and clear-cutting forests—are all examples of
density-independent limiting factors
• In response to such factors, many species show a
characteristic crash in population size
• After the crash, the population may soon build up
again, or it may stay low for some time
Density-Independent Limiting Factors
• For some species, storms or hurricanes can nearly
extinguish a population
– For example, thrips, aphids, and other insects that feed on plant
buds and leaves might be washed out by a heavy rainstorm
• Extremes of cold or hot weather also can take their toll
on a population, regardless of the population's density
– A severe winter frost, for example, can kill giant saguaro
cactuses in the Arizona desert
• In some areas, periodic droughts can affect entire
populations of vegetation
• Such events can, in turn, affect the populations of
consumers within the food web
Density-Independent Limiting Factors
• Environments are always changing, and most
populations can adapt to a certain amount of
change
• Populations often grow and shrink in
response to such changes
• Major upsets in an ecosystem, however, can
lead to long-term declines in certain populations
• Human activities have caused some of these
major upsets
• Human population continues to grow but
many scientists believe that we overconsume resources and exceed the
environment’s capacity to cycle nutrients
and process waste.
• Possible population crash is possible
Factors that can lead to crisis
• Agricultural system –”green revolution”
depends on nonrenewable fossil fuels for
fertilizers, pesticides, and irrigation
• Most countries depend on almost entirely
on petroleum to fuel industrialization and
transportation
Factors that can lead to crisis
• Fresh water supplies are declining due to
pollution and overuse
• Habitat destruction due to agriculture,
development, and mining
• Burning of fossil fuels has brought about
atmospheric change
Sustainability
• We must consider the impact of our
decisions on the next seven generations
• A sustainable activity or state can be
maintained indefinitely, w/out
compromising resources for the future.
Sustainability
• Sustainability of products and services
considers complete life cycles – raw
materials, manufacturing,
transportation/distribution, use and re-use,
maintenance, recycling and disposal.
Sustainability
• Way to estimate sustainability is your
ecological footprint
– The amount of land area you would need to
sustain your current lifestyle
– Considers resources you consume and
pollution you generate then calculates the
amount of land needed to produce equivalent
RENEWABLE resources