Silent Spring – Ecology
Project
Chapter 52
By: Jacqueline Laurenzano , Judene
Mavrikis, Samantha Viscovich, and
Rebecca Wojfnis
Density: A Dynamic Perspective
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A population is a group of individuals of a single species living in the same
general area. Members of a population rely on the same resources, are
influenced by similar environmental factors, and have a high likelihood of
interacting and breeding with one another.
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Once a population’s boundaries, natural or defined by an investigator, are
defined a population can be described in terms of density and dispersion.
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Density- the number of individuals per unit area or volume.
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When determining population density; it is rare to find cases where it is
possible to count all individuals, in most cases it is impossible to count all
individuals. So, ecologists use sampling techniques, such as the markrecapture method, to estimate densities and total population sizes.
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Density is the result of a dynamic interplay between two processes:
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-Immigration- the incoming of individuals from other areas.
-Emigration- the movement of individuals out of a population.
Patterns of Dispersion
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Dispersion- the pattern of spacing among individuals within the
boundaries of the population.
Social interactions between members of the population, which may
maintain patterns of spacing between individuals, can contribute to
variation in population density.
Three Patterns of Dispersion
 Clumped: The most common pattern of dispersion is clumped where
individuals are aggregated in patches. The clumped pattern is
associated with mating behavior, the uneven distribution of resources,
and can increase effectiveness of certain predators
 Uniform: The uniform pattern is not as common as clumped and is
when individuals are evenly spaced. Organisms often exhibit uniform
dispersion because of antagonistic social interactions, such as
territoriality
 Random: The least common pattern of dispersion is random where
there is unpredictable spacing, the position of each individual is
independent of other individuals. Random dispersion occurs when key
physical or chemical factors are relatively homogeneous or where
there is an absence of strong attraction among individuals of a
population.
Demography
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Demography is the study of the vital statistics of populations and how they
change over time
Life tables, a useful way to summarize some of the vital statistics of a
population, are age-specific summaries of the survival patterns of a
population. The best way to construct a life table is to follow the fate of a
cohort, a group of individuals of the same age, from birth until death
A graphic way of representing the data in a life table is a survivorship
curve, a plot of the proportion or numbers in a cohort still alive at each
age.
 Type I Curve: low death rate during early and middle years and then
death rates increase with old age
 Type II Curve: constant death rate over life span
 Type III Curve: high death rates for the young then death declines
for the survivors
A reproductive table, or fertility schedule, is an age-specific summary
of the reproductive rates in a population. The best way to construct a
reproductive table is to measure the reproductive output of a cohort
from birth until death
Life History Diversity
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Life Histories are made up of three traits : a) when reproduction begins, b) how often
the organism reproduces, and c) how many off-spring are produced during each
reproductive episode.
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Semelparity or “Big Bang Reproduction” is a type of one-shot reproduction
where the female only reproduces once in her lifetime.
 Example of Semelparity: Pacific Salmon: hatch in stream, migrate to open
waters for 4 years to mature, travel back to stream to reproduce and then die
 Environments which favor Semelparity reproduction: Semelparity is
favored where the survival rate of off-spring is low, in highly variable or
unpredictable environments, since production of large numbers in those
environments increases the probability that some will survive.
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Iteroparity or Repeated Reproduction: this is a type of repeated
reproduction in which the female reproduces more than once in her lifetime.
 Example of Iteroparity Reproduction: Some lizards produce a very large amount
of eggs during their second year of life, they continue this reproductive act
annually until death
 Environments which Favor Iteroparity Reproduction: Iteroparity is favored
in dependable environments, where competition for resources could be intense,
since a few relatively-large healthy off-spring will have a better chance of
surviving to reproductive age.
Per Capita Rate of Increase
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Per Capita Rate of Increase is represented as the variable ( r )
The per capita rate of increase indicates whether a given population is
growing (r > 0), declining (r < 0), or remaining constant (r = 0)
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You find the Per Capita Rate of Increase by subtracting the Per Death rate
from the Per Capita Birth Rate.
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Per Capita Birth rate: is the number of offspring produced per unit time
by the average member of the population It is represented by the variable
B.
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B=bN (Where B is the number of births, b is the per capita birth rate, and N is
the population size)
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Per Capita Death rate: allows for calculation of the expected number of
deaths per unit time in a population. It is represented by the variable m
for mortality.
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The Per Capita rate of Increase equation is r = b – m
Zero Population Growth occurs when r=0, meaning the per capita birth
and death rates are equal to each other.
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Logistic and Exponential Models
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Exponential growth or (geometric population growth) is population
increase under ideal and unlimited conditions.
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Under ideal conditions the per capita rate of increase is not restricted and may
assume the maximum rate of any specific species. This is called the intrinsic rate
of increase (rmax).
When graphed it assumes a J shape because even though the rate is constant,
over time, there will be more individuals present per unit time when it is large,
resulting in increasing steepness.
Characteristic of some populations that are introduced into a new or unfilled
environment or populations whose numbers have been drastically reduced and
are rebounding.
The Logistic Model: The Logistic Growth Model displays exponential
growth with limiting conditions
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The Logistic Growth Model shows that the per capita rate of increase declines as
carrying capacity is reached.
It assumes an S-shape because the population growth slows dramatically as the
population size nears carrying capacity.
The Logistic Model and Real
Populations
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The logistic model assumes that populations adjust
instantaneously to growth and approach carrying capacity , usually
there is a lag time before the negative effects of an increasing population
are realized in most natural populations
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The logistic model also incorporates the idea that regardless of
population density, each individual added to a population has the
same negative effect on population growth rate.
 however, some populations show an Allee Effect, in which individuals
may have a more difficult time surviving or reproducing if the
population size is too small
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The logistic model is a useful starting point for thinking about how
populations grow and for constructing more complex models, it is
also useful in conservation biology for estimating how rapidly a particular
population might increase in numbers after it has been reduced to a small
size
The Logistic Model and Life
Histories
The logistic model predicts different per capita growth rates for
populations of low and high density relative to the carrying
capacity of the environment
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High Densities:
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Low Densities:
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per capita resources are relatively abundant, and the population can grow rapidly
selection favors adaptations that promote rapid reproduction
*different life histories are favored under each condition*
K- selection (density-dependent selection) – selection for life history traits that
are sensitive to population density
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each individual has few resources available, and the population grows slowly, if at all.
selection favors adaptations that enable organisms to survive and reproduce with few
resources
tends to maximize population size and operates in populations living at a density near the
limit imposed by their resources ( the carrying capacity, K)
R-selection (density-independent selection) – selection for life history traits that
maximize reproductive success in uncrowded environments
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tends to maximize r, the rate of increase, and occurs in environments in which population
densities fluctuate well below carrying capacity or individuals are likely to face little
competition
Density Dependent Population
Regulation
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Density-dependent birth and death rates are examples of negative
feedback, without some type of negative feedback, a population would not
stop growing. At increased densities birth rates decline and/or death rates
increase, providing the needed negative feedback. The mechanisms
causing these changes involve many factors.
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Competition for Resources- in crowded populations, increasing population density
intensifies Interspecific competition for declining resources, resulting in a lower birth
rate.
Territoriality- when territory space becomes the resource for which individuals
compete. The presence of non-breeding individuals is indication that territoriality is
restricting population growth.
Health- a disease’s impact may be density dependent, if the transmission rate of a
disease depends on a certain level of crowding in a population
Predation- if a predator encounters and captures more food as the population
density of the prey increases, then predators may feed only on that species,
consuming a higher percentage of individuals
Toxic Wastes- the accumulation of toxic wastes can contribute to densitydependent regulation size. An example would be in laboratory culture of small
organisms metabolic by-products accumulate as the populations grow, poisoning the
organisms within the environment
Intrinsic Factors- for some animal species, intrinsic (physiological) factors, rather
than extrinsic (environmental) factors, appear to regulate population
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Population Dynamics
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Population dynamics focuses on how the interactions between
biotic and abiotic factors cause variation in population size.
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Populations undergo periods of stability and fluctuation
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Large mammals are usually more stable than other populations but in
some cases this is not always true.
 Example: Moose from the mainland colonized Isle Royale around
1900, being isolated from immigration and emigration their population
should stay stable. Yet because of abiotic factors (harsh winters) and
biotic factors (wolves as predators) the moose population was
extremely unstable
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While the moose were fluctuating, the Dungess crab, a much smaller
species, located at Fort Bragg varied between 10,000 and hundreds of
thousand over a 40 yr period
 Severe temperature extremes and cannibalism can caused fluctuation
in the Dungess Crab population.
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A metapopulation is a group of linked populations
This concept shows the significance of immigration and emigration in
contrasting populations
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Population Cycles
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Some populations follow regular and predictable “boom and bust” cycles.
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While some populations fluctuate at unpredictable intervals, some
fluctuate with extreme regularity and pattern.
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Ex. (voles and lemmings have 3 to 4 years cycles, while the ruffed goose has a 9
to 11 year cycle)
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For predators that depend heavily on a single prey species, the availability
of that prey is the major factor influencing their population changes
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Some causes of rises and falls in populations can be food shortage,
excessive predator/prey interactions, or both.
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The Hare cycle relies greatly on the predation but also partially relies on
the food especially in the winter
Global Human Population
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The global population now numbers over 6 billion people and is increasing
at a rate of about 73 million each year.
Zero population growth = High birth rate – High death rate
or
Zero population growth = Low birth rate – Low death rate
Demographic transition - a shift from zero population growth in which
birth rates and death rates are high to zero population growth
characterized instead by low birth and death rates
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Age Structure: is the relative number of individuals of each age, is
commonly represented in pyramids
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reduced family size is the key to the demographic transition
age-structure diagrams predict a population’s growth trend and illuminate social
conditions
Infant Mortality and Life Expectancy
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infant mortality is the number of infant deaths per 1,000 live
life expectancy at birth is the predicted average length of life at birth
these differences reflect the quality of life faced by children at birth
Global Carrying Capacity
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The United Nations estimated that the global population IN 2050 WILL BE
FROM 7.5-10.3 BILLION PEOPLE. Just how many people can our biosphere
support?
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Estimates of Carrying Capacity: Some researchers use a logistic curve
to predict the future maximum of human population. Others predict this
by looking at existing “maximum” population density and multiplying this
by habitable land. Still other make prediction based on a simple necessary
factor such as food.
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Ecological Footprint: The ecological footprint summarizes the
approximate land and water used by each nation to produce all the
resources it consumes and absorbs all the waste it generates. – How close
we are to the maximum carrying capacity
 U.S. – 8.4 ha per person – maximum is 6.32 ha per person
 New Zealand – 9.8 ha per person – maximum – 14.3 ha per
*Perhaps food would be a main factor in limiting our growth Perhaps we
will be limited by space, or we could run out of nonrenewable
resources such as metal and fossil fuels. Or we may even run out of
the renewable resource of water
The Dangers of DDT
Effects of DDT on Density and
Dispersion
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The density and dispersion of different populations of species can be very
fragile and easily disrupted by changes in the environment.
The web of life and the balance of nature is extremely fragile and
harmful chemicals such as DDT can completely disrupt that
balance.
Certain species are abundant in certain areas because of a certain quality,
feature, or resource that, that environment contains.
If a needed resource is damaged, destroyed, or poisoned by a powerful
chemical such as DDT, the food chain and eating patterns as well as
predator/prey relationships are disrupted.
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Other relationships such as Commensalism, Competition, Parasitism, Mutualism,
etc. can be disrupted by the poisoning of the resources in the environment.
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The disruption of the food chain and interspecific relationships, will
eventually deplete and destroy the native populations. This is because the
death and emigration rates will increase ; while birth rates and
immigration rates will decline.
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In the end result, The use of harmful chemicals such as DDT will disrupt
the relationships in a population and will affect the density and dispersion
of a population in a given area.
DDT effects on Density-Dependent
Population Regulation
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DDT that is sprayed filters down from the plants or trees that were sprayed, and
some of it reaches the ground and leeches into the flowing streams and ground
water.
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Health: When DDT has been ingested into an animals systems, it will be stored in
their fat tissue and will greatly harm them. If an animal doesn’t die immediately it
will pass the chemical on to whomever it gets eaten by. As DDT moves up the food
chain its abundance grows immensely.
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Toxic wastes: DDT is an extremely toxic and harmful waste, as DDT leeches into
soil and water it will contaminate there reservoirs with toxic wastes. Any animal
living in or crop produced in these reservoirs will become infected and be harmed
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Predation: Any animal which feeds on smaller animals that live in environments
which were spayed with DDT will not only become infected by DDT as well, by will be
infected with a larger amount of DDT that the animal which was eaten
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Competition for resources: When DDT traveling through soil and water bodies it
either infects or kills the life those ecosystems support. Herbivores and Carnivores
may lose their supply of food if that species has been killed off or infected by the
toxic DDT. This will increase competition because there are very limited amount of
resources.
DDT affects on Global Carrying
Capacity
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The carrying capacity of Earth for humans is uncertain. The ecological
footprint concept summarizes the aggregate land and water appropriated by each
nation to produce all the resources it consumes and to absorb all the waste it
generates.
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DDT residues are found on the ground and the water, affecting all organisms
living within, or on the ground and the organisms living within the waters.
These organisms are consumed by humans, and the DDT is found in the
tissues or humans, causing illnesses and may even causing death.
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Through biological magnification, any animal in which humans eat that contains toxic
chemicals such as DDT will harm humans drastically. The amount of toxins that
animal contains will be multiplied in the human body.
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An overall analysis suggests that the world is now at or slightly above its
carrying capacity. We can only speculate about Earth’s ultimate carrying capacity
for the human population or about what factors will eventually limit our growth.
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DDT may cause damage to the factors that could potentially limit our growth
or limit our resources needed for survival, such as water, air, and soil. We need
water to survive, air to breathe, and soil for agriculture. DDT could affect all of
resources and more
Acid Precipitation
Acid Precipitation Effects on
Density and Dispersion
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Acid Precipitation is formed from the excess of Carbon Dioxide and Sulfur
in the atmosphere which combines with water vapor and falls down to the
earth as acid rain.
Acid rain damages the ecosystems in which it falls, by:
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Contaminating water supply
Contaminating water ecosystems
Killing animals and plants
Changing the composition of soil
Raising the pH levels in water and soil ecosystems
Causing key nutrients to leech out of soil, destroying forests
Causing health problems
Destroying possible habitats
All of these effects of Acid Precipitation negatively affect density and dispersion
With less available habitats and less water availability competition for resources
will increase, this will affect the patterns of dispersion within an ecosystem. A
random dispersion ecosystem can be transformed into a clumped or uniform
patterned ecosystem.
The density of an ecosystem can be negatively affected by acid rain because the
rain diminishes some of the necessary resources that are required by individuals.
These individuals will have to die off or look elsewhere to survive, lessening the
density in certain ecosystems.
Acid Rain effects on Density-Dependent
Population Regulation
Acid rain has been shown to have immense affects on forests,
freshwaters and soil ecosystems, killing insect and aquatic life
forms as well as causing damage to buildings and having impacts
on human health
Acid precipitation has negative effects on the many aspects that regulate
the density of a population.
Competition: Acid rain contaminates water supply and damages aquatic
and terrestrial biomes. Animals competing for water or for suitable
habitats will have to compete more for suitable water supply and habitats.
Territoriality: Territoriality is affected by acid rain, because territoriality
comes into affect when the density of a population is high and there aren't
enough suitable habitats. Acid Precipitation destroys some of the aquatic
and terrestrial habitats and therefore lessens the amount of suitable
habitats.
Health: Small fragments which are mainly formed from the same gases
which form acid rain, have been shown to cause illness and premature
deaths such as cancer and other diseases
Toxic wastes: Just as garbage is accumulating on earth, and carbon
dioxide is accumulating in the atmosphere, acid precipitation is
accumulating in the aquatic ecosystems and soil raising the pH levels of
these ecosystems and killing off their inhabitants.
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Acid Precipitation effects on The
Global Carrying Capacity
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Acid Precipitation destroys many of the earth’s resources either by raising
the pH level of our resources, making our resources unsuitable for life, or
by destroying the resource all together.
Water: Ecologists believe that water could be a limiting factor on the
Global carrying Capacity. When acid precipitation enters lakes, pond, or
rivers it sometimes raises the pH level to such a degree that it is unsafe to
consume
Food: Food is also believed to be a possible limiting factor. Acid
precipitation destroys a great deal of plant life when falling to earths
surface.
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Ecologists have observed severe leaf damage that can be attributed to acid rain,
the acid rain limits the plants ability to sustain itself.
Acid Precipitation can also raise the pH level or change the composition in soil
making it unsuitable for plant life.
Many ecosystems, such as forests are destroyed because the acid
precipitation in the soil causes essential nutrients to leech out of the soil
making it unsuitable to support plant life.
Acid Precipitation negatively affects many of the factors in which ecologists
predict will limit the Global Carrying Capacity of earth. It is imminent that
there is an international consensus to limit the amount of Carbon Dioxide
released into the atmosphere.
Ms. S. AP Bio A Period
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Silent Spring – 10962- By Rachael Carson
 AP Edition Biology – Campbell Reece