Silent Spring – Ecology Project

Chapter 52

By: Jacqueline Laurenzano , Judene Mavrikis, Samantha Viscovich, and

Rebecca Wojfnis

Density: A Dynamic Perspective

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.

Once a population’s boundaries, natural or defined by an investigator, are defined a population can be described in terms of density and dispersion.

Density- the number of individuals per unit area or volume.

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 mark-recapture method, to estimate densities and total population sizes.

Density is the result of a dynamic interplay between two processes: -Immigration- the incoming of individuals from other areas. -Emigration- the movement of individuals out of a population.

Patterns of Dispersion 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 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 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.

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.

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

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)

You find the Per Capita Rate of Increase by subtracting the Per Death rate from the Per Capita Birth Rate.

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.

B=bN (Where B is the number of births, b is the per capita birth rate, and N is the population size)

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.

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.

Logistic and Exponential Models

Exponential growth or (geometric population growth) is population increase under ideal and unlimited conditions.

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

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

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

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

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 High Densities:

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 Low Densities:

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 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 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

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.

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 density-dependent 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

Population Dynamics Population dynamics focuses on how the interactions between

biotic and abiotic factors cause variation in population size. Populations undergo periods of stability and fluctuation

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

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.

A metapopulation is a group of linked populations This concept shows the significance of immigration and emigration in

contrasting populations

Population Cycles Some populations follow regular and predictable “boom and bust” cycles.

While some populations fluctuate at unpredictable intervals, some fluctuate with extreme regularity and pattern.

Ex. (voles and lemmings have 3 to 4 years cycles, while the ruffed goose has a 9 to 11 year cycle)

For predators that depend heavily on a single prey species, the availability of that prey is the major factor influencing their population changes

Some causes of rises and falls in populations can be food shortage, excessive predator/prey interactions, or both.

The Hare cycle relies greatly on the predation but also partially relies on the food especially in the winter

Global Human Population 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

reduced family size is the key to the demographic transition Age Structure: is the relative number of individuals of each age, is

commonly represented in pyramids age-structure diagrams predict a population’s growth trend and illuminate social

conditions Infant Mortality and Life Expectancy

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 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?

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.

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

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.

Other relationships such as Commensalism, Competition, Parasitism, Mutualism, etc. can be disrupted by the poisoning of the resources in the environment.

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.

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 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.

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.

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

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

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

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.

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.

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.

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.

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

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: 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.

Acid Precipitation effects on The Global Carrying

Capacity 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.

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

Silent Spring – 10962- By Rachael Carson AP Edition Biology – Campbell Reece