Download Chapters • Lesson 17

Chapters • Lesson 17
Population Dynamics
Objectives: 2,1.3, 2.1.4
Key Terms Words
• abiotic factor • biotic factor • population • immigration • emigration • limiting factor
• density-dependent limiting factor • population density • density-independent limiting factor
• carrying capacity
Getting the Idea
You have learned how predator-prey relationships affect the sizes of populations. Many
other factors also cause populations to increase or decrease. These factors include all the
ways in which organisms interact with the living and nonliving parts of their ecosystems.
Understanding how population sizes change helps humans preserve healthy ecosystems.
Abiotic and Biotic Factors
The abiotic factors in an organism's environment are all the nonliving components of its
ecosystem. Abiotic factors include the amount of sunlight an area receives, temperature,
precipitation, soil type, salinity, and materials such as water, nitrogen, and oxygen.
Biological factors, or biotic factors, are all the living things in an environment. Biotic factors
are all the other organisms with which an organism may interact—bacteria, plants, animals,
fungi, and protists. Biotic factors may include the other organisms that the organism eats or
that try to eat it, those with which the organism competes, and any other organism with
which the organism has a symbiotic relationship.
Population Growth and Decline
Recall that a population is made up of all the organisms of a species that live in an area at
the same time. The size of a population is affected by births, deaths, immigration, and
emigration. Immigration is the movement of organisms into an area. Births and immigration
increase population size. Deaths and emigration decrease population size. Emigration is
the movement of organisms out of an area. When births and immigration exceed deaths
and emigration, the population grows. When deaths and emigration exceed births and
immigration, the population shrinks.
Birth and death rates of human populations are affected by many factors. For example, birth
rates in underdeveloped nations tend to be much higher than those in developed nations.
The higher birth rate is due in part to a need for more people to do work such as farming. At
the same time, death rates in many developing nations are higher than those in developed
nations. The higher death rates often result from a lack of food and water, both of which
may be scarce because of droughts. People in those nations may also lack access to
medical care and modern sanitary conveniences (such as sewer services and clean water).
Poor sanitation and lack of medical care enable diseases to spread rapidly through a
population.
Death rates in developed nations have generally declined since 1900. These decreases are
the result of better access to food and clean water, improvements in personal hygiene,
improved sanitation, and medical progress. Birth rates in many of those nations have also
declined, partly because of the increasing costs of raising children. The graph below shows
how birth rates and death rates in Europe have changed since 1900 in response to these
factors.
In natural environments, changes in either abiotic or biotic factors can greatly affect
population sizes. If a population cannot adapt to changes in its environment, some
members will die or move away, and the population size will decrease. Alternatively, if
organisms are already well adapted for the changed environment, their numbers may
increase.
Limiting Factors
Population size changes over time. However, most populations stabilize rather than grow
endlessly. This is because as a population grows, it puts more demands on its ecosystem.
For example, a large population of lions in a savannah needs more food than a smaller
population. If the ecosystem does not contain enough resources to feed all the members of
the larger population, some lions will move out of the area or die. As a result, the population
will decrease. The lion population is limited by the food supply.
Anything in the environment that can limit the size of a population is called a limiting
factor. In addition to food, limiting factors include the supply of water, oxygen, and sunlight
as well as relationships with other organisms. Limiting factors can be either biotic or abiotic.
The table gives examples of each kind of limiting factor.
Density-Dependent Limiting Factors
There are two main types of limiting factors-density-dependent and density-independent.
A density-dependent limiting factor is a limiting factor that depends on the size of a
population
living in a certain amount of space. Population density is a measure of the number of
organismsper unit of area—for example, per square kilometer. Density-dependent limiting
factors havemore effect when population density is high than when it is low.
Density-dependent limiting factors include competition, predation, and parasitism, which you
learned about in Lesson 16. Recall that organisms compete when they try to use the same
resource. Competition can occur between organisms of the same species as well as
between members of differing species. Predation is a feeding relationship in which one
animal kills and eats another. Parasitism is a symbiotic relationship in which one animal
benefits at the expense of another. All three of these limiting factors can have a greater
effect in a dense population in which organisms are crowded together.
Diseases spread by pathogens are also density-dependent limiting factors. In dense
populations, such diseases can dramatically disrupt an ecosystem. For example, Dutch elm
disease, which is caused by a fungus, was first observed in the United States in 1930. At
the time, elm trees were common in forests and along tree-lined streets in towns and cities.
Dense populations of elms provided an ample habitat for beetles that spread the fungus.
Because the trees were so close together, the disease spread rapidly and has killed almost
half the elm trees in the northern United States. The fungus can also spread from the roots
of a diseased tree to the roots of other elms nearby.
Diseases caused by pathogens do not affect only plants and wildlife. Such diseases can
also spread rapidly through dense human populations and may increase the death rate in
these populations. For example, tuberculosis (TB) is a disease caused by bacteria that
usually affects the lungs. Many people who become infected with TB do not become ill from
the infection. However, these people can easily spread the bacteria to others through the air
when they cough, sneeze, or simply exhale while talking.
Because of its mode of transmission, TB can spread rapidly in dense populations. Although
many people who become infected with TB do not show symptoms of the disease, nearly
10 percent of those infected develop an active infection. The World Health Organization
(WHO) estimates that more than 8 million persons throughout the world develop active
cases of TB each year. Almost 2 million people with active TB die of the disease each year.
Density-Independent Limiting Factors
Density-independent limiting factors limit the growth of a population regardless of its
density. Abiotic factors including weather and natural disasters such as forest fires,
droughts, and tsunamis are density-independent limiting factors. These events can sharply
reduce population sizes or even wipe out some populations. For example, a large forest fire
can destroy entire plant and animal populations. Populations of organisms that fed on those
species will decline or die out. Some surviving populations may remain small, leading to a
new stable ecosystem with fewer organisms than before. Other populations will recover to
their previous sizes over time. Pollution produced by humans is another density-
independent limiting factor. You will learn more about how some density-independent
limiting factors caused by humans affect ecosystems in the next lesson.
Carrying Capacity
The largest population that an environment can support over a long period is its carrying
capacity. The carrying capacity of an environment is different for each species because the
habitats and resource needs of each species differ. However, two or more species in an
ecosystem can compete for resources such as food or nest sites. For example, an
environment may be able to support a limited number of seed-eating birds, regardless of
species. If conditions in an environment change, its carrying capacity may also change. In
the case of the seed-eating birds, a sudden decrease in the supply of seeds will reduce the
carrying capacity of the ecosystem. By contrast, if the plants start to produce more seeds,
the ecosystem's carrying capacity will increase.
A population stops growing when it reaches the carrying capacity of its ecosystem. In many
cases, as the carrying capacity is reached, the population size stabilizes, or levels off. The
graph below shows the effect of carrying capacity on a fly population. Initially, the population
is small and resources are plentiful. The death rate is low, meaning most of the flies survive,
and the birth rate is high because the surviving flies reproduce. These conditions enable the
fly population to grow rapidly. However, the flies reach the carrying capacity of the
ecosystem in about a month. The environment no longer contains enough resources to
support a growing population. At this point, the population stops growing and levels off. The
leveling off indicates that the total number of births and immigrants is about equal to the
total number of deaths and emigrants.
The carrying capacity of an ecosystem depends on the number of organisms living there,
the size of the ecosystem, and the available resources. The resources humans need
include energy, water, oxygen, food, medical care, and sanitary living conditions. Different
ecosystems can support different numbers of humans as well as of other organisms. An
ecosystem with many resources, such as a tropical rain forest, has a large carrying
capacity. In contrast, a desert has a much lower carrying capacity because much less water
is available. Deserts also tend to have poor soil. The size of an ecosystem also affects
carrying capacity. A large area of rain forest has a larger carrying capacity than a small plot
of rain forest.
Effects of Resource Use on the Environment
As you have read, a growing population places an increasing demand on the resources of
its environment. This increased use of resources can sometimes cause changes in other
abiotic factors in the environment. For example, the growing human population needs more
energy, much of which is provided by burning fossil fuels.
Recall from Lesson 15 that burning fossil fuels plays a key role in Earth's carbon-oxygen
cycle. Combustion removes oxygen from the air and returns carbon dioxide to the air as a
waste product. Over time, the increased demand for energy has resulted in an increase in
the amount of carbon dioxide in the air. Carbon dioxide in the atmosphere plays an
important role in regulating temperatures on Earth. Carbon dioxide keeps some of the heat
that is reflected from Earth's surface from escaping back into space. Trapping heat, known
as the greenhouse effect, helps keep temperatures at Earth's surface in a range that is
suitable for life. However, as the graph below shows, both atmospheric CO2 and average
global temperatures have increased since the mid-1800s. Most scientists think these
increases are due to the increased use of fossil fuels. You will learn more about the impact
of rising worldwide temperatures, or global warming, in the next lesson.