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.

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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.

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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.

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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.

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