CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

15. Ecosystem Dynamics. The Flow of Energy and Matter

Fertilizer on Lawns Causes Water Pollution

You may be able to help solve the problem.

In urban areas, lawn care is big business. Millions of dollars are spent annually for fertilizer and pesticides and landscape services to maintain lawns. It is estimated that 15-30% of the fertilizer applied to lawns is washed from the soil and ends up in local streams, ponds, and lakes. Most fertilizers contain nitrogen, phosphorus, and potassium. Of these three components, phosphorus is the most detrimental to aquatic ecosystems because it stimulates the growth of algae and aquatic plants that foul the water and make it unappealing for recreational purposes. Furthermore, in the winter the algae and plants die and decay. The bacteria that bring about their decomposition use oxygen from the water and often lower the oxygen level of the water so much that aquatic animals die from a lack of oxygen.

In order to control pollution of local water bodies in urban areas, the State of Minnesota (“Land of 10,000 Lakes”) has passed a law that prohibits the use of phosphorus in fertilizer that is to be applied to lawns. This ban does not extend to agriculture.

• Can lawns be maintained without the use of fertilizer?

• Why does phosphorus stimulate the growth of algae and aquatic plants more than nitrogen?

• In order to improve local water quality, would you vote to restrict the use of fertilizer on lawns?

ü  Background Check

Concepts you should already know to get the most out of this chapter:

• What energy is and how it is related to matter (chapter 2)

• How the atoms of various elements differ in structure (chapter 2)

15.1. What Is Ecology?

People often use the terms ecology and environment as if they meant the same thing. Students, homeowners, politicians, planners, and union leaders speak of “environmental issues” and “ecological concerns.” They speak of products that are “green” or “environmentally friendly” and activities that will do “ecological damage.” However, scientists use the terms ecology and environment in a more restricted way.

Ecology is the branch of biology that studies the relationships between organisms and their environments. This is a very simple definition for a very complex branch of science. Throughout the next four chapters, we will explore many of the aspects of this interesting topic. Because the word environment is included in this definition of ecology, it is important to have a clear understanding of how an ecologist uses the term. Most ecologists define the word environment very broadly as anything that affects an organism during its lifetime. These environmental influences can be divided into two broad categories: biotic environmental factors and abiotic environmental factors.

The field of environmental science is related to ecology. While environmental science is based on ecology, it is an applied science that looks at the impact of humans on their surroundings. Thus, politics, social interactions, economics, and other aspects of human behavior are important aspects of environmental science.

Biotic and Abiotic Environmental Factors

Biotic factors are living things that affect an organism. You are affected by many different biotic factors. Your classmates, disease organisms, the food you eat, and the trees you seek for shade are all biotic factors. Abiotic factors are nonliving things that affect an organism. Common abiotic factors are wind, rain, the composition of the atmosphere, minerals in the soil, sunlight, temperature, and elevation above sea level (figure 15.1).

FIGURE 15.1. Biotic and Abiotic Environmental Factors

(a) The sticks and branches bald eagles use to build their nest are part of their biotic environment. The pine tree in which this nest is built is also part of the eagle’s biotic environment. (b) The irregular shape of the tree is the result of wind, an abiotic factor that tends to sandblast one side of the tree and prevent limb growth on that side.

Characterizing the environment of an organism is a complex and challenging process. There are many things to be considered, and everything seems to be influenced or modified by other factors. For example, consider a fish in a stream; many environmental factors are important to its life. The temperature of the water is extremely important as an abiotic factor, but the temperature may be influenced by the presence of trees (biotic factor) along the stream bank that shade the stream and prevent the Sun from heating it. Obviously, the kind and number of food organisms in the stream are important biotic factors as well. The type of material that makes up the stream bottom—mud, sand, or gravel—and the amount of oxygen dissolved in the water are other important abiotic factors, both of which are related to how rapidly the water is flowing.

Similarly, a plant is influenced by many factors during its lifetime. The types and amounts of minerals in the soil, the amount of sunlight hitting the plant, and the amount of rainfall are important abiotic factors. The animals that eat the plant and the fungi that cause disease are important biotic factors. Each item on this list can be further subdivided. For instance, because plants obtain water from the soil, rainfall is studied in plant ecology. But even the study of rainfall is not simple. In some places, it rains only during one part of the year; in other places, it rains throughout the year. Some places experience hard, driving rains, whereas others experience long, misty showers. The kind of rainfall affects how much water soaks into the ground—heavy rains tend to run off into streams and be carried away, whereas gentle rain tends to sink into the soil.

Levels of Organization in Ecology

Ecologists study ecological relationships at several levels of organization. Some ecologists focus on what happens to individual organisms and how they interact with their surroundings. Others are interested in groups of organisms of the same species, called populations, and how they change. Interacting populations of different species are called communities, and community ecologists are interested in how various kinds of organisms interact in a specific location. The highest level of ecological organization is the ecosystem, which consists of all the interacting organisms in an area and their interactions with their abiotic surroundings. Figure 15.2 shows how these levels of organization are related to one another.

FIGURE 15.2. Levels of Organization in Ecology

Ecology is the branch of biology that studies the interactions between organisms and their environments. This study can take place at several levels, from the broad ecosystem level through community interactions to population studies and the study of individual organisms. Ecology also involves the study of the physical environment, which makes up the nonliving parts of an ecosystem.

Understanding the ecological relationships of any species of organism involves the accumulation of large amounts of detailed information about the organism and how it interacts with its surroundings. Although this task may seem impossible, ecologists recognize three broad concepts that help simplify the task:

1. Each organism is part of one or more food chains.

2. The organisms in food chains can be separated into functional units called trophic levels based on how they obtain energy.

3. It is possible to trace the flow of energy and matter through ecosystems.

15.1. CONCEPT REVIEW

1. List three abiotic factors in the environment of a fish.

2. List three biotic factors in the environment of a songbird.

3. How do an ecosystem, a population, and a community differ?