Lesson 15.1: The Science of Ecology

Lesson Objectives

State what ecologists study, and identify levels of organization in ecology.

Define ecosystem, niche, and habitat, and explain how the concepts are related.

Describe methods of ecology, such as field studies, sampling, statistical analysis, and modeling.

 

Introduction

Ecology is the scientific study of the interactions of living things with each other and their relationships with the environment. Ecology is usually considered to be a major branch of biology. However, ecology has a more broad scope, because it includes both organisms and their environments. Examining the interactions between organisms and the environment can provide a basic understanding of the richness of life on earth and can help us understand how to protect that richness, which is increasingly threatened by human activity. Regardless of the challenges associated with conducting research in natural environments, ecologists often carry out field experiments to test their hypotheses.

 

 

Organisms and the Environment

Ecology is guided by a number of basic principles. One principle is that each living organism has a continual relationship with every other element in its environment. In this context, the environment includes both living and nonliving components.

 

 

Organisms

An organism is a life form consisting of one or more cells. All organisms have properties of life, including the ability to grow and reproduce. These properties of life require energy and materials from the environment. Therefore, an organism is not a closed system. Individual organisms depend on and are influenced by the environment.

 

The Environment

To the ecologist, the environment of an organism includes both physical aspects and other organisms. These two components of the environment are called abiotic and biotic compo- nents, respectively.

 

 

Abiotic components, or abiotic factors, are the non-living physical aspects of the environment. Examples include sunlight, soil, temperature, wind, water, and air.

Biotic components, or biotic factors, are the living organisms in the environment. They include organisms of the same and different species.

 

Biotic components can be very important environmental influences on organisms. For ex- ample, the first photosynthetic life forms on Earth produced oxygen, which led to the de- velopment of an oxygen-rich atmosphere (see the History of Life Chapter). This change in Earth’s atmosphere, in turn, caused the extinction of many life forms for which oxygen was toxic and the evolution of many other life forms for which oxygen was necessary.

 

Levels of Organization

Ecologists study organisms and their environments at different levels. The most inclusive level is the biosphere. The biosphere consists of all the organisms on planet Earth and the areas where they live. It occurs in a very thin layer of the planet, extending from about 11,000 meters below sea level to 15,000 meters above sea level. An image of the biosphere is shown in Figure 15.1. Different colors on the map indicate the numbers of food-producing organisms in different parts of the biosphere. Ecological issues that might be investigated at the biosphere level include ocean pollution, air pollution, and global climate change.

Ecologists also study organisms and their environments at the population level. A popula- tion consists of organisms of the same species that live in the same area and interact with one another. You will read more about populations in the Populations chapter. Important ecological issues at the population level include:

 

rapid growth of the human population, which has led to overpopulation and environ- mental damage;

 

 

Figure 15.1: This image of Earth’s surface shows the density of the chief life forms that produce food for other organisms in the biosphere. Plants are the chief food producers on land, and phytoplankton are the chief food producers in the ocean. The map shows the density of plants with a measure called the normalized difference vegetation index and the density of phytoplankton with the chlorophyll concentration. (14)

 

rapid decline in populations of many nonhuman species, which has led to the extinction of numerous species.

 

Another level at which ecologists study organisms and their environments is the community level. A community consists of populations of different species that live in the same area and interact with one another. For example, populations of coyotes and rabbits might interact in a grassland community. Coyotes hunt down and eat rabbits for food, so the two species have a predator-prey relationship. Ecological issues at the community level include how changes in the size of one population affect other populations. The Populations chapter discusses population interactions in communities in detail.

 

Ecosystem

A community can also be defined as the biotic component of an ecosystem. An ecosystem is a natural unit consisting of all the living organisms in an area functioning together with all the nonliving physical factors of the environment. The concept of an ecosystem can apply to units of different sizes. For example, a large body of fresh water could be considered an ecosystem, and so could a small piece of dead wood. Both contain a community of species that interact with one another and with the abiotic components of their environment. Another example of an ecosystem is a desert, like the one shown in Figure 15.2.

Like most natural systems, ecosystems are not closed, at least not in terms of energy. Ecosys- tems depend on continuous inputs of energy from outside the system. Most ecosystems ob- tain energy from sunlight. Some obtain energy from chemical compounds. In Lesson 2, you will read how energy is transferred in ecosystems. In contrast to energy, matter is recycled in ecosystems. Elements such as carbon and nitrogen, which are needed by living organisms, are used over and over again. You will read how elements and water are recycled through ecosystems in Lesson 3.

 

 

Niche

One of the most important ideas associated with ecosystems is the niche concept. A niche refers to the role of a species in its ecosystem. It includes all the ways species’ members interact with the abiotic and biotic components of the ecosystem.

Two important aspects of a species’ niche include the food it eats and how it obtains the food. Figure 15.3 shows pictures of birds that occupy different niches. The various species eat different types of food and obtain the food in different ways. Notice how each species has evolved a beak that suits it for these aspects of its niche.

Figure 15.2: This desert ecosystem in southern California has fewer species than most other types of ecosystems, but it is still home to a community of interacting species (such as the cacti and grasses shown here) and potent environmental factors such as extreme heat and dryness. (12)

 

Habitat

Another aspect of a species’ niche is its habitat. A species’ habitat is the physical environ- ment to which it has become adapted and in which it can survive. A habitat is generally described in terms of abiotic factors, such as the average amount of sunlight received each day, the range of annual temperatures, and average yearly rainfall. These and other factors in a habitat determine many of the traits of the organisms that can survive there.

Consider a habitat with very low temperatures. Mammals that live in the habitat must have insulation to help them stay warm. Otherwise, their body temperature will drop to a level that is too low for survival. Species that live in these habitats have evolved fur, blubber, and other traits that provide insulation in order for them to survive in the cold.

Human destruction of habitats is the major factor causing other species to decrease and become endangered or go extinct. Small habitats can support only small populations of organisms. Small populations are more susceptible to being wiped out by catastrophic events from which a large population could bounce back. Habitat destruction caused the extinction of the dusky seaside sparrow shown in Figure 15.4. Many other bird species are currently declining worldwide. More than 1,200 species face extinction during the next century due mostly to habitat loss and climate change.

 

 

Figure 15.3: Each of these 11 species of birds has a distinctive beak that suits it for its particular niche. For example, the long slender beak of the Nectarivore allows it to sip nectar from flowers, and the short sturdy beak of the Granivore allows it to crush hard, tough grains. (1)

 

 

Figure 15.4: The dusky seaside sparrow, which used to live in marshy areas of southern Florida, was declared extinct in 1990. (16)

 

Competitive Exclusion Principle

A given habitat may contain many different species, each occupying a different niche. How- ever, two different species cannot occupy the same niche in the same geographic area for very long. This is known as the competitive exclusion principle. It is another basic principle of ecology. If two species were to occupy the same niche, they would compete with one another for the same food and other resources in the environment. Eventually, one species would outcompete and replace the other.

Humans often introduce new species into areas where their niches are already occupied by native species. This may occur intentionally or by accident. Consider the example of kudzu. Kudzu is a Japanese vine that was introduced intentionally to the southeastern United States in the 1870s to help control soil erosion. The southeastern United States turned out to be a perfect habitat for kudzu, because it has no natural enemies there. As a result, kudzu was able to outcompete native species of vines and take over their niches. The extent to which kudzu has invaded some habitats in the southeastern United States is shown in Figure 15.5.

 

 

Figure 15.5: Kudzu covers the trees in this habitat near Atlanta, Georgia, in the southeastern United States. Native species of vines cannot compete with kudzu’s thriving growth and lack of natural enemies. (15)

 

Methods of Ecology

Ecology is more holistic, or all-encompassing, than some other fields of biology. Ecologists study both biotic and abiotic factors and how they interact. Therefore, ecologists often use methods and data from other areas of science, such as geology, geography, climatology, chemistry, and physics. In addition, researchers in ecology are more likely than researchers in some other sciences to use field studies to collect data.

Field Studies

Ecological research often includes field studies because ecologists generally are interested in the natural world. Field studies involve the collection of data in real-world settings, rather than in controlled laboratory settings. The general aim of field studies is to collect observations in wild populations without impacting the environment or its organisms in any way.

Ecologists commonly undertake field studies to determine the numbers of organisms of par- ticular species in a given geographic area. Such studies are useful for a variety of purposes. For example, the data might help an ecologist decide whether a given species is in danger of extinction.

 

Sampling

In field studies, it usually is not possible to investigate all the organisms in an area. Therefore, some type of sampling scheme is generally necessary. For example, assume an ecologist wants to find the number of insects of a particular species in a given area. There may be thousands of members of the species in the area. So, for practical reasons, the ecologist might count only a sample of the insects. In order to select the sample, the ecologist could divide the entire area into a grid of one-meter-square test plots. Then the ecologist might systematically select every tenth (or other numbered) test plot and count all the insects in the plot.

 

Statistical Analysis

Like other scientists, ecologists may use two different types of statistical analysis to interpret the data they collect: descriptive statistics and inferential statistics. Descriptive statistics are used to describe data. For example, the ecologist studying insects might calculate the mean number of insects per test plot and find that it is 24. This descriptive statistic summa- rizes the counts from all the test plots in a single number. Other descriptive statistics, such as the range, describe variation in data. The range is the difference between the highest and lowest values in a sample. In the same example, if the numbers of insects per test plot ranged from 2 to 102, the range would be 100.

Scientists often want to make inferences about a population based on data from a sample. For example, the ecologist counting insects might want to estimate the number of insects in the entire area based on data for the test plots sampled. Drawing inferences about a population from a sample requires the use of inferential statistics. Inferential statistics can be used to determine the chances that a sample truly represents the population from which it was drawn. It tells the investigator how much confidence can be placed in inferences about the population that are based on the sample.

Modeling

Ecologists, like other scientists, often use models to help understand complex phenomena. Ecological systems are often modeled using computer simulations. Computer simulations can incorporate many different variables and their interactions. This is one reason they are useful for modeling ecological systems. Computer simulations are also working models, so they can show what may happen in a system over time. Simulations can be used to refine models, test hypotheses, and make predictions. For example, simulations of global warming have been used to make predictions about future climates.

 

Lesson Summary

Ecology is the scientific study of living things and their relationships with the environ- ment. Levels of organization in ecology include the biosphere, population, community, and ecosystem.

An ecosystem is a natural unit consisting of all the living organisms in an area function- ing together with all the non-living physical factors of the environment. Each species has a unique role in an ecosystem, called its niche. The physical environment where a species lives is its habitat.

Ecologists use field studies and sampling schemes to gather data in natural environ- ments. Like other scientists, ecologists use statistics to describe and make inferences from data. They also use computer simulations to model complex phenomena.

 

Review Questions

 

Further Reading / Supplemental Links

Define abiotic and biotic components of the environment.

What does the biosphere consist of?

How do ecologists define an ecosystem?

What does the competitive exclusion principle state?

Assume an ecologist is studying interactions among different species in an ecosystem. What level of organization should the ecologist study? Why?

Why are field studies and computer simulations important methods of investigation in ecology?

Compare and contrast the ecosystem concepts of niche and habitat.

Desonie, Dana, Biosphere: Ecosystems and Biodiversity Loss. Chelsea House Publica- tions, 2007.

Novacek, Michael, Terra: Our 100-Million-Year-Old Ecosystem and the Threats That Now Put It at Risk. Farrar, Straus, and Giroux, 2007.

http://estrellamountain.edu/faculty/farabee/biobk/BioBookcommecosys.html

http://estrellamountain.edu/faculty/farabee/biobk/BioBookpopecol.html

http://green.nationalgeographic.com/environment/global-warming/gw-overview. html

http://www.science.doe.gov/ober/CCRD/model.html

http://interactive2.usgs.gov/learningweb/explorer/topic_eco.htm

http://www.dmoz.org/Science/Biology/Ecology/

http://www.enviroliteracy.org/category.php/3.html

http://www.sciencedaily.com/news/earth_climate/ecosystems/

http://www.soinc.org/events/ecology/index.htm

http://www.topix.net/science/ecology

http://en.wikipedia.org

 

Vocabulary

abiotic components The non-living physical aspects of the environment; includes sun- light, soil, temperature, wind, water, and air; also known as abiotic factors.

 

biosphere The areas of Earth where all organisms live; extends from about 11,000 meters below sea level to 15,000 meters above sea level.

 

biotic components The living organisms in the environment; also known as biotic factors.

 

community Populations of different species that live in the same area and interact with one another.

 

competitive exclusion principle States that two different species cannot occupy the same niche in the same geographic area for very long.

 

descriptive statistics Statistical analysis used to describe data.

 

ecology The scientific study of the interactions of living things with each other and their relationships with the environment.

 

ecosystem A natural unit consisting of all the living organisms in an area functioning together with all the nonliving physical factors of the environment.

 

field studies Studies that involve the collection of data in real-world settings, rather than in controlled laboratory settings; allows observations of wild populations without im- pacting the environment or its organisms in any way.

habitat The physical environment to which an organism has become adapted and in which it can survive.

 

inferential statistics Statistical analysis that draws inferences about a population from a sample; used to determine the chances that a sample truly represents the population from which it was drawn.

 

niche The role of a species in its ecosystem; includes all the ways species’ members interact with the abiotic and biotic components of the ecosystem.

 

organism A life form consisting of one or more cells.

 

population Organisms of the same species that live in the same area and interact with one another.

 

range Statistic used to describe the difference between the highest and lowest values in a sample.

 

Points to Consider

An ecosystem needs continuous inputs of energy in order for its organisms to survive. In most ecosystems, this energy comes from sunlight.

 

Which organisms in an ecosystem capture the energy from sunlight? How do they transform the energy so that other organisms in the ecosystem can use it? Why is the energy that enters an ecosystem eventually used up?