4

Principles of Ecology: How

Ecosystems Work

Chapter Outline

Humans and Nature: The Vital Connections

Are Humans a Part of Nature?

Ecology: The Study of Natural Systems

What is Ecology?

The Structure of Natural Systems

The Biosphere and the Importance of Recycling and Renewable Resources

Biomes and Aquatic Life Zones

Ecosystems: Abiotic and Biotic Components

Ecosystem Function

Food Chains and Food Webs

The Flow of Energy and Matter Through Ecosystems

Trophic Levels

Nutrient Cycles

The Carbon Cycle

The Nitrogen Cycle

Key Terms

ecology tundra detritivores

abiotic taiga food web

biotic temperate deciduous forest biomass

biosphere grassland nutrient cycle

photosynthesis range of tolerance nitrogen fixation

closed system zones of physiological stress competitive exclusion principle

biome zones of intolerance biomass pyramid

aquatic life zone endangered species energy pyramid

limiting factor population pyramid of numbers

niche competition carnivores

consumers producers herbivores

omnivores food chain

Objectives

1. Define “biosphere,” “biome,” and “ecosystem.”

2. Discuss “abiotic and biotic factors” and their significance in ecosystems.

3. Define the terms “range of tolerance,” “zones of physiological stress,” and “zones of intolerance.”

4. Compare the terms “habitat” and “niche.”

5. Discuss the competitive exclusion principle.

6. Diagram and label the trophic levels of a typical food web.

7. Explain why there is a pyramid of biomass, energy, and numbers in ecosystems.

8. Discuss the carbon and nitrogen cycles.

9. List the major biomes and, for each, give distinguishing characteristics and identify the major threats.

10. Explain why humans are a part of nature and are dependent on natural systems for survival.

Lecture Outline

Humans and Nature: The Vital Connections

A. Humans are a part of nature. Like all other species, humans depend on the soil, air, water, sun, and a host of living organisms to survive.

Ecology: The Study of Natural Systems

A. Ecology is the study of organisms and their relationships to one another and to the environment.

B. Environment is a term referring to our surroundings; it is not synonymous with ecology.

III. The Structure of Natural Systems

A. The biosphere or ecosphere refers to the life-supporting portion of the earth. The biosphere is a closed system because, within it, all materials are recycled and reused. Humans break these cycles at great risk.

B. Biomes are terrestrial areas with distinctive climate, soil characteristics, and plant and animal associations.

1. Precipitation and temperature are the primary controlling factors in biomes.

2. North America’s five major biomes are: tundra, taiga, temperate deciduous forest, grassland, and desert.

C. Aquatic Life Zones

1. These zones, similar to biomes, are determined primarily by sunlight penetration and nutrient availability.

2. Coral reefs, estuaries and coastal wetlands, deep ocean areas, and continental shelves are distinct aquatic life zones.

D. Ecosystems or ecological systems are dynamic networks of interdependent plants and animals in a particular environment. Ecotones are transitional areas of overlap between adjacent ecosystems. All ecosystem components are either biotic (living) or abiotic (nonliving).

1. Abiotic Factors - These include sunlight, physical factors, and chemical components. Each organism can survive only within the limits of its range of tolerance to abiotic factor fluctuation and will thrive only within the optimum range. An abiotic element, such as rainfall, temperature, or nitrate, may be the limiting factor that is primarily responsible for restricting the growth or reproduction of key organisms in an ecosystem.

2. Biotic Factors - These include all living things in an ecosystem. Groups of the same species in an area form a population; several populations living together form a community. Organisms within a community may interact through predation, commensalism, mutualism, neutralism, parasitism, or interspecific/intraspecific competition. Biotic factors can be potent forces in shaping the structure of biological communities.

a. Humans are one of the fiercest competitors in the biotic community. Our activities affect both the abiotic and biotic components of ecosystems.

IV. Ecosystem Function

A. Food Chains - A food chain is a series of organisms, each feeding on the preceding. Food chains may be either the grazer or decomposer (detritus) type.

1. Each organism is either a producer (autotroph, self-feeder) or a consumer (heterotroph, other-feeder).

2. Depending on their primary feeding habits, consumers may be classified as herbivores, carnivores, or omnivores.

B. Food Webs - A food web is a complex network of feeding interactions involving many organisms and food chains. Food web is a term that more accurately describes the feeding relationships in an ecosystem. Generally, the more complex a food web, the more stable it is.

C. Classifying Consumers - Organisms occupy feeding or trophic levels.

1. The first trophic level is occupied by producers; the second, third, and subsequent levels are occupied by primary, secondary, etc. consumers.

D. The Flow of Energy and Nutrients Through Food Webs

1. All ecosystems ultimately depend on photosynthesis by green plants for their energy. Food chains are biological avenues for the flow of energy and the cycling of nutrients in the environment.

2. Energy flows in one direction through food chains, but nutrients are recycled.

3. Nutrients in the food chains reenter the environment through waste or the decomposition of dead organisms.

E. Biomass and Ecological Pyramids

1. Dry organic matter produced by living things is termed biomass. Little biomass, usually 5-20%, actually passes from one trophic level to the next. As a result, graphic representations of ecosystems show pyramids of biomass, energy, or numbers. These pyramids have important ecological and human implications.

2. Eating lower in the food chain (grains instead of meat) has advantages for food production since 80-90% of the available energy is lost at each trophic level.

F. Nutrient Cycles - Nutrients move through the biosphere in biogeochemical cycles. Each cycle involves an environmental and an organismic phase.

1. The Carbon Cycle - Photosynthesis and cellular respiration (an oxidative process) cycle carbon and oxygen in the biosphere. Humans affect the carbon cycle through deforestation (reducing photosynthesis) and burning (oxidizing) fossil fuels.

2. The Nitrogen Cycle - Atmospheric nitrogen is made available to plants by a variety of organisms capable of nitrogen fixation. Modern farming practices are interfering with the natural cycling of nitrogen.

3. The Phosphorus Cycle - Phosphorus-rich rocks slowly release phosphates to soils and water. Artificial fertilizers replace phosphates lost to runoff; this practice often causes pollution problems in nearby aquatic systems.

Suggestions for Presenting the Chapter

· An examination of local endangered species and their habitat needs may illustrate the impact of humans on natural systems. Focus can be placed on the abiotic and biotic factors necessary for the survival of a particular endangered species.

· A field trip to explore local aquatic habitats (marshes, lakes, streams, etc.) will aid in understanding of aquatic life zones. An exercise focusing on the difference in habitat types and the adaptations necessary for life in these habitats is useful in illustration of several concepts. Food chains/webs, trophic levels, and the concept of niche can all be explored during this activity.

· Instructors might supplement their lectures with videos focusing on biomes and aquatic habitats. Students can also be assigned video viewing outside of class time in conjunction with assigned readings or worksheets.

· Students can be asked to identify threatened ecosystems. Many fine articles are found in common environmental periodicals (Audubon, National Wildlife, etc.).