Systems and Cycles of the Biosphere

Chapter 22

Systems and Cycles of the Biosphere

OVERVIEW

This chapter is the first of three chapters on biogeography. Biogeography explores the distribution of plants and animals on the Earth. This chapter examines how organisms live in ecosystems and the cycling of energy and matter through ecosystems.

• Ecology studies the interaction between life forms and their environments.

• An ecosystem is defined as a group of organisms and the environment with which they interact. These systems import and export matter and energy.

• The food web, or food chain, refers to the flow of energy from one level to another in an ecosystem.

• Primary producers are plants and animals that are able to create carbohydrates from carbon dioxide and water and light energy through the process of photosynthesis.

• In the food web, consumers feed on the primary producers or on other consumers and transfer energy through different levels in this manner.

• Decomposers (microorganisms and bacteria) feed on decaying organic matter at all levels in the food web.

• Solar energy is absorbed initially by the primary producers and stored as chemical energy which is digested by consumers. Only ten to fifty percent of the energy at any level is passed on to the next level; consequently, the amount of organic matter and consumers must decrease with each level.

• Photosynthesis is a biochemical reaction which results in the production of carbohydrates and oxygen using water, carbon dioxide, and light energy. A simplified chemical reaction is:

H2O + CO2 + light energy = —CHOH— + O2

• In the respiration process carbohydrate is broken down and combined with oxygen to create carbon dioxide, water, and chemical energy. A simplified chemical reaction is

—CHOH— + O2 = CO2 + H2O + chemical energy

• Photosynthesis is dependent on light and heat. Photosynthesis only occurs when light is available, so longer days produce more plant growth. Photosynthesis also increases with temperature to about 20°C and then levels off.

• Net photosythesis is measured as the carbohydrate remaining after respiration takes up carbohydrate to feed the plant. Net photosynthesis increases with temperature until approximately 18° C, after which it declines as the rate of respiration increases faster than the rate of photosynthesis.

• Net primary production is the annual amount of useful energy produced by an ecosystem. It is controlled by light intensity and duration, temperature, and water availability. Net primary production is measured as biomass, the dry weight of organic matter per unit area within an ecosystem.
• Biomass is an important source of renewable energy. Using biomass involves releasing solar energy that has been stored in plant tissue through photosynthesis. Energy can be obtained by burning firewood, or through intermediate products such as charcoal, methane gas, and alcohol.

• Biochemical cycles are the pathways of particular nutrients or materials through the Earth's ecosystem.

• The macronutrients hydrogen, carbon, and oxygen account for 99.5% of all living matter.

• The Carbon Cycle

• most carbon lies in storage pools as carbonate sediments

• only 0.2% is available as CO2 or as decaying biomass in active pools.

• carbon exists as carbon dioxide in the atmosphere and oceans, carbohydrate in organic matter, hydrocarbon compounds in rock, and as mineral carbonate compounds.

• CO2 is added to the Earth system by volcanic eruptions and by industry: it is taken out of the Earth system by plants in photosynthesis and by phytoplankton in the oceans.

• The Oxygen Cycle: oxygen is added to the Earth system by volcanic activity and is lost to the system through organic respiration, mineral oxidation, industrial and natural combustion, and dissolved in ocean water.

• The Nitrogen Cycle

• the atmosphere is a large storage pool of nitrogen.

• nitrogen can only be utilized through nitrogen fixation and is lost to the biosphere through denitrification.

• human influence has increased the amount of nitrogen in the biosphere through the use of nitrogen fertilizers and fuel combustion.

• Sedimentary cycles involve many macronutrients such as calcium, magnesium, iron, potassium, sodium, and phosphorus which move from the land surface to the ocean and subsequently return to land surfaces by tectonic uplift. Storage pools include sea water, sediments, and sedimentary rocks. Eventually these macronutrients are released into the Earth system through weathering.

KEY TERMS

STUDY QUESTIONS

1. Why are geographers interested in the biosphere?
2. What plants and animals would be found at the different levels of the food web in an ecosystem close to your home?
3. What inorganic compounds would be part of this ecosystem?
4. Describe how photosynthesis works. How is photosynthesis affected by heat and light?
5. Describe the process of respiration. How it respiration affected by heat and light?
6. How does net photosynthesis respond to increasing light intensity and increasing heat? Why?
7. What climatic factors affect rates of net primary productivity?
8. What is a biogeochemical cycle? Define sedimentary cycle, gaseous cycle, active pool, and storage pool.
9. Where are the storage pools of carbon in the Earth system? Where are the active pools of carbon?
10. How does human activity reduce the amount of oxygen in the atmosphere?
11. Why is nitrogen fixation important? What is denitrification?
12. What impact has the use of fertilizers had on the nitrogen cycle?

CHAPTER QUIZ

Multiple Choice Questions

1. Plants and animals that can use light energy to convert CO2 and H2O to carbohydrate are:

a)consumers

b)primary producers

c)decomposers

d)detritus

1. The highest average rates of biomass production on land occur:

a)in the midlatitude forest

b)on agricultural land

c)in the midlatitude grassland

d)in the equatorial rainforest

1. The most abundant element in global living matter is:

a)oxygen

b)carbon

c)hydrogen

d)nitrogen

1. Which of the following macronutrients moves only in sedimentary cycles?

a)nitrogen

b)carbon

c)calcium

d)oxygen

1. An ecosystem is a(an) ______system with respect to energy and a(an) ______system with respect to matter.

a)closed; closed

b)closed; open

c)open; open

d)open; closed

True/False Questions

1.Normally, there are four levels of consumers in a food web. (T/F)

2.Net photosynthesis increases steadily as light and heat increase. (T/F)

3. At present, nitrogen is accumulating in the life layer: (T/F)

4. The carbon cycle is an example of a sedimentary biogeochemical cycle. (T/F)

Short Answer Questions

1.Why are there very few top-level consumers in an ecosystem?

2.Why does primary productivity in the polar seas decline dramatically during the winter season?

3.Why does net primary production level off in very wet climates?

Short Essay Questions (1 - 2 paragraphs)

1.Why is biomass production highest in the wet equatorial climate?

2.How has human activity influenced the carbon cycle?

Internet Resources

1. The Carbon Cycle (a student project): <
2. Understanding the global carbon cycle: <
3. Carbon – an element on the move: <
4. Issues in Ecology (reports, in language understandable by non-scientists: the consensus of a panel of scientific experts on issues relevant to the environment): <
5. Biomes and Ecosystems Resources – ecology-related information on particular places and issues: <
6. Life and biogeochemical cycles: <
7. Photosynthesis and respiration: <
8. Biodiversity and ecosystem processes in terrestrial herbaceous ecosystems – results and relevance: <
9. Balancing the carbon cycle: <
10. Remote sensing net primary production: <