CONCEPTS IN BIOLOGY
PART IV. EVOLUTION AND ECOLOGY
15. Ecosystem Dynamics. The Flow of Energy and Matter
15.5. Human Use of Ecosystems
The extent to which humans use an ecosystem is tied to its productivity. Productivity is the rate at which an ecosystem can accumulate new organic matter. Because plants are the producers, it is their activities that are most important. Ecosystems in which the conditions are the most favorable for plant growth are the most productive. Warm, moist, sunny areas with high levels of nutrients in the soil are ideal. Some areas have low productivity because one of these essential factors is missing. Deserts have low productivity because water is scarce, arctic areas because temperature is low, and the open ocean because nutrients are in short supply. Some terrestrial ecosystems, such as forests and grasslands, have high productivity. Aquatic ecosystems, such as marshes and estuaries, are highly productive, because the waters running into them are rich in the nutrients that aquatic photosynthesizers need. Furthermore, these aquatic systems are usually shallow, so that light can penetrate through most of the water column.
The Conversion of Ecosystems to Human Use
The way humans use ecosystems has changed dramatically over the past several thousand years. Initially, humans fit into ecosystems as just another consumer. These kinds of societies are known as hunter-gatherer societies because they collect food and other needed materials directly from the plants and animals that are a natural part of the ecosystem. There are still examples of peoples who live this way (figure 15.13).
FIGURE 15.13. A Hunter-Gatherer
This Australian aboriginal hunter functions as a carnivore and uses the natural ecosystem as a source of energy and materials.
However, the development of agriculture has changed how humans interact with other organisms in ecosystems. We have altered certain ecosystems substantially to increase productivity for our own purposes. In so doing, we have destroyed the original ecosystem, with all of its complexity, and have replaced it with a simpler agricultural ecosystem. In addition, many of the crops we plant are not native to the region. For example, nearly all of the Great Plains region of North America has been converted to agriculture. The original ecosystem included the Native Americans, who used buffalo and other plants and animals as a source of food. There was much grass, many buffalo, and few humans. Therefore, in the Native Americans’ pyramid of energy, the base was more than ample. However, with the exploitation and settling of America, the population in North America increased at a rapid rate. The food chain (prairie grass-buffalo-human) could no longer supply the food needs of the growing population. Because wheat and corn yield more biomass for humans than the original prairie grasses could, the settlers’ domestic grain and cattle replaced the prairie grass and buffalo. This was fine for the settlers, but devastating for the buffalo and Native Americans (figure 15.14). In similar fashion, the deciduous forests of the East were cut down and burned to provide land for crops. The crops were able to provide more food than did harvesting game and plants from the forest.
FIGURE 15.14. The Conversion of Prairie to Agricultural Production
North America’s Great Plains changed from a natural prairie ecosystem to an agricultural ecosystem either for raising crops or for grazing livestock.
Associated with modern agriculture is the use of fertilizer and other agricultural chemicals. Fertilizers usually contain nitrogen, phosphorus, and potassium compounds. The numbers on a fertilizer bag indicate the percentage of each in the fertilizer. For example, a 6-24-24 fertilizer has 6% nitrogen, 24% phosphorus, and 24% potassium compounds. In addition to nitrogen, phosphorus, and potassium, other elements including calcium, magnesium, sulfur, boron, copper, and zinc are cycled within ecosystems. In an agricultural ecosystem, these elements are removed when the crop is harvested. Therefore, farmers must not only return the nitrogen, phosphorus, and potassium to the soil but also analyze for other, less-prominent elements and add them to their fertilizer mixture. Aquatic ecosystems are also sensitive to nutrient levels. High levels of nitrates or phosphorus compounds often result in the rapid growth of aquatic producers. In aquaculture, such as that used to raise catfish, fertilizer is added to the body of water to stimulate the production of algae, which is the base of most aquatic food chains (see Outlooks 15.2).
Many ecosystems, particularly the drier grasslands, cannot support the raising of crops. However, they can still be used as grazing land to raise livestock. Like the raising of crops, grazing often significantly alters the original grassland ecosystem. Some attempts have been made to harvest native species of animals from grasslands, but the primary species raised on grasslands are domesticated cattle, sheep, and goats. The substitution of the domesticated animals displaces the animals that are native to the area and alters the kinds of plants present, particularly if too many animals are allowed to graze.
Even aquatic ecosystems have been significantly altered by human activity. The Food and Agriculture Organization of the United Nations states that nearly all the fisheries of the world are being fished at capacity or overfished. Overfishing in many areas of the ocean has resulted in the loss of some important commercial species. For example, the codfishing industry along the East Coast of North America has been destroyed by overfishing.
OUTLOOKS 15.2
Dead Zones
Dead zones are regions of the ocean bottom that have little or no oxygen dissolved in the water. Throughout the world there are about 400 such zones; most located near the mouths of rivers. Two things contribute to the development of dead zones: poor mixing of the water in the area and an input of nutrients from rivers. Rivers carry nutrients from the lands they drain and the nutrients stimulate the growth of phytoplankton (microscopic, single-celled, photosynthetic organisms) in the upper regions of the water column. When these organisms die they sink to the bottom where bacteria bring about their decay. The bacteria use oxygen from the water in the decay process and the amount of oxygen falls. As the oxygen level falls, animals are stressed. Those that are able to swim or crawl leave the area, and those that cannot, die. The result is an area on the ocean floor that is devoid of life.
Dead zones typically develop in the summer months when the water is warm. The combination of warm water and abundant nutrients results in rapid growth of phytoplankton. It appears that the primary source of the excess nutrients is related to the way humans use the land drained by rivers. Fertilizer from agriculture and lawns runs off the land into streams and rivers. Animal waste from cattle feedlots, hog farms, and chicken-raising facilities is often spread on land as fertilizer and washes into streams and rivers. Other animal waste enters streams as a result of poorly designed containment lagoons that fail. Nutrients from human wastes can also enter from sewage treatment plants. All of these sources of nutrients contribute to the problem. A major dead zone develops every year in the Gulf of Mexico near the mouth of the Mississippi River. Most years the area affected is about the size of the state of New Jersey.
The problem is not just of concern to those worried about the environmental impact. It is important economically, since large dead zones impact commercial and recreational fishing.
The Energy Pyramid and Human Nutrition
Anywhere in the world the human population increases, natural ecosystems are replaced with agricultural ecosystems. In many parts of the world, the human demand for food is so large that it can be met only if humans occupy the herbivore trophic level, rather than the carnivore trophic level. Humans are omnivores able to eat both plants and animals as food, so they have a choice. However, as the size of the human population increases, it cannot afford the 90% loss that occurs when plants are fed to animals that are in turn eaten by humans. In much of the less-developed world, the primary food is grain; therefore, the people are already at the herbivore level. It is only in the developed countries that people can afford to eat large quantities of meat. This is true from both an energy point of view and a monetary point of view. Meat, fish, poultry and other sources of animal protein are more expensive than grains. (Most of the corn raised in the United States is used as cattle feed). Figure 15.15 shows a pyramid of biomass having a producer base of 100 kilograms of grain. The second trophic level has only 10 kilograms of cattle because of the 90% energy loss typical when energy is transferred from one trophic level to the next. The consumers at the third trophic level—humans, in this case—experience a similar 90% loss. Therefore, only 1 kilogram of humans can be sustained by the two-step energy transfer. There has been a 99% loss in energy: 100 kilograms of grain are necessary to sustain 1 kilogram of humans. Because much of the world’s population is already feeding at the second trophic level, we cannot expect food production to increase to the extent that we could feed 10 times as many people as exist today.
FIGURE 15.15. Human Pyramids of Biomass
Because approximately 90% of the energy is lost as energy passes from one trophic level to the next, more people can be supported if they eat producers directly than if they feed on herbivores. Much of the less-developed world is in this position today. Rice, corn, wheat, and other producers provide most of the food for the world’s people.
It is difficult for most people to fulfill all their nutritional needs by eating only grains. Although protein is available from plants, the concentration is greater from animal sources and people who rely primarily on plants for food often experience protein deficiency. People in major parts of Africa and Asia have diets that are deficient in both calories and protein. These people have very little food, and what food they do have is mainly from plant sources.
These are also the parts of the world where human population growth is the most rapid. In other words, these people are poorly nourished, and as the population increases they will probably experience greater calorie and protein deficiency. It is important to realize that currently there is enough food in the world to feed everyone, but it is not distributed equitably for a variety of reasons. The primary reasons for starvation are political and economic. Wars and civil unrest disrupt the normal foodraising process. People leave their homes and migrate to areas unfamiliar to them. Poor people and poor countries cannot afford to buy food from the countries that have a surplus.
15.4. CONCEPT REVIEW
15. Explain why poor people in countries with limited food must eat primarily grains. Explain this from both an economic and ecological point of view.
16. Define the term productivity.
17. What is the primary reason for humans destroying natural ecosystems like prairies and deciduous forests?
Summary
Ecology is the study of how organisms interact with their environment. The environment consists of biotic and abiotic components, which are interrelated in an ecosystem. All ecosystems must have a constant input of energy from the Sun. Producer organisms are capable of trapping the Sun’s energy, through photosynthesis, and converting it into the energy in biomass. Consumers (herbivores, carnivores, and omnivores) eat other organisms. Herbivores feed on producers and are, in turn, eaten by carnivores, which may be eaten by other carnivores. Each level in the food chain is known as a trophic level. Other kinds of organisms involved in food chains are omnivores, which eat both plant and animal food, and decomposers, which break down dead organic matter and waste products.
All ecosystems have a large producer base with successively smaller amounts of energy at the herbivore, primary carnivore, and secondary carnivore trophic levels. This is because, each time energy passes from one trophic level to the next, about 90% of the energy is lost from the ecosystem.
The amount of matter in the world does not change but, rather, is recycled. The carbon cycle involves the processes of photosynthesis and respiration in the cycling of carbon through ecosystems. Water is essential to living things, most of the cycling of water involves the physical processes of evaporation and condensation. The nitrogen cycle relies on the activities of nitrogen-fixing bacteria, nitrifying bacteria, and decomposers to cycle nitrogen through ecosystems. The phosphorus cycle involves the deposition of phosphorus-containing compounds into oceans and the geologic processes of uplift and erosion to make phosphorus available to organisms.
Humans use ecosystems to provide themselves with necessary food and raw materials. As the human population increases, most people will be living as herbivores at the second trophic level, because they cannot afford to lose 90% of the energy by first feeding it to an herbivore, which they then eat. Humans have converted most productive ecosystems to agricultural production and continue to seek more agricultural land as populations increase.
Basic Review
1. Which one of the following is an abiotic factor?
a. a nest in a tree
b. the water in a pond
c. the producers in an ecosystem
d. the fish in a pond
2. Which one of the following categories of organisms has the largest total energy and biomass?
a. eagles, which eat fish
b. herbivores, which eat plants
c. organisms that carry on photosynthesis
d. fish that eat insects
3. The carbon that plants need for photosynthesis comes from _____.
4. Symbiotic nitrogen-fixing bacteria
a. live in association with the roots of certain plants.
b. convert ammonia to nitrate.
c. are found in the atmosphere.
d. are rare.
5. The process of absorbing water from the soil and releasing it from leaves is called _____.
6. In the phosphorus cycle, phosphorus enters plants through the roots. (T/F)
7. When energy flows from one trophic level to the next, about _____ percent of the energy is lost.
8. An herbivore is at the second trophic level. (T/F)
9. Nitrogen is important in which one of the following organic molecules?
a. sugars
b. fats
c. water
d. proteins
10. Decomposers break down organic matter and release _____ and _____.
11. Which one of the following is a producer?
a. earthworm
b. algae
c. yeast
d. fungus
12. Which of the following populations in an ecosystem would have the highest biomass?
a. insect-eating birds
b. fish-eating birds
c. fungi
d. plants
13. A vegetarian is at the _____ trophic level.
14. Which of the following is the largest conceptual unit?
a. ecosystem
b. community
c. decomposers
d. producers
15. Humans have altered most ecosystems of the world. (T/F)
Answers
1. b 2. c 3. carbon dioxide 4. a 5. transpiration 6. T 7. 90% 8. T 9. d 10. carbon dioxide and water. 11. b 12. d 13. herbivore 14. a 15. T
Thinking Critically
Understanding Interrelationships
Construct a diagram on a piece of paper that includes the following items to show their levels of interaction. Which is the most important item? Which items are dependent on others?
People are starving.
Commercial fertilizer production requires temperatures of 900°C.
Geneticists have developed plants that grow very rapidly and require high amounts of nitrogen to germinate during the normal growing season.
Fossil fuels are stored organic matter.
The rate of the nitrogen cycle depends on the activity of bacteria.
The sun is expected to last for several million years.
Crop rotation is becoming a thing of the past.
The clearing of forests for agriculture changes the weather in the area.