Case Study Conclusion: Farming for Balance
These organic tomatoes look delicious, but is it worth choosing them over less expensive conventionally-produced tomatoes? Camille, who you read about in the beginning of the chapter, asks herself questions like this whenever she goes food shopping. If organic agricultural practices are significantly better for the environment, she would like to buy organic food products at least some of the time. But are they better? And if so, how?
For foods to be officially labeled as organic in the U.S., they must meet detailed requirements from the USDA about how they are produced. One major aspect of organic agriculture is that most synthetic pesticides and fertilizers cannot be used on crops, unlike in conventional agriculture. As you have learned, pesticides can be harmful to human health if appropriate safety precautions are not taken. But besides direct exposure to humans, pesticides can also have negative effects on the broader ecosystem. For instance, some pesticides are toxic to bees. In fact, in addition to the varroa mite, which you learned about in this chapter, scientists think that pesticides are one of the factors that have contributed to the recent dramatic reduction of honey bee populations.
What effect does loss of honey bees have on the ecosystem? As you have learned, bees and flowering plants have a mutualistic symbiotic relationship, where bees benefit by obtaining food from flowers, and the plants benefit from pollination by bees. Recall that the honey bee is the most important pollinator of crops, providing over a billion dollars of pollinating services to agriculture in the U.S. By contributing to the loss of bee populations, pesticides can disturb the ecological balance between plants and their pollinators. When pollinators are reduced, so is food yield, which affects humans and other animals in the food chain that rely on foods such as fruits, vegetables, and nuts.
This is such a serious issue that in 2017, the U.S. Environmental Protection Agency (EPA) implemented a policy to reduce the risk of pesticides on bee populations, including banning the use of certain pesticides when bees are most likely to be present. Although organic agriculture prohibits the use of most synthetic pesticides, the use of some naturally derived pesticides is allowed. Some of these compounds may also be toxic to bees, other pollinators, and natural pest predators such as ladybugs. So before Camille decides to purchase an organic food product over a conventional one because of concerns about the negative effects of pesticides on the environment, she should research whether naturally derived pesticides have been used on that product, and if so, whether they have any negative effects.
Organic agriculture also generally prohibits the use of synthetic fertilizers. These fertilizers are concentrated sources of chemical elements such as nitrogen and phosphorus. Why would a farmer want to add these elements to their crops? As you have learned, both nitrogen and phosphorus are nutrients that plants need to produce organic compounds. In fact, nitrogen is a limiting factor of plant growth in most terrestrial ecosystems. Therefore, adding these elements can increase plant growth and crop yield.
But there can be too much of a good thing. Recall that nitrogen and phosphorus are recycled through the biotic and abiotic factors in the ecosystem, as part of their respective biogeochemical cycles. Ecosystems have a delicate balance of complex interactions, and when one component changes significantly in an ecosystem, it usually causes a variety of other effects. In the case of synthetic fertilizers, the excess nutrients can run off into waterways from irrigation and rain. This is called nutrient pollution and it can seriously upset the balance in aquatic biomes. For instance, excess nitrogen and phosphorus in bodies of water such as rivers and lakes can cause the overgrowth of algae. The overgrowth of algae can clog waterways, block sunlight to deeper levels, and use up dissolved oxygen. In turn, this can kill other aquatic organisms such as fish. This process is called eutrophication, and you can see an example in the picture below. Besides seriously disrupting the ecosystem, eutrophication can directly harm human health because some large overgrowths of algae (algal blooms) can produce potent toxins. These toxins become increasingly concentrated as they move up the food chain, and people can become ill or even die when they consume fish or shellfish from areas where there are algal blooms.
What about organic fertilizers? Organic fertilizers are generally better for the ecosystem than synthetic fertilizers. Organic fertilizers come directly from plant or animal sources, such as compost or manure. They tend to contain lower concentrations of nutrients such as nitrogen and phosphorus than synthetic fertilizers. Organic fertilizer often needs to be broken down by decomposers before many of its nutrients can be used by plants, which limits the speed at which these nutrients become introduced to the environment and allows them to be retained by the soil for longer periods of time, increasing soil quality. It also encourages biodiversity in the soil by providing food for a variety of decomposers, which, as you have learned, are critical to matter recycling in ecosystems. Organic fertilizer also helps maintain soil structure, makes the soil more resistant to erosion, and helps with water infiltration. In these ways, organic fertilizers can help keep nutrient and water cycling balanced in the ecosystem.
There are many other aspects of organic agriculture beyond the types of pesticides and fertilizers used. For instance, organic agriculture promotes biodiversity through techniques such as crop rotation, the planting of cover crops, and encouraging the growth of plants and maintenance of habitats that attract beneficial pest predators and pollinators. These techniques add nutrients to the soil, improve soil structure, reduce pest damage, and promote pollination while also providing benefits to many species in the ecosystem. In general, organic agriculture tends to promote more natural ecosystem interactions than conventional agriculture, but as you have seen from the example of organic pesticides, “organic” isn’t always necessarily better in all respects. Therefore, there isn’t one easy answer about whether Camille should choose organic over conventional foods, but by learning about balance in ecosystems and the impact of specific farming practices, she — and you — can make more informed decisions at the grocery store.
In this chapter, you learned about the science of ecology and the ecosystems and biomes on Earth. Specifically, you learned that:
- Ecology is the study of how living things interact with each other and with their environment. All organisms need energy and matter that must be obtained from the environment, so organisms are not closed systems. The environment of an organism includes biotic factors, which are the living aspects of the environment, and abiotic factors, which are the nonliving aspects of the environment.
- Ecologists generally organize the biological world in a nested hierarchy. Above the level of the individual organism, from the least to most inclusive, the levels of this hierarchy are the population, community, ecosystem, biome, and biosphere. The biosphere consists of every part of Earth where life exists, including the land, water, and air.
- Basic ideas in ecology include the ecosystem, niche, habitat, and competitive exclusion principle. An ecosystem consists of all the biotic and abiotic factors in an area and their interactions. A niche is the role of a species in its ecosystem, and a habitat is the natural environment in which a species lives and to which it is adapted.
- An ecosystem is a set of interacting components that form a complex whole, including its community of living things (biotic components) and nonliving environmental factors such as water and sunlight (abiotic components).
- Ecosystem processes move energy and matter through the biotic and abiotic components of an ecosystem, starting with primary production by producers such as plants. Through photosynthesis, plants capture energy from sunlight and make organic molecules from inorganic compounds.
- Nutrients, including carbon and nitrogen, are recycled among the biotic and abiotic components of ecosystems. In most terrestrial ecosystems, nitrogen is a limiting factor on plant growth and primary production. A limiting factor is any factor that constrains the population size of one or more of an ecosystem’s species.
- Ecosystems provide a variety of goods and services upon which our species depends. Ecosystem goods range from foods to recreational opportunities. Ecosystem services range from providing oxygen to the air to pollinating crops.
- A community is the biotic part of an ecosystem. It consists of all the populations of all the species that live in the ecosystem and their relationships with each other. There are three major types of community relationships: symbiosis, predation, and competition.
- Symbiosis is a close relationship between two organisms of different species in which at least one of the organisms benefits. Types of symbiosis include mutualism, commensalism, and parasitism.
- Mutualism is a symbiotic relationship in which individuals from both species benefit. An example is the relationship between clownfish and sea anemones.
- Commensalism is a symbiotic relationship in which an individual from one of the species benefits while the individual from the other species is unaffected. An example is the relationship between cattle egrets and grazing mammals, in which the egrets benefit and the mammals are unaffected.
- Parasitism is a symbiotic relationship in which an organism from one species, called the parasite, benefits, while the organism from the other species, called the host, is harmed. An example is the relationship between parasitic roundworms and human hosts.
- Predation is a community relationship in which an organism of one species, called the predator, consumes tissues of an organism in another species, called the prey. An example is snake predators that consume prey animals such as lizards.
- A predator-prey relationship tends to keep the populations of both species in the balance because each population is a limiting factor on the other population.
- Both predators and prey are likely to have adaptations to predation such as camouflage that evolves through natural selection.
- Interspecific competition is a community relationship in which organisms from different species rely on the same limiting resource.
- Interspecific competition is the basis of the competitive exclusion principle and may lead to the extinction of one species or greater specialization in both species.
- All organisms need energy. There are two basic types of organisms in terms of how they obtain energy: autotrophs and heterotrophs.
- Autotrophs (producers) use energy directly from the sun or from chemicals to produce organic molecules. Photoautotrophs such as plants use energy from sunlight to make organic compounds by photosynthesis. Chemoautotrophs such as certain bacteria use energy from chemicals to make organic compounds by chemosynthesis.
- Heterotrophs (consumers) obtain energy by consuming other organisms. Heterotrophs include all animals and fungi as well as many protists and bacteria. They can be classified on the basis of what they consume as carnivores, which eat animals; herbivores, which eat plants; omnivores, which eat both animals and plants; and decomposers, which consume organic wastes and dead organisms.
- The flow of energy in an ecosystem can be represented with a food chain or food web. A food chain represents a single pathway through which energy flows in an ecosystem. A food web represents multiple pathways through which energy flows in an ecosystem.
- Feeding positions in a food chain or food web are called trophic levels. The first trophic level is producers; the second trophic level is consumers that eat producers; third and higher trophic levels are consumers that eat organisms from the trophic level below them. There are rarely more than four trophic levels. Most consumers actually feed at more than one trophic level.
- Only about 10 percent of the energy at one trophic level actually passes on to the next higher trophic level. The rest of the energy is used up at the lower trophic level or lost to the environment as heat or incompletely digested food. Generally, there are fewer organisms and less biomass at higher trophic levels.
- Water and the chemical elements that organisms need continuously cycle through ecosystems. Cycles of matter are called biogeochemical cycles because they include both biotic and abiotic components and processes. Components that hold matter for short periods of time are called exchange pools, and components that hold matter for long periods of time are called reservoirs.
- The water cycle involves water changing state as it moves from one exchange pool or reservoir to another.
- Carbon cycles quickly between organisms and the environment through cellular respiration and photosynthesis. Carbon in organic compounds moves through food chains and webs and some is released back to the environment by decomposers. Human actions such as burning fossil fuels release huge amounts of additional carbon dioxide into the atmosphere.
- Most natural processes cycle carbon more slowly. Running water slowly dissolves carbon in rocks and carries it to the ocean, and the top layer of ocean water dissolves carbon dioxide out of the atmosphere. Carbon in ocean water may gradually settle to the bottom, and some of this carbon may eventually be changed to fossil fuels or sedimentary rocks that can store carbon for millions of years.
- Nitrogen gas in the atmosphere cannot be used by plants, but nitrogen-fixing bacteria in soil or on plant roots change nitrogen gas to nitrates, which plant roots can absorb. Decomposition of organic matter releases nitrogen as ammonium ions that plants can also absorb or that nitrifying bacteria in soil can change to nitrates for use by plants. Denitrifying bacteria release nitrogen gas from unused nitrates, and this nitrogen enters the atmosphere and completes the cycle.
- Since our species first evolved 200,000 years ago, a total of 108 billion human beings have lived on Earth, with 7.4 billion of them alive in 2017 and many more predicted in the future. People permanently live on a large scale on every continent except Antarctica. The rapid increase and spread of the human population raise concerns over our species’ continued existence.
- The scientific study of human populations is called demography. It includes the study of population size, distribution, and structure. It also includes the study of population dynamics, including population growth and changes in population structure.
- The age-sex structure of a population is the number of individuals of each sex and age group in the population, typically represented by bar graph called a population pyramid. The shape of a population pyramid reflects past births, deaths, and migrations; and it may provide insights into political and socioeconomic change.
- In 2015, the global human population had an average growth rate of 1.2 percent, but the growth rate varied among nations from less than zero to greater than 3 percent. A 3 percent growth rate is high for human populations, leading to a doubling time of just 23 years. Rates of population growth much higher than 3 or 4 percent or much lower than zero are generally caused by high rates of migration.
- Population growth rates may change over time. With exponential growth, the larger the population becomes, the faster it grows. With logistic growth, population growth slows and levels off as the population size reaches its carrying capacity (K), which is the largest population size that can be supported by available resources without harming the environment.
- For most of its existence, the human population grew very slowly. It started growing exponentially a few centuries ago. It is currently adding more than 80 million people per year.
- Since the time of Malthus, there has been a concern that the human population is growing so rapidly that it will soon outstrip food production and crash due to increased warfare, famine, and disease. Estimates place the human population carrying capacity at 7.7 billion people, which we are expected to reach by 2020, suggesting that there is an overpopulation problem.
- Many environmental problems are aggravated by the size of the human population, although over-consumption and waste by populations in wealthy nations may be worse problems than overpopulation per se. A widely accepted goal is zero population growth (ZPG), which occurs when birth rates match death rates because women average only enough children to replace themselves and their partner. This is called the replacement fertility rate, and it depends on the death rate.
- Climate change refers to any change in average weather conditions on Earth that lasts for at least several decades. The most important cause of recent and ongoing climate change is human actions, which cause an enhanced greenhouse effect and global warming.
- Direct evidence for global warming comes from measurements of global land and ocean temperatures, which show an overall warming trend since the late 1800s. Indirect evidence for global warming comes from observations of its effects, such as the shrinking of glaciers over the past century or so.
- Even if greenhouse gas emissions are reduced in the future, the climate will continue to get warmer because of the greenhouse gases already present in the atmosphere. Projections for future increases in the mean global temperature range from less than 1 to almost 5 degrees C, depending on future greenhouse gas emissions. Warming has been and will continue to be greater over land than the ocean and greater in the Arctic than anywhere else on Earth.
- Future global warming is projected to have a wide range of impacts, many of which are already occurring. Impacts are likely to include the continued retreat of glaciers, greater weather extremes, expansion of deserts, shifts toward the poles in natural habitats, loss of biodiversity, changes in food production, rising sea levels, displacements of human populations, and increases in human violence.
- Most climate experts agree that future global warming should be limited to less than 2.0 degrees C. Otherwise, human and natural systems may be unable to adapt. Controlling global warming requires first and foremost deep cuts in greenhouse gas emissions by phasing out fossil fuels and replacing them with energy resources that do not produce greenhouse gases.
Chapter Summary Review
1. Which of the following have abiotic components, in addition to biotic components? Choose all that apply.
2. True or False. Earth has several types of biospheres.
3. True or False. There is one species of plankton, and it lives near the surface of the water.
4. Is a niche the same thing as a habitat? Why or why not?
5. Which of the following are considered abiotic factors?
A. Primary producers
D. All of the above
6. Are organisms and ecosystems open or closed systems? Explain your answer.
7. If there is an unusually cool week in the summer, does that mean there is a change in climate? Explain your answer.
8. The concept that two species cannot occupy the same niche in the same place for very long is called the ___________________ .
9. Where are aquatic organisms that are bioluminescent, meaning that they can produce light, most likely to be found? Explain your answer.
10. When an organism is said to be “marine,” what does this mean?
A. That it is an aquatic mammal
B. That it lives in running water
C. That it lives in the ocean
D. That it can swim
11. Can there be an ecosystem within an ecosystem? Why or why not?
12. Explain the importance of decomposers within ecosystems, and relate them to biogeochemical cycles.
13. True or False. A symbiotic relationship is a close relationship where both species benefit.
14. True or False. Most of the gross primary production that is not used by producers themselves are broken down by decomposers.
15. Is a mosquito that feeds on human blood a parasite or a predator? Explain your answer.
16. For each statement below, choose which type of bacteria best fits the description. Each type of bacteria is used only once.
Types of bacteria: denitrifying; nitrifying; nitrogen-fixing
A. Takes a product of decomposition and turns it into a molecule that plants can use.
B. Takes nitrogen from the atmosphere and turns it into a molecule that plants can use.
C. Releases nitrogen gas back into the atmosphere.
17. When a prey population decreases, its predator population usually:
C. Does not change
D. Becomes heterotrophic
18. What are two reasons why two locations that are at the same latitude might have different temperatures?
19. Species A and species B are living in the same location and eat the same prey. When members of species A encounter members of species B, they will attack them, often resulting in the death of members of species B. Answer the following questions about this scenario.
a. What kind of interspecific competition is this?
b. Over time, what are some possible outcomes of this interaction on species B?
20. True or False. Increased sublimation would cause more water vapor to be emitted into the atmosphere.
21. True or False. There is generally less biomass at higher trophic levels.
22. Identify a reservoir for carbon, and explain why it is considered a reservoir.
23. What does “eating low on the food chain” mean?
24. Fish are usually:
C. primary producers
25. Give one example of how plants are involved in each of the biogeochemical cycles: the water cycle, carbon cycle, and nitrogen cycle.
26. What is the difference between dogs and cats in terms of how they obtain their energy and nutrients?
27. The U.S. population was approximately twice as high in 2010 as it was in 1950, based on census numbers. Use this information to answer the following questions.
a. Can you conclude that new births were solely responsible for the increase in population size? Explain your answer.
b. What other types of data would help you determine the reasons for this increase in population size?
28. True or False. Some countries have a negative growth rate.
29. What is the current population of the Earth?
A. Around 5 billion
B. Around 6 billion
C. Around 7 billion
D. Around 9 billion
30. Explain why population growth slows down as the population size approaches the carrying capacity when there is logistic growth.
32. True or False. Carbon monoxide is the most significant greenhouse gas.