Ch 42: Ecosystems and Energy

Question 1

List trophic levels and give examples of organisms that can be found at different levels

Answer 1

• Trophic level: Composed of all organisms that feed at a particular link in a particular food chain.
• Level 1: Primary Producers: An autotroph, usually a photosynthetic organism. Autotrophs make up the trophic level of an ecosystem that ultimately supports all other levels. (Photosynthesizers)
• Level 2: Primary Consumers: An organism that eats plants or other autotrophs (Decomposers, Mutualists, Pathogens, Parasites, Root-feeders)
• Level 3: secondary consumers: A carnivore that eats herbivores. (Shredders, Predators, Grazers)
• Level 4: Tertiary consumers: A carnivore that eats other carnivores.
• Level 5: Apex predators: a predator at the top of a food chain

Question 2

What is the energy flow within an ecosystem? (define and explain)

Answer 2

Energy enters most ecosystems as sunlight. This light energy is converted to chemical energy by autotrophs, passed to heterotrophs in the organic compounds of food, and dissipated as heat.

Question 3

Why is energy transfer not 100% efficient? What happens to the energy as you move from one trophic level to the next?

Answer 3

Energy decreases as it moves up trophic levels because energy is lost as metabolic heat when the organisms from one trophic level are consumed by organisms from the next level.

Question 4

Why are there typically so many more primary producers in an ecosystem than top level consumers?

Answer 4

The progressive loss of energy along a food chain limits the abundance of top-level carnivores that an ecosystem can support. Only about 0.1% of the chemical energy fixed by photosynthesis can flow all the way through a food web to a tertiary consumer, such as a snake or a shark. This helps to explain why most food webs include only about four or five trophic levels

Question 5

Be able to calculate/ evaluate how many calories are available in a particular trophic level assuming a 10% transfer efficiency.

Answer 5

10% moves from each trophic level so just divide by 10 each time you move up a level (ex: primary 10 kJ → secondary 1 kJ)

Question 6

Explain why consuming tuna is potentially problematic while pregnant when consuming other fish in the same water is not.

Answer 6

Tuna bioaccumulates mercury at a higher rate than other fish

Question 7

How is the movement of nutrients (nutrient cycle) through ecosystems different from the flow of energy through ecosystems?

Answer 7

Chemical nutrients and energy tend to flow in the same direction for most of an ecosystem, but the main difference is thatthe nutrient cycle is recycled in the ecosystem while the energy flow is ultimately lost from the ecosystem to the universe at large.

Question 8

Describe the water cycle

Answer 8

The main processes driving the water cycle are evaporation of liquid water by solar energy, condensation of water vapor into clouds, and precipitation. Transpiration by terrestrial plants also moves large volumes of water into the atmosphere. Surface and ground water flow can return to the oceans, completing the water cycle.

Question 9

Describe the short term and long term carbon cycles.

Answer 9

• Short Term Carbon Cycle: Timescale of decades; driven primarily by biological activity
  
  • In the carbon cycle plants in both terrestrial and aquatic ecosystems take up carbon dioxide (CO2) from the atmosphere through photosynthesis.
  • Carbon is incorporated into food used by autotrophs and heterotrophs.
    
    • Cell respiration returns a portion of this carbon to the atmosphere as carbon dioxide (CO2).
• Long Term Carbon Cycle: Timescale of centuries or longer; driven primarily by physical processes
  
  • Living and dead organisms contain organic carbon and serve as reservoirs for the carbon cycle.
    
    • The biotic components, especially trees, contain 800 billion tons of organic carbon.
    • An additional 1000-3000 metric tons are estimated to be held in remains of plants and animals.
    • Some of the remains undergo a physical process that transforms them into coal, oil, and natural gas.
      
      • These are know as fossil fuels.

Question 10

Describe the nitrogen cycle. Explain what nitrogen fixation is and why it is important.

Answer 10

• Plants cannot use nitrogen gas (N2) so nitrogen is a limiting inorganic nutrient for plants N2 needs to be transformed:
• Nitrification: the production of nitrates (NO3-) during the nitrogen cycle.
  
  • Two ways:
    
    1. Nitrogen gas is converted to nitrate in the atmosphere by lightning, meteor trails, or cosmic radiation.
    2. Ammonium ions in the soil are converted to nitrate by nitrifying bacteria.
       
       • First, nitrite-producing bacteria convert ammonium to nitrite (NO2-).
       • Then nitrate-producing bacteria convert nitrite to nitrate.
• Denitrification: the conversion of nitrate back to nitrogen gas (N2) by denitrifying bacteria.
  
  • The nitrogen gas will enter the atmosphere.
  • These bacteria live in the anaerobic mud of lakes, bogs, and estuaries and carry out this process as part of their own metabolism.
  • Under natural conditions, denitrification would counterbalance nitrogen fixation.
• Nitrogen fixation: The conversion of N2 to forms that can be used to synthesize organic nitrogen compounds.

Question 11

Describe the phosphorus cycle and why it is biologically important

Answer 11

• Phosphorus moves from rocks on land to the oceans, where it gets trapped in sediments.
• Phosphorus moves back onto land following a geological upheaval.
• Phosphate is used by producers to form molecules such as phospholipids, ATP, and nucleotides used in DNA and RNA.
• Phosphate is usually a limiting resource for plants, which means their growth is limited by the amount of available phosphorous.

Question 12

Explain the human impact on each cycle

Answer 12

• Water: Aquifers are being drained faster than they can be naturally replenished. Groundwater may be depleted within a few years. Fresh water makes up 3% of Earth’s water -> considered a renewable resource since the water cycle produces it. It is possible to run out of fresh water if supply not adequate and/or becomes polluted -> not usable.
• Carbon: More carbon dioxide is being deposited in the atmosphere than is being removed. The increased CO2 in atmosphere contributes to global warming. Global warming is contributing to climate change.
• Nitrogen: Humans nearly double the nitrogen fixation rate when fertilizers are made from N2. Nitrogen in fertilizers contribute to eutrophication. Fertilizer use also results in the release of nitrous oxide (N2O), a greenhouse gas which contributes to acid rain and ozone depletion
• Phosphorus: Phosphates are used in fertilizers, animal feeds, and detergents. Excess phosphates in water supplies can lead to cultural eutrophication.

Question 13

What is biomagnification?

Answer 13

Toxic substances accumulate at increasingly high concentrations in progressively higher trophic levels

Question 14

How do we know that humans are impacting the carbon cycle?

Answer 14

The increased CO2 in atmosphere contributes to global warming. Global warming is contributing to climate change.

Question 15

What is eutrophication and how does eutrophication result in the formation of dead zones?

Answer 15

Eutrophication is a general term describing a process in which nutrients accumulate in a body of water, resulting in an increased growth ofmicroorganismsthat may deplete the water ofoxygen. Dead zones are hypoxic (low-oxygen) areas in the world’s oceans and large lakes.

• Algal blooms
• Algae dies off → decomposers increases
  
  • decomposers use up all O2 → can lead to massive fish kills.