1. A week after returning from a backpacking trip, four teenagers developed nausea and severe diarrhea. All had consumed untreated water from a stream which doctors concluded was infested with the organism, Giardia. Think about the symbiotic relationships that you studied in this unit. What type of relationship exists between the teens and the organism? 2. As humans burn fossil fuels for energy, the percent of carbon stored in fossils is reduced while the carbon in the atmosphere is increased. If green plants are also being removed, how might the carbon cycle and greenhouse effect be influenced?

3. How are decomposers critical to the carbon cycle?

Explanation:

1) The relationship that exists between the teens and the organism (Giardia) is Parasitism. Because in such symbiotic relationship one species (the parasite-Giardia) benefits while the other species (the host- the teens) is harmed.

2) Plants remove Carbon dioxide (a major green house gas) from atmosphere during photosynthesis. But, if plants are also being removed with increase in use of fossil fuels, the CO2 that escapes the fossil fuels will remain in the atmosphere for longer as there are not enough plants to "suck" them back into a stored form of Carbon. Carbon Dioxide increases green hourse effect thus green house effect will increase and earth's temperature might rise due to this.

3) Without decomposers we would not be able to continue the carbon cycle because we could not release the stored carbon from plants and animals into the atmosphere. Decomposers break down dead material from organism and release carbon dioxide into the atmosphere, where it's available to plants for photosynthesis. Without them, we could not recycle nutrients or carbons in our ecosystem and thus, they are critical for the carbon cycle.

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c. heat from the poles back down towards the equator.