Unit 8 practice test: Ecology

This is biomagnification. Persistent, fat-soluble PCBs stay in the body; since only about 10% of food becomes new tissue, each predator eats far more prey than its own mass and keeps the PCBs from all of it.

• PCBs are made inside the bodies of predators from chemicals in their prey, so more PCBs form at each higher level.: PCBs are industrial chemicals; animals do not make them.
• Larger animals drink more lake water than smaller ones, so they take in more of the PCBs dissolved in the water each day.: The water holds only 0.000004 ppm; drinking cannot explain 4.5 ppm in trout, which take in PCBs mainly in food.
• Correct: PCBs stay in body fat instead of being excreted, and each consumer eats many times its own mass, so they concentrate.: Correct: storage plus the large amount of food eaten.
• PCBs break down slowly in small organisms but speed up as they move into larger ones, which makes them more concentrated.: PCBs break down slowly everywhere; faster breakdown would lower concentrations, not raise them.

Biomagnification turns a tiny concentration in water into a high one in top predators; harm happens where the concentration is high, such as in gull eggs.

• The claim is supported, because 0.000004 ppm is far too low to harm any organism.: The water level is low, but animals at the top of the food web are exposed to far more.
• Correct: The claim is not supported, because gull eggs carry 90 ppm, over 20 million times the water level.: Correct: the risk is at the top of the food web.
• The claim is supported, because PCB levels in phytoplankton are below those in fish.: Low levels in producers fit biomagnification; they do not show the top predators are safe.
• The claim is not supported, because the water level is higher than in the phytoplankton.: The water level (0.000004 ppm) is lower than in phytoplankton (0.02 ppm).

Long-lived top predators hold PCBs accumulated over years in their fat and keep eating prey that still carry them, so their levels fall slowly. Short-lived plankton are replaced quickly by new, cleaner individuals.

• Phytoplankton, because they are closest to the water and absorb PCBs directly: Phytoplankton live days to weeks, so new cells soon reflect the cleaner water.
• Zooplankton, because they reproduce faster than the other consumers in the lake: Fast reproduction replaces zooplankton quickly, so their levels drop sooner.
• Correct: Lake trout and gulls, because they are long-lived and keep stored PCBs in their fat: Correct: long life and fat storage keep PCBs in top predators.
• Lake water, because PCBs dissolve more easily in water than in living tissue: The stem says the water levels fall within a few years; PCBs are fat-soluble, not water-soluble.

Energy stored in phytoplankton biomass that zooplankton do not take in either is never eaten or is eaten but not absorbed. Both end up as detritus, the food of decomposers.

• It is released as heat by the phytoplankton's respiration after the cells have grown.: Phytoplankton respiration is already subtracted: 13,800 kJ was released as heat, leaving 10,200 stored.
• It is destroyed by the zooplankton's digestive enzymes as they break down the algae cells.: Energy is not destroyed; digestion breaks molecules down so they can be absorbed.
• Correct: Algae that die uneaten, and eaten algae lost in zooplankton feces, pass to decomposers.: Correct: uneaten algae and undigested food feed the decomposers.
• It is stored as fat by the zooplankton, a store that the table does not include.: Energy taken in already includes everything the zooplankton absorb, fat stores included.

Endotherms spend most absorbed energy on respiration that produces body heat (39.2 of 40 kJ here), leaving little for growth. Ectotherms like the fish do not pay that cost.

• Correct: The birds are endotherms, so they burn much of their food to make heat that keeps the body warm, and that energy leaves the food chain.: Correct: the cost of endothermy shows up as heat released by respiration.
• The birds are at a higher trophic level, and the second law of thermodynamics destroys more energy at each higher level of a chain.: Energy is never destroyed, and trophic level alone does not explain why one animal stores 20% and another 2%.
• The birds digest fish less fully than the fish digest zooplankton, so less of the fish they eat is ever absorbed into the body.: The comparison already uses energy taken in (absorbed), so differences in digestion are not part of it.
• The birds store most of their energy as fat, and the table counts the energy in fat as heat released by cellular respiration.: Fat is stored biomass and would be counted in the last column, not as heat.

If it stored a share like the small fish (about 20%), it would store about 8 kJ, ten times as much. An ectotherm's body temperature follows the water, so less of its food is burned for heat.

• About 0.8 kJ, the same as the birds, because both feed at the same trophic level: Trophic level sets how much energy is available (40 kJ), not what share an animal stores.
• Correct: Much more than 0.8 kJ, because an ectotherm does not spend energy making body heat: Correct: ectotherms store a larger share of what they take in.
• Less than 0.8 kJ, because large fish need more energy to swim than birds need to fly: The data show fish storing 20% of their intake; nothing suggests a fish's swimming costs exceed an endotherm's heat production.
• Zero, because a fourth trophic level has no energy left to store as new biomass in a lake: The birds at this level already store 0.8 kJ, so a fourth level can store energy.