1. Which statement best describes the relationship between free energy and the trophic levels of an ecosystem?

Transcription

1 1. Which statement best describes the relationship between free energy and the trophic levels of an ecosystem? (A) Since approximately 10% of the free energy available at one trophic level is available to the organisms at the next trophic level, the number of organisms at each trophic level is approximately 10% of the number at the previous level. This answer suggests the student may understand that, as a general rule, approximately 10% of the free energy available at one trophic level is available at the next trophic level, but does not understand that the number of organisms is different than the amount of free energy and that the amount of free energy needed by each organism combined with the total amount of available free energy determines the number of organisms that can be supported at each trophic level. (B) Since all ecosystems receive the same amount of free energy from the sun, all the organisms at the same trophic level receive about the same amount of free energy, which is approximately 10% of the free energy available to the organisms at the previous trophic level. This answer suggests the student may understand the 10% rule at a general level, but does not understand that it is a broad rule and not true for all organisms/trophic levels, that each ecosystem receives a different amount of free energy, that not all free energy comes from the sun, and that the amount of energy transferred between trophic levels depends on many different factors, including energy requirements, reproductive rates, and environmental conditions, and is usually more or less than 10%. (C) The amount of free energy that is transferred to the environment as thermal energy at each trophic level is approximately 10%, which means that the total number of organisms that are supported at each successive trophic level is less because of several factors, including a reduced biomass and a decreased reproductive rate. Page 1 of 6

2 This answer suggests the student may understand that less free energy is available to organisms at the higher trophic levels than to organisms at the lower trophic levels and that the biomass of each successive trophic level is less, but does not understand that the majority of the energy available at each trophic level is converted to thermal energy and only a small portion is actually transferred to the next trophic level or that the number of organisms a trophic level can support depends on the size and energy demands of the organisms. (D) The amount of free energy available to the organisms at a trophic level is approximately 10% of the free energy available at the previous trophic level, but the actual amount of free energy transferred between trophic levels depends on many factors, including the organisms energy requirements and environmental conditions. This answer suggests the student understands that the 10% rule is an oversimplification, that it does not apply to all energy transfers between trophic levels, and that the actual amount of free energy that is transferred depends on many different factors. 2. A small island is hit by a massive hurricane. The only two species that survive the hurricane are frogs and mosquitoes. Frogs feed on mosquitoes and mosquitoes feed on frogs. Which statement describes what will most likely happen to the frog and mosquito populations over time? (A) The frog and mosquito populations will remain in equilibrium because the individual organisms in each population have a steady supply of free energy. This answer suggests the student may understand that some energy is transferred between the frogs and mosquitoes, but does not understand that there is less available free energy with each transfer because some energy is converted to thermal energy (heat) by each organism and is no longer available for use, and that no free energy is entering the system to replace that converted into heat. (B) The mosquito population will start to decline first, followed by the frog population, because free energy is only transferred in one direction, from smaller organisms to larger organisms. Page 2 of 6

3 This answer suggests the student may understand that energy is transferred in one direction in an ecosystem and that both populations will decline without free energy entering this system, but does not understand that energy may be transferred between organisms of any size, or that the populations of mosquitoes and frogs will decline together as the available free energy is converted to thermal energy (heat) and is no longer available for use. (C) The frog population will decrease in response to a decrease in the mosquito population because frogs and mosquitoes have a predator-prey relationship and depend upon one another for food. This answer suggests the student may understand that the frog population will decrease in response to a decrease in the mosquito population, but does not understand that frogs and mosquitoes do not have a predator-prey relationship, that no free energy is entering this system, and that some energy is converted into thermal energy (heat) with each transfer between the organisms and is no longer available for use. (D) The frog and mosquito populations will both decline because individual organisms will convert some available free energy into thermal energy, and a constant input of free energy is needed to maintain the system. This answer suggests the student understands that energy transfer between organisms always involves a transfer of some usable energy into thermal energy (heat) by cellular processes and is no longer available for use, and that the two species cannot feed off each other indefinitely because no free energy is entering this system and all the free energy will eventually be converted into thermal energy. Page 3 of 6

4 3. The diagram below shows the flow of energy through some species in an ecosystem. Which statement best supports the information in the diagram? (A) The individuals in species K have a high reproductive rate and a fast maturation rate because they are a food source for all consumers in the ecosystem. This answer suggests the student understands that a species that is a source of food for many other species needs to have a relatively high growth and reproductive rate in order to sustain its population due to the high rate of predation. (B) Species J has the greatest total biomass of all species in the ecosystem since it obtains energy from both primary and secondary consumers. This answer suggests the student may understand that species J obtains energy from both primary and secondary consumers, but does not understand that producers generally have the greatest biomass in an ecosystem because the amount of free energy is greatest at the producer trophic level and can support the most organisms. (C) Species F has the least amount of available free energy because it obtains its energy from decomposers rather than producers. This answer suggests the student may understand that species F obtains some of its energy from decomposers, but does not understand that species F is a primary producer and has the greatest amount of available free energy of all species in the food web. Page 4 of 6

5 (D) The removal of species G would be catastrophic to the other species in the ecosystem because species G is a predator. This answer suggests the student may understand that predators often play a vital role in keeping other populations in balance within an ecosystem, but does not understand that there is no evidence to support the claim that the removal of species G would be catastrophic for the ecosystem (species G is not the only predator or even the top predator), or that species H and J are also predators in the ecosystem and they also help keep other species in balance within the ecosystem. 4. The diagram below represents a partial food web for an ecosystem. Which statement describes the most likely impact if species A is removed from the ecosystem? (A) Species C, D, and E will all be equally affected because they all receive free energy from species A. This answer suggests the student may understand the general flow of energy in a food web, but does not understand that species with multiple sources of free energy (food) will be less affected than species that connect within the food web and depend on only one primary food source. Page 5 of 6

6 (B) Species B will rapidly evolve to fill the niche left by species A and will become the only primary producer. This answer suggests the student may understand that species A and B have the same role in the food web and that a species can adapt to expand into another species niche when the competing species is removed, but does not understand that this expansion depends on many factors, does not always happen and is not supported by the food web as being the most likely impact. (C) Species C and G will be most affected since the free energy for these species comes directly or indirectly from species A. This answer suggests the student understands how free energy is transferred through food webs, that some species obtain energy from multiple sources but that other species may rely on only one source, and that a species that relies on only one source is affected more if its only food source is removed than a species with multiple food sources. (D) Species F, G, H, and J will be minimally affected because they receive free energy from other sources rather than from species A. This answer suggests the student may understand that a primary consumer is directly impacted by the removal of part or all of its food source, but does not understand that this reduces the available free energy for the higher trophic levels in the food web and that the reduction in free energy at a lower trophic level has a direct impact on those species in the higher trophic levels. Page 6 of 6