3.3 Study Guide

Question 1

Explain how trophic levels describe relations between species and the transfer of energy.

Answer 1

Each ascending trophic level has a consumatory relationship with the level below it in which it acquires energy from the lower level. The base of the trophic levels is the Producers, which ‘produce’ their own energy without needing to take it from another organism. Above the Producers are the Primary Consumers which feed on the Producers for energy, then the Secondary Consumers which feed on the Primary Consumers for energy, and so on (as well as this, each subsequent level acquires a smaller fraction of the original energy produced from the level below it.

Question 2

Explain the differences between autotrophs and heterotrophs. What are their general roles in a community?

Answer 2

Autotrophs (or Producers), create their own energy, usually through photosynthesis, without having to absorb it from another organism. Heterotrophs, on the other hand, cannot create their own energy and therefore must absorb it from another organism, typically by consuming it. Autotrophs’ role in communities is to introduce energy to be spread to all organisms. Heterotrophs’ role is to initiate the spread of that energy throughout.

Question 3

What is the differentiating factor between an omnivore and an herbivore or a carnivore? How is a detritivore distinct from all three?

Answer 3

While herbivores and carnivores eat solely plants or animals respectively, omnivores are characterized by the consumption of both plants and animals (doesn’t have to be exactly half and half; it is really a spectrum). Detritivores, on the other hand, can eat any of these things, but their distinctive feature is that everything they eat is dead matter, or detritus, instead of living organisms.

Question 4

Between food webs and trophic cascade diagrams, solid lines between two species represent different things. Explain the two different meanings. (EC: What about the lines is changed between the two diagrams due to this difference in meaning?)

Answer 4

In food webs, solid lines between two species represent the natural transfer of energy that occurs from one to the other through predation and/or consumption. In trophic cascades, solid lines between two species indicate one species’ direct impact on the other (will have a + or - based on whether the impact is positive or negative). EC: In food webs, the direction of the line is from the affected organism to the affecting organism, but in trophic cascades the direction is swapped so that the line goes from the affecting organism to the affected organism.

Question 5

In a given food web, there are 6 organisms. Organism A does not have any arrows pointing at it. Organisms B and C both have an arrow pointing at them from Organism A and Organism C also has an arrow from B. Organisms D and E both have arrows from Organism B while E also has an arrow from Organism C. Finally, Organism F has arrows from Organisms B, D, and E. Label each organism’s role/position in the food web.

Answer 5

Organism A: Producer/Autotroph
Organism B: Primary Consumer
Organism C: Primary and Secondary Consumer
Organism D: Secondary Consumer
Organism E: Secondary and Tertiary Consumer
Organism F: Secondary, Tertiary, and Quaternary Consumer

Question 6

In a forest ecosystem, a non-native species of bird is introduced that preys primarily and heavily on squirrels. These squirrels help native oak trees to reproduce by burying their acorns during the fall and forgetting to dig them up. The presence of these oak trees helps strengthen and drain the forest soil and prevent flooding in the forest.
Describe whether each organism’s direct impact on the organism on the trophic level below it (or on an abiotic factor) is positive or negative. Then, explain in a sentence the indirect impact of the bird species’ presence on flooding of the forest.

Answer 6

Bird to Squirrel Impact: Negative
Squirrel to Oak Tree Impact: Positive
Oak Tree to Flooding Impact: Negative
Because the bird species reduces the squirrel population which increases the oak tree population which reduces flooding of the forest, the presence of the bird species has an indirect positive impact on the amount of flooding that occurs in the forest. In short, the more of that bird species there are, the more often and more intensely the forest will flood.