Question 1 · 8.1 Responses to the Environment
Flatworms are placed in a shallow dish with a lamp at one end. They crawl in straight paths toward the shaded end and gather there. In a second dish lit dimly and evenly throughout, the same flatworms turn often, crawl slowly, and spread across the whole dish. Which pair of terms describes the behavior in the first dish and in the second dish?
- A Taxis in the first dish and kinesis in the second
- B Kinesis in the first dish and taxis in the second
- C Taxis in both dishes, at two different intensities
- D Kinesis in both dishes, at two different intensities
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A — Both terms name movement responses to a stimulus, and they are separated by whether the movement has a direction relative to the stimulus. In the first dish the worms travel along a straight heading away from the lamp, so the light gradient sets the direction of travel: that is a taxis. In the second dish the light carries no gradient, so there is nothing to steer by; what changes is the rate of turning and the speed of crawling, and the worms end up scattered. A change in activity level rather than heading is a kinesis. A useful test is to ask whether the animal's path would still point somewhere meaningful if the stimulus source were moved.
Question 2 · 8.2 Energy Flow Through Ecosystems
A tropical rainforest averages 2,200 g m^-2 yr^-1 of net primary productivity while the open ocean averages 125 g m^-2 yr^-1, yet the open ocean contributes more to the biosphere's total annual production. Which fact explains this?
- A Ocean producers have a longer growing season than rainforest trees
- B The open ocean covers a far greater area of Earth's surface
- C Ocean producers respire a smaller fraction of what they photosynthesize
- D Ocean water dissolves carbon dioxide, raising the supply for photosynthesis
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B — Both figures are rates per square meter, so multiplying by area converts them to totals. Open ocean spans roughly 65 percent of the planet's surface while tropical rainforest covers a few percent, and that area advantage of more than twenty to one outweighs an eighteenfold deficit in rate. Comparing per-area productivity to total productivity without accounting for area is one of the most common errors in this topic.
Question 3 · 8.3 Population Ecology
A rancher doubles the number of water tanks spread across a rangeland. Over the next five years the resident pronghorn population levels off at a number well above its former plateau. Which interpretation of carrying capacity does this outcome illustrate?
- A Carrying capacity is a fixed trait of the species itself, so the herd must have evolved a higher one
- B Carrying capacity is set by whichever resource runs short first, so easing that shortage raises it
- C Carrying capacity depends only on habitat area, which the added tanks effectively enlarged for the herd
- D Carrying capacity is simply the largest number a habitat has ever held, so the old record was replaced
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B — Carrying capacity is a property of a habitat for a particular species, set by whichever requirement becomes limiting first. On dry rangeland that requirement is usually access to water, and adding tanks relieves it, so the population settles at a higher equilibrium until some other requirement, such as forage, becomes limiting instead. The value is therefore not a constant: change the supply of the limiting resource and the equilibrium moves. This is the same reasoning that predicts a decline in carrying capacity when habitat is degraded.