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How would you figure out what the keystone species is in the following textbook question? (questions 1 and 2 at the bottom of the image)

Based on my knowledge, a keystone species helps to maintain biodiversity therefore I think it is either the otter or the kelp that is the keystone species. I say this because when either one of them decreases in number, the other species decreases in number as well (less biodiversity).

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closed as off-topic by rg255, kmm, Oreotrephes, fileunderwater, March Ho Apr 4 '16 at 3:06

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  • $\begingroup$ What book is the picture/questions from? $\endgroup$ – fileunderwater Mar 31 '16 at 7:58
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From http://education.nationalgeographic.org/encyclopedia:

A keystone species is a plant or animal that plays a unique and crucial role in the way an ecosystem functions. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether.
[...]
The sea otter is an example of a keystone species in the Pacific Northwest. These mammals feed on sea urchins, controlling their population. If the otters didn't eat the urchins, the urchins would eat up the habitat's kelp. Kelp, or giant seaweed, is a major source of food and shelter for the ecosystem. Some species of crabs, snails, and geese depend on kelp for food. Many types of fish use the huge kelp forests to hide from predators. Without sea otters to control the urchin population, the entire ecosystem would collapse. 1

  1. The organism I would therefore hypothesize to be the keystone species is the sea otter.
  2. The data supports the hypothesis as when there is an decrease in the number of sea otters there is a increase in the biomass of sea urchins, which is accompanied by a decrease in the density of kelp. The lack of otters feeding on sea urchins causes an increase in their abundance, and this leads to an increased consumption of kelp by the sea urchins, which leads to the decrease in the density of kelp. Therefore, the sea otter is the keystone species since a change in their abundance has a significant impact onto the ecosystem and the organisms living in it.
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