To analyze which of the options would NOT increase the population of urchins in a kelp forest, let's consider the effects of each scenario on the dynamics of the ecosystem:
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Increased predation by orca whales on sea otters: Sea otters are a major predator of sea urchins. If orca whales increase their predation on sea otters, this would lead to a decrease in sea otter populations, resulting in less predation on sea urchins. Consequently, the population of urchins would likely increase.
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Overfishing of the sheephead fish: Sheephead fish are another predator of sea urchins. If sheephead fish are overfished, this would reduce the number of these predators in the ecosystem, likely allowing the urchin population to rise due to reduced predation.
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A decrease in kelp density: A decrease in kelp density means that there is less habitat and food available for many marine species, including sea urchins. While sea urchins do feed on kelp, a significant reduction in kelp can lead to lower population support for urchins over time and possibly a decline in their population if the kelp ecosystem becomes too degraded. Thus, this situation does not directly contribute to an increase in urchin populations.
Based on this analysis, the correct answer is: 3) a decrease in kelp density would NOT increase the population of urchins in a kelp forest.