An immediate decrease in the deer population in the forest would be most likely caused by a great increase in the wolf population.
In the scenario where a large population of deer lives in the forest and wolves kill and eat the deer as their primary food source, the event that would most likely cause an immediate decrease in the deer population is if the wolf population greatly increases. This increase in wolf population would lead to an enhanced predation pressure on the deer, resulting in a rapid decline in the deer population due to increased hunting by the wolves.
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