In a forest ecosystem, a species of bird was introduced. The birds had few predators, and for a period of time its population rose steadily and then roughly leveled off. Which of the following conclusions provides the best explanation for this phenomenon?
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A predator of the birds entered the ecosystem, causing the bird population to level off.
Migration of birds into the ecosystem began to decline, and emigration began to increase.
Another species of bird was introduced, and the two species competed for nesting sites and food.
The ecosystem reached its carrying capacity, and was not able to support continued population growth