The animal whose absence would have the greatest negative effect on the ecosystem is the wolf.
The reasons are:
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Wolves are apex predators that help regulate the populations of foxes and deer. Without wolves, the populations of these species could increase unchecked, leading to overgrazing or overbrowsing and eventually harming the vegetation in the ecosystem.
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The absence of wolves would also allow fox populations to increase, which could further impact rabbit populations, thereby disrupting the prey-predator balance further down the food chain.
Overall, the removal of wolves would have cascading effects on the ecosystem, making them critical for maintaining ecological balance.