In this ecosystem, wolves play a crucial role as apex predators. They not only control the populations of foxes and deer but also influence the population of rabbits, as foxes and wolves both prey on them.
If wolves were absent, there would likely be an overpopulation of foxes, which could lead to a decline in the rabbit population due to increased predation. Additionally, without wolves, deer populations could increase significantly, which would result in overgrazing and a subsequent negative effect on the vegetation and overall ecosystem health.
While the absence of any of these animals can impact the ecosystem, the removal of wolves would likely have the greatest negative effect due to their role in maintaining the balance between various species and controlling the populations of both prey and competing predators.
Therefore, the answer is:
wolf