The animal whose absence would have the greatest negative effect on the ecosystem is the wolf.
Wolves are apex predators in this scenario and play a crucial role in maintaining the balance of the ecosystem. Their absence would lead to an increase in the populations of both foxes and deer, which could result in overgrazing and depletion of vegetation, negatively impacting the habitat for other species. Additionally, without wolves to control their numbers, fox populations could rise unchecked, further disrupting the ecosystem's balance.