The animal whose absence would have the greatest negative effect on the ecosystem is the wolf.
Here's why:
- Wolves are top predators in this ecosystem and control the populations of both foxes and deer. Without wolves, the populations of these species could increase unchecked, leading to overgrazing and depletion of vegetation, which could have cascading effects on the entire ecosystem.
- Foxes and rabbits are also interdependent, but they are part of the lower trophic levels compared to wolves. The absence of foxes would impact rabbits, but they are not as crucial for maintaining balance in the ecosystem.
- Deer mainly affect their own populations and do not directly control other species as wolves do.
Thus, the absence of wolves would disrupt the balance significantly more than the removal of any of the other species.