Based on the predator-prey relationships you've provided:
- Rabbits are eaten by both foxes and wolves.
- Deer are eaten by wolves.
- Foxes are eaten by wolves.
In this ecosystem, wolves have the most significant role. They are at the top of the food chain, impacting the populations of both deer and foxes and indirectly affecting the population of rabbits by controlling the number of foxes. If wolves were to disappear, the populations of both deer and foxes could increase uncontrollably, leading to overgrazing and further ecological imbalance.
Therefore, the animal whose absence would have the greatest negative effect on the ecosystem is the wolf.