Plasmolysis is the reason. The reason that the Elodea cell prevents from collapsing completely is that the Plasmolysis is the main reason because it always has water to keep it full. The answer in this question is Plasmolysis is the reason because it always has water to keep it full.
The disappearance of the deer from the forest indicates the EXTINCTION OF THE DEER.
The extinction of the deer from the forest will throw the ecosystem out of balance and has negative impacts on the ecosystem. After all the deer has been eliminated, the elephants too will either have to find another source of food or be wiped out.