Plants spread seeds and if that seed find a place and conditions are good, a new plant will grow. The same with trees. If you give them time forest will cover the terrain again. Seeds also come from outside the map with the wind. This way even if you cut every tree, eventually new trees will grow again.
Animals also enter and leave the map. If you hunt them all, more will come while they have free space to roam.
Other resources can be limited or virtually unlimited.
