You need players to be trees, players to be squirrels (half of tree total plus a few) and a few players to be the foxes. For a class size of 22, assign 12 kids be trees. Each of those players will pair up with another to be one tree, which will give you 6 trees. There will be 8 squirrels. This is to create the problem of not enough trees for all the squirrels. There are also two foxes to catch those loose squirrels. For a group of 40, have 22 kids be trees giving you 11 trees, 14 squirrels and 4 foxes.
The trees go out into the play area first. Two people become a tree by standing face to face and holding hands with arms raised over their heads. Once the trees are into position you send out the squirrels to find a home. A squirrel must stand underneath the raised arms to be in a tree. If a squirrel has no home he goes to another squirrels home tap them on the shoulder. That squirrel must leave his home out the other side of the tree and find a new home. Give the squirrels a few minutes of establishing a cycle of tree swapping. Once this has been done, send in the foxes and they are to tag any loose squirrels. Those squirrels tagged are to sit out of the game. After a few squirrels are tagged, start taking away one or two trees (Have someone come in with a chain saw to remove the trees – they will notice that this is changing the squirrel’s habitat. After the kids have played this game then let them choose what they would like to be. You’ll find most players will want to be squirrels and foxes, and few will be trees if any at all. Play out the game and then talk about what went wrong.