In order to survive is there any kind of plant, animal, bird, fish or insect that humans could not eat if normal food resources expired?

Ha!  ..okay

Right I guess some plant species can be poisonous. 

indeed  , but  .. not to worry  ..  the antidote  for many  poisonous plants  usually  grows right beside it , or near by.  if you ever get into   poison ivy  for  example  ,  go  back to where you came in contact  with it  . and look  for  a plant called  jewelweed  . that is  the cure  ;-)

Good to know. 

tell em a witch told ya  ;-)