The members of the same species share some morphological features. They are able to interbreed and produce fertile progeny. if two or more organisms share common morphological traits and are able to interbreed, they belong to the same species.
The organisms from different species can not interbreed; if they interbreed, the produced progeny is sterile.
personally I think that since there no oxygen on mars the extraterrestrial creatures or alien plants may have alternative means of survival even if there ain't water
