R. Eliezer b. Yaakov says: This (verse) is not needed (to tell us this). It is written (Bamidbar 18:21) "And to the sons of Levi I have given all the tithe in Israel as an inheritance" — Just as an inheritance does not "move" (i.e., is not rescinded), so, the first-tithe.
I might think that leket, shikchah, and peah, too, (see Vayikra 19) are subject to the tithe; it is, therefore, written "And the Levite shall come, for he has no portion and inheritance with you." (For) things in which he has no portion or inheritance with you you give him (a tithe), to exclude those things (the above) in which he does have a portion and an inheritance with you, (viz. Vayikra 19:10) "To the poor and to the stranger shall you leave them."