Dear Rabbi Kornfeld,
According to the gemorrah in Taanis 17B, we are told that Biblical laws do NOT reqire Rabbinic support. How does one explain this statement, bearing in mind what it states in Pirkei Avos, "That one should make a fence around the TORAH.
Beis Din must make Takanos where necessary, in order to protect the Torah. We learn this from the Pasuk u'Shemartem Es Mishmarti (va'Yikra 18:30). The Gemara in Ta'anis is saying that in this case, the Din Torah is not at risk, so there is no need for a Takanah. However, since people tend to be more lax where a Rabbinic law is involved, the Chachamim found it necessary to protect the Din d'Rabanan, even though there was no need to protect the Din Torah.