I could be wrong here, i gave my copy to my sister and havn't gotten it back in a few months, so I can't provide exact page numbers.  I think in the beginning of FoC, the scene where Tavi and Bernard first encounter the herdbane, Bernard mentions that it's unusual for them to be there.  Something about them being found near the feverthorn forest, I think to the south.  It's been a while since I read it, but that would mean a least one other land connection to the south, if not a continuous one.  Maybe a mountain range or the forest itself makes it hard for large numbers of either side (Marat or Aleran) too get through.  If someone could check their book, I'd appreciate it.  Thanks.