The Harpy's looked pretty well established though, Hdawg gets captured just after the Ship crashes. The Grove was already pretty much out of food at that point he really messed up on the logistics i dont think he cut out for being grove head, he pretty much says he hated it after he bails to join you
There were only a few harpies and a couple nests. They could've only appeared in the last month or so. Which would align with all the refugees who the druids seem to think are attracting more creatures to the grove than they've ever had to deal with before.
Which also makes sense. Harpies and gnolls like to eat sapient creatures. No large population of people on the roads no gnolls and harpies. Granted we know the gnolls, goblins, and drow seem to be a product of the Absolute, which Halsin had been studying for quite a while according to Nettie. So all this has been going on for longer than we've been in the area.
As for food, that is just kind of what happens when a huge population of foreign people move into a region without an abundance of farmlands. The druids likely lived off the land and what their magic could provide. Druid groves tend to be sparsely populated. They wouldn't be able to sustain however many tieflings showed up on their front door for long, and the food shortage is probably what encouraged Halsin to risk his life to get to the bottom of this sudden rise of monsters in his region in the first place.