You may have a 3rd tree.
The 3rd source of food is a spoiler: (highlight to read it) you need level 2 of your faction to make a potion with the special plant and get rid of the sharks (end of spoiler)
You may put the chief making magic food every 24 hours (our hours)
And there are the mushrooms!

And you may look at this thread in Forums List » The Games » Virtual Villagers 3: The Secret City » My villagers are dying!! What am I doing wrong? by LadyCFII

Also, don't quit the game in Fast Speed! But put it in Slow or in Pause until the tomorrow.

With 9 villagers, you get enough food but try to not do more babies!

Good luck!