So I have a fortress that just went from a population of 60-something in its second year to a population of 2. This is my third fortress; the second one is doing well and I decided to have some fun in the first one (digging into the river and experimenting with walls).
(I'm new, obviously.)
I usually pick a world with the default "everything is easy" layout. I selected a temperate environment because working with ice is easier than digging in water.
It started at embark, when I started up a plump helmet farm and I found plump helmet spawn all across the tavern/dining hall. I found a single rope reed plant near the river, pulled it up, and started a rope reed farm with auto processing.
Then, I designated an area for gathering. I set up a few stockpiles for food, queued a bunch of barrels, and let it run. After the first summer, I queued up another 25 rock pots.
Fast forward to the second winter. The river has just froze over and I've started digging around to set up a well. Suddenly, I start to get a steady list of "Urist McDwarf, Dwarf, has been found dead, dehydrated" messages.
Turns out that when I queued the rock pots, my entire misc. stockpile had been completely filled with rope reed cloths and threads. Also, when I checked my food stockpiles, every single tile was occupied by either a barrel of fish or a single walnut. Thus, my dwarves had nowhere to store their pots, and the booze supply dried up fast.
Moral of the story: Walnuts are bad for your dwarves. Don't use them.
(At least the two remaining dwarves are male and female, if migrants get scared off by the 60 miasma generators in the doorway.)