The short version

Shells, food, wood, imp essences, etc. (anything that wasn't specifically designed to be left outside on display) that are left on the ground anywhere outside your tent will eventually get deleted by the game. Basically, if it isn't a pet, plant, or man-made structure then it's not safe if you leave it lying around outdoors on the ground.
- Note: Version 4.0.0 made it safe to leave these items on the ground inside your tent. In prior versions inside the tent is no safer than outside the tent.
- Shells on the ground will prevent new shells from spawning, regardless of whether they're in your tent or outside. Please see this page for more information.
If it was specifically designed to be kept on display (pets, trees, flowers, furniture, etc.) then you should be able to leave it on the ground without worrying about permanently losing it. We wouldn't design such an item and then make you keep it in your backpack/storage for safety!
- Shifts in the island's layout may slightly relocate such an item, or may make it disappear. If it disappears, it shouldn't be permanently lost—you should receive it back in your mailbox on the next Campaign Day.

The long version

There are two systems at work that will possibly move or destroy items lying on the ground.

There is a system in place to ensure that stuff does not lie around forever.

"Consumable" items will be deleted if left on the ground anywhere outside your tent for long periods of time, regardless of where they are. If this happens, the item is gone for good.
- Don't assume that, just because such-and-such item lasted a particular number of days without being deleted, that other similar (or even identical) items will last the same length of time.
- Note: Version 4.0.0 made it safe to leave these items on the ground inside your tent. In prior versions, inside the tent is no safer than outside the tent.
- Shells on the ground will prevent new shells from spawning, regardless of whether they're in your tent or outside. Please see this page for more information.
Decorations and pets stick around forever. Pets are birds, deer, etc., that you feed and harvest from. Decorations (e.g., lamps, plants, trees, and furniture) are clearly labelled as decorations. This stuff is safe to put on the ground and leave there forever.
- Despite being pretty, shells and food aren't Decorations in this respect!

The island of Cozy Grove often undergoes changes in appearance and layout. It is a limbo-esque island of ghosts, after all.

There's a sectioned-off rectangle around Flamey. Items placed inside this rectangle are safe from layout changes (more details in the next paragraph) but could still be destroyed via the clean-up system described above.
You will likewise find other similar sectioned-off areas as you progress through the game and the island expands. Like the sectioned-off area around Flamey, items placed here are safe from layout changes (more details in the next paragraph) but could still be destroyed via the clean-up system described above.
The interior of the tent is safe from both layout changes and the clean-up system described above.

Items outside of the "safe areas" described above will be affected by layout changes in the following manner:

If it's a permanent item (i.e., a pet or decoration), one of three things will happen:
- It will remain in the same place
- It will be relocated to another location nearby
- It will be be removed from the island but will show up in your Postal Inbox on the next campaign day. I.e., you don't permanently lose it.
If it's a consumable item (note that shells and food are in this category!), one of two things will happen:
- It will remain in the same place
- It will be deleted and is gone for good
Items near a bear are less likely to remain in the same place than items far from a bear. But ultimately any item can possibly be affected by island spookiness if it is outside of the safe areas.
There is some randomness involved. So just because a particular item remained in the same place this time, doesn't guarantee that the same will happen next time.