I attempted to make a terrestrial biosphere a few months ago out of
one of those plastic containers that grocery stores use for holding
pasteries and such.
It had a 2.5cm of dirt in it X-x I think theres clay in it because it
held water really well. The surface area of the dirt was about 11cm X
10cm.
I placed in it a few small leafy plants, two different dandelion
species, the one that most people are familiar with, and a smaller
one that has "Hairy leaves". There was also a lot of moss.
I had place a small bottlecap in one corner and filled it with water
to make a sort of pond or pool of drinking water ^-^; (I remember
from a guide on caring for millipedes that they need a small amount
of drinking water). I had even made a mechanism from a chamber I
attached to the roof of the container and a sheet of plastic with
holes scattered throughout it to simulate rain.

For animals, I had:
4 wood lice(AKA "Pill Bugs")
3 millipedes
1 white slug X-x don't know the species

The animals all seemed pretty content with their environment for the
entire 4 months they lived in it @___@ but two things went wrong that
caused a total die-off of the biosphere(or at least I believe these
are the two main causes):
1- The rain simulator broke X-x and the biosphere started slowly
drying out(looking at it now theres some small holes in the roof from
where the rain simulator broke)
2- All of the moss started releasing spores at once, and untill
things started drying out, it looked like the spores were going to
choke everything out. Only one of the pillbugs seemed showed any
unhealthiness after it tho.

So now I have a biosphere full of dry, fairly loose dirt that could
hold water if I added some, full of dry dead plants, and animals.
One idea I was considering was filling dumping all of into a larger
container, adding some new dirt as well, perhaps some gravel or
rocks, more animals, maybe an earthworm to turn the dead stuff into
nutrients for the soil + new plants?

any advice anyone can give me to prevent another die-off, or at least
to prevent a total die-off for the same reasons as before?