The short answer is: Yes.
The longer answer is: Eh, mostly.
If you want to be technical, nothing is really self-sustaining. In the end, entropy will win every time… but that’s a nihilistic way to look at things. It’s entirely possible to create a terrarium that lasts longer than you do!
Defining the “Self Sustaining Terrarium”
We’ve been throwing around this term “self-sustaining” for a while now, but what does it actually mean?
In this case, it’s a concept from ecology – the study of ecosystems. An ecosystem itself is the interaction between biotic and abiotic factors (living things and non-living things, respectively). A self sustaining ecosystem, then, is one that will last forever without adding or removing anything from the system.
More specifically, there shouldn’t be any matter entering or leaving the system. That includes the obvious things like taking out dead plants or adding new bugs… but it also means less obvious forms of matter like gases (air, for example).
And since we’re talking about matter, it’s only fair we look at energy as well. Trying to prevent the movement of energy in and out of the system would be a lost cause. Besides, we need light energy to run the thing! Almost all life on the planet ultimately derives energy from the sun, so it follows that your terrarium would as well. Secondly, energy will be lost in the form of heat being radiated out of the container (just as the Earth bleeds heat into space). Not too much you can do about that.
The idea is that whatever you put in the terrarium during its construction is all that it has to sustain itself. That’s the crux of the self sustaining terrarium and that careful balancing act is the challenge and joy of the hobby.
The Earth as a Terrarium
The Earth is a terrarium or, more accurately, a paludarium (which has both aquatic and terrestrial elements). As far as we know, it’s been pretty self sustaining since life first developed. It’s quite unlikely that at any point all life died out and then had to be reseeded by God or aliens or chemical caprice.
That’s not to say that Earth’s planetary ecosystem has always been stable. On the contrary, Earth has undergone some dramatic shifts in its short 4.5 billion year life.
During the earliest days of the planet, there was no atmosphere at all. But there were a lot of volcanoes. All that lava released tons of gases as it came to the surface. It built our first atmosphere! Granted, that atmosphere was composed of all kinds of nasty gases like methane, carbon monoxide, ammonia, hydrogen sulfide, etc., but it was a start.
It only took about 100 million years for the volcanoes to chill out a little and for all that water vapor in the air to rain down and form oceans. Another hundred million or so and we have the first semblance of life in some prokaryote microbes. Things get really interesting when the Earth turns 1 billion and cyanobacteria, or bluegreen algae, evolve.
All of the sudden we have photosynthetic organisms and guess what? BAM, oxygen. It took a couple billion years for it to really take off, but it marked the switch from environmental conditions repressing life to life determining the environmental conditions.
During the journey to the present day, the percentage of atmospheric oxygen fluctuated between 15% and 35%. Those are huge swings that totally changed the ecology of the planet, both the plant and animals that were dominant as well as biogeochemical processes that underpin it all.
But why are we talking about the history of the Earth? In order to illustrate that ecosystems aren’t static.
If you were watching at Earth as if it were a terrarium, you might be concerned to see that all the dinosaurs suddenly died. (That was super lame, you should have stopped the asteroid.) If you kept watching, you would have noticed that mammals slowly emerged as the dominant life form after reptiles vacated the position. That’s pretty neat, too.
Likewise, a terrarium you build will also be a dynamic system. It’s possible that some of your plants will die or that some will grow out of control, choking out the others. An animal might die and spike the ammonia in the system, killing other organisms.
Be prepared for that eventuality, and don’t be too disappointed when it happens. Know that life won’t necessarily adhere to your plans, but it will be interesting to observe either way.