In early versions of Ars Magica, the pawn of vis was a quantum: you wither had a pawn, or you did not. There were no fractions of a pawn of vis. I can’t recall how early this was broken, but I certainly recall a vis source being given which was a cave of bats, with one pawn of vis for 100 pairs of bat wings. As you collected them, you had something you could not use. This meant that in a game sense, the pawn quantum was maintained. In a world-building sense, however, it meant that pawns were divisible into smaller units, which the Order could identify magically, but not use. This eventually culminated in the spark magic of the Soqotrans, which has a usable amount of of one tenth of the Hermetic.pawn, but recognises that even tiny amounts of vis have flickers of power in them, which can be bundled together for use, or eaten by magical creatures.
Eating vis has often been presented as a good idea. To make vis sources useful to classes of character other than magi, many of the early ones had magic effects, so they were consumed as single use magic items. A typical example was a tree of apples that contained Corpus vis, but if eaten healed wounds permanently. This didn’t formally creating warping, oh these many years ago, but something like warping occurred in the colour text, and so eating too much vis was probably not good for a person.
If the bats are somehow concentrating vis out of the world’s aura, then what happens to it if they die, but are not collected by magicians?
You might make an argument that the bats need to be collected alive. A parallel case might be that a magical fire needs to be collected before it goes out. In this case, vis cannot bioaccumulate: death makes the subtle connections between the magical energy and the physical material decay.
A counter might be that things need only be collected while they resonate with the Form. A fire that goers out is not longer a fire, but a bat that is killed and dried is, one might argue, still a bat. As such the form may be maintained and the vis retained.
If the vis remains in the dead object, then can it accumulate through biological processes? If, for example, dead bats pile up on the floor of the cave, can a magus collect them before they rot? If they are something a bit more durable, could there be a vast horde of vis? I’d argue that the vast hordes of vis are eaten by faeries, which explains why they are so rarely found. A cache of this type is, however, suitable as a starting point for a covenant.
If the object is destroyed, outside of laboratory conditions, can the form be maintained? You might argue that dissolution within the form may not cause harm. So, if I have vis enriched wine in a glass, and pour it into a barrel of wine, there seems a stronger argument that a magus could extract it again than if, for example, I mixed the wine into a bucket of clay. Might a similar argument be made for trophic biomagnification? As the snake is an Animal, does it suffice to retain the vis of the dead bats? Is there a loss in this process, or can it gather all of the vis? If a kingfisher then starts eating the snakes, does it in turn bioaccumulate vis? Is there loss at each level? Do the magi care if it means they don’t need to wade into guano to collect rotting bat corpses?
This might be a way of introducing the Hooks which provide the covenant with surprising resources. The player characters discover that their vis source is an accumulation from a lower trophic level, and they can get more by tracing back the creatures they have been harvesting to their feeding grounds.