Why would a mage choose to harvest weak residual anima when wisps anima-filled capsules waiting to be taken?

Question

In my world, wisps are more or less vessels for souls on their journey to the afterlife or reincarnation. These wisps coat the soul in a shell of anima so to speak, allowing their movement and energy. This anima is present within every living being, to the smallest of weeds to the largest of animals, and even the sentient races in between. It is this world's equivalent to mana, relying upon a force of will and mental fortitude.

Anima, without a host, can be a great boon within the magitech of the world, when placed within a Paeric Shard, a crystal known for their great potential as mana batteries, can allow a variety of machinery to function.

Wisps can contain the souls present from each form of life upon death, but like the anima found in each. A Wisp of a typical animal, or a plant, is easily discerned from that of a sentient self-aware being, or elemental spirit.

Knowing Wisps can be great sources of Anima when captured and emptied into a Paeriic Shard, despite containing souls, what could prevent them from being harvested en masse with non-sentient wisps being easily distinguished?

Answer 1

Quality and Harmonic Dissonance.

Lower sentience produces lower quality Anima. Thus, you would need many lower quality Wisps to power the same machines. However, Wisps are naturally Harmonically Dissonant. This causes a negation of Anima generation if too many wisps are harvested into the same Paeriic Shard. Many low quality cannot outdo a single higher quality wisp in this instance because the Anima naturally fights against itself.

Answer 2

Plants, insects, birds and many animals are innately aggressive. Plants grow outward as quickly as they can. Insects, birds and predators spend their entire lives hunting for prey.

In these pre-sentient beings, these instinctive drives take the place of will power, governing the anima, focusing it to a single purpose, the survival of its host. Even after the being's life ends, the taint of that governing instinct remains within the anima. This makes the anima uncooperative to the will of a magitech caster, with each small morsel of soul power wanting to pursue its original life's work.

It is too much work to overpower these diverse instinctive drives. Even when one manages to retrain a sufficiently large amount of life-harvested anima, there remains the risk of reversion, where one aberrant morsel of power reminds the entire flock of their original purposes. Chaos follows.

Using life-harvested anima, either sentient or pre-sentient, in complex magic is akin to training a family of fleas to perform circus acts. The results are usually catastrophic. Everyone ends up itching.

Answer 3

If your world includes religion of a sort, it almost absolutely includes injunctions against harvesting souls. To do so would be wildly taboo. If your wizard is power-hungry and doesn't care about that sort of thing, then perhaps the real physical consequences of harvesting floating souls make themselves abundantly clear when other anima see what he/she is doing and punish them. It's up to you to decide what that punishment might look like.