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?