Years ago I climbed trees to sample insects high in the canopy. Here is a picture of (part of my boot and) colleague Tim Dilworth, now a PhD. We suspended sampling devices (sticky boards) a different levels and recorded species at each level through an Ohio hardwood. We found unknown insects.
Importantly, the experience gave me true “new perspective.” I had lectured on "the world from 5-feet up," a common human viewing height. To understand the natural world requires snorkeling and digging in earth pits and laying prone on the forest floor for extended periods for things change rapidly. It requires seeing the blended layers of the forest canopy for just as mammals and some birds defend territory and are area and habitat specific, many insects, bats, and birds are layer specific. The world is not flat to them but three dimensional and they know the bounds inside of their “box.”
Animal density, the numbers per unit area, may not be the appropriate expression for analyzing populations animals per unit of available quality volume may be the measure. Its use will reduce variation in studies and help understand and predict future populations.