All living things form communities, passing material and energy back and forth. Almost all communities on land rely on plant-like beings for their energy.
The key to understanding an ecosystem is understanding interdependence as a question of probability and long-term dynamic equilibrium in materials and energy.
Plants use the light energy of the sun to drive chemical reactions which build carbon dioxide and water into more complex molecules, ending in sugars.
The key to a healthy ecosystem is biodiversity: while many organisms may be lost, some are keystone species, others act as buffers in lean years.
Plants and animals break down a variety of chemicals including sugars, originally derived from plants, to get the energy they need to carry out life functions.
The economist's view of a forest sees productive trees and non-productive trees as the only components, and regards non-productive trees as expendable.
The ecologist's view of a forest sees many complex webs of life forms, all interacting with and depending on each other to maintain the forest's integrity.
The ecological model of a forest sees a so-called non-productive tree as one which acts as a key shelter resource to many other parts of the forest ecosystem.
In any given ecosystem, many of the existing species would be replaced (so far as their role is concerned) by other species if they were entirely removed.
In any given ecosystem, there are some species, called keystone species, and if one of these is removed, the system will crash because they cannot be replaced.
In most ecosystems, because producers are more interchangeable, the keystone species is a top-order predator, referred to sometimes as the keystone predator.
In most ecosystems, the only certain way of identifying keystone species is to remove them, and then wait to see if the system crashes disastrously.
Some of the rivets in a bridge can be removed without the bridge being harmed, but nobody ever tests this principle by removing rivets until a bridge collapses.
These are the considerations which need to be taken into account when the extremely complex question of logging old growth forest is being considered.