Regulators and terminators: The importance of biotic factors to a healthy forest

Of all the extraordinary numbers of species of insects and other arthropods, nematodes, fungi, bacteria, and viruses that exist in the world's forests, only a relatively small percentage affect trees or forests to such an extent that they are considered to be problematic. Most have beneficial or at least harmless functions in the ecosystems in which they occur. What is it about this troublesome minority of species that makes them damaging to forests? The answers to this simple question are complex, and have far-reaching ramifications in the way we view forest ecology and management. At the broadest level, the answer begins with recognizing that native organisms living in their natural environments are only problematic when a human dimension is introduced. The human role can take many forms including (1) management practices (e.g., commodity production) that create favorable conditions for insects and disease, (2) management objectives that view natural ecological processes as undesirable (e.g., insect outbreaks or diseases that damage forest resources) even though these processes occurred in the absence of human influence, and (3) the movement of insects, pathogens and/or trees out of their native ranges into new regions or continents; and thus the creation of new and often unpredictable interspecific interactions. Absent the human dimension, forest insects and pathogens act as natural thinning agents causing the tree mortality that is needed to cull the weak competitors and release resources that are needed to support the growth of the surviving trees (see Chapter 8).