Deadwood Decomposers 
On their way to becoming piles of humus, fallen Douglas fir trees 
support an insect community numbering more than 300 species 
by Mark Deyrup 
In every forest ecosystem there is 
an insect community that is instru- 
mental in the death and rapid decom- 
position of trees. A year after its death, 
an insect-infested tree in the proper 
environment has been profoundly 
changed. The inner bark has been re- 
duced to a conglomeration of insect 
frass; the outer bark has loosened and 
is profusely perforated, allowing ex- 
tensive leaching by rainwater; the sap- 
wood contains numerous insect gal- 
leries from which fungal hyphae 
spread, extracting and concentrating 
nutrients from the xylem. In succeed- 
ing years, as the tree proceeds on its 
way to becoming an elongate pile of 
humus, it is host to innumerable in- 
sects, which are sustained by the en- 
ergy and minerals hoarded by the liv- 
ing tree for years before its death. 
This community of insects does 
more than mediate decomposition in 
dead trees, for some species attack 
and kill weakened trees in the manner 
of carnivores removing infirm ungu- 
lates from the herd. Insects usually 
give the coup de grace to suppressed, 
diseased, or injured trees and branches 
and to trees in unsuitable sites. Re- 
moval of these permanently stressed 
trees usually promotes the growth and 
stability of forest stands by reducing 
competition and recycling nutrients. 
Insects that attack weakened trees 
may also attack trees temporarily 
stressed by drought or defoliation. Re- 
moval of these basically healthy trees 
can drastically alter a forest stand and 
does not normally contribute to the 
health or stability of the stand. Over- 
stocked even-aged stands are com- 
posed largely of stressed trees that 
may be readily attacked by insects. 
If only the weakest trees are attacked, 
the stand is gradually thinned, freeing 
the stronger trees from competition. 
If the entire stand is attacked, it is 
likely to be replaced by another over- 
stocked even-aged stand that in turn 
may be devastated by insects. 
Inspired by the insect community’s 
dual role of architects and janitors 
of forests, I began a study of dead 
trees in the Douglas fir forests of west- 
ern Washington. I started with the 
idea that the insect community of 
dead Douglas fir was relatively simple, 
composed of a couple of dozen species 
prominent in the literature, but as I 
read further and dissected more and 
more dead trees, my list of species 
grew until it surpassed three hundred. 
My early elegant diagrams showing 
the relationships among all the Doug- 
las fir insects grew into monstrous tan- 
gles and were eventually discarded. 
Specimens of insects — identified, un- 
identified, and unidentifiable — piled 
up by the thousands. Some of the spe- 
cies had never been described. A sim- 
ple manual of insects living in dead 
Douglas fir, a manual I envisioned 
as a small handbook, swelled to more 
than six hundred pages. The proposed 
compact study of a few kinds of bark 
beetles, a few wood borers, and a ter- 
mite or two grew into an enormous 
project easily capable of absorbing a 
lifetime of research. 
The size and complexity of this in- 
sect community indicate that insects 
have found a great many ecological 
niches in the bark and wood of dead 
Douglas fir. There are two ways in 
which insects have carved a multitude 
of niches in what seems to be a simple 
resource. First, there is a hierarchy 
of trophic levels: the phytophagous 
species, which feed on bark or wood, 
support parasites, predators, and scav- 
engers. Second, many insects have 
unique feeding habits due to the nu- 
tritional and environmental variables 
in dead trees. 
A good example of a phytophagous 
species with a well-developed group 
of associated insects is provided by 
Scolytus unispinosus , a small bark 
beetle that breeds in the inner bark 
of recently killed Douglas fir, seldom 
attacking any other species of conifer. 
This species is restricted to small trees 
and branches that are partly or com- 
pletely exposed to sunlight. Its com- 
plex life history is similar to that of 
many other bark beetles. Host trees 
or branches are located by a female, 
which begins boring into the bark, usu- 
ally taking advantage of the shelter 
provided by bark fissures or twigs. 
As the female penetrates the succulent 
inner bark, she apparently releases one 
or more odor signals (pheromones) 
highly attractive to both sexes. Soon 
after beginning her gallery, she is 
joined by males and numerous females 
Each type of deadwood in a stand of 
Douglas fir — fallen logs, stumps, 
and branches — supports a vast array 
of insects. This faunal complexity 
multiplies because the different 
insect communities change 
as the wood decays. 
S.J. Krasemann; Peter Arnold, Inc. 
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