Larger vertebrates may form a cru- 
cial link in the process of vegetation 
recolonization. Where heavy ash or 
mudflows dried to form a hard, uni- 
form crust, there are few cracks to 
shelter germinating seeds. But large 
animals wandering in search of food 
or water make tracks that trap seeds 
and provide microsites suitably miti- 
gated for germination and growth. 
Insects suffered incalculable tolls, 
primarily from the ash effects of the 
May 18 eruption and from sizable 
blasts that occurred on May 25 and 
June 12. Abrasion of the exoskeleton 
and ingestion of ash during preening 
were major causes of death. Studies 
of agricultural areas in central Wash- 
ington have revealed that although 
honeybees and other beneficial insects 
suffered greatly, most insect popula- 
tions recovered quickly. Nearer 
Mount St. Helens, where higher al- 
titudes mean a later spring, fewer in- 
sects were exposed at the time of the 
eruption. In the blowdown area, many 
species have been encountered, but 
except near water, their numbers are 
reduced. I noted that seed set in lu- 
pines, which are dependent on insects 
for outcrossing (the production of off- 
spring from individuals of the same 
species but different strains), was poor 
even where the vegetation received lit- 
tle damage. I believe this poor seed 
production was a result of the relative 
paucity of pollinating insects. 
Insect colonists will come from 
pools of insects outside the blast zone, 
and new insect communities will even- 
tually develop. Will these communi- 
ties be similar in species number and 
composition to communities in similar 
but unaffected habitats? Daniel Sim- 
berloff’s experiments during the 
1960s, in which all insects were re- 
moved from a series of very small 
Florida keys, indicate that such com- 
munities will soon return to the same 
number of species but that the com- 
position, largely a matter of chance, 
will be quite different. In the Mount 
St. Helens blast zone, the habitat is 
larger than in the island experiments 
and there is no significant barrier to 
migration. Will this alter the results? 
Will novel, stable assemblages of in- 
sects be formed? 
Food limitations present surviving 
insects and early immigrants with 
unique challenges. Species with the 
most generalized requirements are 
predicted to be successful, and some 
early observations in the blowdown 
area support the idea that survival 
requires adaptability. In the weeks 
after the big eruption, when ash was 
ubiquitous and aphids were rare, Jerry 
Franklin and I observed a ladybug, 
normally an aphid predator, feeding 
directly on the sap of a bracken fern 
(Pteridium aquilinum) that had 
emerged from the ash. Such short- 
circuited food chains should return to 
normal as the vegetation, on which 
aphids and other herbivores depend, 
recovers. 
Because insects have short genera- 
tion times and are often early colo- 
nizers, entomologists are likely to ob- 
serve microevolutionary events, which 
involve shifts in gene frequency rather 
than speciation. For example, where 
biotic recovery requires immigration, 
the first individuals of a species to 
invade an area are, not surprisingly, 
Far left: Twelve miles or so west of 
Mount St. Helens (visible in the 
distance), clear-cut and forested 
areas along the North Fork of the 
Toutle River escaped direct effects 
of the eruption. Large, rapid 
mudflows, however, buried the 
river's flood plain. Left: Closer to 
the volcano, trees were blown down 
and a hot debris flow, placed on top 
of the mudflow, swept all life from 
the upper Toutle Valley. 
usually the best able to disperse. Re- 
duced competition and the absence 
of predators in their new environment 
may permit a “founder effect” to oc- 
cur, that is, the genetic differences 
between founding individuals and the 
average members of the donor pop- 
ulation become fixed. Such differ- 
ences will probably be minor, but if 
they occur repeatedly in many species, 
biogeographical processes that are 
known to operate between islands will 
have been shown to be significant for 
evolutionary processes in terrestrial 
situations as well. 
The effects of the eruption on vege- 
tation in various zones are also under 
study. In forests within a few miles 
of the mountain, the initial ash de- 
posits fell wet because of eruption- 
induced thunderstorms and covered 
the trees and ground with a thick goo, 
which soon dried to form an imper- 
vious, cementlike layer. Ash on vege- 
tation interrupts gas exchange and 
curtails photosynthesis. Jini Seymour 
of the University of Washington mea- 
sured temperature in ash-coated silver 
firs ( Abies amabilis) and found them 
to be more than 30° F warmer than 
adjacent leaves from which the ash 
had been removed. Fortunately, since 
heavy ash fell prior to bud burst in 
higher elevations, much ash-free new 
growth is now present in most of the 
surrounding forest. As ash-covered 
leaves are washed clean by precipi- 
tation or replaced by new growth, co- 
nifer productivity should return to nor- 
mal. Some species, such as red cedar 
( Thuja plicata), retain ash more te- 
naciously than others, such as Douglas 
fir ( Pseudotsuga menziesii), but 
39 
