Page 110 - Environment: The Science Behind the Stories

seeds as they chronicled the area’s 25 other arthropods. Most individuals
recovery. 20 Mudflow soon died, but the nutrients from their
Plant regrowth was very slow at 15 bodies enriched the soil, helping the
Dale’s study plots for several years and Number of species 10 Pumice community to develop.
then accelerated. After 20 years, 150 5 Once plants took hold, animals
species of plants covered 65% of the Ridge began exerting influence through
ground. One important pioneer species 0 herbivory. Caterpillars fed on plants,
was the red alder. This tree germinates 0 5 10 15 20 25 occasionally extinguishing small popu-
on debris, grows quickly, deals well with Years since eruption lations as they began to establish. As
browsing by animals, and produces (a) Species richness elk from surrounding forests moved into
many seeds at a young age. As a result, the region, Dale and her team fenced
it has become the dominant tree spe- 80 off “exclosure” plots to compare how
cies on the debris avalanche. Because 60 plants grew within the ungrazed exclo-
it fixes nitrogen (pp. 141, 144), the red Ridge sures versus in plots grazed by elk out-
alder enhances soil fertility and thereby Percent plant cover 40 Mudflow side the exclosures. Both types of plots
helps other plants grow. Research- 20 saw increases in plant cover and similar
ers predict that red alder will remain 0 Pumice amounts of species diversity, because
dominant for years or decades and elk herbivory can spur plant growth that
that conifers such as Douglas fir (which 0 5 10 15 20 25 compensates for what they eat. Non-
today are moving in and beginning to Years since eruption native species did best in the grazed
seed) will eventually outgrow them and (b) Percent plant cover plots, whereas species important for
establish a conifer-dominated forest. Figure 2 Plants recovered differently forest recovery did best in the ungrazed
Patterns of plant growth have at mudflow, ridge, and pumice sites at exclosures. Overall, elk herbivory did
varied in different areas. Roger del Mount St. helens. In the 25 years after not stall plant regrowth.
Moral of the University of Washington the eruption, (a) species richness of A surprising number of amphibians
and his colleagues compared eco- plants and (b) percentage of ground cov- and small mammals survived in refugia
logical responses on a variety of ered by plants both increased. Data from in the impact zone, researchers found.
surfaces, including barren pumice, del Moral, R., et al., 2005. Proximity, microsites, Many of these species were under-
mixed ash and rock, mudflows, and and biotic interactions during early succession. ground or dormant at the time of the
the “blowdown zone” where trees were Pp. 111–127 in V. Dale et al., eds., Ecological blast, and enough were able to survive
responses to the 1980 eruption of Mount St.
toppled like matchsticks. Numbers Helens. Springer, New York. once they made their way to the surface
of species and percent of plant cover that populations slowly rebuilt and
increased in different ways on each sur- Which substrate gained the spread from these few chance survivors.
face (Figure 2), affected by a diversity of greatest species richness, and All told, research at Mount St.
factors. Windblown seeds accounted what was its highest number of species? Helens has shown that succession is
On which substrate was the increase in
for most regrowth, but plants that hap- percent plant cover the slowest? not a simple and predictable process.
pened to survive in sheltered “refugia” Instead, communities recover from dis-
within the impact zone helped to and John Edwards of the University turbance in ways that are dynamic, com-
repopulate areas nearby. of Washington showed that insects plex, and highly dependent on chance
Chance played a large role in and spiders arrived in the impact zone factors affecting which species survive to CHAPTER 4 • S PEC i ES i n TERA CT i on S A nd Co mmuni T y E C ology
determining which organisms survived in great numbers before plants did. repopulate the new landscape.
and how vegetation recovered, del Insects fly, while spiders disperse by The results from Mount St. Helens
Moral and others found. Had the erup- “ballooning” on silken threads, so in also show life’s resilience. Even when
tion occurred in late summer instead summer the atmosphere is filled with the vast majority of organisms perish in
of spring, there would have been no an “aerial plankton” of windblown a natural disaster, a few may survive,
snow, and many of the plants that arthropods. Trapping and monitoring and their descendants may eventually
survived would have died. Had the at Mount St. Helens in the months build a new community.
eruption occurred at night instead of in following the eruption showed that Ecological change at Mount St.
the morning, nocturnal animals would insects and spiders landed in the Helens is still in its early stages and will
have been hit harder. impact zone by the billions. Research- continue for many decades more. All
Animals played major roles in the ers estimated that over 1500 species along, ecologists will continue to study
recovery right from the beginning. In arrived in the first few years, surviv- and learn from this tremendous natural
fact, researchers such as Patrick Sugg ing by scavenging or by preying on experiment.
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