Download The Resilience of Ecological Systems

The Resilience of
Ecological Systems
In the Cascade Mountains of
Washington State, the morning of
Sunday, May 18, 1980 was bright
and sunny. From his ridge-top
observation post, David Johnston
had a clear view of Mt. Saint
Helens, 10 km away. Only 30, he
had been one of the first geologists
on the scene when Mount St. Helens
started stirring after 123 years of
inactivity. At 8:32 AM, Johnston’s
voice crackled excitedly over the
USGS radio, "Vancouver!
Vancouver! This is it!"
Hot magma and gases exploded out
of the side of the mountain with the
force of 500 atomic bombs.
Traveling at 1000 km per hour, the
wall of superheated air and debris
obliterated 600 square kilometers of
forest in less than a minute. David
Johnston and 60 others died in the
blast.
Thanks to the dedication of
scientists like David Johnston, Mt
Saint Helens has provided a treasure
trove of information about volcanic
eruptions. But the lessons learned
didn’t stop at geology. Over the
past 25 years, field work at Mount
St. Helens has helped revolutionize
our understanding of how
ecosystems recover from large scale
disturbance.
Mt. St. Helens May 18, 1980.
Twenty-five years ago,
conventional theories of succession,
predicted that the devastated
ecosystems around the volcano
would slowly rebuild, starting with
colonizations by a few tolerant
pioneers from surrounding areas.
These early species would make the
disturbed area more amenable for
invasion by another set of species.
After a predictable series of
intermediate communities, the
successional changes would
eventually culminate in a stable
climax community similar to the
one that existed before the eruption.
What actually happened was far
more interesting and complex.
Organisms from peripheral habitats
did invade, but the colonizers were
much more diverse than expected,
ranging from early to late
successional species. Early
colonizers made some sites more
viable for later arriving species, but
just as often climax trees established
themselves on bare lava flows.
Chance played a major role in what
species survived the blast and in the
subsequent recovery process. If the
eruption had occurred in midsummer, for example, succession
would have taken much longer and
the surviving set of species would
have been very different. In May,
lakes were still covered with a
protective layer of ice and numerous
snow banks sheltered organisms
from excessive heat. These areas
provided oases of life that
expanded outward into more
disturbed areas after the eruption.
In general, the Mount St. Helens
ecosystem recovered much more
rapidly, via more diverse pathways,
than expected.
In the natural world, all ecosystems
are subjected to continuous
disturbance from both biotic and
abiotic factors. Diseases and
parasitic infections may sweep
through a community causing major
reductions in population
abundances. Alien predators or
competitors may invade and cause
the extinction of native species.
Physical disturbances, such as
hurricanes, fires, and droughts, can
Fig. 1. Scaling relationships of natural
disturbances with respect to
frequency, severity, and area.
occur over a range of different
frequencies, severities, and sizes
(Fig. 1). To predict the ecological
impact of disturbance, we need to
understand how disturbance on
different scales affects the structure,
diversity, and recovery of a given
community. Large, infrequent
disturbances are especially
important because their effects
occur over large areas and may
persist for thousands of years.
Other major disturbances
intensively studied by ecologists
include the 1988 fires in the
Yellowstone ecosystem that burnt
36% of the forest, and the
prolonged, widespread flooding in
the mid-western United States in
1993. Because these disturbances
affected such large areas, spatial
variation in disturbance was much
greater than that occurring in
smaller perturbations.
The Yellowstone fires burned over a
complex topography and left a
mosaic of severely burned and
relatively unscathed patches of
forest. Similarly, at Mount St.
Helens, areas sheltered by hills and
snow banks were much less affected
than more exposed parts of the
landscape, while areas covered by
hot, pyroclastic flows were
essentially devoid of life. In the
midwest floods of 1993, the
duration of flooding varied with
differences in elevation and
Top: Seedling-sapling stage of the
Yellowstone lodegpole pine forest 8
years after the 1980 fires.
Bottom: The mature successional
stage of lodgepole pine forest 80-150
years after fire.
drainage. Again, some areas were
relatively unscathed while others
lost virtually all their vegetation.
The major consequence of this
spatial variation in disturbance
intensity is a corresponding
heterogeneity in surviving
organisms and seeds. The
abundance and distribution of these
leftovers from the pre-disturbance
community greatly influence the
speed of successional change, as
well as the structure, composition,
and diversity of the recovered
communities. Ecologists now view
large, infrequent disturbances as
“vast editing processes” in which
some biotic elements are deleted,
others transformed, and still others
unaffected.
Studies of large disturbances over
the past twenty-five years have
shown that ecosystems have a
remarkable ability to return to their
former state after severe
perturbations. Ecologists call this
property resilience. Resilience is a
measure of the magnitude of
disturbance a system can absorb
before it fails to return to its
previous state and, instead, enters
another state. Under certain
conditions, ecosystems can be
pushed beyond their resilience
limits and flip into an alternate state.
When the new state represents a
seriously degraded system with
fewer species and impaired
functioning, we call the change in
state a collapse or catastrophic shift.
In some cases catastrophic shifts are
precipitated by two or more large
disturbances, with the second
occurring before the system has
recovered from the first. The coral
reef community near Discovery
Bay, Jamaica, provides an example.
In the late 1980s a collapse of the
reef system occurred as it shifted
from a coral dominated state to an
algae dominated state. Coral cover
declined from 30-60% to 5%, while
algal cover climbed from 5% to
70%. The severe disturbances that
led to the shift were two major
hurricanes in 1980 and 1988 and the
mass die off of a major algal
predator, the sea urchin Diadema,
from 1982-1984.
alternative state characterized by
turbid water, very high levels of
phytoplankton, and loss of the
submerged plant community.
This state shift is caused by
increases in nutrients, primarily
phosphorous, that result from
human activities. The pristine state
is highly resilient to changes in
nutrient levels because the large
aquatic plants promote several
feedback mechanisms that stabilize
the system. Most importantly, they
absorb excess nutrients from the
water column, provide hiding places
where phytoplankton grazers such
as Daphnia are protected from fish
predation, and prevent nutrients at
the bottom of the lake from reentering the water column and
fertilizing algal growth.
Nutrient levels can increase
substantially with no visible change
in a lake until a threshold is reached
which exceeds the capacity of the
aquatic plants to stabilize the
system. For example, when a storm
causes excessive nutrient runoff
from surrounding farms, sewers,
and storm drains, shallow lakes can
quickly collapse as phytoplankton
populations increase exponentially.
Phytoplankton densities become so
high that they reduce light levels
Hurricanes, pollution, over fishing,
and the die off of sea urchins have
led to catastrophic shifts in some
coral reefs to a state dominated by
algae.
Given that the larval recruitment of
all coral species ceased in the mid
1980s, the algae-dominated system
appears to be quite resilient, and,
barring major increases in
herbivore populations, the return of
the system to its former state
appears unlikely.
Shallow lakes provide another
example of catastrophic shifts in
natural systems. Their undisturbed
condition is typified by clear water
and abundant submerged
vegetation. However, abrupt
transitions can occur to an
2
Shallow lakes can rapidly shift from
clear water with large bottom plants
to an alternative state with extremely
high phytoplankton densities.
and the submerged plants die. As
the bottom plants disappear,
Daphnia and other herbivores lose
their hiding places and their
populations plummet from
increased predation.
If a disturbance does cause a shift
to state 2, the system would gain a
new set of properties, including new
species, a different resilience, more
or less productivity, etc. Finally, if
the system exists under condition 3,
it is again quite resilient, but will
now return from even large
disturbances to state 2.
Ecologists have identified a
growing number of catastrophic
shifts in ecosystems including
desertification in China, dead zones
in the Gulf of Mexico, the loss of
woodlands in Africa, and the
Fig. 2. Graphical model illustrating
resilience and alternative stable states collapse of the Bering Sea food
(see text for explanation).
web. These ecological changes are
Figure 2 shows a simple graphical caused by disturbances resulting
from human activities. Global
model illustrating the relationship
warming, the introduction of alien
between resilience, disturbance,
and alternative stable states. Think species, massive deforestation, and
of the marbles as containing all the the extermination of top predators
are pushing many ecosystems into
properties of a given ecosystem
less resilient states. As the human
collapsed into three dimensions.
population climbs towards ten
The valleys in the grey cross
billion, ecosystem collapses are
sections represent stable states of
likely to increase significantly.
the system. The arrows symbolize
disturbances. The steepness of the
valley is an indicator of how fast the References
system will return to a stable state
Gunderson, L. 2000. Ecological
after a disturbance, and the depth
Resilience in Theory and Practice.
and width of the valleys are
Annu. Rev. Ecol. Syst. 31: 425-439.
indicators of the system’s resilience. Schefer, M., S. Carpenter, J. Foley
Under environmental conditions 1, and B. Walker. 2001. Catastrophic
the system is highly resilient. It will shifts in ecosystems. Nature 413:
591-596.
return to stable state 1 (red marble
Turner, M., W. Baker, C. Peterson,
at low point of valley) even if a
disturbance pushes it far to the right. and R. Peet. 1998. Factors
Influencing Succession: Lessons
As it returns, it will regain its
from Large, Infrequent Natural
former properties (i.e. successional Disturbances. Ecosystems 1:511changes will occur).
523.
Now imagine that the environment
has changed and the system exists
Brad Lister
within conditions 2. Under this set
RPI, June 2006
of conditions a second stable state
© Thomson Learning, Inc.
exists. The valley representing state
1 is also narrower and more
shallow, and a smaller disturbance
could cause the system to shift into
alternative state 2 (blue marble at
bottom of right hand valley). In
other words, the system’s resilience
has been reduced.
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