Transcript Slide 1

When Large, Infrequent Disturbances
Interact
Dahl Winters
October 28, 2005
Outline
•What are LIDs? Single LIDs and Their Consequences
•Examples of Interacting LIDs and Their Consequences
- Compounded Perturbations Yield Ecological Surprises (Paine et al
1998)
- Interactions of Large-Scale Disturbances: Prior Fire Regimes and
Hurricane Mortality of Savanna Pines (Platt et al 2002)
- Comments on the P-T Extinction
•How Human Disturbances Compare With Natural LIDs
•Questions for Future Research
Disturbance Interval / Recovery Interval
What are LIDs?
10.0
5.0
1.0
equilibrium
or
steady state
A
0.5
0.1
stable,
low
variance
stable,E
very high
variance
stable,
high
variance
B
C
stable,
low
variance
“A disturbance is any
relatively discrete event in
time that disrupts
ecosystem, community, or
population structure and
changes resources,
substrate availability, or the
physical environment.”
(White and Pickett 1985)
D
0.05
F
0.01
unstable
system,
bifurcation
or crash
0.25
0.50 0.75
Disturbance Extent / Landscape Extent
Turner et al 1993
Large, infrequent
disturbances (LIDs) are
unusually catastrophic,
but ecosystems recover
from them.
Examples of LIDs – Extent and Severity
Foster DR et al. 1998. Landscape Patterns and Legacies Resulting from Large, Infrequent Forest Disturbances.
Ecosystems 1: 497-510.
Examples of LIDs - Extent and Duration
Foster DR et al. 1998. Landscape Patterns and Legacies Resulting from Large, Infrequent Forest Disturbances.
Ecosystems 1: 497-510.
Comparison of LIDs – Spatial and Temporal Characteristics
Examples of Interacting LIDs and Their
Consequences
Why Study Interacting LIDs
We know that ecosystems are always
recovering from the last disturbance, but
how might recovery be affected after a
flurry of intense disturbances?
This is an important question, given the
increasing frequency of LIDs due to
both climate change and human land
use.
Also, large anthropogenic disturbances
can interact with natural LIDs to yield
greater detrimental effects.
Example of human-caused LID. For scale, note
the Himalayas at right.
MODIS Imagery: northwest India (10/24/05) –
intense agricultural burning in the Punjab region
south of Kashmir, which produces 2/3 of the
country’s food.
Disturbance Interactions Are Common
Lecture by Peter White, Sept. 9, 2005
Compounded Perturbations
If no other disturbances occur
during recovery from a single
LID, the ecosystem can
rebound to its previous
condition.
If not, it can transition to a new
stable state.
Summary of Six Examples
ENSOs, storms, kelp bed recovery – warmest waters led to kelp
extinctions, and replacement by a different kelp species, preventing
recovery
Climatic extremes and exotic species in San Francisco bay – physical
disturbance (drought followed by flood) made way for a biotic
disturbance (establishment of P. amurensis); declines in zooplankton
and fish
Boreal forest wildfires, forest fragmentation, and logging – climate-driven
fire frequency changes did not change forest community composition,
but increased fire frequency from homesteading and logging did
Summary of Six Examples
Early succession and exotic species – ashfall and invasion of Myrica,
changing N cycling and ecosystem composition to favor other exotics
Hypoxia in northern Gulf of Mexico – eutrophication and adverse
hydrological/meteorological events shifts community composition to
mostly ruderal species
Phase shifts in Jamaican coral reefs – 2 major hurricanes reduced urchin
and fish grazers = algae takeover, preventing corals from recovering
Effects on Succession
Supporting study by Turner et al
1998:
Gray to black: Single LIDs to
compounded LIDs.
As disturbance frequency
increases, succession
pathways run the risk of being
qualitatively altered.
Turner MG, Baker WL, Peterson, CJ, and Peet
RK. 1998. Factors Influencing Succession:
Lessons from Large, Infrequent Natural
Disturbances. Ecosystems 1: 511-523.
Fires, Hurricanes, and Pine Mortality
MODIS Imagery: Fires in Southeastern US (10/18/05), and Hurricane Wilma a day after
it was the strongest hurricane on record (10/20/05).
Research Summary
Divided remnant Everglades pine savannas into unburned (natural),
burned-wet season (natural), and burned-dry season (anthropogenic)
areas
Measured direct mortality during Hurricane Andrew 10 years later
Measured extended mortality 24-30 months after that
Bayesian model averaging used to determine the best model to explain
observed mortality patterns.
Hurricane Mortality Due to Prior Fire Regime
Direct Mortality
<30% of trees in unburned or wetseason burned sites
50% in dry-season burned sites
Extended Mortality (next 24-30 months)
35% of trees in unburned and wetseason burned sites
90% in dry-season burned sites.
In the long term, natural wet-season
burning actually increases tree survival.
However, human dry-season burning
causes an extremely high mortality (90%).
Conclusions
Mortality found to increase with tree size and dry-season fire, and decrease
with stand area (direct mortality) and wet-season fire (extended mortality).
 These results weren’t predicted from fires or hurricanes alone.
Concluded that altered fire timing (anthropogenic dry-season fires) strongly
influenced the effects of subsequent hurricanes on pine mortality in this
ecosystem.
 Differences in initial LIDs can change the effects of subsequent
LIDs.
The P-T Extinction: Interacting LIDs?
1. Pangaea formed ~250 Mya:
species evolved in isolation were
brought together.
2. Millions of years later, a bolide
impact, off the NW coast of today’s
Australia. Recovery time: millions of
years.
3. Within 100 kya later, the Siberian
Traps - enough lava erupted to cover
the Earth’s surface 20 feet deep.
= loss of 96% of all species on Earth.
Images: NASA, Wikipedia
Mundil R et al. 2004. Age and Timing of the Permian Mass Extinctions: U/Pb Dating of Closed-System Zircons. Science
305(5691): 1760-1763.
How Human Disturbances Compare With
Natural LIDs
Same Spatial Extents, Longer Time Spans
August P. et al. 2002. Human Conversion of Terrestrial Habitats. In Gutzwiller, KJ. Ed.
Applying Landscape Ecology in Biological Conservation. Springer. p. 198-224.
Dependent on Population
Linear relationship between
human population density and
disturbed habitats.
Disturbance increases in scale
as population density
increases (C vs. A).
August P. et al. Human
Conversion of Terrestrial
Habitats. In Gutzwiller, KJ.
Ed. Applying Landscape
Ecology in Biological
Conservation. Springer. p.
198-224.
Human Disturbances Over Time…
To protect natural
ecosystems while
maintaining
population growth,
current farmlands
will need to feed
more people, who
will need to be
crowded into
already present
cities.
Foley JA et al. 2005.
Global Consequences of
Land Use. Science 309:
570-574.
…and Over Space
Today, croplands and
pastures together cover
~40% of the Earth’s land
surface.
Over the past 40 years,
population growth fueled
by a ~700% increase in
global fertilizer use.
Over the past 300 years,
7-10 million km2 of forest
lost—the largest
hurricanes only affect an
area of 0.1 million km2
(Foster et al 1998).
Foley JA et al. 2005. Global
Consequences of Land Use.
Science 309: 570-574.
Questions for Future Research
Questions for Future Research
- The results of interacting disturbances are unpredictable from those of single
disturbances. Is this true, or might there be some information we can take from
single-LID or other research to predict the effects of interacting LIDs?
- Differences in initial LIDs can change the effects of subsequent LIDs. With the
huge disturbances required by growing modern industries, can we realistically
expect to maintain stable ecosystems over the long run?
- What kinds of “ecological surprises” might we expect from interactions
between human disturbances and natural LIDs?
- What are the management consequences of ecosystems shifting to new stable
states?
- Could humans reverse state shifts deemed irreversible by natural means over
the long term? What would this take?