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
What are LIDs? • “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) • Large, infrequent disturbances (LIDs) are unusually catastrophic, but ecosystems recover from them. Turner et al 1993
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.
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 wet-season burned sites • 50% in dry-season burned sites • Extended Mortality (next 24-30 months) • 35% of trees in unburned and wet-season 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.
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 - 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?