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Transcript
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. 3