SEE-U 2000: Atlantic Forest - Module 12

Module 12 at:

• | Biosphere 2 Center (Arizona)
• | Black Rock Forest (New York)

Exercises for Module 12:

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Exercises for Module 12:
Module 12 at:

# Atlantic Forest ## Module 12: Disturbances
Printer format ### Disturbances - Historical and Recent #### By Dr. James A. Danoff-Burg, Columbia University ### Disturbances in Scientific Thought Until just recently, it was assumed among most European cultures that disturbances were not a common occurrence, with the exception of a single major Noachian flood. Consequently, until around 250 years ago science assumed that the Earth has been more or less as it has always been. Earthquakes, glaciation, tidal waves, catastrophic flooding and other massive terrestrial catastrophies had not entered the scientific community, except as myth. The earliest geologist to recognize that the Earth has been drastically different from its current construction was Georges Cuvier in 1812, and even he did not conceive of major catastrophes, major disturbances that simultaneously affect most of the Earth. Instead he thought that all catastrophes (or "revolutions" as he called them) were sudden and local events. He also thought that these local catastrophies led to novel creation events, thereby explaining the differences in fossil forms that are encountered in different geological strata. Potentially catastrophic events such as glaciation, plate tectonics, global warming (Module 15), the ozone hole (Module 15), and other global phenomena could not have been a part of science. Charles Lyell subsequently transformed our understanding of both catastrophes and their impact on biodiversity. As part of the philosophy of uniformitarianism, Lyell and his adherents a few decades later asserted that catastrophes have repeatedly happened across the earth, that the same types of catastrophes continue to happen, and they are important transformative events. Under uniformitarianism, there are not separate creations of new life forms. Instead, the catastrophes merely change the environment. Darwin was greatly influenced by Lyell and his emphasis on catastrophes in reshaping and restructuring the Earth's biodiversity. So influential was Lyell that Darwin cited his Principles of Geology as being one of the most significant books that he had read before writing the Origin of Species. An important phenomenon that greatly impacted on our understanding of the role of the impacts of disturbances on restructuring ecosystems was the 1883 eruption of the Krakatau volcano in Indonesia. This volcano erupted with such force that the explosion was heard on 1/13th of the Earth's surface. It was said to be 21,547.6 times as powerful as the atomic bomb dropped on Hiroshima and displaced approximately 9 km3 of debris. It killed more than 36,000 people around the world through tidal waves and eliminated more than 2/3 of the island. Climate and the atmosphere were changed all over the world for five years afterwards, with the average global temperature being reduced by 0.5°C. Catastrophic disturbances, such as major volcanic eruptions or huge asteroid strikes, have been said to be among the most prominent causes of most of the major extinction paroxysms of the past. Chief among these are the one approximately 65 million years ago that wiped out the dinosaurs and enabled the rise of mammals, and thus, humans and also the one at the end of the Permian, 250 million years ago that caused the extinction of 90% of all species. To Top of Page ### Smaller Scale Disturbances We have recently come to the realization that disturbances do not have to be on such a massive scale as Krakatau to significantly alter the local ecosystem. We are only now beginning to realize the impact of many smaller natural occurrences (e.g., tree falls, storms, tornadoes) as well as human-caused disturbances (e.g., clear-cutting, strip-mining, road construction, sudden introduction of exotic species) on the distribution of organisms. Given that this is our working definition of the science of ecology, disturbance effects are an integral part of our studies in the SEE-U program. We define disturbances as "any process or condition external to the natural physiology of living organisms that results in the sudden mortality of biomass in a community on a time scale significantly shorter (e.g., several orders of magnitude faster) than that of the accumulation of the biomass" (Huston 1998, p. 215). By this definition, a disturbance can kill anywhere from a few to all of the organisms in the local ecosystem. Therefore, a catastrophe like Krakatau would be at one extreme end of the disturbance scale and a tree fall producing a light gap in a tropical canopy would be at the other end. The suddenness of the definition is crucial. Gradual senescence ("natural" death) of a community of trees would not be considered a disturbance. To Top of Page ### Effects of Disturbances The effects of disturbances are usually not irrevocable, but the traces can take decades or centuries to erase. Extreme disturbances usually take many more years to overcome than do more mild disturbances. Usually very little of an ecosystem is apparently undisturbed. More common, are ecosystems that are a patchwork of areas that have been disturbed at different times and in different ways. As an example, the BRF is a disturbed secondary mixed-deciduous forest in upper New York State. Trees from various patches of it have been cut back for timber harvest at different times, fires that were triggered by human activity and lightning strikes have swept through it, and parts of it have been cleared for farming. The lingering effects of all these disturbances are still evident in the community composition. As a consequence, the BRF is said to be a mosaic of recovering patches. Disturbances are almost never randomly distributed either geographically or in time. Tree falls tend to be concentrated on windward side of hills, floods occur in lowland areas, tornadoes are most common in large open areas, and roads are most common near large cities. The geographic distribution of disturbances is fairly predictable as a consequence. The temporal distribution of disturbances is similarly predictable, with most natural disturbances being tied to the seasons. The balance between the disturbance frequencies, disturbance intensity, and the subsequent plant regrowth determines the state of dynamic equilibrium in which the community exists. If a community is undisturbed for a long time, it will proceed to its climax community. The climax is characterized by a lower level of biodiversity, but a great increase in endemic species that are found only there, relative to the earlier stages in succession. If the disturbance frequency or intensity increases, the recovery time slows and the community is kept at an earlier successional stage. As we will discuss during Modules 13 and 14, highly disturbed areas tend to be relatively depauperate in terms of species and also disproportionately comprised of introduced species. Therefore, these areas not only have fewer species than do less disturbed areas, but they have fewer native and other rare species. To Top of Page ### Conservation Implications Many people interested in conservation biology, when faced with the impacts of disturbances, at first think that it would be best to eliminate disturbances as much as possible. However, some level of disturbance would be beneficial. Some disturbances help to increase the heterogeneity of an area, which in turn increases the diversity of habitats and niches that are available. This then increases the number of species that can coexist in a given unit of area. The idea that some moderate level of disturbance is good for maximizing biodiversity, and possibly the health of the local ecosystem, is termed the intermediate disturbance hypothesis. This hypothesis can be summarized as a little bit of disturbance is a good thing. The intermediate disturbance hypothesis is well supported by the available data. However, the emphasis in this idea should be placed on the "Intermediate" term. In this case, you can have too much of a good thing, and we are presently proving it. Humans are the cause of the most severe disturbances that are currently affecting the Earth, including drought, desertification, habitat loss, pollution, global warming, and the ozone hole (Module 15). The levels of disturbance, both in terms of intensity and frequency, presently are abnormally high. Therefore, reducing the frequency and intensity of most human-caused disturbances would be a boon for species conservation. We are causing an extinction event that is every bit as large as that recorded in geological history and it is happening because of the disturbances that we are creating. To Top of Page ### Additional Relevant Online Resources The archive of the Talk.Origins online newsgroup has many interesting discussions of the tension between scientifically discredited, but still recurring views of evolution, the origin and age of the Earth, and the importance of catastrophies in shaping both. Most relevant to our discussion today is a FAQ entitled Changing Views of the History of the Earth by Richard Harter. A brief biography of Georges Cuvier is available from the University of California Berkeley Museum of Paleontology. A more critical, and slightly biased, version of Cuvier's life is available from Strange Science. Georges Cuvier and Catastrophism, a page explaining catastrophism and has several links to pages discussing this outmoded version of geological change, is available from Kumari Fernando's Exploring Scientific Controversies website. The full text of Charles Lyell's influential book Principles of Geology III & IV is available from The "Contradictions of the Enlightment" section of Fordham's Modern History Sourcebook, an excellent resource with many full texts or excerpts from key early scientific texts such as those by Darwin, Spencer, Lyell, Huxley, and many others. The entire text of Darwin's Origin of Species is available from Literature.org, as is The Voyage of the Beagle and The Descent of Man. The USGS has a thorough discussion and set of images of pages detailing the types and effects of volcano hazards, the location of major eruptions, the deadliest volcanic eruptions since 1500 A.D., and other background reading. A discussion of the After Effects of Krakatau's Explosion is available from San Diego State University. Sandia National Laboratories have a news release that discusses what an asteroid strike would look like as well as what effect an asteroid impact near New York City would have, as inspired by the movie Deep Impact. The original research paper that describes a possible 120km diameter impact structure caused by an asteroid that could have caused the extinction of 90% of all life 250 million years ago. National Geographic has a page briefly discussing the asteroid impact at the end of the Permian, 250 million years ago, along with several links to similar pages. The Smithsonian Institution's Department of Paleobiology has a page discussing the asteroid hypothesis of the extinction that led to the extinction of the dinosaurs 65 million years ago. Brittanica.com has an excellent discussion of community ecology, as well as the concepts of a climax community and ecological succession. R. J. Peacock, J. E. Williams, and J. F. Franklin have an interesting paper online discussing Disturbance ecology of forested ecosystems: implications for sustainable management from the Proceedings of the 1997 Conference of the Ecological Society of Australia, from which several other related papers addressing conservation biology and biodiversity are available. Quinlan Consultants have an excellent discussion of the intermediate disturbance hypothesis. The United National Convention to Combat Desertification has a fact sheet discussing the causes of desertification as well as many other aspects of the problem. Sierra Club has a series of FAQ sheets discussing several components of environmentalism, including habitat loss. To Top of Page ##### All Materials Copyright © 2000 by James Danoff-Burg ##### All Rights Reserved.