Statistical Outliers as Ecological Benchmarks and Legacy Leavers

Mean annual precipitation (Silver lake, Co.) with 68.3 inches (1735 mm) in 1921. This illustrtion was composed from data in Greenland (1995).

David Greenland (AND and NWT) recently sent along a copy of his paper on a giant of an NWT snowstorm that was a wimp in its impact. It got me to thinking about disturbances and "who" does the work in ecosystems: giants or dwarfs. I knew just where to go in the literature.

"Perhaps the state of knowledge as well as the geomorphic effects of small and moderate versus extreme events may be best illustrated by the following analogy. A dwarf, a man, and a huge giant are having a woodcutting contest. Because of metabolic peculiarities, individual chopping rates are roughly inverse to their size. The dwarf works steadily and is rarely seen to rest. However, his progress is slow, for even little trees take a long time, and there are many big ones which he cannot dent with his axe. The man is a strong fellow and a hard worker, but he takes a day off now and then. His vigorous and persistent labors are highly effective, but there are some trees that defy his best efforts. The giant is tremendously strong, but he spends most of his time sleeping. Whenever he is on the job, his actions are frequently capricious. Sometimes he throws away his axe and dashes wildly into the woods, where he breaks the trees or pulls them up by the roots. On the rare occasions when he encounters a tree too big for him, he ominously mentions his family of brothers all bigger, and stronger, and sleepier." (from Wolman and Miller, 1960)

Disturbance is an LTER core area. Each LTER site investigates the role of disturbances in structuring ecosystems and in ecosystem dynamics. What about an event that is more than 5 standard deviations above from the long-term mean. At Niwot Ridge 5 standard deviation snow fall in 1921 produced 76 inches of snow in 24 hours (Greenland, 1995). It was a world record! Nonetheless, David concluded that "there is no evidence of any major or long-lasting impacts to the alpine tundra or subalpine forest ecosystems." David also suggested that "higher temperatures and persistent drought, as might be found with global climatic warming, are proposed as potentially more important disturbance factors to these systems." This Niwot giant was a brother bigger and sleepier but weak in leaving a lasting ecosystem legacy.

At the Virginia Coast Reserve we had a coastal storm, the Ash Wednesday Storm of March 5, 6, 7 1962 that was the storm of the century (based on a 25 year record!). It too was many standard deviations away from a run-of-the-mill storm of average stature. However in this case we can still see the structuring impact of this storm on the vegetation of the islands. The storm changed the landscape and the kind of community that could be sustained. This VCR giant left a legacy easy to see.

Earlier at the VCR we had two giants of different clans that synergistically left the entire system fundamentally changed. In the early 1930 a slime mold, most people think, caused the extinction of seagrasses all around the North Atlantic and in the VCR Lagoons in particular. Following the seagrass extinction we had a major, major hurricane landfall at just the critical spot. The then unvegetated lagoon bottoms of fine sediments were mixed into the water column. The VCR went from a clear water, phytoplankton, seagrass and scallops estuarine ecosystem to a turbid water, bacterial-detritus-ecosystem. That legacy is now 6 decades old and hasn't reverted back to its former state. The seagrasses have returned elsewhere and we expect them to return to the VCR and perhaps erase or cover the legacy of the 1930s.

One of David's most interesting conclusions was that the regional synoptic weather patterns for the 1921 snowblitz were not unusual and such a large storm was not expected. In this sense, the 1921 disturbance giant came from a rather ordinary weather system family. Down the line we will have to have a workshop on ecosystems responses to disturbance giants and dwarfs.

Greenland, D. 1995. Extreme Precipitation during 1921 in the Area of Niwot Ridge Long-Term Ecological Research Site, Front Range, Colorado, U.S.A. Arctic and Alpine Research, Vol. 27(1):19-28.

Wolman, M. G. and J. P. Miller, 1960. Magnitude and Frequency of Forces in Goemorphic Processes. J. of Geology 68:54-74.