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Lecture

lecture note 12 for BGYB50

by

Department
Biological Sciences
Course Code
BIOC50H3
Professor
Herbert Kronzucker

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LECTURE 12:
- An examination of atmospheric CO2 levels over the Phanerozoic reveals that current
CO2 levels are exceptionally low, even considering the projected increases due to
anthropogenic emissions; current levels near 360 ppm are contrasted by 1,000-2,000 ppm
during the reign of the dinosaurs, and levels as high as 7,000 ppm during the Cambrian
species explosion; this parameter, too, has experienced constant change throughout the
history of life
- Mount St. Helens: much has been learned about the ecological responses to drastic
disturbances from the eruption of this volcanic peak of the Northern Cascade mountain
range; the mountain erupted on May 18, 1980, with the largest debris avalanche in
documented geologic history (~2.9 km3), creating some 600 km2 of denuded forest land,
and some 20 km2 of near complete devastation in the immediate blast zone, knows as the
‘pumice plains
- Life was found to return very quickly in the areas near the crater, including plants, such
as fireweed (Epilobium angustifolium), whose seeds blew into the pumice plains from the
surrounding areas within a season of the eruption
- Over longer time scales, the recolonization of species following glacial retreat gives an
excellent example of ecological response to disturbance (examples: maple and hemlock
trees gradually re-conquered more northerly reaches from their southern refugia [Florida
and Mississippi Valley, respectively] after the pinnacle of the last major ice age in North
America ended ~10,000 years ago; this process is still ongoing!)
- In general, following a disturbance, vegetation, microorganisms, and animals recolonize
the affected site, often with significant predictability, in a process known as succession
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