Unit 3

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CONCEPTS:  The global fossil record shows 5 mass extinctions in the geological past, each of which is estimated to have caused the loss of 50% to 90% of all species and required 5 to 10 million years for recovery to the same number of species as before.  The currently documented global rate of bird and mammal species extinctions, with supporting evidence from large global losses of major terrestrial ecosystems, strongly suggests that Earth is entering a 6th mass extinction.  This event is qualitatively different from the others because it is related to the activities of humans rather than to abiotic factors such as shifts of tectonic plates and collisions with meteorites.  Over the past 150 years since homesteading began, the Denver Metro Area (DMA) has experienced large losses of natural grassland ecosystems to agriculture.  Much more recently, large scale urban and semi-urban development in the DMA has resulted in rapidly increasing loss of natural grassland, shrubland, and forest ecosystems.  The most important threats to current global biodiversity are natural ecosystem loss and degradation (including pollution and habitat fragmentation), nonnative (“exotic” or “alien”) species, and overexploitation of plant and animal species (e.g., large-scale removal of old growth forest trees and excessive harvest of marine mammals).  These threats are, in turn, driven by human population expansion, large per capita increases in human consumption, and development of modern technologies that permit massive changes in natural ecosystems and their conversion to other uses.


The overhead “The Five Global Mass Extinctions” is not included on the website. To order a copy of this overhead, please contact CU Science Discovery. Send your name, school, address, phone number, activity name, and check or money order to:

Karen Hyde Edgerly

Science Discovery – Biodiversity reprints

University of Colorado

UCB 448

Boulder, CO  80309


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