Environmental Problems, Their Causes, and Sustainability 1

LAURASIA120° 80° 40° 80° 120°APA N G A E120° 80° 80° 120°G O N D W A N A L A N D225 million years ago135 million years agoNORTH AMERICAEURASIASOUTHAMERICAAFRICA120° 80° 120°MADA-GASCARA N TA R C T I C A65 million years agoINDIAAUSTRALIA120° 0° 40° 120°PresentFigure 5-8 Continental drift, the extremely slow movement of continents over millions of years onseveral gigantic plates. This process plays a role in the extinction of species and the rise of newspecies. Populations are geographically and eventually reproductively isolated as land masses floatapart and new coastal regions are created. Rock and fossil evidence indicates that about 200–250million years ago all of the earth’s present-day continents were locked together in a supercontinentcalled Pangaea (top left). About 180 million years ago, Pangaea began splitting apart as the earth’shuge plates separated and eventually resulted in today’s locations of the continents (bottom right).peatedly shrunk as continents have been flooded, andexpanded when the world’s oceans have shrunk. Atother times much of the planet’s land has been coveredwith ice.The earth’s life has also had to cope with volcaniceruptions, meteorites and asteroids crashing onto theplanet, and releases of large amounts of methanetrapped beneath the ocean floor. Some of these eventscreated dust clouds that shut down or sharply reducedphotosynthesis long enough to eliminate huge numbersof producers and, soon thereafter, the consumersthat fed on them.Populations of existing species in some placeshave been also been reduced or eliminated by newlyarrived migrant species or species that are accidentallyor deliberately introduced into new areas. More recentlyour species arrived and began taking over ordegrading more and more of the earth’s resources andhabitats. Today’s biodiversity represents the speciesthat have survived and thrived despite environmentalupheavals.What Is the Difference between BackgroundExtinction, Mass Extinction, and MassDepletion? Wiping Out Large GroupsAll species eventually become extinct, but sometimesdrastic changes in environmental conditions eliminatelarge groups of species.Extinction is the ultimate fate of all species, just asdeath is for all individual organisms. Biologists estimatethat 99.9% of all the species that ever existed arenow extinct.As local environmental conditions change, a certainnumber of species disappear at a low rate, calledbackground extinction. Based on the fossil record andanalysis of ice cores, biologists estimate that the averageannual background extinction rate is one to fivespecies for each million species on the earth.In contrast, mass extinction is a significant rise inextinction rates above the background level. It is a catastrophic,widespread (often global) event in whichlarge groups of existing species (perhaps 25–70%) arewiped out.http://biology.brookscole.com/miller1495