Biodiversity
Biodiversity: the number of different species in an environment
Extinction: most species become extinct eventually, mostly replaced by other species (natural selection; catastrophic events)
Humans accelerated extinction ever since present
Prehistoric extinction: mostly big mammals (mammoth, saber tooth, North-American horses, camels) due to hunting; 74-86% of big animals extinct in North America after humans arrived
Historical times: extinction rates exploded during last 200 years: 1600-1700: ca. 1 species/100 a 1850-1950: ca. 1 species/a 1986-1990: ca. 4 species/a (for birds & mammals only)
Recovery from extinction might not occur this time