The primary reason for these extinctions is environmental change or biological competition. Since the beginning of the industrial revolution, a large number of biologically classified species have gone extinct due to the actions of humans. Scientists can only estimate the number of unclassified species that have gone extinct. Using various methods of extrapolation, biologists estimate that in 1991 between 4000 to 50,000 unclassified species became extinct, mainly in the tropics, due to our activities. The continued extinction of species on this planet by human activities is one of the greatest environmental problems facing humankind.
Scientists are unsure of the causes of both background extinction and mass extinction.
Possible explanations for mass extinctions include climate changes or catastrophes such as the Earth being hit by a meteor. Most mammals, birds, turtles, crocodiles, lizards, snakes, and amphibians were unaffected. Assessment of the number of different organisms that live on this planet is plagued with difficulties. First and foremost, biologists lack a precise definition of what exactly defines a species. The concept of a species often refers to a population of physically similar individuals that can successfully mate between each other, but cannot produce fertile offspring with other organisms.