A majority of biologists are convinced that a "mass extinction" of plants and animals is underway that poses a major threat to humans in the next century. The rapid disappearance of plant and animal species is ranked as one of the planet's gravest environmental worries, surpassing pollution, global warming and the thinning of the ozone layer, according to the 1998 survey of 400 scientists commissioned by New York's American Museum of Natural History.
From an evolutionary perspective extinctions have no single or simple cause. The mechanisms by which species become extinct are not solely external (extrinsic) factors (e.g. impacts, volcanism, floods, competition, etc.) but also depend on internal (intrinsic) factors (e.g. longevity of individuals, population size, body size). Many traditional evolutionary palaeontologists promote catastrophic explanations for species extinctions within evolutionary theory. Among modern ecologists, the developmental approach in evolutionary theory to species extinctions involves the modelling of ecosystems where complexity works against species stability. Modern evolutionary palaeontologists are unable to explain relations between impacts and the continuous character of extinctions. But the developmental approach on a planetary scale reveals that the extinction processes in the history of the Earth do involve the puzzling phenomenon of ageing in species and higher taxa, planetary biological mega-rhythms, as well as the extraterrestrial influences themselves.