Any period when the extinction rate is in excess of 12 times normal. The Library standard definition for an Earth-like biosphere is on the order of 223 extinctions of macroscopic animals (size over one cm) over a period of less than 228 hab-years. These are caused by such factors as ice-ages, asteroid impact, significant oscillations in planetary orbit elements and axial tilt, major geological events, uplift failures, and Nontraditional exploitation of viable habitats by mature sapient species --may the Ancestors prevent their Retirement.
Terragens claim that Galactic Institute for Migration measures significantly understate total species numbers, and hence extinction rates (see bibliography). Some Terragen researchers still use the "Linnean" classification system, but this is not recognized by the Galactic Library. Hence, there can be no extinction levels for Linnean type taxonomic categories. Mass extinctions in plants are not strongly correlated with those in animals, and are less paleontologically detectable. They are also more difficult to assess historically since Galactics mostly consider them to be of secondary interest, although several recent species of note have been semi-plants. This has lead to an upswing of interest in plant-based biospheres at the current time. This trend is, however, unlikely to last more than a few million hab-years.
Note that variations in the Earth's orbital and rotational properties are damped by the Moon, and distance from Jupiter. For many worlds orbital properties can undergo significant change. The eccentricy, orbital radius, and axial tilt may oscillate over a wide range on a billion year time scale. On Earth minor oscillations trigger ice ages (Milkanov cycles). Large oscillations will place some fraction of current Garden Worlds in class A within 227 (134 million) hab-years, triggering Permian level extinctions.
Extinctions up to Cretacian level are allowed to happen on
Fallow worlds, as they promote potential. Even if no potential results, they are generally not harmful
when looked at over the standard period of fallow for most worlds. However, Permian
level extinctions are believed to happen very rarely under natural conditions. They are recorded
very rarely in the Galactic Library, and about 78_128 of these
disasters are thought to be caused by external forces --illegal settlers, illegal dumping, machine life
outbreaks or other gross violations of the Traditons of the Ancestors. Any worlds that have undergone
this sort of disruption should immediately be reported to the appropriate authorities.
A mass extinction effecting 64 to 96 per_128 of all species. All species over 512 kg are affected and 127 per_128 of those over 48 kg. All ecological specialists are removed, and ecologies are grossly simplified. The winnowing produced by these events often results in increased flexibility among the newly evolved organisms and tends to promote the appearance of potential.
Earth's Cetaceous-Tertiary event is
thought to have been caused by asteroid impact on the Yucatan Peninsula.
The exact event sequence remains unclear, but the principle mechanisms are thought to include prolonged
darkness and rapid global temperature change.
Mass extinction affecting 112 per_128 or more of all species. At this level, the negative feedback loops essential for the maintenance of biosphere integrity can be compromised.
The causes of the Permian-Triassic extinction are
unclear, but are thought to include a combination of several geological events; ice ages, large shallow seas
prone to evaporation, and the existence of only one continent. It has been suggested that the
Edicaran fauna consisted of independently evolved multicelluar life, which was
utterly eradicated. Animal life would have been reinvented from scratch.
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by R. Shaw, edited by A. Crystall, Biology Editor (July 1999)