Extinction. Ultimate causes for insect extinction most likely involve environmental degradation, although their immediate manifestation is in the context of a local ecological community involving associations with other organisms. Because there is a distinct range in the severity of extinction in the fossil record, the background extinction level is typically separated from mass extinction. Although data from marine invertebrates indicate that a continuum may link these two modes (Raup 1991), for insects the records of background and mass extinction are very different, and impart differing evolutionary dynamics separating taxa with the ambient level of extinction from those that succumb to mass dieoff at major events. Specifically, background extinction is very flat and consists of a persistent Mesozoic plateau of about 10 families per stage, interrupted occasionally by an extinction spike (Fig. 4b). These four extinction events range from about a two- to more than fivefold increase above background levels that encompass one to three geologic stages.