State and federal endangered species laws allow the protection ofcritical habitat for listed plant taxa. In the case of extreme ecologicalspecialists with restricted geographic distributions, significant proportions ofunusual ecosystems could thus be afforded protection as a byproduct of listingand subsequent restrictions on land use or management practices. This studysurveyed federally and state-listed plants to determine: (1) which taxaconserved a significant proportion of a distinctive ecosystem, (2) which taxaprovided a protective, regulatory 'umbrella' to unlisted rare orrestricted plants and animals, and (3) the taxonomic, life history or othercharacteristics correlated with the greatest secondary benefits. Protection of'whole' ecosystems was uncommon among listed taxa, but the relativedegree of protection depended on critical delineation of boundaries. The numberof associated, but unlisted taxa was considerable in some cases. In general,taxonomic distinctiveness and non-biological factors generated the greatestsecondary effects. Management and monitoring problems prevent the assessment ofwhether such single-species approaches can effectively conserve ecosystems.