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F RAY E 0 SA F E T Y NET S ..........................................................................................................................................................................; MSCP is implemented. This is especially true for "pre-listing agreements," where conservation plans are developed in order to avoid listing species that clearly deserve the protections of the ESA (e.g., the Atlantic Salmon Conservation Plan). Management Techniques To address the needs of listed and unlisted species, a conservation plan requires not only solid preserve design, but active management based upon validated management techniques. A comprehensive preserve management plan should aim to maintain suitable habitat over time and possibly to restore disturbed areas. In particular, active management must redress the problem of species and habitats suffering from (1) exotic species invasions, (2) suppression of natural disturbance regimes and (3) adverse edge effects. Invasion from exotic species threatens habitat in many areas. More than half of all federally listed species are adversely affected by interac- tions with non-native species, and nine of the 21 most endangered ecosystems in the United States are significantly affected by exotic invasion (Flather, Joyce and Bloomgarden 1994; Noss and Peters 1995). For some habitats addressed by plans in this report, the conservation plan may not succeed without success in controlling exotic invasion. For the San Bruno Mountain HCp, large portions of butterfly habitat are taken over by such exotic plants as gorse, broom, eucalyptus and fennel. For the lower Colorado River, many rare native fish species suffer from the introduc- tion of exotic fish species. For HCPs that permit residential development, it may be vitally impor- tant to curtail introduction of an exotic predator, the domestic cat (e.g., the Fel-Kran Plumbing HCP for the Perdido Key beach mouse). For many plans, habitat and species also can- not persist without active management to mimic or allow natural disturbance regimes (e.g., peri- odic fire or flood). In fact, the disruption of nat- ural disturbance regimes affects numerous habi- tats nationwide, including at least ten of the 21 most endangered ecosystems in the United States (Noss and Peters 1995). For example, several HCPs reviewed here concern the red-cockaded woodpecker, a resident of longleaf pine habitat in the southeastern United States. Longleaf pine forests are maintained by periodic fire that clears out the less fire-resistant hardwoods (Bridges and Orzell1989), and without that occasional fire disturbance, habitat becomes unsuitable for woodpeckers. In developing a multiple species conservation plan for the lower Colorado River, it will be essential for the agencies involved to address the importance of annual floods to riparian vegeta- tion and aquatic communities up and down the river. Historically, flooding occurred along the lower Colorado and its tributaries each spring, and flooding conditions are necessary for seed germination of native cottonwoods and other vegetation (Ohmart et al. 1977). Moreover, many of the native aquatic species are adapted to warm, fast-flowing water and periodic flood con- ditions. But many of these native species are being outcompeted by exotics that thrive in the cold, relatively still waters being released from the huge dams that have been constructed along e ....................................................................................................................................................................................................... ..........................