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500 S.M. NELSON AND D.C. ANDERSEN at revegetated sites may preclude site use by butterflies. Premature abscission of cottonwood leaves from water stress at xerified sites could affect butterflies dependent upon these food sources. Although the presence of another phreatophyte-dependent butterfly-N. antiopa-at the No Name Lake and Havasu Refuge revegetation sites suggests a trend toward the `natural' Bill Williams River site, this species could be a holdover using relictual willows. Unlike most other resident butterfly taxa, N. antiopa can undergo seasonal migrations (Shapiro, 1986), a trait that would promote colonization of disjunct revegetated sites. Despite differences in chronological age, the No Name Lake and Havasu Refuge sites may both attract this butterfly because each offers moist soils and areas of deep shade from closed canopies. Woodland butterflies often prefer shade (Warren, 1985) and Debinski and Brussard (1994) found N. antiopa to prefer moist soils. Microclimate is important in butterfly habitat restoration (Weiss and Murphy, 1990). Impacts to assemblages Butterflies associated with First Bottom vegetation have undergone a dramatic decline in numbers because of the virtual elimination of First Bottom environment. The entire historic butterfly assemblage may not be supported at revegetation sites because compositional (presence of larva] host plants), structural (closed canopies), and functional (hydrology, flowering phenology) diversity is lacking. In some cases the original intent of revegetation was to decrease water use by plant communities. This was done by replacing dense stands of exotic tamarisk with native plants that would transpire less but provide .increased wildlife values (Anderson and Ohmart, 1985). The depauperate butterfly assemblages at revegetated sites, however, suggest that these few plantings at widely-spaced floodplain sites are insufli- cient for restoration. Potential butterfly source areas, along with being extremely rare, may be too far from revegetation sites. The difficulties to butterflies of colonizing distant habitats are illustrated by Thomas et al. (1992) who showed that some taxa.may take 2-3 years to colonize patches just 300-700 m from source populations. The Bill Williams is one of the last regional areas containing the historic butterfly assemblage's major elements. Butterflies dispersing from. BW -must travel large distances through, inhospitable habitat, a consequence'of the isolation of the Bill Williams and Colorado River floodplains by the creation of Lake Havasuy a reservoir with little native woody riparian vegetation along its shoreline. The time required for colonists from BW to reach a revegetated site on the lower Colorado River probably exceeds the life span of the site. Additionally, although areas of habitat needed by butterflies are largely unknown, the relatively small size of revegetated sites may negatively affect the viability of butterfly populations. It is possible that butterfly habitat was patchy and fragmented in the historic Lower Colorado River; however, these patches were part of a heterogenous landscape, rather than surrounded by near monocultures of tamarisk and agricultural crops as at present. Assemblage restoration The usefulness of created habitats to butterflies may not be fully discernable until appropriate taxa are artificially introduced. Successful introductions could also protect remaining regional populations of some butterfly species from chance extinctions. The introduction of phreatophytes such as willows and cottonwoods at some revegetation sites suggests that reintroduction of phreatophyte dependent butterflies (e.g., Calephelis nemesis, Limenitis archippus, Nymphalis antiopa) might be possible. The dependence of the first two of these butterflies on nectar sources, however, argues that they cannot survive at any of the xerified revegetation sites. Saline soils and lowered water tables may also preclude the introduction of C. nemesis's host plant, B. salicifolia. Because the annual disturbance caused by flooding and seed sources are now absent, recreating the original spatial mix of riparian vegetation patches along the lower Colorado River is probably impossible. With canopy closure, however, Nymphalis-antiopa might be reintroduced or repopulate additional revegetated sites. This butterfly differs from the other two in that populations can apparently persist without extensive nectar plants and may use sap for nutrition (Scott, 1986). Autecological studies of these riparian butterflies are needed to determine whether successful introductions are likely. Copyright ~ 1999 John Wiley & Sons, Ltd. Regul. Riuers: Res. Mgml. 15: 485-504 (1999)