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496 S.M. NELSON AND D.C. ANDERSEN to the Bill Williams such as Calephelis nemesis and Limenitis archippus. A third grouping contained the No Name Lake site (R87) and the Havasu Refuge site revegetated in 1993 (R93-H). Both of these sites had few taxa in common with the Cibola sites, and each harbored Nymphalis antiopa, a species otherwise found only at Bill Williams. Although their ages differed, sites R87 and R93-H both featured moist soils and cottonwood canopy closure. The reduction in light (shading) due to canopy-forming trees at these two sites averaged 96.3% (n = 4) and was similar to forested areas of the Bill Williams (95.5%, n = 2). Other sites (R78, R90, R93-94) featured less canopy development and only a slight light reduction (16.9%, n = 5) because of planting arrangement (single line of trees), poor tree growth, or because trees were young. The presence of Pyrgus communis at the Cibola area is likely related to the abundance of alkali mallow-the species' larval host plant-at many of these sites. The pattern in June (Figure 8b) was broadly similar to that found in March, with many Cibola sites placed opposite the Bill Williams site in the ordination along the first axis. Hesperopsis gracielae and Apodemia palmeri were particularly common at Cibola, sites and separated these sites from Bill Williams sites, which again contained unique but abundant taxa such as Calephelis nemesis and Limenitis archippus (Table III). Some separation among the sites may have been due to temporal shifts in assemblage makeup _ caused by between-year variation in- rainfall (Figure 4). Overall species richness and Leptotes marina abundance- were both .relatively low in 1996 and 1997 compared with 1994 and 1995 (Table III). At Bill Williams, Junonia coenia was not detected during tle~ low precipitation years of 1996 and 1997. Taxa differences. such as these are likely responsible for "the separation, of Bill Williams samples by wet and dry years and are perhaps related to drying patterns of_the river. Rankings of revegetated and.. tamarisk sites -,along -Axis- II .were-correlated (r = 0.60; p < 0:01)- with a simple moisture index [sites ranked as 1-4 with __ ~~1 the wettest winter and 4 the driest winter (from=iFigure 4) and.-with irrigated sites. ranked as lj, lending some credence. to this interpretation. ~ ~ ;- . - -,August assemblages separated basedion Calephelisn@me~sis, abundant solely at Bill Williams (Figure-8c and Table Imo. Revegetafed and tamarisk~sifes"cl'ippe~i tsgether (Figure 8c) with some separation from.;.. the resence of Lerodea eu ala _ p f (Tab1e~ I~ at Cibola ~ites aril rthe-Reclamation site (R78) with nearby ,_ , ;irrigation ditches. Larvae of this species feed on B~rirtuda and Ja~lilson-grass (Sorghum halepense), ovally -common exotics associated with agricultural' featur~e~-jSites along the middle of-the first axis contained- ..only- a few ubiquitous taxa. ~ ~ 25 Mean and range of 20 values from the Bill` Williams River M rA - - ... m t 15 _U y 00 O ~ ~ - m 10 0--------o-- ~ ~_ /'=_0 2483 __ o 0 0 0 5 0 0 OI 0 5 10 15 20 25 Site Age Figure 5. Rank correlation of butterfly species richness with site age. The full range of richness values observed at Bill Williams is presented for comparison. Tamarisk sites are assigned age = 0 Copyright ©-1999 John Wiley & Sons, Ltd. Regut. Rivers: Res. Mgm[. 15: 485-504 (1999) ~~ ~~ 2 4 i . ;:.-=.