Page 125 - Land Snails of New Mexico

species is not encountered, so that species diversity, overall, shows a progressively marked decline, with decrease in elevation. Hoff (1962:61-62) noted that "Land molluscs are scarce in the pinyon-juniper community . . . ," which he i d e n t i f i e d a s o c c u p y i n g e l e v a t i o n s f r o m 6 , 5 0 0 t o 7 , 5 0 0 ft. H e related this to a decrease in availability of moisture, although noting that the more immediate effect on snails likely would be the decrease in "broad-leaved trees and other suitable mesic
plants" at lower elevations.
As withtheotherlifezones,thereareno speciesthatcould
be considered as specific indicators of the Upper Sonoran Zone alone. In the transects discussed here, Vallonia perspectiva w a s found only in the Upper Sonoran Zone. However, in other mountains itcommonly occurs also athigher elevations, as in the OrganandSanAndresMountains,whereMetcalf(1984:42) considered it a species typical of the Transition Zone. Most Upper Sonoran Zone species are shared with the Transition Zone, above, and a few with the Lower Sonoran Zone, below. Several species, which seemed to find their optimal habitat in the middle Transition to middle Canadian Zones, descend into t h e u p p e r p a r t o f t h e U p p e r S o n o r a n Z o n e , b u t it s e e m s l i k e l y that they gradually are approaching their limits of tolerance at lower elevations. As shown in Table 1, a majority of species extend at least a short distance down into the Upper Sonoran Z o n e , b u t m o s t o n l y t o 6 , 8 0 0 o r 6 , 4 0 0 ft, e x c e p t a l o n g t h e W B transect, a special case discussed below. The only species of low
elevations, typical of the combined Lower and Upper Sonoran Zones, which occurred in these transects, was Gastrocopta pellucida.
There is considerable variation in regard to the physiography of the lower parts of canyons, and this isreflected in the nature of canyon biotas. In some cases, canyons gradually widen as they descend, and biotas show a corresponding gradual change. This isthepatternseenintheSB,LP,andMT transects.Whitewater Canyon (WB), on the other hand, exemplifies a type of canyon t h a t b e c o m e s e n t r e n c h e d i n h i g h - w a l l e d " n a r r o w s " a s it n e a r s t h e mountainfront.Thisiswellshown alongthe"Catwalk,"where the lowermost canyon is constricted greatly. Just below the Catwalk segment, the narrow canyon abruptly gives way to a broadvalley,sothatthereisastrikingchangebothin physiography and in the biota over a short distance. Along the narrow, deeply shaded, lower part of Whitewater Canyon, m a n y elementsofamontanebiotaextendasanarrowtonguedown to the C a t w a l k area. T h i s includes m o n t a n e trees a n d other plants
and the land snails commonly associated with them. Table 1 showsthatseveralspeciesdescendtolowerelevationsalongthe W B than they do along other transects studied. Euconulus
fulvus, Helicodiscus eigenmanni, Vertigo gouldii, and Zonitoides arboreus were collected 800 ft lower and Discus
whitneyiandMicrophysulaingersolliwerefound1,200ftlower thanalongothertransects.Ashmunellamogollonensiswastaken 1,200ftlowerthanitsrelative,A. rhyssa,intheSB transect.A similar situation w a s noted in a distributional study of land snails of the Grand Canyon area, Arizona, by Spamer and Bogan
(1993). They observed (p. 21) that "specialized riparian communities" at lower elevations were inhabited by species "normally found in higher Life Zones."
The species Discus whitneyi, Vitrina pellucida,
Microphysula ingersolli, and Zonitoides arboreus seem especially tolerant of a broad altitudinal range, occurring verticallythroughatleast4,400ftin2 differenttransects,each, and the first 2 species through at least 5,080 ft in the S B transect. With all transects considered, Discus whitneyi occurs t h r o u g h a v e r t i c a l r a n g e o f 6 , 2 8 0 ft, M i c r o p h y s u l a i n g e r s o l l i through 6,000 ft, Euconulus fulvus through 5,880 ft, Vertigo modesta through 5,480ft, and Pupilla blandi, Vitrina pellucida, and Zonitoides arboreus, through 5,200 ft. The altitudinally tolerant Discus whitneyi occurs at still lower elevations both in N e w Mexico and in Arizona (Bequaert and Miller, 1973, Dillon,
1980).
SUMMARY OF ZONATION IN NUMBERS OFSPECIESAND SPECIMENS
For the transects discussed above, numbers of species that were taken and numbers of specimens that were calculated for the standardized sample (three liters) are summarized in Table 2 for each elevation collected. Both of these show patterns of altitudinalzonation.Again,theWB transectdisplayshigher values at lower elevations, which are not shared by the other threetransects.AlongWB, thelargestnumbersofspecieswere
at 6,400 ft(16) and 9,200 ft(15). More than 10 species were foundatallstationsfrom6,000to10,000ft;11 speciesat6,000, 8,800,9,600,and10,000ft.
AlongWB, thelargestnumberofspecimenscalculatedper sample also was from a low-altitude station: 999 at 6,400 ft, withsecondandthirdlargestnumbersbeing867 at7,600ftand 6 2 7 a t 9 , 2 0 0 ft. I n s u m m a r y , t h e r e i s a r e l a t i v e l y u n i f o r m l y r i c h fauna over some 4,000ft, from 6,000 to 10,000 ftalong transect WB.
For the other three transects, highest numbers of species and specimenscalculatedpersamplealloccurinthe8,000-8,800ft r a n g e - f o r s p e c i e s : 2 2 a t 8 , 4 0 0 f t ( S B ) , 1 9 a t 8 , 0 0 0 ft ( L P ) , a n d 10 at 8,800 ft (MT), and for calculated number of specimens: 1,306at8,000ft(SB),898 at8,400ft(LP),and 1,017at8,800 ft(MT).On SB,afteradecreaseinnumber ofspecies(only8 to
11) from 9,200- to 10,000-ft levels, a resurgence is exhibited higherup,with13speciesat10,400and10,800ft.Thesecond largestnumberofspecimens,persample,fortheSB transect was taken at 10,400 ft. It seems that at these 10,400- and 10,800-ftlocalitiesthereisalocalhabitateffectthatproduced assemblages that are exceptionally rich for these elevations. This effect is similar to, but in the reverse direction from that noted forthelowerWB transect.Disregardingthesetwohigher stations of the S B transect and the lower stations of the W B transect,thereisafairlyconsistentpattern,whereinlargest numbers of species (20-28 with all transects considered) and greatest densities per sample (1,355-2,138, totals for all transects)occurbetween7,600-and 10,000-ftelevation.
I n s u m m a r y , i n t e r m s o f life z o n e s , t h e r i c h e s t f a u n a s o c c u r from the middle Transition to the middle Canadian Zones.
Above 10,800 ft, a Hudsonian Zone fauna becomes progressively impoverished in terms of both number of species and specimens, with relatively few species appearing to be tolerantofthesehighestelevations.A similardecreaseoccurs downwardthroughtheUpperandLowerSonoranZones.
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