Page 126 - Land Snails of New Mexico

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G E O G R A P H I C P A T T E R N S
significance,asbotharewidespreadspeciesthatextendinto Arizona.
One objectiveinvolvedinchoosingtransectsthatroughly define the four corners of a square within N e w Mexico was to ascertain ifthere might be discerned any noteworthy patterns of geographicdistribution. AsindicatedinTable2,thereisa general trend of reduction in number of species and abundance of snails calculated per sample (where containing 300 or more specimens) from southeast to northwest, as shown below:
Fifteenspeciesarefoundinallfourtransects.Becausethe faunallistfromtheMT transectcomprisesonly18species,itis clearthatmostofthesearewidespreadspeciesinNew Mexico and probably ecologically tolerant as well.
Number oflocalitieshaving N u m b e r specimens per sample indicated:
SacramentoMountainstothesouth.Physiographically,thetwo
Transect ofSpecies >1000 700-999 500-699 300-499
other in places. Petrologically, however, the ranges are
S B W B LP M T
2 6 24 22 1 8
2 1 - 2 - 1 1 1
1 4 1 3 1 4 1 1
distinctive, the Sierra Blanca Mountains being of Tertiary
The greaterfaunaldiversityofthetwo southernversusthe two northerntransectsmightrelatetogreaterproximitytothe summerMexicanmonsoonalbelt.Ofthefour,theSB transectis closesttotheGulfofMexico,asourceofmonsoonalmoisture. However,incomparisontoWB,SBascends1,000fthigher, which also may be related to its greater faunal diversity and richness. Nevertheless, in regard to diversity, the two transects
Pennsylvanian and Permian age, comprising mainly limestone andcalcareousshaleunits.Structurally,themountains areblock faulted and tilted.
are similar (26 and 24 species, respectively). MountTaylor,ontheotherhand,may belocatedtoofareast
Because of the geographic proximity of the Sierra Blanca and Sacramento Mountains, but of their geologic dissimilarity, it is of interest to compare transects in the two ranges. Collections were made along transects in two canyons in the Sacramento Mountains in the late 1960s and early 1970s. The Peñasco Canyon (PC) transect heads in the central part of the mountains and descends eastward. The Rio Peñasco ultimately flowsintothePecos River. The Tularosa Canyon (TC) transect heads in the northwestern part of the Sacramento Mountains and
to benefit significantly from western sources of winter Pacific precipitation,andtoofarnorthwestforGulfofMexico summer sources.Mueller(1989)notedthatthispartofNewMexicohas less access to moist and unstable air from the Gulf of Mexico
descends north to Mescalero and then west to Tularosa, beyond whichtheTularosaRiverdebouchesintotheTularosaBasin.
during the s u m m e r thunderstorm season. L a k e Peak, the eastern counterpart of M o u n t Taylor, is in a better position in regard to Gulf moisture sources, and is also part of the high chain of the Sangre de CristoMountains, which may sap summer moisture supplies from mountains to their west, such as Mount Taylor. The comparative aridityoftheMount Taylortransectprobably relates to the depauperate land-snail fauna found in the area below ca. 7,000 ft.
The Sacramento Mountains do not reach the heights of the Sierra Blanca Mountains. The Peñasco Canyon transect culminates at the crest of the range at some 9,600 ft, the Tularosa Canyon transect culminates at 8,400 ft at the head of the canyon, which forms a "pass" to Turkey Pen Canyon, tributary to Silver Springs Canyon. Collections were made at 300-ft intervals, rather than at the 400-ft intervals used in the transectsdiscussedintheprecedingsections.An exceptionwas the lowermost station on the P C transect, at 5,280-ft rather than the expected 5,400-ft elevation. Along the TC transect, collections extended from 5,400 to 8,400 ft.
Excluding the records of Vallonia gracilicosta from the M T transectandofPupillamuscorum fromtheSB transect,bothof which are widespread elsewhere in the state, the remaining taxa thatwererecordedfromonlyonetransectare:MT -none,LP Ashmunella thomsoniana and Oreohelix strigosa depressa; WB-Ashmunella mogollonensis, Oreohelix barbata, and O. subrudis, SB-Ashmunella rhyssa, Oreohelix nogalensis, Paralaoma caputspinulae, and Pupilla sonorana. Radiodiscus millecostatus and Vallonia perspectiva are recorded for only the two southern transects, and Discus shimekii for only the 2 northern. This modest elevation in number of taxa towards the
A t e l e v a t i o n s o f 7 , 2 0 0 - 9 , 0 0 0 ft a l o n g P C , c o l l e c t i o n s w e r e made both on the alluvial floodplain of Peñasco Canyon and along adjoining valley walls, where collections consisted of ca. t w o liters o f leaflitter, a s a m p l e s i z e s m a l l e r t h a n t h a t u s e d i n t h e t r a n s e c t s a b o v e . O n t h e f l o o d p l a i n , it w a s o f t e n n o t p r a c t i c a l t o obtain leaf-littersamples, and these collections relied heavily on hand-pickingsnailsfromunderdeadwood andstones.InTable 3, collections from the two habitats are shown separately for elevations 7,200-9,000 ft and indicated as PCF (Peñasco
southmay conformtoamodelbyMetcalf(thisvolume)which relates greater diversity in southern New Mexico to paleoclimatic factors causing extinctions that have encroached from north to south in the Cordilleran region.
Canyon Floodplain) and P C V (Peñasco Canyon Valley walls). AlongtheTC transect,leaf-littersampleswerefrom1.5to 2.0 liters in quantity and were augmented by hand-picking. In middle elevations of the canyon, valley wall and floodplain samples were combined. At higher elevations, the floodplain was poorly developed or absent. At lowest elevations, the
There seem to be no significant differences between eastern andwesterntransects.No speciesaresharedbythewestern transects,WB andMT,toexclusionoftheeasternones.The
floodplainwas broad, and collections were not made from valley
records of Gastrocopta pellucida and Vallonia cyclophorella from only the eastern transects are of no zoogeographic
walls, which were widely separated and far back from the Tularosa River.
TRANSECTSONSEDIMENTARYBEDROCK
The Sierra Blanca Mountains, discussed above, abut the
ranges are conjoined and are scarcely discernible from each
igneousandtheSacramentoMountainsofPaleozoicsedimentary bedrock. Rocks of the Sacramento Mountains are of