Page 11 - Land Snails of New Mexico

As we cansee,aspectsoftheecologicalnicheofasnailare many and complex, and their analysis laborious and time consuming. Much as Bequaert and Miller observed over 20 years ago (1973), there is, to date, little detailed ecological information available about southwestern snails. At a less
Precipitationisgreateringrasslandsoftheeasternquarterof the state. Dick-Peddie (1993:37–38) described occurrence of both grassland and scrub components at lower elevations. Except for the occurrence of some relatively unaltered riparian or marshy habitats, mentioned below, few species of land snails are found in any of these arid-to-semiarid habitats. In the grasslandorscruboftheChihuahuanDesert,themainspecies foundareGastrocoptapellucida,Pupoides albilabris,Succinea grosvenori, and Thysanophora hornii. These same species, sans T.hornii,alsooccurintheeasternplainsalongwith Gastrocopta cristata, G. procera, and Helicodiscus singleyanus.
detailedlevel,threeaspectsofecosystemsofspecialsignificance toNew Mexicolandsnailsarethosepertainingtovegetation, effective precipitation, and elevation, all closely interrelated. Theseecosystemparametersvarygreatlywithintheconfinesof NewMexico,andmeritfurtherdiscussion.Asvegetationisof greatimportancetosnails,itisstressedbelow,givingspecial attention to Dick-Peddie's (1993) analysis of New Mexico vegetation. Effective precipitation is also of m u c h importance to landsnailsand,intypicalecosystemfashion,involvesnumerous factors such as temperature, wind, evaporation, gradient, ground cover,andnatureofthesoil.Some notesregardingprecipitation have been drawn from articles by Bennett (1986) in the publicationNew Mexico inMaps. BennetthasnotedthatNew Mexicoisoneofthemostaridstates,90%ofthelandsurface h a s a n a n n u a l p r e c i p i t a t i o n o f < 2 0 i n , w i t h 2 0 - 3 0 % - 1 0 in.
U p p e r S o n o r a n L i f e Z o n e — 4 , 0 0 0 - 7 , 0 0 0 ft o n c o o l e r s l o p e s and 4,500-8,000 ft on warmer slopes. Average annual precipitationinthiszoneisgenerallysome 12-16in.Thiszone commonly is referred to as the Pinyon-Juniper Zone. Dick Peddie (1993:87-90) proposed fairly equivalent zones that he termed Pinyon-Juniper Woodland and Mixed Woodland. Like theprecedingzone,theUpper Sonoranisnotveryproductiveof land snails, although a few hardy species succeed very well in hillier habitats. For example, in the San Andres Mountains, mainly within this zone, many minute pupillid snails of the species Gastrocopta pellucida and G. ashmuni were found, along with larger species such as Rabdotus dealbatus neomexicanus, Glyphyalinia indentata, Hawaiia minuscula, and Thysanophora hornii. In this zone, on the sides of arid appearing mesas in eastern Colfax Co., great numbers of Gastrocopta pilsbryana and Vallonia gracilicosta occurred alongwithfourorfivelesscommon species.
M a n y abiotic and biotic factors vary with elevation. N e w Mexico has an altitudinal span of ca. 10,000 ft,from the Pecos Valley, at the southern state boundary, to the high summits of the Sangre de Cristo Mountains. Examples of how ecosystems change with elevation are noted, even by the casual observer, as when driving up the steep western face of the Sacramento Mountains from Alamogordo to Cloudcroft or up the eastern faceoftheSangre de CristoMountains from Cimarron toEagle Nest. Rather presciently, C. Hart Merriam, Vernon Bailey, and others of their school of natural history of the early 20th century anticipated the concept of ecosystems to some extent in their definition and description of "life zones" for parts of the western UnitedStates,includingBailey'sstudy(1913)ofNew Mexico. For the various lifezones, these authors described characteristic plants and animals, and related their occurrences to such abiotic factors as elevation, slope exposure, and precipitation. Elevation, in fact,played a central role in their models, in which they distinguished zones, each of which occupied a particular
T r a n s i t i o n L i f e Z o n e - - 7 , 0 0 0 - 8 , 5 0 0 ft o n c o o l e r s l o p e s a n d 8,000-9,500ft on warmer slopes. Average annual precipitation m a y reach or slightly exceed 2 0 in. This has been referred to as a Pine-Oak Belt (Elmore and Janish, 1976:109), and to most casual observersitis probably defined mainly by the presence of extensive stands of Ponderosa Pine. Dick-Peddie (1993:66) identified a Lower Montane Coniferous Zone "generally below about 8,500ft.," which seems to correspond, in large part, to the Transition Zone. H e listed five "series" within this forest type, four of which incorporate Ponderosa Pine plus a kind of oak in their identifying name (the fifth incorporates Chihuahua Pine). Although pines and oaks are dominant throughout most of this zone, as on hillsides and interfluves, there are narrow corridors of riparian habitat in which deciduous trees and shrubs dominate. These deciduous elements, in turn, supply leaflitter and decaying logs that are attractive to land snails. Narrow riparian habitats support a snail fauna much like that of the Canadian Zones, whereas habitats in this zone, other than riparian, tend to share species with the Upper Sonoran Zone.
altitudinalspan.FromlowesttohighestinNew Mexico(Bailey, 1913), these were: Lower Sonoran, Upper Sonoran, Transition, Canadian, Hudsonian, and Arctic-Alpine Life Zones. The life zones described by Bailey (1913) for N e w Mexico also can be
interpretedasecosystems,andwe treatthemassuchbelow.The n a m e s o f t h e life z o n e s h a v e t h e a d v a n t a g e o f l o n g u s a g e , a n d o f beinga"neutral"terminologythatdoesnotemploy thenames of either plants or animals, as do some other classifications. This classification also has been employed in a study of montane land-snail distribution by Dillon and Metcalf (this volume). Bailey (1913) indicated elevations (in ft) of life zones for cooler (northeastern) and warmer (southwestern) slopes, which are given below, except for the Lower Sonoran Zone.
C a n a d i a n L i f e Z o n e — 8 , 5 0 0 - 1 1 , 0 0 0 ft o n c o o l e r s l o p e s a n d 9,500-12,000 fton warmer slopes. Annual precipitation in this, zone may range from 20-30 in. In the Dick-Peddie system ( 1 9 9 3 : 5 1 , 5 8 ) , m o s t o f this z o n e w o u l d fall i n h i s U p p e r M o n t a n e C o n i f e r o u s F o r e s t ( 8 , 0 0 0 - 1 0 , 0 0 0 ft), b u t B a i l e y ' s w a r m e r - s l o p e aspect would be equivalent to his Subalpine Coniferous Forest ( 9 , 5 0 0 - 1 2 , 0 0 0 ft). T h i s o f t e n h a s b e e n r e f e r r e d t o a s a " s p r u c e fir"zone. Blue and Engelmann Spruce, White and Douglas Fir, and Limber Pine are important conifers, and Gambel Oak and Aspen are important deciduous species.
Lower Sonoran Life Zone--below 4,000 -4,500 ft in Bailey's scheme, but extended here to include the somewhat higher desert (or plains) of northwestern N e w Mexico. This zone comprises most areas of low gradient in the state, and is alsothedriestzone. Average annual precipitation in an arc from the northwestern part of the state southeast to Sandoval Co. and downtheRioGrandeValleyrangesfrom8to10in.
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