Page 115 - Land Snails of New Mexico

Metcalf,A.L.,andSmartt,R.A.,eds.,1997,LandsnailsofNewMexico. NewMexicoMuseumofNaturalHistoryandScienceBulletinNo.10.
ALTITUDINAL DISTRIBUTION OF LAND SNAILS IN S O M E M O N T A N E C A N Y O N S IN N E W M E X I C O
TIMOTHY J.DILLON andARTIEL. METCALF
University of Texas at El Paso, Department of Biological Sciences, El Paso, Texas 79968-0519
ABSTRACT: LandsnailswerecollectedalongtransectsinsixcanyonsinNewMexico mountains. Four west-descending canyons of Whitewater Baldy, Sierra Blanca, Lake Peak, and Mount Taylorwereinareasofigneousbedrock.Two transectsalongTularosaandPeñasco Canyons in the Sacramento Mountains were in areas of sedimentary bedrock. Altitudinal zones were categorized interms of the traditional Merriam-Bailey Life Zone schema for the Southwest. All transects extended from Canadian, through Transition, to the Upper Sonoran Zones, and three ascended into the Hudsonian Zone. In alltransects, the greatest numbers of species and specimens werefoundfromaboutmid-Transitiontomid-to-upperCanadianZones,fromca.2,286m (7,500 ft)to3,048m (10,000ft).Bothdiversityanddensitydecreased,progressively,atelevationslower and higher than the above. Species tolerant of harsh conditions of higher elevations extended into the Hudsonian Zone, but there were no species that were restricted to highest elevations. Along canyons species descended to lower elevations than they did in inter-canyon areas--the degree depended on variables such as configuration of canyons and the extent to which tongues of mesic, riparianvegetationextendedtolowerelevations.A "ripariandepression"wasespeciallyevident along Whitewater Canyon, below Whitewater Baldy. Although shorter, altitudinally, the two transects (together) of the Sacramento Mountains yielded more species (35) than the longer transectalongThreeRiversCanyon (26)intheadjacentSierraBlancaMountains. Itislikelythat
the widespread occurrence of calcareous bedrock in the Sacramento Mountains contributes to this difference.
I N T R O D U C T I O N
Elevation is clearly a factor of environmental importance to the biotas of mountains. Elevation impinges upon a large n u m b e r of intricately interrelated ecological factors, physical a n d biotic. In New Mexico's mountains, elevation is related to physical factors such as temperature, precipitation, evaporation, insolation, wind, and air pressure. T h e physical factors, in turn, relate to geologic aspects such as physiography, slope inclination
and aspect, weathering of bedrock, soil erosion, and soil
chemistry. All of these are variously interrelated with the biotic factors of the niches of the land snails that are considered here.
The bioticfactors include vegetation (both living and occurring as leaf litter), microbes and fungi of soil and leaf litter, and animals acting as parasites, predators, or competitors, and in conspecificroles.
Analyzedmore closely,"elevation,"initsown right,isless an ecological factor than a rough, but useful, index of a complex of factors that can be expected to operate predictably. The greater this predictability, the greater the usefulness of the term "elevation." Its usefulness is similar to that provided by geographic locations, i.e., relation to longitude and latitude. However, the effects of elevation on biotas may be evidenced over much shorter distances than those of longitude and latitude, and this is especially the case in mountains. In a mountainous
regionsuchasNew Mexico,itseemsuseful,then,toexamine h o w land snails are related to elevation.
Insomestudies,anattempthasbeenmadetoidentifyand analyze ecological factors related to elevation, such as those noted above, and which pertain to altitudinal distribution of land snails. Grime and Blythe (1969) suggested thatmicroclimatic differences were responsible for distributions of two species of snails living on opposing slopes of the same "pass" in Great Britain. In Sweden, Waldén (1981) found that increasing soil p H correlated closely with snail diversity, while slope aspect had a modest influence. In an intensive study in Tennessee, Coney et al. (1982) found that microhabitat differences were more important, ecologically, formore species studied, than were rock type,elevation,slope,orsoilpH. Dillon(1980),workinginone canyon in Arizona, used multivariate and principal component analyses in interpreting distributions of two species of land snails--thisbasedupontheenvironmentalvariablesofelevation, slope angle, slope aspect, percent of vegetational cover, and substrate type. Dillon found correlation among variables to be high, with vegetational cover accounting for most of the variability. In a study by Cameron (1986), characteristics of litter, soil, and associated vegetation explained most of the diversity observed in snail faunas of Vancouver Island, British
Columbia.No doubt,suchmultivariateapproacheswillbecome increasingly sophisticated and useful and will receive wider applicationinfuturestudies.Still,elevationpersemay retain considerable utility as a general, broad index.
For the most part, itis recognized that land snails inhabiting the southern Rocky Mountains and the Basin and Range
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