Page 12 - Land Snails of New Mexico

The studyofDillonandMetcalf(thisvolume)showedthe Canadian Zone to be the most productive of kinds and of numbers of land snails of any ecosystem in the mountains surveyed in the study. In the Canadian Zone, both riparian and nonriparian habitats are favorable for land snails. In the Sacramento Mountains, for example, the snail Ashmunella rhyssa may be found widely, as along creeks, on slopes, or in mountaintopforests with low relief.
Of special importance to land snails are stands of Aspen, which have their greatest development in the Canadian Zone. The leaflitterand decaying logs and branches found in Aspen grovesareextensivelycolonizedby snails.The term "colonized" is p e r h a p s a d v i s a b l e i n a s m u c h a s A s p e n s c o m e a n d go. D i c k Peddie discussed (1993:70 ff) what he termed "Aspen disturbance forest." In the simplest model, Aspens establish themselves after fire has destroyed coniferous forest, a n u m b e r oftreesarisingasclonesfromacommonAspenrootstockthat had existed within the former conifer-dominated forest, and w h i c h h a d s u r v i v e d t h e fire. R e s u l t i n g s t a n d s o f A s p e n s m a y persistformany decades or for over a century, but gradually fall victimtopathogens,insects,and browsing ofyoung growth, and eventually give way once more to reestablishment of conifer forest. Presumably, populations of land snails fluctuate in
accordance with the cycles of Aspen versus coniferous forest. Althoughforestfireskillmany snailsdirectly,suchfiresmay be of long-term benefit insofar as Aspens establish themselves subsequently. However, factors that contribute to decline in stands of Aspens produce a concomitant decline in prime snail habitat.
H u d s o n i a n L i f e Z o n e - 1 1 , 0 0 0 - 1 2 , 0 0 0 ft o n c o o l e r s l o p e s and 12,000-13,000 ft on warmer slopes. In this and the next higherzone,precipitationmay exceedslightly30 in perannum. T h i s is a n a r r o w z o n e i n t h e B a i l e y L i f e Z o n e s c h e m e , b e i n g t h e timberline ecosystem. In the study of Dillon and Metcalf (this volume), a definitediminution in diversity and numbers of snails was notedinprogressingfromtheCanadiantotheHudsonian Life Zone. In other words, the sharp decline in larger trees, which occurs at timberline, seems to be reflected clearly in a decline in land-snail density and diversity.
Arctic-Alpine Life Zone-12,000 ftand above on cooler slopes and above 13,000 ft on warmer slopes. This zone corresponds to the Alpine Tundra category of Dick-Peddie (1993:47). The trend towards decreasing numbers and kinds of land snails continues here, with only a few species to be found, such as Pupilla blandi, Columella columella, Vertigo modesta, Discus shimekii (in the north), Vitrina pellucida, Euconulus
fulvus, and Microphysula ingersolli.
In summary, density and species diversity of snails are
distributed along a bell-shaped curve in N e w Mexico, with lowerendsofthecurveintheLower Sonoran andArctic-Alpine Zones, and the apex in the Canadian Life Zone.
R i p a r i a n a n d M a r s h H a b i t a t s - C u t t i n g a c r o s s all b u t t h e highest montane zones, noted above, are corridors of riparian (stream-bordering) vegetation. These have the effect of lowering life zones. This is marked especially in the montane life zones where, as noted, Canadian Zone vegetation and associated fauna occur down into the Transition Zone and the latter into the
lowerthantheywouldotherwise.Thus,alife-zonedepressionis produced, comparable in this small-scale context, to life-zone depressions of much larger scale that apparently took place during times of Pleistocene glaciations.
Atlowerelevations,Dick-Peddie(1993:151ff)recognized several kinds of riparian situations: floodplains of major rivers, "Plains Riparian" along intermittent streams in eastern N e w Mexico, arroyo-related riparian, and vegetation associated with the ponded waters of closed basins and their feeder streams. Fossilsindicatethatmany oftheselowland ripariancommunities supported a sometimes rich land-snail fauna in the past, but most such communities are gone today. The floodplains of major rivers are inhabited by humans and cultivated. In many places themainriparianwoodyplantisSaltCedar(Tamarixspp.),and dense Tamarix bosques generally are not inhabited by land snails. Before arroyo entrenchment occurred (related to grazing by domestic animals),itseems likelythattherewas better retention of pools of water in the low-banked arroyos, and that there was a higher local water table alongside the arroyo. This would have enabled moisture-requiring plants to grow, which, in turn, would have encouraged populations of some small species of land snails, like those that n o w occur as fossils in arroyo-bank sediments.
Similar to the fate of a previous "arroyo ecosystem" is one associated with marshes at lower elevations, often called
cienegas in N e w Mexico. Fossils indicate former presence of marshesassociatedwithspringsatlowerelevations.Many seem to have existed during the Pleistocene and disappeared with progressive Holocene warming. Others seem to have disappeared concomitantly with human settlement and subsequent farming, diversion of waters, and the effect of livestock. Iflarge numbers of livestock concentrate at a small springbrook and marshforwater, predictably they will overgraze associated vegetation and trample the area into something of a mud hole. The fortuitously well-preserved area around and below Blue Spring, Eddy Co., allows some perception of the former appearance of other spring-related areas that have not been so fortunate. Here, the brook-side snail Vertigo ovata seems to retain one of its few remaining populations in N e w Mexico. There are numerous indications that marshes formerly existedmore widely in the state. For example, fossils show that marshes along the Tularosa River, which presently extend downstream to near Mescalero, formerly extended over 10 miles and1,000ftbeyondandbelowMescalerotothelevelofthe Tularosa Basin. Historical accounts attest to presence of marshes a l o n g t h e e a s t e r n s i d e o f t h e T u l a r o s a B a s i n , r e f l e c t e d , it s e e m s , inthenamesCarrizozo(reedy)andTularosa(fromtule,applied to various marsh-inhabiting plants). The disappearance of spring-related wetland ecosystems is, of course, part of a problem of national dimensions. For marshes at higher elevations, especially those o n national lands, the prognosis m a y be better, but these also are vulnerable, being of such limited extent.We believethatadetailedsurveyofthemolluscanfauna ofthestate'sremainingmarsheswouldbeofvalue.
ENDEMISM INMONTANE LAND SNAILS
Upper SonoranZone,allowingspeciestothriveatelevations
There are numerous species of land snails in N e w Mexico