Mammals of the Peloncillo Region: Bridging the Tropical and Temperate
The Peloncillo Mountains in southwestern New Mexico and southeastern Arizona are unique in that while they are comparatively low and dry in contrast to other Sky Island mountain ranges, they are part of a region that is biogeographically important for mammals because it provides a linkage between temperate and tropical faunal zones, most notably between the Sierra Madre to the south and the Rocky Mountains to the north. A classi c example of this is the 1996 sighting of a jaguar in the Peloncillo Mountains, presumably coming from a breeding population some 150 miles south. Small rodent distributions are arguably more affected by the region’s low elevation “underpass” between eastern and western faunas than they are by it’s higher elevation “bridge” between Northern and Southern mountain chains.
Outstanding Features:
Perognathus pocket mice demonstrate this blending of eastern and western faunas; P. baileyi is a western species from Baja California and Sonora that spills east over the continental backbone here. P. flavus is a species of the eastern plains, spilling westward through this same continental low spot. The region also harbors several species that are rare in the US and/or in Mexico, as well as a handful of mammals with very limited geographic ranges (i.e., narrow endemics).
As highlighted in previous chapters, the Peloncillos and adjoining landscapes are located at the intersection of six major biomes, including the Chihuahuan Desert, Great Plains, Great Basin, Sonoran Desert, Rocky Mountains, and Sierra Madre. This meeting of biomes and associated topographic variation results in phenomenally high mammal diversity for a semi-arid zone, with 91 species documented for New Mexico’s Hidalgo County, which contains much of the Peloncillo Mountains and all of the Animas Mountains and Animas Valley (Appendix E). The Animas Mountains, better surveyed for mammals than other parts of the Peloncillo region, report 76 mammal species. This is a full third more than the much larger Yellowstone National Park, North America’s park best-known for its mammals. Even local, relatively small-scale sites can possess high levels of diversity. On a 49-acre (20 ha) study site in the San Simon Valley near Portal, Arizona, 25 species of small mammals have been documented since 1977– more mammal diversity than is contained in the entire State of Pennsylvania. Bats comprise nearly one quarter of the region’s mammal diversity, with 18 species known from the Animas Mountains alone. Five additional species have been reported from Hidalgo County, and eight (including two of the Hidalgo County species) from other parts of the Peloncillo region, for a grand regional total of 26-29 species. Few other U.S. or Borderland locations can boast bat species numbers anywhere near this, with the closest we found being the Big Bend region of Texas with 19 species, and Saguaro National Park in Arizona with 20 species. As with many of the mammals of the Peloncillo region, the high diversity appears to be the result of overlap among species that are typically associated with divergent faunal provinces. For example, several tropical species such as the Leptonycteris bats discussed below are at the northern extent of their range here, where they overlap with boreal species.
This region’s unique mammal elements make it more than just a high-diversity crossroads. The southern pocket gopher Thomomys umbrinus emotus is considered to be endemic to the Animas Mountains. The white-sided jackrabbit Leppus callotis is known in the US only from the Animas Valley and adjacent Playas Valley; its distribution in Mexico is often depicted as extending into central Mexico (the Animas’ subspecies gaillardi reaching northern Durango), but is poorly documented and may have shrunk dramatically with habitat degradation as has happened in the US. Several other species are reported in the US either exclusively or primarily from the Peloncillo region, including the Arizona cotton rat Sigmodon arizonae cienegae (a species mainly known from Sonora and southern Arizona, near its eastern range edge here), Yellow-nosed cotton rat Sigmodon ochrognathus (a Sky Island and Sierra Madre specialty, known also from the nearby Big Burro Mountains), and Arizona shrew Sorex arizonae (known from a handful of other Sky Island mountain ranges). Mearns’ pocket gopher Thomomys bottae mearnsii is also a Sky Island specialist. The fulvous harvest mouse Reithrodontomys fulvescens canus is known in New Mexico only from the Peloncillo and Animas Mountains.
Geographic scope. The core data for this chapter center around mammal lists from Hidalgo County, New Mexico, and from studies of the Animas Mountains specifically, but extrapolations and interpretations for the entire Peloncillo region including Mexico, are also discussed.
Historical Data
Mammalogists and other biologists have been drawn to the Borderlands of southwestern New Mexico, southeastern Arizona, and adjacent Sonora and Chihuahua for over 100 years. E. A. Mearns noted in his mammal survey of the Mexican boundary that he and F. G. Irwin traveled to the region in 1892, though no specimens were collected. The earliest mammal specimens from southwestern New Mexico were obtained by E. A. Goldman, C. Birdseye, and V. Bailey of the United States Biological Survey in 1908. Subsequent reports on mammals, plants, and birds relied principally on the data collected in that earlier fieldwork. In the late 1950s and early 1960s, J. S. Findley and students from the University of New Mexico made periodic trips to the area. Specimens from these mammalogists are reported in Findley.
More recent surveys (from ~1950’s through 1980’s) conducted in the Animas Mountains, as well as the nearby Chiricahua, Huachuca, and Pinaleno (Graham) Mountains, provide regional information on mammal composition to the west. The Big Hatchet and Alamo Hueco mountains east of the Peloncillos were also surveyed for mammals in the 1970s. The above papers are reviewed in Cook.
Because detailed reviews of borderland mammals already exist, the rest of this discussion will not list the mammals of the Peloncillo region separately by species. Instead, discussion will focus on engineering and keystone species that are of fundamental importance in structuring vegetation of the Borderlands, and discuss federally listed species of conservation importance.
Landscape Context: Dramatic Vegetation Changes
The Peloncillo region and surrounding areas have undergone considerable vegetation change since European settlement. While few exotic species exist in the landscape compared with other regions of the county, contrasts of surveyor notes from the 1870s and satellite imagery from the 1990s indicate considerable increases in native woody vegetation. Landscape change in the Southwestern US and Northwestern Mexico is often associated with over-grazing and drought in the late 1800s and early 1900s. There is increasing evidence, however, that continued or accelerating increases in woody vegetation in the 20th century may be associated with fire suppression and high levels of winter rainfall.
Since the late 1990s less seasonal and drier climatic patterns associated with Pacific Decadal Oscillation have displaced recent strong El Nino patterns associated with woody plant growth. It is not clear if current climatic patterns will result in a decline in woody species as happened in the 1950s drought, or if recent patterns of shrub and woody vegetation establishment will be sustained.
Since 2000 the region of the Peloncillos has experienced considerable drought,31 yet the response by small mammal populations has been mixed. Long-term studies near Portal, Arizona have recently attained record high numbers of mammals. Mammal populations on the McKinney Flats study site 12 miles (20 km) east of the Peloncillos, however, show considerable declines since initial censuses in 1998, though these populations had been stable through the early 2000s. Recent work by J. H. Brown and associates analyzing over 25 years of mammal data indicates that no simple causation exists between mammal populations and climate with interactions independent of climate and considerably more complex than the simple associations with drought and rainfall.
Engineering Species
American Bison (Bison bison)
Prior to European settlement, bison were the major engineering species throughout the North American continent with a key role in recycling nutrients and creating patch mosaics through disturbance. Bison are recognized as a big game animal in Arizona, but not in New Mexico. A bison herd of roughly 200 animals exists in the Playas Valley east of the Sierra San Luis (12 miles/20 km east of the Peloncillos), and there are no physical barriers to prevent the animals from ranging to the Peloncillo region.
Bison appear to have been a key part of the prehistoric fauna of the Borderlands with reports of bison remains in southern Arizona dated from the 1200s. Prehistoric bison remains are regularly located on the Glenn Ranch in the San Bernardino valley just west of the Peloncillo Mountains. A remnant of America’s wild bison herds also existed in the Palo Duro River Valley of the Texas Panhandle during the 1880s. In the 1950s, 20 head of bison were moved from the Raymond Ranch near Flagstaff, Arizona to the Mexican state of Sonora. It was widely believed until recently that the Playas Valley herd was a part of these Arizona animals (previously from the National Bison Range and therefore of non-local genetic stock). New evidence from Mexico, however, suggests that bison have been a considerably larger part of the recent history of the Borderlands.
From 950 to the time of Spanish settlement, bison appear to have been a major source of protein for the Casas Grandes culture that inhabited the region southeast of the Peloncillos. Since Spanish settlement, bison have been regularly reported starting in 1565 and continuing to the present. Though recently penned on the Hurt Ranch in the Playas Valley, the animals now appear to again be ranging on both sides of the Mexico/U.S. border. Initial genetic work being conducted at Texas A & M University is encouraging in supporting the idea that this may well be a native herd, though conclusive analysis is still pending. This information is of crucial importance not only to conservation in the Peloncillo region, but how we perceive the entire Borderlands region. In the past it has been argued that because bison and other large grazers did not inhabit the Borderlands that ranching and other agrarian land-uses involving cattle could never be an appropriate conservation strategy because the land and the vegetation were fundamentally maladapted to grazing. In contrast, this historic information suggests that large grazers may still have an important role in maintaining the structure and function of these semi- arid ecosystems.
Family Heteromyidae, kangaroo rats, pocket mice, and allies
Heteromyid rodents are small-bodied rodents with long tails, often modified for jumping, with elongated hind limbs and shortened front limbs. The group is characterized by bi- pedal movement (e.g., hopping) compared with most other rodents that move primarily on all four legs. Other characteristic features are fur-lined cheek pouches that allow them to carry food and auditory bullae (an enlarged audio portion of the skull) that give them remarkable hearing. As a highly desert-adapted organism, they are reported to never drink water and obtain water primarily through eating seeds. They have highly evolved predator defenses. Some species have been reported to have sensitive-enough hearing that they can detect owls in flight. They also appear to be moderately immune to snake venom. Members of this family mainly occupy grasslands and savannas, or lowland desert habitats.
Of particular note for their importance as engineering species are the three species of kangaroo rat that inhabit the Peloncillo region. Ord’s kangaroo rat is a plains inhabitant at the southwest edge of its range, which extends north and east across the Great Plains to Canada. Merriam’s kangaroo rat is a desert organism found in shrublands and is primarily distributed across the Chihuahuan desert, eastern Sonoran Desert, and the southeastern Great Basin. Finally, of greatest ecological importance, is the larger mound-building banner-tailed kangaroo rat, which is nearly twice the size of the other species at 3 – 3.5 ounces (90 to 100 grams). The banner-tail is at the eastern edge of its range, which extends across the northwestern Chihuahuan desert and across much of the Sonoran Desert.
Kangaroo rats are perhaps the single most important engineering or keystone species in the Peloncillo region (Figure 2.4.2). Many of the biodiversity hotspots for both vertebrate and invertebrate species appear to be associated with kangaroo rat burrows.38 When banner-tailed kangaroo rats declined in the San Simon Valley adjoining the Peloncillo Mountains in the late 1980s and early 1990s, populations of other organisms such as burrowing owls and some species of rattlesnake also declined, apparently due to habitat loss.39 Brown and Heske40 documented this guild’s crucial importance in structuring vegetation, while Chew and Whitford illustrated how ecotonal boundaries between grasslands and shrublands are influenced by kangaroo rats with the abandoned mounds serving as important focal points for shrub establishment. While these studies were sometimes interpreted as evidence that kangaroo rats had a negative role in facilitating desertification, later research illustrates that kangaroo rats and other Heteromyids are crucial to preserving existing vegetation composition in face of recently climatically driven vegetation change.
Curtin et al. illustrate that in the absence of kangaroo rats, shifts in vegetation structure and patch composition would have been 16-fold between 1979 and 1995 on research plots in the Peloncillos, whereas in the same time period, vegetation in plots with kangaroo rats changed 3-fold. Work by Curtin and Kelt presented in Curtin and Brown45 indicates that rather than directly facilitating shrub increases as suggested by Brown and Heske, kangaroo rats dampen environmentally driven vegetation changes by disproportionately impacting the most common species. The tremendous loss of grassland habitats in western North America speaks to the value of any species that can slow the impacts of shrub invasion in remaining grasslands.
Black-tailed Prairie Dog (Cynomys ludovicianus)
Prairie dogs are ground squirrel-like rodents with a short tail (15 - 30% of body length), hind foot as long as the tail, which is dark-tipped. Prairie dogs are frequently cited as the classic keystone or engineering species in North American grasslands48 and crucial to preserving the biodiversity of grasslands.
As with kangaroo rat mounds, prairie dog burrows provide essential habitat for numerous species ranging from lizards to burrowing owls, while recycling nutrients and increasing overall biodiversity. Prairie dogs have been demonstrated to reduce shrubs and sustain grasslands, and increases in shrubs in the Southwestern US and Northwestern Mexico has been partially attributed to loss of prairie dog colonies.
Although a grassland species and not found in the mountains proper, prairie dogs were documented by surveyor records from the 1870s as being on bajadas west of the Peloncillos in the San Simon Valley, while field notes from Vernon Bailey written in 1908 document widespread prairie dog colonies in the Animas Valley. While these sites were extirpated in the early 20th Century, the largest remaining prairie dog colony in the world remains near Janos, Chihuahua, Mexico. An active colony occurs north of the border in the Playas Valley, and colonies of recently translocated individuals were established on the Gray Ranch 12 miles (20 km) east of the Peloncillo Mountains.
Most of the information on prairie dogs comes from the central and northern plains. Two fundamental assumptions of the literature are: 1) Prairie dogs are essential to sustaining the structure and function of grasslands, and 2) That cattle grazing and prairie dogs are fundamentally in conflict. Yet these assumptions have rarely been carefully examined, especially in desert grasslands. Work from the vicinity of Janos, Mexico documents the landscape-level contributions of large colony complexes to overall diversity and system function. Results from experimental studies on the Gray Ranch illustrate that at the local scale of meters, prairie dog contributions to biodiversity are mixed with some factors increasing (lizards and vegetation biomass) and others declining (vegetation diversity and small mammals). Yet the effect of prairie dogs on organisms at a landscape-level are profound, even from small experimental colonies with cattle traveling miles across hot, dry pastures to forage in the vicinity of the prairie dog towns. Burrowing owls (Athene cunicularia), once rare in the area, are now common in the pasture. On a 26-mile breeding bird survey the only place the regionally declining Ferruginous hawks (Buteo regalis ) are found is adjoining the prairie dog towns.
While cattle and prairie dogs have long been considered competitors, the reliance of prairie dogs on large herbivores was recognized as early as the 1850s. Recent studies on the Gray Ranch indicate a positive interaction between cattle and prairie dogs, with cattle important for maintaining low grass and reducing predation pressure on prairie dogs, while prairie dogs reduce shrubs, increase vegetation biomass, and increase vegetation nutrient content. These studies indicate that much of the biodiversity bene fits ascribed to prairie dogs are really the result of interactions between prairie dogs and large grazers such as bison or cattle. In grasslands of the Southwestern US, the benefits of prairie dog restoration are as least as tangible for ranchers as they are for conservationists. The reintroduction of prairie dogs represents a potentially important tool for restoring degraded grassland structure while boosting and sustaining landscape-level biodiversity.
Rare or Endangered Species
Leptonycteris Bats
The principal bats of concern are the two federally listed species of Leptonycteris bats that have been located in the vicinity of the Peloncillos including the Mexican long-nosed bat (Leptonycteris nivalis ) and the lesser long-nosed bat (Leptonycteris yerbabuenae; formerly known as L. sanborni and L. curasoae yerbabuena and also referred to as the North American long-nosed bat). The core habitat of these species is in Mexico. For each the Mexican border region of the United States represents the extreme northern edge of their range.
The New Mexico State-listed western yellow bat ( Lasiurus xanthinus) is another Mexico species that appears to be extending its range northward. This organism is not included in this discussion of endangered and rare species because, while historically caught in Guadalupe Canyon in the Peloncillos, this is another species at the northern edge of its range, and there is no evidence that it is under specific threat across its range.
During the short time that Leptonycteris bats are in the U.S. in the summer, one of their principal natural foods is agave nectar. A key question for bat conservation has been whether agaves are a limiting resource and therefore must be protected to preserve the bat. To answer this question in 1997, Dr. Liz Slauson of the Desert Botanical Garden chose an agave site near a known Leptonycteris bat roost in the Peloncillos. At a site less than one-half mile from the known roost, while the roost was occupied, Slauson bagged one portion of the flowering agave, while leaving adjacent flowers unbagged. Her results in the Peloncillos, and in 1998 studies in the Huachuca Mountains, showed no measurable difference in nectar present between those agaves accessible to the bats, and those inaccessible. Dr. Slauson’s conclusion was that there was so much nectar being produced within the bat’s feeding range that the amount consumed by foraging bats had no effect on nectar availability, indicating that agaves are not a limiting resource.
In a companion study Dr. Peter Scott of Indiana State University in 1997 monitored agaves weekly from mid-July to August 6, and at two-week intervals after that until September 10 in the Peloncillos and Chiricahua Mountains. No bat visits were recorded in Peloncillos until late August and early September when agaves were nearly finished flowering. Similarly, in the Chiricahuas almost no bat observations were recorded from mid-July to mid-August, while between August 20 and September 10, both observation sessions yielded frequent visits. This study, as with the ones by Dr. Slauson, indicates that agave nectar is not likely to be a limiting resource to Leptonycteris bats in the Arizona-New Mexico borderlands, and what usage does occur happens during a brief period at the end of the bolting season.
Mexican long-nosed bat (Leptonycteris nivalis)
The Mexican long-nosed bat has no documented occurrences in the Peloncillo Mountains. It has been found in the Animas Mountains in very small numbers (a few individuals), with reports of roosting bats in Pine Canyon. Its population size and dynamics in Arizona and New Mexico are unknown. Its global population size is unknown. The only substantial roosting site known in the U.S. is in Big Bend National Park, Texas, some 400 miles southeast of the Peloncillos.
Lesser long-nosed bat (Leptonycteris yerbabuenae)
The lesser long-nosed bat's total population in the Southwestern US is roughly 100,000 individuals. This estimate is based on the known numbers from a maternity roost in the Pinacate region of Sonora with over 65,000-75,000 bats (which disperse into the U.S.), and another 10,000 - 26,000 females at maternity roosts in the Organ Pipe National Monument area. Some authors postulate severe declines in these bats’ population in historic times, while others argue that populations of lesser long-nosed bats in Arizona are large, appear to be stable, and may even have increased in the 20th Century as a result of increases in habitat due to mining.
The largest known U.S. summer roost for lesser long-nosed bat is in a Patagonia, Arizona cave, which up to 41,500 individuals may use on an occasional basis. Other large summer roosts of several thousand bats are found in the Huachuca and Chiricahua Mountains. The Peloncillo Mountains contain a summer roost, which was discovered in the mid-1990s. This natural rock shelter site contained fewer than 100 individuals and therefore represents a fraction of one percent the known U.S. population. It is unknown what proportion of the southern Arizona maternity population stays in Arizona for the summer, and what proportion returns to Mexico. During this summer dispersal period, the primary food source in the U.S. is the nectar of various species of agave.
Additionally, a much larger and more diverse food supply exists for these animals in their core habitat in Mexico. It is recognized in the literature that if there is a shortage of food in any one area, these highly mobile animals can move to where adequate food supplies exist.
Mexican Gray Wolf (Canis lupus baileyi)
The Mexican gray wolf is a large canid with ears that are not markedly pointed, and a weight of up to 125 pounds (60 kg). Color is variable, and males are considerably larger than females. Bogan and Mehlop reviewed the wolves of Arizona, New Mexico, and west Texas and northern Mexico and concluded that two distinct subspecies were present in the region: Canis lupus youngi and C. l. baileyi. The Borderlands were inhabited by Canis lupus baileyi, which was considered the smallest wolf sub-species in North America, with 16 males from Chihuahua averaging 78 pounds (35 kg) and 16 females averaging 63 pounds (28 kg).
The Peloncillo region was perhaps the last stronghold for wolves in the Southwestern United States. From the 1910s through the 1960s government trappers worked out of the OK Bar Ranch 6 miles (10 km) west of the Peloncillo Mountains, with Red Hill in the Animas Valley a favorite wolf denning area. The last wolf was taken in 1965 in Pine Canyon in the Animas Mountains. Although I could find no records of wolves specifically from the Peloncillo Mountains, a number of authors referred to a major wolf runway that appears to have crossed a significant portion of the southern Peloncillos. Wolves reintroduced to the Blue Range Wilderness in east-central Arizona could easily dispersed into the Borderlands, as they could from rearing sites in Mexico’s Sierra del Nido. Turner Endangered Species Fund biologist and wolf expert Mike Philips regards the greater Peloncillo region as perhaps the best wolf repopulation site left in North America, whether by direct reintroduction or by natural dispersal.
Jaguar (Panthera onca)
The jaguar is a large felid with cinnamon-buff color spotted with black. Many of the black spots form broken circles or rosettes with one or more black spots in the center. Tail is about 40 to 45% of the head-body length. Overall length is six to eight feet (1,700 to 2,400 mm). The jaguar is another species that is at the northern extent of its range in the Borderlands.
Even in the time of settlement from the 1880s through 1905, only 30 specimen records exist, with none in New Mexico prior to 1996. With core habitats 150 miles south, the sightings appear to be primarily dispersing young males. E. A. Goldman’s 1908 field notes refer to a jaguar having been killed in the “Cloverdale Mountains east of the Animas Valley” five years earlier, but even then they were considered occasional visitors from the Río Yaqui Valley. Sightings in Arizona and New Mexico occur several times a decade with one of the better-documented sightings in 1996 when rancher Warner Glenn photographed a young male in the central Peloncillos.
Jaguars have historically been associated with moist riparian areas, yet recent work by Carlos Lopez Gonzales and David Brown in Mexico indicates that in many areas mid- elevation scrub sites are more typical habitat. This information makes the Peloncillo region appear to be better jaguar habitat than formerly thought, though debate continues over whether the Peloncillo Mountains proper have the prey base to support jaguars. The individual photographed in 1996, however, made its living in the region for nearly a year after this first sighting. The major current threat to jaguar is hunting in Mexico; for example, Carlos Lopez Gonzales reports that the skin of the animal photographed by Warner Glenn was later seen in Mexico.
Other Species of Concern
The federal “Species of Concern” category includes species that were formerly considered Category 2 and 3 Candidates for federal endangered or threatened listing. Several such mammal species are known from the Peloncillo region, with varying amounts of information available for each.
Townsend's big-eared bat (Corynorhinus townsendii) earned its Species of Concern listing by virtue of its low reproductive rate, extreme sensitivity to human disturbance, historic and continuing vandalism of roost caves by recreationists, and disturbance of abandoned mine shafts. Both the pallid subspecies Plecotus townsendii pallescens found here and the more northerly subspecies P.t.townsendii qualify under this listing.
The primary threats to this bat are disruptions of roost sites. Luckily, efforts by federal land managers and private landowners to control access into caves, and to secure mine shafts without blocking bat movement, has begun to ameliorate these threats. These predominantly moth-eating insectivores are also vulnerable to generalized insecticides, but toxic dumping in water sources (e.g., from mining, oil and gas exploration, and agricultural activities) is probably a greater threat in this sparsely populated region.
The Western red bat Lasiurus blossevillii roosts in trees and shrubs, especially in well- developed riparian cottonwood, sycamore, and willow gallery forests. These insectivorous animals are solitary roosters that often give birth to twins but may raise up to four pups at a time. Historic loss of riparian habitat is thought to be responsible for declines in Western red bat populations throughout the Western US, but population trends are poorly documented.
The White-sided Jackrabbit Lepus callotis is of particular interest here since most of its US range falls within the Peloncillo region. This large jackrabbit can be recognized by virtue of sides that are whitish rather than grayish, with the white areas extending far up the sides; ears without black tips; and black or grayish coloration behind the ears and nape. The type -specimen was collected by Mearns in 1896 near Boundary Monument 63 on McKinney Flats within what is now the Gray Ranch.
Recent unpublished studies by Traphagen and Schmitt indicate that the presence of these jackrabbits is typically associated with buffalo grass (Buchloe dactyloides) and negatively associated with shrub cover. This animal seems to have gone through a considerable constriction in its abundance and range. In his 1908 field notes, mammalogist Vernon Bailey reports from the Watkins Ranch northeast of Hachita “two shot and saved and half a dozen more seen; half as many as of the gray.” This is at least 40 miles (64 km) northeast of the apparent current range. Even in the best of habitats, white-sided jackrabbits only occur with a fraction of the abundance of the common Lepus californicus. While extending south into Mexico, L. callotis now only occupies two valleys in the United States, the Playas Valley 12 miles (20 km) west of the Peloncillos and the Animas Valley adjoining the Peloncillos (Bailey also reported L. callotis to be common here in 1908).
L. callotis currently occupies an area of less than 26,116 acres (10,568 ha) within the lower Animas Valley (though several unconfirmed sightings have recently occurred in the Playas Valley). This estimate, based on 1977 studies by Jim Bednarz, is probably optimistic because this area has experienced considerable shrub encroachment since the 1970s. Regular surveys since 1976 have documented a steady decline. This trend appears to not just be occurring north of the border, but also in Mexico. Of the rare and threatened mammals in the Borderlands, the white-sided jackrabbit may be the most threatened with extinction.
The Arizona shrew Sorex arizonae was proposed for endangered species listing on the basis of being apparently very rare, with extremely localized occurrences in disjunct montane sites. This species is known from four places in the United States—the Chiricahua, Santa Rita, and Huachuca Mountains in Arizona, and Animas Mountains in New Mexico—plus one specimen caught in the Sierra Madre Occidental. Within these mountain ranges, the shrew is known from montane conifer forest, encinal, and Mexican pine-oak woodland, tending to be found in relatively moist canyons with downed logs and well developed tree canopies. Future surveys in appropriate habitats might also reveal Arizona shrews in the Peloncillo and San Luis ranges, though the latter seems more likely given its larger area of high-elevation forest.
Knowledge Gaps
Because of the importance of the Peloncillo region as a corridor for wildlife, more focus must be placed on understanding the sections of the Peloncillos north of Interstate 10 in Arizona and New Mexico. These northern sections of the range are largely unknown by biologists and conservationists. A major knowledge gap remains in understanding the resources of, and threats to, these areas. In addition, very little is known about local populations of several species of concern such as the endangered Mexican long-nosed bat (Leptonycteris mexicana). Better local information will be vital to understanding the conservation needs of these species.
Conservation Targets
Though the region’s narrow endemics tend to be found in upper elevations, the majority of engineering and species listed as threatened or endangered occur in foothill and valley grasslands. This, and the biogeographic importance of the Peloncillos as a corridor between the Rocky Mountain to the north and Sierra Madre to the south, highlights the importance of focusing conservation efforts not just in upland areas, but in a mosaic of habitat types.
Long-term conservation strategies must therefore focus on not just isolated low-human- use core habitats held by public agencies, but also on preservation of the semi-natural matrix of habitats with a range of human uses. Critical issues needing active conservation planning are:
Maintaining and expanding the prairie dog complex near Janos adjoining the Sierra San Luis, the Playas Valley, and the San Simon Valley.
Ensuring that areas adjoining the Peloncillo Mountains, Animas Valley and Gray Ranch are not eventually isolated by development. Without a conservation strategy that includes the preservation of these lower-elevation habitats contained within the larger landscape matrix, critical linkages between Sky Islands in the Borderlands will cease to exist.
Ensuring that other developments such as border fences and high-speed roads do not further divide the Peloncillo region or isolate it from other parts of its faunal provinces that both feed it and are connected by it.
Additionally, specific species in urgent need of conservation are: White-sided jackrabbit, which of the rare and threatened mammals in the Borderlands may be the most threatened with extinction.
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Section Authors:
MAMMALS - Charles Curtin, Ph.D. Conservation Biologist, Arid Lands Project and Malpai Borderlands Group. Founder of the nonprofit research institute Arid Lands Project, which conducts landscape-level experimental studies of the interaction of human and natural systems, Charles has coordinated a science program to support the efforts of the Malpai Borderlands Group.
