Sonora Tiger Salamander

Ambystoma tigrinum stebbinsi

Description

The Sonora tiger salamander, Ambystoma tigrinum stebbinsi, is a large salamander with a dark venter and light colored blotches, bars, or reticulation on a dark background. Snout/vent lengths of metamorphosed individuals vary from approximately 2.6-4.9 in (6.5-12.2 cm) Larval salamanders are aquatic with plume-like gills and well-developed tail fins. Larvae hatched in the spring are large enough to metamorphose into terrestrial salamanders from late July to early September, but only an estimated 17-40% metamorphose annually. Remaining larvae mature into branchiates, aquatic and larval-like but sexually mature salamanders that remain in the breeding pond, or over-winter as larvae.

The Sonora tiger salamander was discovered in 1949 at the J. F. Jones Ranch stock tank in Parker Canyon, San Rafael Valley, Arizona. In 1954, based on color patterns of metamorphosed animals, the Sonora tiger salamander was described from southern Santa Cruz County, Arizona as the sub-species stebbinsi of the broad-ranging tiger salamander (Ambystoma tigrinum ). Color patterns were again used to make the next taxonomical determination. In 1965 and 1967, Ambystoma tigrinum stebbinsi and Ambystoma tigrinum tahense were equated, from the Rocky Mountains region, with Ambystoma tigrinum nebulosum, from northern Arizona and New Mexico. Nevertheless, Ambystoma tigrinum stebbinsi continued to be recognized in the scientific literature.

Findings were published in 1988 that established the description of color patterns in Ambystoma tigrinum stebbinsi was only accurate for recently metamorphosed individuals. About 40% of metamorphosed adults exhibit a unique reticulate pattern, while 60% are marked with light colored blotches, spots, or bars on a dark background that is indistinguishable from Ambystoma tigrinum mavortium, found in the central United States and adjacent portions of Mexico. Starch gel electrophoresis of 21 presumptive gene loci of Ambystoma tigrinum stebbinsi were compared with gene loci of Ambystoma rosaceum (from Sonora), Ambystoma tigrinum mavortium, and Ambystoma tigrinum nebulosum. Based on this analysis, distinctive reticulate color patterns, low heterozygosity, and apparent geographic isolation, subspecific designation of Ambystoma tigrinum stebbinsi was considered warranted. Further analysis of mitochondrial DNA the same year reaffirmed subspecific designation. Color pattern and allozyme data suggests that Ambystoma tigrinum stebbinsi is closely related to Ambystoma tigrinum mavortium ; however, the Ambystoma tigrinum stebbinsi haplo-type is derived from Ambystoma tigrinum nebulosum. The most likely explanation for these observations is that Ambystoma tigrinum stebbinsi arose from a hybridization between Ambystoma tigrinum mavortium and Ambystoma tigrinum nebulosum.

The grassland community of the San Rafael Valley and adjacent montane slopes, where all extant populations of Ambystoma tigrinum stebbinsi occur, may represent a relict grassland and therefore a refugium for grassland species. Tiger salamanders in this area became isolated and, over time, genetically distinct from ancestral Ambystoma tigrinum mavortium and Ambystoma tigrinum nebulosum.

Based on color patterns and electrophoretic analysis, Ambystoma collected in Mexico at one site in Sonora and 17 sites in Chihuahua were all Ambystoma rosaceum, not Ambystoma tigrinum stebbinsi. Reanalysis of reported Ambystoma tigrinum stebbinsi collected in Sonora and at Yepomera, Chihuahua revealed that these specimens were actually Ambystoma tigrinum rosaceum.

Behavior

Larval and adult Sonora tiger salamanders are predators that feed voraciously on aquatic invertebrates, minnows, larval amphibians, and other small animals. Terrestrial adults also feed on land invertebrates. Terrestrial adults migrate to breeding ponds in the springtime.

Habitat

All sites where Sonora tiger salamanders have been found are located in the Santa Cruz and San Pedro river drainages, including sites in the San Rafael Valley and adjacent portions of the Patagonia and Huachuca mountains in Santa Cruz and Cochise counties, Arizona. All confirmed historical and extant aquatic populations are found in cattle tanks or impounded cienegas within 19 mi (30.4 km) of Lochiel, Arizona. If the Los Fresnos population is the subspecies stebbinsi, it is the only population known to occur in a cienega. Historically, the Sonora tiger salamander probably inhabited springs, cienegas, and possibly backwater pools where permanent or nearly permanent water allowed survival of mature branchiates.

Distribution

Research through the end of the 1980s had documented 18 sites for the Sonora tiger salamander, and additional extensive survey work from 1993 through 1996 revealed another 18 sites, bringing the confirmed site-total to 36. Salamanders tentatively identified as the Sonora tiger subspecies have also been found at Portrero del Alamo at the Los Fresnos cienega in the headwaters of the San Pedro River, San Rafael Valley, Sonora, Mexico and at the lower Peterson Ranch Tank in Scotia Canyon, Cochise County, Arizona. No salamanders have been observed in recent visits to Scotia Canyon; thus, this population may be extirpated. A single terrestrial Sonora tiger salamander was found near Oak Spring in Copper Canyon of the Huachuca Mountains. This individual likely moved to this site from a population at the "Game and Fish Tank" located approximately 0.6 mi (960 m) to the southwest.

A total of 79 aquatic sites in the San Rafael Valley and adjacent slopes of the Huachuca and Patagonia mountains have been surveyed for salamanders. These include most potential aquatic habitats on public lands. However, private lands in the center of the San Rafael Valley have not been surveyed intensively.

Thirty sites in northeastern Sonora and 26 sites in northwestern Chihuahua, Mexico, were surveyed by Collins and Jones in 1987. No Sonora tiger salamanders were found at these sites. Ambystoma rosaceum and Ambystoma tigrinum velasci occur at localities in Sonora and Chihuahua to the south and east of the extant range of the Sonora tiger salamander. Ambystoma tigrinum mavortium occurs at scattered localities to the east in the San Pedro, Sulphur Springs, and San Simon valleys of Arizona, but at least some of these populations were introduced by anglers and bait collectors.

Populations of this salamander fluctuate greatly under the influence of drought and disease, which periodically extirpate or greatly reduce populations. Several tanks supporting aquatic populations went dry during drought in 1994 and again in 1996. As tanks dry out, some larval and branchiate salamanders metamorphose and leave the tanks; others desiccate and die. Disease killed all aquatic salamanders at least three sites in 1985, and also was evident in aquatic populations at seven tanks in 1995-1996. Tanks in which salamanders have been eliminated may be recolonized through reproduction by terrestrial metamorphs. Drying of tanks also may eliminate non-native predators and create sites suitable for salamander colonization.

Because populations are dynamic, the number and location of extant aquatic populations change over time, as exhibited by the differences between survey results in 1985 and 1993-1996. It can be very difficult to determine whether a population is extant or not. If salamander numbers are low, isolated individuals may not be detected during sampling; also, aquatic salamanders may have been recently eliminated due to drought or disease, but terrestrial salamanders may still be present in the area. Of the 36 sites where aquatic Sonora tiger salamanders were recorded since the mid or early 1980s, no salamanders have been found at four tanks during the last three visits from 1993 to 1996. Salamanders were probably extirpated from these locations. Salamanders also were found to be extirpated from the J.F. Jones Ranch Tank, the type locality. Salamanders have not been found during the last three visits from 1993 through 1996 at five other tanks, and they may well be extirpated from these sites. Another three sites where salamanders were found from 1980 to 1983 have not been surveyed since that time. The status of populations at these tanks is unknown. At the remaining 23 tanks, salamanders have been found during one or more of the last three visits from 1993 through 1996. These populations are probably extant.

Populations of aquatic salamanders include as many as several hundred individuals. However, 10 or more salamanders in any one visit were found at only 16 of 32 occupied sites examined by Collins from 1993 through 1996. Large, reproducing populations of Sonora tiger salamanders were more concentrated in the southeastern portion of the San Rafael Valley in the 1990s as compared to the 1980s. Sampling during 1993-1996 revealed few populations and low numbers of salamanders in the northern portion of the valley.

Threats

A variety of factors threaten the Sonora tiger salamander. Disease and predation by introduced non-native fishes and bullfrogs (Rana catesbeiana ) are probably the most serious and immediate threats, both of which have been implicated in the elimination of aquatic populations. Tiger salamanders are also widely used in Arizona as fishing bait, and this collection of them for bait could extirpate or greatly reduce populations. Other subspecies of tiger salamander introduced into habitats of the Sonora tiger salamander for bait propagation or by anglers could, through interbreeding, genetically swamp distinct Ambystoma tigrinum stebbinsi populations. The possibility of the transmission of disease increases as salamanders are moved among tanks by anglers or bait collectors. Additional threats include habitat destruction, reduced fitness resulting from low genetic heterozygosity, and the increased probability of chance extirpation characteristic of small populations.

Stream headcutting threatens the presumed Sonora tiger salamander populations at Los Fresnos cienega in Sonora. Erosion is occurring in Arroyo Los Fresnos downstream from the cienega and the headcut is moving upstream. The causes of this erosion are uncertain, but are presumably livestock grazing and roads in this sparsely populated region. If the causes of this erosion are left unchecked and headcutting continues, it is likely that the cienega habitat will be lost within the foreseeable future. The loss of Los Fresnos cienega may extirpate this tiger salamander population. If the salamanders at the Los Fresnos cienega are Sonora tiger salamanders, this would represent the only known natural cienega habitat occupied by an aquatic population of this species.

All confirmed Sonora tiger salamander populations have been found in stock tanks or impounded cienegas constructed to collect runoff for livestock. Many tanks probably date from the 1920s and 1930s when government subsidies were available to offset construction costs; however, some tanks were constructed as early as the 1820s and as late as the 1960s. These stock tanks have, to some degree, created and replaced permanent or semipermanent Sonora tiger salamander water sources. Although the tanks provide suitable aquatic habitats, current management and the dynamic nature of these artificial impoundments compromise their ability to support salamander populations in the long term. The tanks collect silt from upstream drainages and must be cleaned out periodically, typically with heavy equipment. This maintenance is done when stock tanks are dry or nearly dry, at an average interval of about 15 years. As the tanks dry out, a proportion of aquatic salamanders typically metamorphose and migrate from the pond. If, however, water is present during maintenance, eggs, branchiate, and larval salamanders may be present and would be lost as a result of the excavation of remaining aquatic habitat. Aquatic salamanders also may occur in the mud of dry or nearly dry tanks and would be affected. Any terrestrial metamorphs at the tank or in areas disturbed would be lost during maintenance activities.

Flooding and drought pose additional threats to stock tank populations of Sonora tiger salamanders. The tanks are simple earthen impoundments without water control structures. Flooding could erode and breach downstream berms or deposit silt, resulting in a loss of aquatic habitat. Long-term drought could dry up stock tanks, as happened in 1994 and 1996. Fires in watersheds above the tanks may lead to increased erosion and sedimentation following storms and exacerbate the effects of flooding.

Sonora tiger salamanders have persisted in stock tanks despite periodic maintenance, flooding, and drought. If the tanks refill soon after drought or other events that result in loss of aquatic habitat, they could presumably be recolonized through terrestrial metamorph reproduction. However, if a tank was dry for several years and isolated from other salamander localities, insufficient terrestrial salamanders may remain and immigration from other populations may be inadequate to recolonize the stock tank. Potential grazing practice changes also threaten aquatic Sonora tiger salamander populations. Stock tanks could be abandoned or replaced by other watering facilities, such as troughs supplied by windmills or pipelines. Troughs do not provide habitat for Sonora tiger salamanders.

Ambystoma tigrinum stebbinsi is considered a species of special concern by the State of Arizona, but this designation affords the species and its habitat no legal protection. Collecting the Sonora tiger salamander in the San Rafael Valley is prohibited, except under special permit. The transport and stocking of live bullfrogs, fishing with live bait fish, and fishing with Ambystoma within the range of this salamander in Arizona is also prohibited. Despite these prohibitions, some illegal collecting occurs. Bullfrogs and non-native fish are present at numerous extant and historical Sonora tiger salamander localities, suggesting continued illegal introductions. In 1987, it was reported that tiger salamanders were illegally collected from the San Rafael Valley and transported to at least two tanks in the northern Patagonia Mountains. This incident led them to suspect that bait collectors and anglers often move salamanders among stock tanks.

Furthermore, abandonment, modification, and breaching of stock tanks is allowed on private and public lands, and such actions could eliminate Sonora tiger salamander populations. The extent of these practices and their consequent threats to Ambystoma populations are not presently known. Since Sonora tiger salamander populations are relatively small, collection and associated activities may significantly reduce recruitment, the size of branchiate or larval populations, and genetic diversity within a tank. All these results would increase the likelihood of extirpations.

Arizona anglers and commercial bait dealers also often introduce larval tiger salamanders into ponds and tanks for future bait collecting. The very act of moving larval salamanders to different locations could eventually establish new populations. It was suggested that transport and introduction of salamanders within the San Rafael Valley may have greatly influenced their present distribution.

Moving salamanders could also transmit disease and cause unintentional introductions of fish or bullfrogs, which might reduce or extirpate populations.

The transportation and introduction of salamanders poses an additional threat. Ambystoma tigrinum mavortium is common in stock tanks and ponds to the east of the San Rafael Valley. Bait dealers and anglers probably introduced many of these populations. If Ambystoma tigrinum mavortium is introduced into Sonora tiger salamander localities, populations of the latter could be lost due to genetic swamping by interbreeding of the two subspecies. This is of particular concern for Sonora tiger salamander populations inhabiting stock tanks that could wash out during a storm or dry out during drought. Additionally, Sonora tiger salamander genetic heterozygosity is among the lowest reported for any salamander. Low heterozygosity indicates low genetic variation, which increases demographic variability and the chance of local extirpations.

Sonora tiger salamander populations are eliminated by non-native fish predation, particularly sunfish and catfish. In laboratory studies, bullhead, mosquito fish, and sunfish ate Sonora tiger salamander eggs, hatchlings, and small larvae. Introduced non-native fish are well-established in the San Rafael Valley and have been implicated in apparent Sonora tiger salamander extirpations from five stock tanks, including the type locality. Non-native fish are known to occur at only one of 23 sites where salamanders have been found during one or more of the last three visits from 1993 through 1996. However, non-native fish occur at seven of ten sites where the salamander is thought to be extirpated or where it has not been found during the last three visits. The effect of native fishes on salamander populations is unknown, although some native species have a potential to prey on Sonora tiger salamanders. No native fish are known to occur with aquatic populations of salamanders.

Bullfrogs occur with Sonora tiger salamanders at 16 of 23 sites at which salamanders have been found during one or more of the last three visits from 1993 through 1996. Adult bullfrogs are known to prey on salamanders; however, bullfrog tadpoles do not eat viable salamander eggs or hatchlings. Bullfrogs were found to be more widely distributed in the San Rafael Valley in the 1990s as compared to 1985. The effect of predation by bullfrogs on salamander populations is unknown, but increased mortality attributable to bullfrog predation would likely reduce population viability.

Virtually no recruitment was noted in recent surveys, as evidenced by a lack of surviving larvae in tanks where eggs were known to have been deposited. Lack of recruitment appeared to be a result of predation by overwintering branchiate and larval salamanders. This predation may occur because degraded habitat has lost the structural complexity of emergent and shoreline vegetation, logs, and rocks that would provide cover and protection from predation. Lack of shoreline and emergent vegetation is at least partially due to trampling and foraging by cattle.

A disease characterized by sloughing of skin and hemorrhaging killed all branchiate salamanders at Huachuca Tank, Parker Canyon Tank #1, and Inez Tank in 1985, and has been detected at seven tanks in 1995-1996. The disease may be caused by a combination of a virus and the Aeromonas bacterial infections. Parker Canyon Tank #1 and Inez Tank were recolonized by 1987, and salamanders were found once again at Huachuca Tank in 1994. These tanks were presumably recolonized by reproducing terrestrial metamorphs that survived the disease or that moved to these tanks from adjacent populations. At the seven tanks where the disease was found in 1995-1996, the effects on the populations will not be known until the disease runs its course. If the disease recurs with enough frequency, populations could be lost due to lack of recruitment of juveniles into the adult cohort. Genetic variability is already very low in this subspecies, and this disease has the potential to depress genetic variability even further, greatly increasing the chances of population extirpations. Bullfrogs, wading birds, waterfowl, and other animals that move among tanks may facilitate spread of the disease.

The ability of Sonora tiger salamanders to move between populations is unknown, but arid grassland, savanna, or pine-oak woodland separate all populations and movement through these relatively dry landscapes is probably limited. Movement would be most likely during storms or where wet drainages are available as movement corridors. The distance between aquatic populations of Sonora tiger salamander is frequently more than 1 mi (1.6 km), and much greater distances separate several sites. Game and Fish Tank, for example, is 6.3 mi (10 km) from the nearest adjacent aquatic population. Even if these salamanders are capable of moving relatively long distances, some populations may be geographically isolated. Small, isolated populations have an increased probability of extirpation.

Disease, predation by non-native predators, and drying of tanks during drought further increase the chance of extirpation. Once populations are extirpated, natural recolonization of these isolated habitats may not occur.

Conservation and Recovery

The Coronado National Forest conferred with the Fish and Wildlife Service on the effects of issuance of grazing permits in the Duquesne, Campini, and San Rafael allotments within the range of the Sonora tiger salamander. The Service determined that issuance of the permits would not likely jeopardize the continued existence of the salamander provided that stock tank maintenance and management plans were promptly developed and implemented for the allotments. These plans would ensure the maintenance of quality aquatic habitat for the Sonora tiger salamander.

Contact

U. S. Fish and Wildlife Service
Division of Endangered Species and Habitat Conservation
2105 Osuna Road N.E.
Albuquerque, New Mexico 87103-1001
Telephone: (505) 346-2525
E-mail: r2esweb@fws.gov
http://ifwes.fws.gov/

Reference

U.S. Fish and Wildlife Service. 6 January 1997. "Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for Three Wetland Species Found in Southern Arizona and Northern Sonora, Mexico." Federal Register 62 (3): 665-689.