|Scientific Name:||Gopherus flavomarginatus|
|Species Authority:||(Legler, 1959)|
Gopherus flavomarginatus Legler, 1959
|Taxonomic Notes:||No synonyms or recent re-assignments.|
|Red List Category & Criteria:||Vulnerable A1cd ver 3.1|
|Assessor/s:||van Dijk, P.P. & Flores-Villela, O.|
|Reviewer/s:||Aguirre Leon, G., Iverson, J.B. & Rhodin, A.G.J. (Tortoise & Freshwater Turtle Red List Authority)|
This species is listed as Vulnerable because it has experienced a population decline of up to 50% over the past 3 generations. It faced catastrophic levels of exploitation during the middle of the 20th century, with subsequent lower levels of exploitation. At present the species is protected from direct exploitation and part of its extent of occurrence is protected, but some subsistence collection and habitat degradation impacts likely still occur. With the worst impacts over, it is rated Vulnerable (under A1) rather than Endangered (under A2).
About six separate subpopulations exist, comprising some 7,000 to 10,000 adults, collectively occurring over about 7,000 sq. km.
Generation time is somewhere at 40 to 60 years. The species appears to have weathered its period of worst impacts and its slide into steep decline has been halted; the Bolson Tortoise is probably on the path to recovery due to intensive and sustained conservation efforts dating back to the 1970s, but remaining subpopulations apparently still suffer attrition from incidental take and habitat impacts. The population within the Mapimi Biosphere is recovering in recent years (Oscar Flores-Villela pers. comm. 2005), as a result of continued population protection since mid 1970s, and reserve expansion and law reinforcement through improved reserve administration by the Mexican Government since 2000. However, in the rest of the species' range protection is minimal or non existent (Aguirre Leon in litt. 10 Jan 2007).
|Range Description:||Occurs restricted to a series of disjunct isolated basins collectively known as the Bolson de Mapimi of south-eastern Chihuahua, western Coahuila and northern Durango, Mexico (Aguirre in Groombridge 1982, Morafka et al. 1989, Iverson 1992).
The Bolson de Mapimi covers about 40,000 sq. km; the total geographic range of G. flavomarginatus measures about 150 km across, and the area of occupancy is about 7,000 sq. km (Morafka 1982, Morafka et al. 1989).
The past distribution of the species was much wider - Pleistocene fossils indicate occurrence as far away as south-western Arizona and Oklahoma, US, and Aguascalientes, Mexico (Morafka 1982, Morafka et al. 1989).
|Range Map:||Click here to open the map viewer and explore range.|
Populations are localized and disjunct. Three major populations sustain large reproducing populations, and three further minor populations exist. Total number of adults was estimated at a maximum of 7,000 to 10,000 in 1989. (Morafka et al. 1989).
Maximum known recent density is seven animals per hectare, but normal density is at the order of one animal per hectare. (Morafka 1982, Morafka et al. 1989).
|Habitat and Ecology:||
Bolson Tortoises prefer low grade slopes (0.5% to 2%) of fine textured soil (averaging 48% sand, 32% silt, 10% clay, 10% gravel), vegetated by mixed sclerophyll shrub and desert bunch grass. These areas generally fringe basin floodplains. The area of occurrence is between 1,000 and 1,400 m altitude. (Morafka 1982, Morafka et al. 1989).
Bolson Tortoises dig burrows up to 8 m long and 2 m deep as refuge from predators and extremes of climatic and weather conditions, and surface activity is correlated with rainfall and temperature. Aguirre et al. (1989) calculated that adult Bolson Tortoises spend less than 1% of their entire lives on the surface, either basking or feeding along well-established trails near the burrow. Burrows are constructed in social aggregations, and clusters show social structuring of individuals. (Morafka 1982, Morafka et al. 1989). Radiotracked juveniles preferred to excavate (or opportunistically use) burrows under Opuntia cacti (Tom 1994).
Bolson Tortoises are exclusively herbivorous, feeding on a variety of grasses, shrubs and herbs (Morafka 1982, Morafka et al. 1989).
This is the largest North American tortoise species, approaching 40 cm CL (fossils indicate past size more than double this). Both sexes reach similar size. Sexual maturity probably occurs at CL over 25 cm and 15 to 20 years of age. Females outnumber males, at male/female ratios of 0.43 to 0.83 in different populations. Wild females produce 1 or 2 clutches (average 1.3) averaging 5.2 eggs; infertility rate averages 35%. Thus, an average female will produce only 3.4 offspring in an 8-year period of her reproductive period. With perhaps a survivorship of less than 5% to maturity, replacement time is over half a century.
The extreme reduction of the species' range in geological time has been attributed to a combination of climatic change, seismic activity eliminating whole populations by collapsing burrows, and overexploitation by paleoindians (Morafka et al. 1989).
In recent decades, Bolson Tortoises have been overexploited for food by local residents (particularly resettled people in ejidos) and particularly by railroad workers since the 1940s construction of railroads; tortoises were also collected and shipped by rail to coastal Pacific cities for gourmet consumption. Tortoises are now absent from a 10 km strip on either side of the railroad, and from the vicinity of roads. (Morafka 1982; Morafka et al. 1989).
Ongoing threats include habitat degradation and destruction by overgrazing, ploughing and irrigation, which have apparently contributed to the extirpation of large tortoise colonies through direct mortality and reduced juvenile survival and thus recruitment (Morafka et al. 1989).
The species is protected under Mexican law and is included in CITES Appendix I. A substantial part of the southern area of occurrence of the species is effectively protected by the Mapimi Biosphere Reserve (3,424 sq. km., IUCN Category VI) in Durango; a substantial northern population is protected through cooperative agreement with Rancho Sombreretillo in Chihuahua (Trevino et al. 1997); and a number of ranches in the area afford private protection.
Research concerning the species have led to good understanding of the species' biology. Local awareness projects combined with enforcement of legal protection have had very positive local effects (Morafka et al. 1989). Some in situ and ex situ captive groups exist and some conservation breeding successes have occurred, and headstarted juveniles have been released (Morafka et al. 1989, Aguirre et al. 1997).
Reintroduction / relocation of the species into Big Bend NP (Texas, US) has been proposed (Aguirre et al. 1997) but has found little support with US authorities to date.
Further intensive in situ conservation action for the species is warranted, with ongoing public awareness and outreach and perhaps continuing operation of captive insurance colonies.
|Citation:||van Dijk, P.P. & Flores-Villela, O. 2007. Gopherus flavomarginatus. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <www.iucnredlist.org>. Downloaded on 08 March 2014.|
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