The desert tortoise case is an interesting one. As the linked article points out, this disease was observed in the population at Beaver Dam Slope of Utah in the 1970's, where a large number of captives have been released but it did not cause a mass die off at that time or subsequently. The Kern County population suffered heavy losses between 1988 and 1992 but is the die off caused by the introduction of infected captives to this population? The article is unclear about this. The article also points out that "free-ranging desert tortoises with URDS [are] also widespread in the western Mojave Desert of California, around Las Vegas Valley in Nevada, on the Beaver Dam Slope of Utah/Arizona, and sporadically in low numbers in the Sonoran Desert of Arizona."



Yet there is no report of heavy losses in these other populations. The question remains, what killed so many tortoises in the late 1980s to early 1990s in Kern County but not elsewhere? Sure, tortoises with the disease were seen in this population, but sick tortoises are also seen in many other populations, so a direct causal relationship has not been established. Further, the author points out near the end of the article that "Turtles may remain carriers of Mycoplasma for life with recurrence of the disease at some point in time in the future." Clearly the bacterium, although it most likely causes the disease, does not seem to cause the disease in healthy tortoises, and mass die offs do not occur in many areas even though sick tortoises can turn up virtually anywhere within the range of Gopherus agassizzii. Hence the possibility that wild populations of tortoises carry disease-causing pathogens naturally cannot be discounted.



Nevetheless, the author is correct that captive tortoises should not be released into the wild, just to be on the safe side. That said, it is far from convincing that such releases is responsible for the heavy losses that were observed in Kern County. Most of the tortoises that died in this area during the time period in question may have succumbed to other factors, such as drought, malnutrition or other unknown cause(s).



Below I cite an abstract from a recent study of desert tortoise survival:



"Survival of adult Desert Tortoises (Gopherus agassizii) appears related to site-specific variation in precipitation and productivity of annual plants. We studied adult tortoise survival rates at two closely situated, but physiographically different, sites in the eastern Mojave Desert over a nine-year period (spring 1992 to spring 2001). Survival rates were initially derived from population surveys conducted over a three year period and by radio-telemetry monitoring over a seven-year period beginning in 1994. After a period of initial stability, survival rates on the two sites diverged over the study period, and seven-year survival rates estimated from radio-telemetry monitoring were 0.900 and 0.269, respectively. A die-off in 1996 on the latter site appears to have been triggered by a period of drought, which began in the summer of 1995, coupled with a failure of annual vegetation production in 1996. Depressed survival rates on this site were associated with drought conditions during three of four years. Although the decline had the appearance of an epizootic, there were no clinical signs of disease. Relatively short-term drought, combined with little or no annual biomass, appears to have caused severe reductions in tortoise survival. If periods of drought induced low survival are common over relatively small areas, then source-sink population dynamics may be an important factor determining tortoise population densities."



This study again calls into question the role of disease in the heavy losses suffered by the Kern County population in the late 1980s to early 1990s. Before blaming the losses on disease, the real cause(s) should perhaps be sought.



Literature cited



Longshore, Kathleen M., Jef R. Jaeger, and J. Mark Sappington 2003. Desert Tortoise (Gopherus agassizii) Survival at Two Eastern Mojave Desert Sites: Death by Short-Term Drought? Journal of Herpetology, Vol. 37, No. 1, pp. 169–177
THE DESERT TORTOISE AND UPPER RESPIRATORY TRACT DISEASE

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