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Tortoise Times Two

<em>G. agassizii</em> in the Mojave National Preserve | Chris Clarke photo
G. agassizii in the Mojave National Preserve | Chris Clarke

Biologists have determined that the southwest's desert tortoise population actually consists of two distinct species. Does this finding mean changes in the way we treat those species' habitat?

The biologists, who published their findings recently in the journal ZooKeys, found that tortoises on either side of the Colorado River should be considered distinct species: Gopherus agassizii on the west, and the new species -- Gopherus morafkai, named for the late herpetologist David J. Morafka -- on the east.

As a result of this study Gopherus agassizii, formally listed as Threatened under the US Endangered Species Act (ESA), has in a sense lost more than two-thirds of its range. The new tortoise species' status under ESA is also open to question. Whether the new science will be reflected in land use policy will likely be decided in the courtroom rather than the laboratory. The stakes are high. The tortoises' habitat is under pressure from human activity ranging from urban development to off-road vehicle use to the desert's burgeoning solar power sector.

Evolutionary biologists consider fragmentation of populations, and the resulting "reproductive isolation," to be one of the main drivers of the evolution of new species. The Colorado River would certainly have been a formidable barrier to tortoises seeking mates over the few thousand years likely needed for species to diverge. It's long been known that tortoises on either side of the river are significantly different: Sonoran desert tortoises east of the river tend to be flatter and narrower than their western counterparts, for one thing, and their preferred habitat tends to be farther uphill and rockier than that of the gravel-wash-loving tortoises in the Mojave.

The new desert tortoise species' ranges | Map courtesy ZooKeys
The new desert tortoise species' ranges | Map courtesy ZooKeys

To name a species a biologist must refer to a type specimen, usually a preserved individual of the species being described. The type specimen for Gopherus agassizii, preserved in ethanol, now sits in the collection of the Smithsonian's American Museum of Natural History, collected in 1861 by James Graham Cooper, an M.D. who worked as a Smithsonian field biologist. Researchers extracted DNA from the specimen -- some of it still intact after 150 years of pickling -- and compared it to other DNA samples collected throughout the range of the desert tortoise.

Comparing the torts' DNA confirmed that the two different kinds of tortoise ought to be considered distinct species.

The US Fish and Wildlife Service, the federal agency responsible for determining which species warrant protection under ESA, has historically been slow to upgrade protections on already listed species, or to list new ones. Jeannie Stafford, a public affairs staffer in FWS' Nevada office, told Forbes that her agency plans no changes for the western tortoise species:

Changes in listing under ESA these days are generally propelled by lawsuits from groups such as the Center for Biological Diversity, and the tortoises are unlikely to prove exceptions.

Desert tortoises, which can live for 50-80 years, require a fair bit of room in order to thrive. Living as they do off the flushes of plant growth that occur in spring in the sparse desert, tortoises can range across almost a square mile to gather sufficient food, becoming intimately familiar with each piece of home territory. Relocating tortoises out of the path of development has proved difficult, with subsequent mortality as high as fifty percent.

Meanwhile, the common practice of mitigation -- setting aside and protecting suitable habitat in exchange for destroying habitat elsewhere -- has resulted in an increasing shortage of suitable mitigation lands in the Mojave, with one developer after another pledging to set aside habitat from a finite supply of undisturbed desert. Those developers can spend almost as much money dealing with their tortoises as they do on construction.

Regardless of whether Fish and Wildlife has indeed essentially considered the tortoises as distinct populations equivalent to species, it is near-certain that the ZooKeys study will be a monkey wrench thrown into the Byzantine system of environmental review of desert development proposals. The next few years in tortoise country should prove interesting.