Saving the Desert Tortoise:
A Plan for Managing the Tortoise Population
In the Mojave Desert
Sara Maddison
1 Photo Credit: Red Cliffs Desert Reserve
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Important Traits of the Desert Tortoise:
The desert tortoise, or Gopherus agassizii, is classified by the IUCN as a vulnerable
species and as a threatened species by the Endangered Species Act. Populations of this tortoise
are located in the Mojave Desert, which spans across California, Arizona, Nevada, and Utah
(Figure 2). They are the only large tortoise species in
the area, and no other North American tortoise species
can survive in the extreme conditions of their habitat.
Temperatures in the desert may reach as high as
140°F, and tortoises survive by burrowing into the
sandy soil, making soil type and quality an important
Figure 2 Photo Credit: Desert USA
factor in their designated habitats. It is estimated
that there are about 100,000 desert tortoises in the wild, but this count includes a similar species,
the Gopherus morafkai of the Sonoran Desert. This seems like a large number for a vulnerable
population, but there were millions of desert tortoises in the wild as late as the 1950s, which was
only about two tortoise-generations ago. It is difficult to get an exact count of this species,
because they spend up to 95% of their life burrowed deep in the soil, seeking shelter from the
elements and threats from humans and predators (“Basic Facts”). Because of its specific habitat
requirements and vulnerability to outside influences, the desert tortoise is considered an indicator
species for the Mojave Desert (Doak).
Significant Threats to the Desert Tortoise:
Desert tortoise populations are in danger from increases in human activity, disease, and
the effects of climate change. Possibly because it is the least natural, human activity in the area
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poses the biggest threat. Wide-open spaces with sandy soils are popular for off-road vehicle
enthusiasts, and the tortoises are often the unseen victims of playtime. Many tortoises are hit and
killed by vehicles traveling at high speeds through their habitat, and many others are killed when
humans do not realize that they often seek shade under parked vehicles in the heat of the day.
They are also especially vulnerable when in their fragile burrows, because they can easily
collapse under the weight of a vehicle.
Another less direct threat from humans is the presence of garbage in the desert. The trash
itself poses a risk for the tortoises, because if consumed, it can become lodged in their stomach
or intestines and give them a false sense of fullness, causing them to starve to death. However,
the left-behind litter also attracts scavengers like ravens, whose numbers in the Mojave have
increased about 700% in the last fifty years. Ravens are a natural predator of the desert tortoise
because their hard beaks can easily penetrate the soft shell of juveniles, and they often turn to
young tortoises as a food source. Desert tortoises have a lifespan of fifty to eighty years, so they
are in their juvenile stages for many years. This is a large window of opportunity for dangerous
ravens that are attracted by harmful human activities in the area (“Threats”).
There are only a few major urban areas in the Mojave, but humans may soon pose an
even larger threat to desert tortoises. Most people shy away from life in the desert because of the
extreme heat, lack of development, and brutal sun. Unfortunately for the tortoise, these qualities
that make the desert undesirable to humans also make it the perfect environment for solar power
plants, seeing as that the area receives more than twice the necessary sunlight to host solar
power. There are about ten small plants already in existence, and two very large plants are under
construction at this time. One of the most controversial plants is the Ivanpah Solar Electric
Generating System, which is scheduled to be completed this year. Because the Mojave is
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considered a “hotspot” for endangered and vulnerable species, those seeking to develop in the
area must consider the welfare of wildlife (Lovich). The company behind the plant worked with
wildlife experts to estimate that about forty tortoises would need to be relocated from the area in
Southern California, but they have so far found almost two-hundred, and many of them are
juveniles. The major motivation behind the construction of the plant is combatting the need for
renewable resources that is being escalated by global climate change. It is difficult for
environmental advocates to choose a side in this debate, because both parties will provide a
valuable environmental service: the plant through the creation of clean energy, and tortoises for
their intrinsic values (Wells). It seems that the best option would allow for both the protection of
the tortoise and the creation of the plant, because that would also be the most favored choice by
the public. If the tortoises can be relocated cheaply, then construction of the plant should
continue. Many residents in the area are counting on the energy that will be created, so it is not
realistic to suggest that construction should be halted and expect that plan to receive public
support.
In addition to threats from human activity, desert tortoises are also vulnerable to a more
internal threat: upper respiratory tract disease. URTD was one of the major reasons that the
Mojave population of tortoises was emergency-listed as endangered in 1989. It is widely
believed that this disease is spread either by pathogens or through viral infections, and evidence
suggests that there may even be more than one causative agent. The most severe effect of the
disease is death, and it is highly contagious, so common policy is to euthanize tortoises that are
found to be carriers. When the Las Vegas area experienced a population boom in 2006, about
16,000 tortoises were relocated. All of these were blood-tested for signs of the disease, and
those found to be “suspect” or “positive” were euthanized immediately. Over 3,000 tortoises
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were killed, which is approximately three percent of the estimated total population. However, a
new study hypothesizes that the desert tortoise may have developed immunity to some of the
pathogens and infections that propagate respiratory disease. If this is true, then those tortoises
found to have traces in their system may never actually show symptoms or become contagious.
As a result of this research, and the realization that an alarming number of tortoises were being
killed, the management policy shifted from euthanization to separation. Tortoises that may be
carrying the disease are now separated from others and monitored, and some are used in research
projects to help understand the potential of the immunity and determine which causative factors
are still spreading the disease (Sandmeier). Thousands of tortoises were saved by this change in
procedure, so this is the method that would most benefit the desert tortoise population.
Another problem for the desert tortoise population in the Mojave is the way that they are
distributed. Even if an entire area is suitable for habitat, the tortoises tend to live in high-density
clusters with up to 250 tortoises per square mile, with large open areas between each grouping.
Because of these isolated groups and the fact that there are no other large tortoise species in the
area to allow for cross-breeding, there are many instances of inbreeding in the Mojave Desert
tortoise population (Boarman). This is producing undesirable offspring, but the loss of genetic
variability may also threaten the potential developing immunity to URTD. Finding some way to
connect the isolated populations could put an end to forced inbreeding.
Desert tortoises are also being hindered by the presence of invasive plants in their habitat,
which may be brought in on vehicles in areas where there are typically none. The most direct
result is that the invasive plants eliminate the source of food and water for the tortoises, which
get the majority of their water from the plants that they consume. The indirect effects of the
invasive plants, however, may be having an even larger impact on populations. Desert tortoises
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spend the large majority of their life in their burrows, but when they do move, they travel
relatively large distances searching for water. Invasive plants in areas that were once open desert
can create snagging brush that traps tortoises that are foraging for sustenance. These brush fields
also pose huge fire threats, and many tortoises die each year as a direct result of brush fires
(Brooks).
Population Requirements of the Desert Tortoise:
Many researchers have attempted to do a census or extended survey to determine the
number of desert tortoises in the Mojave. One researcher attempting to do such a thing
discovered that more data was necessary to do a basic census, specifically in years with higher
than average precipitation or areas with significant human interference. He did find, however,
that population numbers have declined from estimates in previous years (Figure 3) (Corn). A
recent study sought to more accurately estimate the number without attempting to count each
individual tortoise. They discovered that areas that were previously estimated to be home to at
least sixty tortoises per square kilometer may now have fewer than twelve. Because desert
tortoises are so elusive, researchers determined that the best surveying practice would be to use
indicators of tortoise activity in the area, rather than actual sightings. By watching
environmental factors and considering tortoise behavior patterns during the active season,
researchers were able to estimate that there are approximately one to twelve tortoises per square
kilometer in the more densely populated areas. This is not a very definite answer, which is why
their more important conclusion was that scientists cannot rely on tortoise population estimates,
because it is almost impossible to get a very accurate count (Inman).
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Figure 3 Photo Credit: Corn
A population viability analysis conducted in the mid-1990s in the Western Mojave Desert
found that the future success of desert tortoise populations relies most heavily on the well-being
of adult females. The study used a stage-based model for the analysis, with special emphasis on
tortoises’ response to environmental changes and the prioritization of future data. Researchers
gathered data through a mark-recapture method, and focused on yearly rates of survivorship,
growth in terms of moving from one size class to another, and reproduction. In addition to the
importance of the adult female, the research also established that aside from needing a specific
soil type, the largest concern for the desert tortoise is finding a habitat with as little human
interference as possible. The tortoises used in the study were all from areas with at least some
human interference, so the scientists used a new matrix and the optimistic survivorship rate of
98% to determine if tortoise survivorship would increase if human interference were eliminated
altogether, and they discovered that it would (Doak).
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Habitat Requirements of the Desert Tortoise:
Based on the diet and water requirements of the desert tortoise and significant threats to
their population, there are certain characteristics that must be present in a habitat to host a
successful population. Desert tortoises must be located in an area with sandy soil, because it is
soft enough for tortoises to dig with their sharp claws, but stable enough to support weight in the
burrows. The burrows are significant for shelter from heat and the birth and rearing of young
tortoises. They will also need an area where no invasive plant species interfere with their food
source, which includes growth of young cacti and xeric grasses and shrubs. They do not have
large water requirements, but they will need access to the grasses that supply most of their water.
Desert tortoises are highly adaptable to the extreme heat of the Mojave summers and the extreme
cold of the Mojave winters, so temperature is of no real concern. In addition to these basic
requirements, it is imperative that the tortoises be in an area free from outside human
interference. This includes recreational activities like off-roading, as well as illegal trash
dumping and other activities that may attract harmful scavengers like ravens. One of the major
concerns for the conservation of the desert tortoise is the physiological damage caused by
translocation. Previous studies suggested that the experience of being relocated was so traumatic
for desert tortoises that many of them would not survive it. However, a recent look into the
physiological effect of translocation on desert tortoises discovered that previous worries about
stress on tortoises may be unnecessary. Scientists tested the presence of total corticosteroid, a
hormone that indicates stress, in tortoises before and after translocation, and found that there is
no significant change (Drake). This suggests that if done correctly, translocation of the desert
tortoises in threatened areas of the Mojave may be a viable option for survival of the species.
Ideally, the tortoises will be transported to an area with a pre-existing population, because that
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would guarantee the presence of suitable soils and plant life, both for nourishment and the
provision of water.
Desert Tortoise Conservation Plan:
Current management of the desert tortoise in the Mojave Desert is governed by the 2011
Revised Recovery Plan, which was implemented in March 2013. The primary goals of the plan
are to protect current habitat of the desert tortoise, reduce human activity in areas where tortoises
are common, and focus on the accelerating effects of climate change, which may include an
increase in fire threat and a lack of water sources for desert tortoises (“DTRO”). In order to
ensure the survivability of the species, it will be necessary to take a more direct approach to
management of the desert tortoise in the Mojave. The best management policy to increase
population numbers will be to combine the tortoises in the desert into one larger area, which can
then be separated into the small, densely-populated clusters that tortoises favor, with corridors
between to allow tortoises to travel and help curb inbreeding. Essentially, this means that
managers will be designing networks of protected areas and monitoring the populations inside.
The highlighted areas of the map (Figure 4) show current estimated distribution of the
desert tortoise in the Mojave, and the circled area would be the ideal location where all tortoises
in outlying areas should be translocated. This area was selected because of its distance from
large urban areas like Las Vegas, Los Angeles, and San Diego. It is also the closest to being a
circular area, which will help curb the negative effects of habitat edge. This area is located in the
Mojave National Preserve, so it is federally protected, although management is difficult on a
tight budget. The selected area is currently two separate population groupings, but they are close
enough in distance that a corridor to connect them will be practical and reasonably cost effective.
The fact that tortoises already exist in the area proves that the soil is of a suitable sandy quality
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for burrowing, which means that tortoises will have shelter and will be able to reproduce
successfully.
Figure 4 Map Credit: Lower Colorado Multi-Species Conservation Program
None of the areas are currently immune to interference from humans. Although the
species is protected under the Endangered Species Act, it is very labor-intensive and expensive
to monitor such large open areas, and many of the humans causing harm to tortoises are not
doing so intentionally. This is why they will be better protected in a more concentrated area.
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Most of the solar plants located in the Mojave are farther west and south of this area, so there
will be few, if any, conflicts between species protection and the demand for renewable energy
sources.
In order to actually implement the conservation plan, managers should carefully
translocate tortoises to the new area. Desert tortoises are not confrontational, so it will be helpful
if they are placed in areas where there is already a high population density. Previously
mentioned research found that areas capable of hosting more than sixty individuals per square
kilometer were only housing about ten, so overcrowding will not be an issue. Although human
interference is a bane for the desert tortoise, it will be necessary for managers to supplement the
area with water and clear out any invasive plant species.
Desert tortoises can survive for up to a year without ingesting any water, so when it does
come, it is vital to their survival. Tortoises sometimes dig small basins to collect water during
storms, and they may even be found waiting beside them when rainfall is imminent (“Basic
Facts”). In years with little rainfall, tortoise managers should fill pre-existing basins with water,
because tortoises check their basins frequently for accumulated water. Managers should not dig
the basins for the tortoises or put out artificial water dishes, because tortoises may lose the
instinctual ability to gather their own water. Managers should also monitor for the presence of
invasive plant species and clear them out when necessary. They should try to keep the area clear
of dry or dead brush in general, because this creates a fire risk that can be fatal to the slowmoving tortoises. In addition to these considerations of the habitat, managers should monitor the
actual tortoise population to ensure that individuals are assimilating well after their translocation
and to watch for the spread of URTD.
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Aside from the responsibilities of the managers, the actual physical habitat is important,
as well. The environment should be as close as possible to the pre-1950s Mojave Desert, when
tortoise populations were at their peak. There should be some areas with dense xeric grasses,
where tortoises can find food sources and sources of water, as well as shelter from predators or
the elements. Other areas should be clear of or sparsely littered with grasses, so that tortoises
can burrow easily into the soil to seek shelter from the extreme heat. This is also where tortoises
will spend their non-active periods and where females will lay their eggs. It is especially
important to protect the adult females since studies found that they are crucial to the survival of
the species. This can be done by keeping humans seeking areas for recreation out of the areas
where females are nesting. This may mean having areas that are fenced in, with signs posted
explaining the situation and discouraging trespassers. The fences will need to have large enough
gaps between the fence and the ground for tortoises to move in and out of the area. It will not be
necessary to do much to the actual physical environment other than the fencing and the removal
of invasive plants. The following shows an ideal section of the proposed protected habitat.
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This is essentially a hotspot approach to conservation of the desert tortoise, because
managers would be grouping all of the existing individuals in the Mojave into one large area in
an attempt to better manage them. In the event of a great loss of habitat, such as a fire or other
natural disaster, there would be devastating effects on the tortoise population. That is why it is
important to use a few good-sized groupings of desert tortoises, rather than one large population.
This provides what can be considered back-up populations, and would also help managers
quarantine any disease that begins to spread. However, the populations should not be completely
isolated, because inbreeding becomes an issue in closed-off populations. Simple corridors
should be managed between the groups to facilitate the movement of individuals. Corridor
management is an expensive endeavor, so it may be something as simple as strips of sandy soil
connecting the areas, with sufficient shrubbery for cover and to find water along the way.
Future Conservation Issues for the Desert Tortoise:
The survival of the desert tortoise relies heavily on the prevention of human activity in
their habitat. In addition to the management of the proposed preserve, the best way to combat
interference is through public education. Tortoises are non-threatening, but they are also large
and visible. They are already an indicator species for the Mojave, but experts should also
attempt to instill them as a flagship species for protection of the area. Educational campaigns
warning of the dangers posed by human recreational activities may be a success if the desert
tortoise is used as a large, friendly mascot. Most humans do not realize the harm they are
causing through their carefree recreational activities, so it should not be difficult to convince
people to stay out of the protected area.
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Like many other species, humans included, desert tortoises will also have to continue to
combat global climate change. Water will become more and more scarce in the area, so it will be
critical for managers to supplement water without encroaching upon instinctual gathering habits.
Climate change will also create an even larger demand for solar energy, which is why the
selected area is as far as possible from current plants. The Mojave National Preserve is located
closer to the Hoover Dam, so it is possible that the area will continue to benefit from that
renewable resource, rather than solar power. Unfortunately, undeveloped land will only become
more of a commodity, so it is crucial that managers continue to instill the importance of
preserving that area for the species located there. Otherwise, the area may be subject to human
development, which would almost certainly eliminate the fragile tortoise populations.
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