Campaigns Against Invasive Species, part two

Happy American Wetlands Month!

One of the biggest threats to wetland ecosystems is, of course, invasive species. In last week’s post I shared a selection of videos that were produced by a variety of organizations to inform the public about invasive species. Many such videos specifically address invasives in wetlands and waterways. Here are a few of those videos.

Invasive Species of Idaho reminds you to “Clean, Drain, Dry” to avoid transporting aquatic hitchhikers:

Purple loosestrife is a “very wicked plant:”

Commander Ben vs. the Saltcedar Bandits:

Michigan’s Department of Environmental Quality urges hunters not to use Phragmites australis to make duck blinds:

More information about Phragmites by National Geographic:

Texas Parks and Wildlife and the Attack of the Zebra Mussels:

The story of Eurasian milfoil told by students at George Williams College:

Water hyacinth – another “very wicked plant”:

Water hyacinth invasion in Africa:

Attack of the Killer Algae by TED-Ed:

Invasive Species vs. The Global Economy

As humans have spread across the globe, other species have followed. The domestication of animals and the advent of agriculture helped speed up this process, but species have been traveling around with humans long before that. Presently, our ability to move species from one corner of the globe to another is unprecedented. As more countries join the global economy, the risk of outsider species establishing themselves in uncharted territory increases. Species introductions via globalization are not likely to decrease, and so the question must be asked: Are we, as a global community, equipped to address this?

A review published in Nature Communications in August 2016 warns that “most countries have limited capacity to act against invasions.” The authors come to this conclusion after analyzing available data about invasive species across the globe and developing a “global, spatial forecast for emerging invasions throughout the twenty-first century.” National responses to invasive species were assessed based on reports to the Convention on Biological Diversity (CBD).

As part of the 2011-2020 CBD Strategic Plan for Biodiversity, nations or states that are parties of the CBD agreed to work towards a series of goals called Aichi Biodiversity Targets. Target 9 addresses invasive species: “By 2020, invasive alien species and pathways are identified and prioritized, priority species are controlled or eradicated and measures are in place to manage pathways to prevent their introduction and establishment.” The authors of the review found that, while most countries have made progress on identifying and prioritizing some of the most prominent and threatening invasive species, “current management practices only target a handful” and “prevention of introduction and establishment lags far behind progress towards the reactive CBD goals.”

Biological invasions are expected to remain high across the globe; however, regions with a high Human Development Index (HDI) face different threats compared to regions with a low HDI. Due to increasing levels of international trade, high-HDI regions will continue to be threatened by introductions via pet and plant imports. Climate change and the coinciding biome shifts and changes in fire frequency are expected to aid in the establishment and perpetuation of invasive species in these regions.

Low-HDI regions have historically been less threatened by invasive species compared to high-HDI regions. As these regions join the global economy, they risk experiencing a much higher level of species introductions. Many of the planet’s biodiversity hotspots are found in low-HDI regions, making these hotspots more vulnerable to invasions as the potential for introductions increases. The authors found that the threat of introductions is at its highest in regions where “high levels of passenger air travel overlap with agriculture conversion.” Low-HDI regions are more limited in their capacity to respond to invasions compared to high-HDI regions and are more vulnerable to food shortages when invasive species disrupt agriculture.

“High risk in low-HDI countries could arise from coincidence between intensifying agriculture sectors and high levels of passenger air travel that is likely to transport arthropod pests. … Low-HDI countries could prioritize screening of passenger baggage for live plants, fruits or vegetables, which could host crop pests and pathogens.” – Early, et al. (2016) – photo credit: wikimedia commons

The authors state: “The intensities and global patterns of introduction and disturbance are changing more rapidly today than at any time during human history.” Introductions are not projected to slow in high-HDI regions, and low-HDI regions will be increasingly threatened as species already well established in high-HDI regions expand their reach. This is grim news, but it also presents an opportunity. Through cooperation and data sharing, our understanding of invasive species can greatly increase, and regions with greater access to resources can share such things with less fortunate regions. This is the hope of the authors as well: “We urge increased exchange of information and skills between regions with a wealth of invasive alien species experts and low-HDI countries that have less expertise.”

For more information about this review, go here. For more information about global trade in the modern era, check out the new podcast Containers.

Learning Lessons from Invaded Forests

In 1946, North American beavers were introduced to the archipelago of Tierra del Fuego at the tip of South America in an attempt to start an industry based on beaver fur. Although this industry has not thrived, beavers have multiplied enormously. By cutting trees and building dams, they have transformed forests into meadows and also fostered the spread of introduced ground cover plants. Now numbering in the tens of thousands in both Chilean and Argentinian parts of the archipelago, beavers are the target of a binational campaign to prevent them from spreading to the mainland of these two nations. — Invasive Species: What Everyone Should Know by Daniel Simberloff

Beavers in South America are just one example of the series of effects a species can have when it is placed in a new environment. Prior to the arrival of beavers, there were no species in the area that were functionally equivalent. Thus, through their felling of trees and damning of streams, the beavers introduced novel disturbances that have, among other things, aided the spread of non-native plant species. Ecologists call this an invasional meltdown, wherein invasion by one organism aids the invasion of another, making restoration that much more difficult.

Complicated interactions like this are explored by David Wardle and Duane Peltzer in a paper published last month in Biological Invasions. Organisms from all walks of life have been introduced to forests around the world, and while many introductions have had no discernable impact, others have had significant effects both above and below ground.

The authors selected forest ecosystems for their investigation because “the imprint of different invaders on long-lived tree species can often be observed directly,” even when the invading organisms are doing their work below ground. Moreover, a greater understanding of “the causes and consequences of invasions is essential for reliably predicting large-scale and long-term changes” in forest ecosystems. Forests do not regenerate quickly, so protecting them from major disturbances is important. Learning how forests respond to invasions can teach us how best to address the situation when it occurs.

The authors begin by introducing the various groups of organisms that invade forests and the potential impacts they can have. This is summarized in the graphic below. One main takeaway is that the effects of introduced species vary dramatically depending on their specific attributes or traits and where they fall within the food chain. If, like the beaver, a novel trait is introduced, “interactions between the various aboveground and belowground components, and ultimately the functioning of the ecosystem” can be significantly altered.

Wardle, D.A., and D.A. Peltzer. Impacts of invasive biota in forest ecosystems in an aboveground-belowground context. Biological Invasions (2017).  doi:10.1007/s10530-017-1372-x

After highlighting some of the impacts that invasive species can have above and below ground, the authors discuss basic tenets of invasion biology as they relate to forest ecosystems. Certain ecosystems are more vulnerable to invasions than others, and it is important to understand why. One hypothesis is that ecosystems with a high level of species diversity are more resistant to invasion than those with low species diversity. This is called biotic resistance.  When it comes to introduced plants, soil properties and other environmental factors come in to play. One species of plant may be highly invasive in one forested ecosystem, but completely unsuccessful in another. The combination of factors that help determine this are worth further exploration.

When it comes to restoring invaded forests, simply eliminating invasive species is not always enough. Because of the ecological impacts they can have above and below ground, “invader legacy effects” may persist. As the authors write, this requires “additional interventions to reduce or remove [an invader’s] legacy.” Care also has to be taken to avoid secondary invasions, because as one invasive species is removed another can take its place.

Nitrogen-fixing plants (which, as the authors explain, “feature disproportionately in invasive floras”) offer a prime example of invader legacy effects. Introducing them to forest ecosystems that lack plants with nitrogen fixing capabilities “leads to substantially greater inputs of nitrogen … and enhanced soil fertility.” Native organisms – decomposer and producer alike – are affected. Simply removing the nitrogen fixing plants does not at once remove the legacy they have left. Examples include Morella faya invasions of forest understories in Hawaii and invasions by Acacia species in South Africa and beyond.

“It has been shown that co-invasion by earthworms enhances the effects that the invasive nitrogen fixing shrub Morella faya has on nitrogen accretion and cycling in a Hawaiian forest, by enhancing burial of nitrogen-rich litter.” – D.A. Wardle and D.A. Peltzer (2017) – photo credit: wikimedia commons

The authors conclude with a list of “unresolved issues” for future research. A common theme among at least a couple of their issues is the need for observing invasive species and invaded environments over a long period. Impacts of invasive species tend to “vary across both time and space,” and it is important to be able to predict “whether impacts are likely to amplify or dampen over time.” In short, “focus should shift from resolving the effects of individual invasive species to a broader consideration of their longer term ecosystem effects.”

This paper does not introduce new findings, but it is a decent overview of invasion biology and is worth reading if you are interested in familiarizing yourself with some of the general concepts and hypotheses. It’s also open access, which is a plus. One thing that is clear after reading this is that despite our growing awareness of the impacts of invasive species, there is still much to be learned, particularly regarding how best to respond to them.


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Eating the Invasives

Happy National Invasive Species Awareness Week! It’s a fine time to get educated about invasive species, and perhaps even play a role in mitigating them. Opportunities for getting involved are myriad and include volunteering with local conservation groups, replacing invasive plants in your yard with non-invasive alternatives, and being mindful when you visit natural areas not to bring along weed seeds and other pests and diseases. Another strategy in the battle against invasive species is to eat them, which is precisely what I plan on doing. If you are interested in doing the same, this revised post (originally published in November 2013) will help get you started.

Invasivore: One Who Consumes Invasive Species

Invasive species are a major ecological concern, and considerable effort is spent controlling them. The ultimate goal  – albeit a lofty one in many cases – is to eradicate them and to prevent future outbreaks. The term “invasive species” describes plants, animals, and microorganisms that have been intentionally or unintentionally introduced into an environment outside of their native range. They are “invasive” because they have established themselves and are causing adverse effects in their non-native habitats. Some introduced species cause no discernible adverse effects and so are not considered invasive. Species that are native to a specific habitat and exhibit adverse effects following a disturbance can also be considered invasive. (White-tailed deer are an example of this in areas where human activity and development have reduced or eliminated their natural predators resulting in considerably larger deer populations than would otherwise be expected.) Defining and describing invasive species is a challenging task, and so it will continue to be a topic of debate among ecologists and conservation biologists for the foreseeable future.

The adverse effects of invasive species are also not always straightforward. Typical examples include outcompeting native flora and fauna, disrupting nutrient cycles, shifting the functions of ecosystems, altering fire regimes, and causing genetic pollution. Countless hours of research and observation are required in order to determine the real effects of invaders. The cases are too numerous and the details are too extensive to explore in this post; however, I’m sure I will cover this topic more thoroughly in the future.

There are many approaches to eradicating invasive species, but one fairly unconventional method is to simply eat them. Why not, right? Historically, the voracious appetite of humans has helped drive several species to extinction, so why not employ our stomachs in the removal of introduced species from their non-native habitats? The folks at Invasivore are suggesting just that. By encouraging people to consume invasive species, they are also promoting awareness about them – an awareness they hope “will lead to decreasing the impacts of invasive species by preventing introductions, reducing spread, and encouraging informed management policies.”

“If you can’t beat ’em, eat ’em!” And so they provide recipes in order to encourage people to harvest, prepare, and consume the invasive species in their areas. Some of the invasive plant species they recommend eating are Autumn Olive (Autumn Olive Jam), garlic mustard (Garlic Mustard Ice Cream), Japanese honeysuckle (Honeysuckle Simple Syrup), purslane (Purslane Relish), and Canada goldenrod (Strawberry-Goldenrod Pesto). And that’s just a sampling. One might ask if we are encouraged to eat invasive species and ultimately find them palatable, won’t our demand result in the increased production of these species? The Invasivores have considered this, and that is why their ultimate goal is raising awareness about the deleterious effects of invasive species. In the end, we should expect to see our native habitats restored. Our craving for Burdock Chips on the other hand will have to be satisfied by some other means.

lonicera japonica

Japanese honeysuckle (Lonicera japonica) – photo credit: wikimedia commons

More about eating invasive species:


What are you doing to celebrate National Invasive Species Awareness Week? Let us know in the comment section below.

Tufty’s Plight, or Saving the U.K.’s Red Squirrel

“There is the great blank area where no red squirrels have returned, and this is where the grey ones first spread and are now permanent inhabitants. Outside it there are plenty of red squirrel populations still, though they have fluctuated, often severely.” — Charles Elton, The Ecology of Invasions by Animals and Plants, 1958

Sciurus vulgaris, or the Eurasian red squirrel, is widespread throughout northern Europe and east into Siberia. It is a small squirrel with a chestnut top and a creamy underside that spends much of its time in the tops of trees. Its tail is large and fluffy, and its ears are adorned with prominent tufts of hair. It enjoys a broad range of foods from seeds, fruits, and leaves to fungi, insects, and birds’ eggs. It is beloved in the United Kingdom, where its survival is being threatened by a North American cousin. This cousin, now established in the U.K. for well over a century, looks to increase its range across Europe, with a growing population in Italy and the potential to spread to neighboring countries.

Eurasian red squirrel (Sciurus vulgaris) - photo credit: wikimedia commons

Eurasian red squirrel (Sciurus vulgaris) – photo credit: wikimedia commons

Sciurus carolinensis, or Eastern gray squirrel, is native to eastern North America but has been introduced to parts of western North America as well as other parts of the world, including the United Kingdom, Italy, South Africa, and Australia. Its fur is typically dark to pale gray with red tones. It prefers mature forests where food and shelter are abundant; however, it is a highly adaptable species and is common in urban areas and disturbed sites. It shares habitat requirements with the red squirrel, but has the advantages of being larger, stronger, and able to digest acorns.

Gray squirrels were first introduced to the U.K. in 1876. Wealthy collectors were enchanted by them and began releasing them on their estates. The first pair made it to Ireland in 1911. Around this time biologists were becoming concerned by how quickly they were spreading as well as the damage they were doing to young trees and the effect they seemed to be having on red squirrel populations. The U.K. Parliament responded in 1937 by banning the possession and introduction of gray squirrels. In an article published in Science in June 2016, Erik Stokstad writes about this “troubling phenomenon: where gray squirrels established colonies, red squirrels sooner or later vanished.” The current population of red squirrels in the U.K. is estimated at around 140,000, while gray squirrels are thought to number more than 2.5 million.

Why red squirrels vanish when grey squirrels are present is not entirely understood. Competition for food is one factor. Grey squirrels seem to have an advantage over red squirrels in mixed deciduous forests, and according to Schuchert, et al. (Biological Invasions, 2014), after colonization by gray squirrels, red squirrels can become restricted to coniferous forests, which are “less favored by grey squirrels.”

But direct competition alone doesn’t explain the plummeting numbers of reds in the presence of grays. Another explanation was identified in 1981 – grey squirrels were spreading a disease. Several years of experimentation confirmed that red squirrels were dying of squirrelpox – a parapoxvirus that gray squirrels carry but show little or no sign of infection. The virus can spread quickly through a population of red squirrels, leaving them lethargic, malnourished, and an easy target for predators. Stokstad writes, “red squirrels are defenseless…as [they] succumb, gray squirrels quickly take over the habitat.

But not all grey squirrels carry the virus, and there are some regions where the virus isn’t a major problem. Habitat loss and fragmentation due to human development also plays a role in the red squirrel’s decline. Add to that, grey squirrels may be more inclined to live among humans, giving them an advantage over the more reclusive reds.

Efforts have been underway for decades now to reduce, and even eliminate, gray squirrels in the U.K. Tens of thousands of grey squirrels have already been trapped and killed, yet they continue to dominate. Schuchert et al. write, “while culling may decrease grey squirrel population size in the short term, their high dispersal abilities makes re-colonization likely.” Funding for culling programs isn’t always available, and protests from animal rights groups like Animal Aide U.K. and Animal Ethics also have an impact. One area that culling has proved successful is Anglesey, an island off the coast of Wales, where the red squirrel population had once been reduced to just 40 individuals. Schuchert et al. analyzed culling data over a 13 year period and determined that trapping and killing efforts “resulted in the sustained and significant reduction of an established grey squirrel population at a regional landscape scale.”

Eastern gray squirrel (Sciurus carolinensis) - photo credit: wikimedia commons

Eastern gray squirrel (Sciurus carolinensis) – photo credit: wikimedia commons

Red squirrels may also be experiencing some relief thanks to another threatened mammal. Martes martes, or the European pine marten, is a member of the weasel family and, as Stokstad writes, “a cat-sized predator [that] was nearly exterminated in the 20th century.” Hunting, both for fur and pest control, and habitat loss reduced pine marten numbers dramatically until it received legal protection in 1988. Since then it has started to rebound, particularly in Scotland and Ireland. Anecdotes suggested that pine marten recovery in these areas was resulting in fewer gray squirrels. A study published in Biodiversity and Conservation in March 2014 confirmed that gray squirrel populations in Ireland were at “unusually low density,” and that the increasing numbers of pine martens played a role in that. Gray squirrels move slower and spend more time on the ground compared to red squirrels, making them easier prey for pine martens.

Efforts are now underway to introduce pine martens to other parts of the U.K. where gray squirrel populations are problematic. However, according to Stokstad, “red squirrel advocates worry that the pine marten could be a false hope, promising a free and uncontroversial solution that could threaten funds for culling.”

Let’s remember that the gray squirrel was deliberately introduced to the United Kingdom by humans, and that human activity is one of the main reasons for the grey squirrel’s explosion and the red squirrel’s retreat. Culling is not likely to ever eliminate gray squirrels completely, yet no one wants to see red squirrels go extinct. Altered landscapes can favor certain species over others, so ensuring that there is plenty of favorable habitat available for the red squirrel is one way to aid its survival. The grays may be there to stay, but let’s hope a compromise can be found so generations to come can benefit from sharing space with the red squirrel (and perhaps the gray squirrel,too).

Tufty Fluffytail, a character developed to help teach kids road safety in the U.K., saves Willy Weasel from getting run over (again).

Red Squirrel Conservation Groups:

Book Review: The New Wild

What if we were to look at invasive species with fresh eyes? Traditionally we have viewed them as interlopers hellbent on environmental destruction, but have we considered the good they can do? Should our efforts to eradicate them be tempered – eliminating them when it seems absolutely necessary, but accepting them when they are doing some good; welcoming them when they have something to offer. What does their presence mean anyway? What does it say about the ecosystems they inhabit and about us? Invasive species are convenient scapegoats, taking the blame for much of the ecological devastation that we started in the first place. Is that justified?

This is, essentially, the theme of The New Wild, a book by Fred Pearce that urges us to reconsider the ways we think, talk, and act towards invasive species. More than that, it is about dumping the idea that pristine nature (a mythological concept anyway, and one that is not all that useful) is the only true wild, and that nature invaded by alien species is a lesser thing that needs to be fixed. The truth is, nature is and always has been in a constant state of flux, and it is unconcerned about the provenance of the species that compose it. As Pearce puts it, if it’s doing “a useful job,” “it matters not a jot where a species comes from.”


Invasion biology is a relatively new field of study, stemming from the publishing of Charles Elton’s book, The Ecology of Invasions by Animals and Plants, in 1958. For thousands of years, humans have had a hand in moving species of all kinds around the planet, but it was in the latter half of the 20th century that our awareness of the ecological damage that some of these species can do really developed. Since then we have made great efforts to remove such species and put things back the way we found them. The zeal with which we have done so hasn’t always been justified or effective, and throughout what at times has felt like an all out war against foreigners, a profound sense of animosity and suspicion towards anything non-native has taken root in our psyche.

Pearce hopes to mitigate these feelings and get us to reconsider some of our actions. To start with, he calls into question the distinction between aliens and natives: “A broad time horizon shows there is no such thing as a native species. All lodgings are temporary and all ecosystems in a constant flux, the victims of circumstance and geological accident.” Also, “many aliens are so well integrated that they are assumed to be native,” and “species come and go so much, as a result of both human and natural forces, that conventional hard distinctions about what belongs where have long been all but meaningless.”

Instead of judging a species by its provenance, “we should treat species on their merits and learn a little tolerance and respect for foreigners.” While “being alien can sometimes be problematic,” it can equally result in the renewal of “flagging ecosystems, creating new space for natives and providing ecosystem services.” Seeing that those services are in place is what should really matter, and “[ecological services] are best done by the species on hand that do it best.” After all, nature is not a system of “preordained perfection,” but instead “a workable mishmash of species, constantly reorganized by the throw of the dice.”

In his criticisms of the field of invasion biology, Pearce investigates some of the “constantly recycled ‘facts’ about alien species.” He finds many of the claims to be unfounded and oft-repeated statistics to be blatant misrepresentations of the original studies. He concludes that “some of the most widely used statistics in the canon of invasion biology do not stand up.” To support his point, he offers several examples of how alien species have added to the biodiversity in certain ecosystems and he shares stories that “show how we instinctively blame aliens for ecological problems that may have a lot more to do with our own treatment of nature.”

Immigrant Killers by Carolyn King, one of many books making a case for the war on alien species.

Immigrant Killers by Carolyn King, one of many books published in the past few decades that makes the case for waging war on alien species.

In so many words, Pearce’s stance is that the classic “aliens are bad, and natives are good” approach is outdated – “nature doesn’t care about conservationists’ artificial divide between urban and rural or between native and alien species,” which means that our perception of aliens should shift from being “part of the problem to part of the solution.” Abandoned farmlands, secondary forests, recolonized waste places, urban sprawl, and other novel ecosystems across the globe offer explicit examples of species from all backgrounds coming together to create functional habitats. This is the new wild.

Pearce is not advocating that we throw in the towel and let invasive species run rampant: “It would be foolish to claim that alien species never do any harm or that efforts to uproot them are always doomed to fail.” His support for the new wild is “not a call to let it rip.” Instead, “conservation in the twenty-first century requires an open-minded assessment of what might work – not a sullen retreat into blinkered orthodoxy.” So, rather than try to stop the flux of nature (an act that is decidedly “anti-nature”), let’s see where it goes, alien species and all; and when we do decide to beat back invasives and intervene “to preserve what we like,” we should be mindful that nature may be “traveling in a different direction.” As Pearce writes, “the new wild is flourishing, and it will do better if we allow it to have its head.”


Obviously this is a controversial topic, but the ideas in this book are worth exploring further. Pearce’s notes are extensive, and I intend to read through many of his resources. Stay tuned for more posts. Meanwhile, you can listen to an interview with Pearce on this episode of Talking Plants. For a more critical veiw of Pearce’s book, check out these reviews by Los Angeles Review of Books and The EEB & Flow.


What Do Desert Tortoises Eat?

Desert conditions are not intuitively conducive to life. In many regards they are extreme. Blistering, bleak, dry, and barren. The desert is a place unsuited for the faint of heart and the ill-equipped. Broadly speaking, life in the desert is reliant on one of two things: technology or evolutionary adaptation. Like many species native to desert environments, the desert tortoise employs the latter. It is at home in the desert because it evolved there. That is not to say that life is always easy for the desert tortoise and species like it, but it is possible, thanks to hundreds of thousands of years of making it work. As John Alcock puts it in, Sonoran Desert Spring, “the tortoise will deal with its environment through evolved design rather than seek to deny the desert its due.”

Perhaps it is because the desert is such a harsh environment, requiring finely tuned adaptations for survival, that sweeping changes can put resident species in peril – threatening their long-term existence. The desert tortoise is an example of this. Since 1990, Gopherus agassizii has found itself listed as threatened under the U.S. Endangered Species Act [it is categorized as vulnerable by the IUCN] due to significant population declines and loss of habitat. Getting there did not happen overnight, and it is impossible to pinpoint a sole cause of the tortoise’s decline. Instead, a suite of things have conspired against it, making it difficult to decide on the best route towards conservation.

In an October 2012 issue of BioScience, Averill-Murray, et al. enumerate some of the human-medaited threats that act both simultaneously and synergistically against desert tortoise populations:

Habitat conversion occurs as a result of urban development, mining, waste disposal, energy development, and road construction. Habitat modification is caused by military training, off-highway vehicle use, utility corridors, livestock grazing, and the proliferation of invasive plants. … Direct losses of tortoises also occur through predation, disease, collection from the wild, and recreational killing.

Apart from climate change, which is projected to substantially reduce the historical range of the desert tortoise in the coming years, the proliferation of introduced grasses is particularly disconcerting. Such grasses tend to increase wildfire frequency in areas where wildfire is historically rare and the native flora is ill-adapted to frequent fire. This can alter plant communities in a way that favors introduced plants over plants native to the region.

Desert Tortoise (Gopherus agassizii) - photo credit: wikimedia commons

Desert Tortoise (Gopherus agassizii) – photo credit: wikimedia commons

The desert tortoise is the largest terrestrial turtle in the United States, measuring up to 15 inches long and weighing up to 15 pounds. Their carapaces are generally dull brown or gray, although those of young tortoises may have orange markings. Their limbs are stocky and elephantine, and their front legs are shovel-like and equipped with claws for digging. They reach sexual maturity between ages 15-20, generally living for at least 35 years and as many as 50-100 years.

Desert tortoises are distributed throughout the Mohave and Sonoran Deserts of southeastern United States and into the Sonaron Desert and Sinaloan foothills of northwestern Mexico. Their habitat varies widely across their range. In general, tortoises prefer sites where the soil is loamy and easy to dig as they spend much of their time in underground dens; however, they also occur in rocky foothills where shelter can be found among the rocks. In the Mohave Desert, they are commonly found in plant communities that are dominated by creosote bush (Larrea tridentata), which they use for shade and an occasional food source.

Creasote Bush (Larrea tridentata) - photo credit: wikimedia commons

Creosote Bush (Larrea tridentata) – photo credit: wikimedia commons

Recently the species known as Gopherus agassizii was determined to consist of at least two (possibly four) distinct species. Desert tortoises that occur north and west of the Colorado River have retained the scientific name G. agassizii and are commonly referred to as Agassiz’s desert tortoise. Desert tortoises occurring east of the Colorado river have been given the name G. morafkai, commonly known as Morafka’s desert tortoise. In light of this, G. agassizii may find itself uplisted to endangered, as its range has been reduced to about 30% of its former self and its southern cousins can no longer be considered a genetic reservoir.

Seeing that desert tortoises have plenty of the right foods to eat ensures their immediate survival and holds them back from the precipice of extinction. The question, “What does a desert tortoise eat?,” was what peaked my curiosity in this subject to begin with. I knew they were herbivores (for the most part), so I assumed they must have a favorite food – something that composed the majority of their diet. Finding an answer to this question led me down a rabbit hole [or should I say a tortoise hole? Some tortoise dens can extend 30 feet or more into the banks of desert dry washes.] that led me to discover the complexity of these creatures. It turns out, there is no easy answer to my initial question. What a desert tortoise eats depends on where in its expansive range it resides, what time of year it is, what plants are available in a particular year, whether or not it’s a drought year, etc.

The desert tortoise is “one of the most studied reptiles in the world,” so hundreds of observations have been made, leading to dozens of reports and studies that examine the diet of the desert tortoise; however, the results are highly variable. Due to such variability, this fact sheet from the San Diego Zoo states matter-of-factly, “an ‘average’ tortoise diet [is] hard to characterize.” But let’s try.

The desert tortoise emerges from its winter den in early spring. At the same time, annual wildflowers are also emerging, taking advantage of warming temperatures and rare soil moisture accumulated during winter precipitation. This is the desert tortoise’s preferred banquet. Because there will be little water available the rest of the year, desert tortoises hydrate themselves mainly through the plants they eat. The lush stems, leaves, and flowers of annual wildflowers provide both nutrients and the water necessary to sustain themselves throughout much of the year and aid in their growth and reproduction.

As spring turns to summer, the tortoises switch to eating herbaceous perennials and grasses. By this point, both introduced annual grasses and native perennial bunchgrasses are drying up, but tortoises are still able to extract some nutrients and moisture by eating their dry stems and leaves. Cactus pads and fruits (particularly those in the genus Opuntia) as well as young leaves of shrubs also help tortoises subsist through the long, hot summers, which are mostly spent deep in their dens away from predators and the blistering heat.

A paper published in a March 1986 issue of Herpetologica follows a group of tortoises over the period of a year and makes a number of lifestyle observations, including their diet. The authors noted that much of their diet consisted of two annual wildflowers (Camissonia munzii and Langloisia setosissima), a perennial bunchgrass (Achnatherum hymenoides), and a non-native annual grass (Bromus rubens). A paper published in a 2010 issue of Journal of Herpetology compared the nutritional quality of four plant species commonly consumed by desert tortoises: a native and non-native grass (Achnatherum hymenoides and Schismus barbatus) and a native and non-native annual forb (Malacothrix glabrata and Erodium cicutarium). They found little difference between the native and non-native species in either catagory, but determined that the forbs were significantly more nutritious than the grasses, which lead them to recommend managament practices that would increase the availability of forbs (regardless of provenance) in tortoise habitat. Numerous studies have documented the frequent consumption of introduced plant species by desert tortoises.

Redstem stork's bill (Erodium cicutarium) is an introduced species commonly consumed by desert tortoises - image credit: wikimedia commons

Redstem stork’s bill (Erodium cicutarium) is an introduced species commonly consumed by desert tortoises – image credit: wikimedia commons

For me, one of the most interesting things to learn was the variety of “non-food” items that tortoises may consume. Tortoises are often observed eating soil and rocks, and are also known to eat bones, arthropods, feces, feathers, hair, and egg shells. The rocks are thought to act as a gastrolith, aiding in digestion. The other items may help supplement minerals and nutrients the tortoises are lacking in their plant-based diet, particularly calcium which is greatly needed for growth and reproduction. Shockingly, a report that appeared in a 2007 issue of The Southern Naturalist details incidences of tortoises eating the skeletal remains of other tortoises.

Desert tortoises are an engrossing subject of study, and so much more could be said about them. For now, I leave you with this passage from Alcock’s book:

To see a tortoise with wrinkled neck and solemn eyes, moving like an animated rock, is an essential part of the experience of the desert. The removal of even a single adult extinguishes a presence that was meant to persist for years to come and snuffs out a prehistoric spark of life in a spartan environment where life, so hard-won, should be celebrated.

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