Executive Orders on Invasive Species

As of January 20, 2017 we have a new administration in the White House, and with that has come a slew of executive orders. For better or worse, U.S. Presidents have the authority to issue orders that direct the policies and procedures of the agencies that make up the Federal government. Such orders have the same effect as federal laws. Thankfully, the U.S. Constitution sets up a system of checks and balances that keep one branch of government from exercising too much power over the others. In this way, executive orders can be challenged and, if necessary, overturned.

Of course, I don’t intend for this to be a political debate. There are plenty of other places out there that can take you down that rabbit hole. This is simply an introduction to invasive species and their run-ins with executive power. As evidence has mounted against invasive species, demonstrating the threat they can pose to human health and safety as well as to the economic well being of the nation, both state and federal governments have created laws and issued orders concerning them. Examples of such legislation include the Federal Noxious Weed Act of 1974 (which was superseded in 2000 by the Plant Protection Act) and the National Invasive Species Act (which deals specifically with ballast water management). Legislation such as this directs the actions of state and federal agencies in an effort to minimize the introduction and spread of invasive species and reduce the harm they may be having.

Executive Order 13112, issued by President Bill Clinton on February 3, 1999, gave further direction to Federal agencies in the nation’s ongoing battle against invasive species. One of its main directives was to create the National Invasive Species Council which, among other things, would be responsible for developing a National Invasive Species Management Plan.

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The order begins by defining a few terms, including “alien species” (“any species…that is not native to that ecosystem”), “introduction” (“the intentional or unintentional escape, release, dissemination, or placement of a species into an ecosystem as a result of human activity”), “invasive species” (“an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health”), and “native species” (“a species that, other than as a result of an introduction, historically occurred or currently occurs in that ecosystem”).

Section 2 of the order describes the duties of Federal agencies “whose actions may affect the status of invasive species.” These include monitoring invasive species, preventing their introduction, controlling them in a “cost-effective and environmentally sound manner,” restoring native species, conducting research on invasive species, and promoting public awareness about the issue.

Section 3 establishes the National Invasive Species Council, and Section 4 describes its duties. The Secretaries of Interior, Agriculture, and Commerce are assigned as Co-Chairs of the council, and several other agencies are listed as council members. The council is directed to “oversee the implementation of this order” and, among other things, “prepare and issue a national Invasive Species Management Plan.”

Section 5 describes what the management plan should include and when it should be completed. The plan will include “performance-oriented goals and objectives and specific measures of success for Federal agency efforts concerning invasive species,” and should also provide a “science-based process to evaluate risks associated with introduction and spread of invasive species.” The Council was also directed to “assess the effectiveness of this order no less than once each 5 years” after its issuance.

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Seventeen years and ten months after this order was issued, President Barack Obama issued an order that amended Executive Order 13112 and directed “actions to continue coordinated Federal prevention and control efforts related to invasive species.” It maintained the National Invasive Species Council while also expanding its membership and updating its duties. It also emphasized the impacts that invasive species can have on human health, specifically calling out “those species that are vectors, reservoirs, and causative agents of disease.” Additionally, it warned that “climate change influences the establishment, spread, and impacts of invasive species,” a subject that wasn’t addressed in the original order, and it encouraged making greater use of “technological innovation,” which had advanced considerably in seventeen years.

Collaboration appears to be a major theme of Obama’s amended order. In a couple different sections, “open data” is advised, as well as a call for Federal agencies to “develop, share, and utilize similar metrics and standards, methodologies, and databases,” and to “facilitate the interoperability of information systems.” The Feds are advised to use “tools such as challenge prizes, citizen science, and crowdsourcing” – a call that encourages more public involvement in the issue.

Both of these orders are worth reading for yourself. Clearly, government agencies take the threat of invasive species seriously. Perhaps in future posts we will explore some of the specific actions that these agencies are taking and the responsibilities they have regarding invasive species.

See Also: Introducing Invasive Species

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.”

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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.”

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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.

 

Alien Plant Invasions and the Extinction Trajectory

One of the concerns about introduced species becoming invasive is that they threaten to reduce the biodiversity of the ecosystems they have invaded. They do this by spreading rampantly, using up resources and space, altering ecosystem functions, and ultimately pushing other species out. In the case of certain invasive animals, species may be eliminated via predation; but plants don’t eat each other (generally), so if one plant species is to snuff out another plant species it must use other means. Presently, we have no evidence that a native plant species has been rendered extinct solely as a result of an invasive plant species. That does not mean, however, that invasive plants are not doing harm.

In a paper published in AoB Plants in August 2016, Paul O. Downey and David M. Richardson argue that, when it comes to plants, focusing our attention on extinctions masks the real impact that invasive species can have. In general, plants go extinct more slowly than animals, and it is difficult to determine that a plant species has truly gone extinct. Some plants are very long-lived, so the march towards extinction can extend across centuries. But the real challenge – after determining that there are no above-ground signs of life – is determining that no viable seeds remain in the soil (i.e. seed bank). Depending on the species, seeds can remain viable for dozens (even hundreds) of years, so when conditions are right, a species thought to be extinct can emerge once again. (Consider the story of the Kankakee mallow.)

On the other hand, there is plenty of evidence that invasive plant species have had significant impacts on certain native plant populations and have placed such species on, what Downey and Richardson call, an extinction trajectory. It is this trajectory that deserves our attention if our goal is to save native plant species from extinction. As described in the paper, the extinction trajectory has six steps – or thresholds – which are defined in the infographic below:

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Downey and Richardson spend a portion of the paper summarizing research that demonstrates how invasive plants have driven native plants into thresholds 1-3, thereby placing them on an extinction trajectory. In New Zealand, Lantana camara (introduced from the American tropics) creates dense thickets, outcompeting native plants. Researchers found that species richness of native plants declined once L. camara achieved 75% cover in the test sites. In the U.S., researchers found reduced seed set in three native perennial herbs as a result of sharing space with Lonicera mackii (introduced from Asia), suggesting that the alien species is likely to have a negative impact on the long-term survivability of these native plants. Citing such research, Downy and Richardson conclude that “it is the direction of change that is fundamentally important – the extinction trajectory and the thresholds that have been breached – not whether a native plant species has actually been documented as going extinct due to an alien plant species based on a snapshot view.”

Introduced to New Zealand from the American tropics, largeleaf lantana (Lantana camara) forms dense thickets that can outcompete native plant species. (photo credit: wikimedia commons)

Introduced to New Zealand from the American tropics, largeleaf lantana (Lantana camara) forms dense thickets that can outcompete native plant species. (photo credit: wikimedia commons)

In support of their argument, they also address problems with the way some research is done (“in many instances appropriate data are not collected over sufficiently long periods,” etc.), and they highlight the dearth of data and research (“impacts associated with most invasive alien plants have not been studied or are poorly understood or documented”). With those things in mind, they make recommendations for improving research and they encourage long-term studies and collaboration in order to address the current “lack of meta analyses or global datasets.” A similar recommendation was made in American Journal of Botany in June 2015.

The language in this report makes it clear that the authors are responding to a certain group of people that have questioned whether or not the threat of invasive plants has been overstated and if the measures we are taking to control invasive plants are justified. The following cartoon that appeared along with a summary of the article way oversimplifies the debate:

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Boy: There are no studies that show weeds cause native plants to go extinct, thus we should not control them. Plant: If we wait until then, we’ll all be gone!!! Girl: Just because no one has demonstrated it does not mean that extinctions do not occur. The problem is not overstated!

It seems to me that a big part of why we have not linked an invasive plant species to a native plant species extinction (apart from the difficulty of determining with certainty that a plant has gone extinct) is that extinctions are often the result of a number of factors. The authors do eventually say that: “it is rare that one threatening process in isolation leads to the extinction of a species.” So, as much as it is important to fully understand the impacts that invasive plant species are having, it is also important to look at the larger picture. What else is going on that may be contributing to population declines?

Observing invaded plant populations over a long period seems like our best bet in determining the real effects that invasive species are having. In some cases, as Downey and Richardson admit, “decreased effects over time” have been documented, and so “the effects [of invasive species] are dynamic, not static.” And speaking of things that are dynamic, extinction is a dynamic process and one that we generally consider to be wholly negative. But why? What if that isn’t always the case? Extinctions have been a part of life on earth as long as life has been around. Is there anything “good” that can come out of them?

Introducing Invasive Species

The terms “invasive” or “invasive species” get thrown around a lot. They are frequently used to describe anything that is “misbehaving,” or acting in a way that doesn’t fit our idealized vision for how a landscape should look and function. Oftentimes a species that is introduced (by humans) or is not native to an area automatically gets labeled invasive, even if it isn’t acting aggressively or having any sort of dramatic impact on the ecosystem. It is an alien species in an alien environment; it has invaded, therefore it is invasive.

image credit: cartoon movement

image credit: cartoon movement

Determining what is actually invasive in what location and at what time is much more complex than that. We do our best to understand the natural features and functions of ecosystems, and we single out any species, whether introduced or not, that is acting to upset things. That species is considered invasive and, if the goal is to restore the natural balance, it must be controlled. To what degree a species should be controlled depends on the degree that it is upsetting things. Ultimately, it comes down to human judgement. Hopefully that judgement is based on the best available evidence, but that isn’t always the case.

But we are getting ahead of ourselves. What I mostly want to accomplish with this post is to introduce the concept of invasive species and point you to a selection of resources to learn more about them. I defined invasive species in a post I wrote back in August 2015, so I will repeat myself here:

“Invasive species” is often used inappropriately to refer to any species that is found outside of its historic native range (i.e. the area in which it evolved to its present form). More appropriate terms for such species are “introduced,” “alien,” “exotic,” “non-native,” and “non-indigenous.” The legal definition of an invasive species (according to the US government) is “an alien species that does or is likely to cause economic or environmental harm or harm to human health.” Even though this definition specifically refers to “alien species,” it is possible for native species to behave invasively.

These terms refer not just to plants but to all living organisms. The term “noxious weed,” on the other hand, is specific to plants. A noxious weed is a plant species that has been designated by a Federal, State, or county government as “injurious to public health, agriculture, recreation, wildlife, or property.” A “weed” is simply a plant that, from a human perspective, is growing in the wrong place, and any plant at any point could be determined to be a weed if a human says so.

Invasive species are easily one of the most popular ecological and environmental topics, and resources about them abound – some more credible than others. Here is a list of places to start:

That should get you started. There are, of course, numerous books on the subject, as well as a number of peer-reviewed journals dedicated to biological invasions. You should also be aware that IUCN maintains a list of the Top 100 World’s Worst Invasive Species and that there is a National Invasive Species Awareness Week, which is quickly approaching. This episode of Native Plant Podcast with Jamie Reaser (executive director of National Invasive Species Council) offers an informative discussion about invasive species, and a search for “invasive species” on You Tube brings up dozens of results including this brief, animated video:

 

I want to believe that we are doing the right thing when we make concerted efforts to remove invasive species and restore natural areas, but I’m skeptical. The reason why I have chosen to spend an indefinite amount of time exploring the topic of invasive species is because I truly want us to get it right. Yet I don’t even know that there is a “right.” It seems to me that there are endless trajectories – each one of them addressing different objectives and producing different outcomes. In a way we are playing God, regardless of which approach we take. We are making decisions for nature as if we know what’s best for it or that there even is a “best.”

Humans have had major impacts on virtually every square inch of the planet and have been placing our fingerprints on every ecosystem we touch since long before we became the humans we are today, and so it is difficult for me to envision a planet sans humans. It is also difficult for me to buy into the idea that our planet should look as though humans haven’t touched it (i.e. pristine). Because we have been touching it – for hundreds of thousands of years. Efforts to rewind time to before introductions occurred or to hold an ecosystem in stasis, securing life for only those species that “belong” there, seem noble yet fanciful at best and misguided, arrogant, and fruitless at worst.

To be the best conservationists we can be, we probably need to find a middle ground regarding invasive species – not a deter and eliminate at all costs approach, but also not a complete surrender/all are welcome and all can stay stance. Somewhere in between seems reasonable, acknowledging that the strategy taken will be different every time based on the location, the species in question, and our objectives. Of course, none of my beliefs or opinions on this topic (or any topic for that matter) are fully formed. I am trying to do my best to maintain an open mind, seeking out the best information available and following the evidence where it takes me. A topic as complex as invasion biology, however, is never going to be easy to finalize one’s opinions on, and so this journey will be boundless. I hope you will join me.

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Last but not least, here are two articles that discuss updating our approach to dealing with invasive species:

 

2016: Year in Review

2016 was another busy year at Awkward Botany headquarters. A major highlight was the response I received from the Help Wanted announcement that I posted early last year. Several people expressed interest in writing guest posts, while several others volunteered to help out in other ways (contributing images, illustrations, logos, etc.). The offer still stands, so please be in touch if you would like to contribute in any way.

Speaking of being in touch, the comments I’ve received and the connections I’ve made through social media and beyond really add to the experience of doing this blog. Not only does it make this more of a conversation, but it is greatly motivating to know that people find this to be a valuable and entertaining resource. Thank you to all who have reached out. And thanks to silent observers as well. Let’s stay in touch.

As I have done in the past, I am including a list of some of the posts from this past year, mainly those that are part of ongoing series. Many posts don’t fall within these categories, so all others can be found in the ‘Archives’ widget on the right side of the screen.

Book Reviews:

Poisonous Plants:

Famous Botanists in History:

Drought Tolerant Plants:

Field Trips:

Ethnobotany:

Botany in Popular Culture:

Tiny Plants:

Rare and Endangered Plants

Podcast Review:

Guest Posts:

What Is a Plant, and Why Should I Care? part one, part two, part three, part four

Along with the great guest posts, I also received Awkward Botany logos from three incredible artists/graphic designers. I loved them all, and I am very thankful for the time and talent that was spent creating them. The logos are featured below. In order of appearance they were created by Franz Anthony, Mesquite Cervino, and Mara McCall. If you have an idea for an Awkward Botany logo, please let me know. I would love to see it.

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And now a heads up…

In the coming months I plan to focus most of my posts on “weeds” and invasive species. These are topics that I have found increasingly intriguing, so I am hoping that writing a long series of posts about them will help satisfy my curiosity. This may or may not be your thing, but I hope you will stick around regardless. I plan to continue to include some guest posts, which will hopefully help break up the monotony. Also, I know I said this last year and it didn’t actually happen, but I will most likely be taking some breaks from my weekly publishing schedule in order to work on some other projects. Those projects and more will be revealed at some point in time, along with other ideas I have rolling around in my head. If the thought of me taking breaks from posting here bothers you, I invite you to join me on twitter and tumblr, where I will continue to post random things regularly.

Until then, I wish you all a splendid 2017. It should be an interesting one, so buckle up.

Yucca in the snow at Idaho Botanical Garden in Boise, Idaho

Yucca in the snow at Idaho Botanical Garden in Boise, Idaho

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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