IN THE LIGHT OF EVOLUTION
Wherein I describe my adventures in the field, opinions about the role of science in society and the importance of inclusion in science, and thoughts on how evolutionary theory informs our understanding of the world.
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Myth: Prairie dogs carry diseases. Mythbuster: All animals, including humans, harbor a suite of bacteria. Some are pathogenic, some are usually harmless but cause illness when an individual’s immune system is weakened, and most bacteria are actually helpful (e.g. help digest food, fight off pathogens, etc). Prairie dogs, like dogs and cattle, do carry bacteria known as Bartonella that may cause illnesses. Prairie dogs are also known to contract sylvatic plague (the same as bubonic plague), caused by the bacterium Yersinia pestis. However, they are not carriers of the disease; they are highly susceptible to it and die within a few days of contracting it. Once plague enters a colony, almost all individuals are exposed and the entire colony will die out within a matter of weeks. Thus, similar to humans, prairie dogs do not carry plague, they just contract it. Plague is transmitted to prairie dogs by fleas, which do carry the disease.  Myth: Prairie dogs and cattle are not compatible. Mythbuster: Studies have shown that bison prefer to graze on land occupied by prairie dogs, probably because of the increased rate of plant growth and nutrient content on prairie dog colonies, or perhaps because of their loud warning system when they spot predators. Cattle appear to prefer prairie dog colonies in mixed-grass prairie as well. Although prairie dogs and cattle may compete for forage (studies have shown that prairie dogs decrease forage available to cattle by 4%; USFS Report), the potential loss to cattle is compensated by the increased protein content of plants on prairie dog colonies, which obtain nutrients brought to the surface by prairie dogs’ burrowing activity. It is estimated that the cost of eliminating prairie dogs exceeds the cost of the loss of forage. There is also a fear among landowners that horses and cattle can step in prairie dog holes and break their ankles, but there is no evidence that this occurs. Here is a nice article about ranchers in Montana who have been enormously successful in their profitable ranching business while keeping prairie dogs on their land: http://www.nwf.org/News-and-Magazines/National-Wildlife/Animals/Archives/2001/Learning-to-Live-With-Prairie-Dogs.aspx  Myth: Prairie dogs overgraze vegetation. Mythbuster: Through their burrowing activity, prairie dogs aerate the soil and bring nutrients closer to the surface, where they are more readily available to plants. Therefore, plants actually have higher growth rates on prairie dog colonies. Prairie dog populations appear to be naturally mobile over time, with a colony moving to areas of higher vegetation and away from areas they have already grazed. This movement prevents overgrazing. However, due to habitat fragmentation and shooting/poisoning, prairie dogs are often restricted to small areas bounded by roads and urbanization. For this reason, colonies are unable to move across the landscape, and instead stay in one area. Thus, in areas of heavy human influence, prairie dogs may overgraze simply because they are unable to move elsewhere. We can solve this problem by creating habitat corridors that allow animals to move from one place to another. 
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 The black-tailed prairie dog (Cynomys ludovicianus), though once found across western North America, was the target for eradication by those considering prairie dogs to be pests. In the last two centuries, they have been reduced to less than 2% of their original numbers (Hoogland 2006). Recently, they became extinct in Arizona. The loss of prairie dogs in such a tremendous portion of their range has altered community structure of many grasslands. Grasslands without prairie dogs may be dramatically different, but consequences of their removal have not been studied.
Because prairie dogs are an important component of grassland ecosystems (see previous post), managers are trying to protect their habitat where they are currently present. In some cases, prairie dog reintroduction efforts have been launched in order to restore the function of grasslands. For example, several reintroductions have occurred or are in the plans in Arizona, where black-tailed prairie dogs no longer occur. You can watch a video of a prairie dog relocation effort here: http://www.youtube.com/watch?v=8pPNd73Jodc&feature=player_embedded Prairie dog reintroductions (like prairie dog conservation in general) are controversial, particularly when destinations are near private land. Ultimately, restoring grasslands to their natural state will require not only prairie dogs, but their natural predators and other species as well. Prairie dogs are a group of fossorial (burrowing) ground squirrels (family Sciuridae) that live in large colonies. They are most closely related to other ground squirrels, and they are split into five species: black-tailed prairie dogs (the most widespread and social species; Cynomys ludovicianus), Mexican prairie dogs (found only in Mexico; C. mexicanus), white-tailed prairie dogs (C. leucurus), Gunnison’s prairie dogs (C. gunnisoni), and Utah prairie dogs (C. parvidens).  Above: Juvenile black-tailed prairie dog in Colorado Prairie dogs are commonly, but mistakenly, referred to as gophers, which are mouse-like rodents in the family Geomyidae (e.g., http://en.wikipedia.org/wiki/File:Geomys_bursarius.jpg). Pocket gophers are more highly fossorial than prairie dogs, and spend almost all of their time underground. They are also solitary, and males interact with females only during mating. In contrast, prairie dogs are highly social. They can live in colonies with thousands of individuals, and prior to human eradication, estimates of some black-tailed prairie dog colony sizes were in the millions (Hoogland 2006). Prairie dogs spend much of their time above ground foraging for food (primarily grasses) and scanning for predators. Because they are numerous and conspicuous, they are an important food source for black-footed ferrets, hawks, coyotes, rattlesnakes, badgers and other predators. Below: Adult white-tailed prairie dog, with painted markings for individual recognition, looking up towards our observation tower from outside its burrow  In addition to their importance as a food item for predators, prairie dogs aerate soil and increase plant diversity and growth rates by bringing nutrients to the surface through their burrowing activity. They also provide habitat for other rodents, rabbits, amphibians and burrowing owls, which share their burrows. Because of their pronounced effects on grassland ecosystems and other species, prairie dogs have been dubbed “keystone species” (Miller et al. 1994). Several species are listed or have been proposed for listing as endangered species. Below: Gunnison's prairie dog, smaller and darker than a white-tailed prairie dog, but also with a white-tipped tail  References:
Hoogland, J.L. 2006. Conservation of the black-tailed prairie dog: saving North America’s western grasslands. Island Press, Washington D.C. Miller, B., G. Ceballos and R. Reading 1994. The prairie dog and biotic diversity. Conservation Biology 8: 863-894.  Welcome to Springerville, AZ. This lovely little town is a nestled in the forested hills of southeastern Arizona, a green oasis in the middle of a desert. Still, the town itself is not much to look at (although Java Blues, the coffeshop/restaurant/bar on the main drag is pretty good, and plays great music). There is a small prairie dog colony up on Forest Service west of town, but the site is remote and the animals cover a large area despite being few in number. Yes, in hindsight, that colony would be better than the one recovering from plague (I presume) and constantly guarding against people (it is at a recreation area). However, only hindsight is 20/20. My actual vision is about 20/600 or so, and my figurative vision is usually better, but sometimes not by a whole lot. AZ4 was the first Gunnison’s pdog site I have set up, and my aspirations may have been too high. Gunnison’s prairie dogs are more shy than black-tails and are less densely populated, so traps need to cover a larger area to capture the same number of individuals. This colony probably has only about 20 individuals in it to begin with, and you can (with a reasonable number of traps) not expect to catch every individual. In addition, there are babies that have not yet emerged, and individuals are probably more wary than usual. This is a long-winded way of leading up to my admission that we captured only 7 different prairie dogs in two weeks– one of my lowest success rates ever. It will be interesting to find out if any of them have been exposed to plague– apparently the colony used to have 100 individuals, but has declined dramatically in the last few years. On the plus side, we caught some short horned lizards (including babies) and have seen hundreds of smaller (fence? earless?) lizards running around, and have trapped several rock squirrels (they LOVE the bait) and a gopher snake (which fortunately did not seem too injured by the trap). And, when there’s lots of down time, we can always visit Java Blues or go get ice cream (the best perk of field work). Some random anecdotes: People in Springerville are nice. I was sitting in my truck returning a phone call when a woman knocked on my window (Zeppelin did not like that). I opened the door to get out and she said, “Oh, you don’t need to get out, I was just wondering if you were hungry.” I was puzzled, but she continued, “I saw you working out there and thought you might want a sandwich.” A bit odd, but maybe that’s what you do in small towns. The Forest Service folks in the local office are very accommodating and even more generous than offering a sandwich– cumulatively, they spent several hours with me discussing prairie dogs and all my requests for water, a freezer, camping, maps, housing, and more.  Field work is supposed to be straightforward, hard work, continue until a job is done, no complications. Just pack up the truck and be prepared, and things will all go smoothly. Right?  Let’s take the first site. I began with a paper map (i.e. not super-imposable on other maps) and drew some transect lines across it, placing points a certain distance apart where I wanted to sample. The first challenge: find out whether prairie dogs exist anywhere near my desired sampling locations. Site one (we’ll call it site NM24) should be somewhere west of Truth or Consequences, NM. [an aside– yes, this is the real name of the town. It used to be called Hot Springs, but in 1950 they decided to rename their town after a game show.] I drove around with my oh-so-patient parents for three days straight looking for prairie dogs– we visited T or C, Socorro, Forest Service and BLM land, private land, and an anonymous wildlife refuge (which will remain unnamed because of the fact that despite their friendly, helpful staff, they somehow managed to reintroduce a species of prairie dog that is probably not native to the area). We went on a wild goose chase following promising leads by people who swore there were lots of prairie dogs in some areas (a common statement: “oh yeah, we have a real problem with them in x location.” Problem? Hm. I’ll leave that issue for now), but many people commented, “Oh, they used to be all over up in y location, but I haven’t seen them there in years.” We found lots of empty burrows that probably belonged to ground squirrels (smaller than prairie dogs) or were abandoned, perhaps due to plague. What we did not find was a single living prairie dog– until we visited the I-25 interchange in Socorro. Yep, there they were, about a half dozen of them, inhabiting the one area that is free of human eradication attempts. Site NM24 has still not been determined. Are there still Gunnison’s prairie dogs in that part of the state? I am told they exist further west by Quemado, but other than the highway prairie dogs and a few along the edge of the highway by the nearby Walmart, I didn’t see any as far east as I-25 (or even 50 kilometers west). Plague has been devastating in the last 100 years, and there have been massive outbreaks in the last 15 years from which prairie dogs have not seemed to recover. Perhaps colonies are so few and so isolated that after plague, there are no nearby animals to recolonize, so local extinctions occur. Discouraged, I thought maybe I had forgotten how to look for prairie dogs and drove back to Boulder, hoping for better luck next weekend. At least I would have my dog (see bed set up next to mine below), my camera, and plenty to read, and I would be in some beautiful places.  I love my job. Well, it has ups and downs… but overall, I am pretty lucky. Today has brought me to Hubbell Trading Post National Historic Site and Canyon de Chelly (pronounced shay) National Monument, two beautiful places in eastern Arizona that are off the beaten track. Hubbell has a collection of historical wagons, farm equipment and supplies used by early settlers (upon looking closely at the wagons, I instantly realized why broken axles were a regular part of the Oregon Trail game… these were built long before the invention of shock absorbers, and any stress was taken directly by the axles).  The drive from Hubbell to Canyon de Chelly was absolutely beautiful—low sunlight stretching over the vast desert landscape, spilling into the canyons but halted by the furrowed canyon walls. A layer of dust hovering above the ground caught the sunlight and appeared to blanket the wide pastures. My breath was taken away upon arrival in Canyon de Chelly. I was suddenly in an oasis overflowing with cottonwood trees and water winding through a wide river basin carved out in wetter years than this one. The water caught the last of the sunlight, and the sun exploded into a bank of clouds, painting them fiery orange as they concealed its descent below the horizon.  Again I get to fall asleep in my tent, close to the outdoors, lying beneath the sleeping warblers that welcomed me to my campsite earlier, and hoping they wake me in the morning. I breathe fresh air and listen to the crickets, and with one zip of my rainfly I narrow my view of the starry sky, and then shut it out completely as I sink into the pillow of my sleeping bag, perfectly content.
The prairie dogs are outsmarting me. Again. The site we are visiting this week is located at a recreation area, and yesterday (a beautiful sunny Sunday), dozens of people were out fishing, kayaking, and swimming at the lake adjacent to the colony. Prairie dogs are already vigilant of potential predators, and their wariness is amplified when many people are around. It is perhaps not surprising, then, that in a colony of low density, we have been unable to catch any prairie dogs in two days… We will keep trying. In the meantime, we have seen a few other critters– different species than we have on black-tailed prairie dog colonies in Boulder. In Boulder, we have seen Thirteen-lined ground squirrels (Spermophius tridecemlineatus), prairie rattlesnakes (Crotalus viridus viridus), bull snakes (Pituophis catenifer sayi), desert cottontails (Sylvilagus adubonii, shown in the image below), deer mice (Peromyscus maniculatus) and many insects and spiders (sorry, I need help with this type of classification…).  On our Gunnison’s prairie dog colony here in eastern Arizona, we have found fence lizards (Sceloporus occidentalis), one horned lizard (Phrynosoma douglassi (?)), rock squirrels (Spermophilus variegatus) and plovers (uncertain Charadrius species; probably the mountain plover, C. montanus). We have also seen Western Meadowlarks (Sturnella neglecta) here (as on prairie dog colonies in Boulder). Rumor has it there are burrowing owls (Athene cunicularia) here too… we’ll keep our eyes out! Tomorrow I will set traps at a colony we’ll visit next week, near beautiful Canyon de Chelly National Monument in Arizona. Lucky us! During the last few weeks, I have spent all my time trying to find prairie dogs. A simple task in Boulder, it has proven to be tricky with the shier Gunnison’s prairie dog (Cynomys gunnisoni), which live in less densely populated colonies and do not clip vegetation the way that black-tails (C. ludovicianus) do. Because of these two life history traits, they are much less conspicuous, and can be difficult to spot even when you know you’re at a colony. Furthermore, plague seems to be taking a heavy toll on populations at the edge of the Gunnison’s prairie dog range– dozens of people pointed me to colonies that had existed within the past few years, and I arrived to find only unmaintained, unoccupied burrows.  Above: Prairie dog researchers at a site that is slowly recovering from plague. Plague is not the only threat to prairie dogs. In many places, prairie dogs are seen as pests and are poisoned continuously over large areas. Eliminating several populations near each other decreases the likelihood that they will be able to recover, because empty colonies can not be re-colonized from nearby animals. The processes of extirpation (extinction of a single population) and re-colonization are known as metapopulation dynamics. In a metapopulation, a series of populations function together, so that when one dies, it can be rescued by individuals from another population. However, when many local populations die out simultaneously, there are no source populations to rescue those that have gone extinct. This can lead to contraction of a species’ range, which may be what is happening to Gunnison’s prairie dogs.  Black-tailed prairie dog in Boulder County, Colorado. Nonetheless, after much searching and asking expert opinion, I was guided to a small colony in Springerville, AZ. In colonies with low population density, deciding where to set up traps is of the utmost importance. Prairie dogs are trap-shy, and will avoid traps if they can. Before attempting to catch any, we bait the traps and hold them open so that the animals can get used to getting free food out of the traps and be less apprehensive about them.  This black-tailed prairie dog is not trap-shy. While my frustration at setting traps in 50+ mph winds mounted, various signs of life reminded me how much fun field work is, particularly in prairie dog colonies: because they build burrows, their colonies are home to many other critters that share the burrows– lizards, rattlesnakes, rabbits, burrowing owls, toads, wyoming ground squirrels and more. Prairie dogs also aerate the soil and move nutrients to the surface where it is more available to plants, so they can increase plant diversity in an area. They also attract predators such as hawks, coyotes and ferrets. Their colonies are therefore places of high species diversity– this is one reason to be concerned about prairie dog declines: many other species would also decline in concert. An assortment of species found on prairie dog colonies is pictured below. L-R and top-bottom (entomologists, please help!): beetles in prickly poppy, Great Plains yucca (Yucca glauca), grasshopper, prickly poppy (Argemone sp.), Hooker's evening primrose (Oenothera elata), scarlet globemallow (Sphaeralcea coccinea), caterpillar, desert plume (Stanleya pinnata), Wyoming ground squirrel (Urocitellus elegans). |
AuthorAn evolutionary biologist with a passion for conservation. Or vice versa. Archives
August 2011
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