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Take Me to the Limit: What Restricts Species’ Ranges?

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A cold opossum. Photo by USFWS.

Recently, I was running, lost in my thoughts, and—WHOOPS—almost tripped over a shivering opossum crossing the bike path! After we both recovered our wits, I jogged in place and watched it waddle away, naked tail dragging through the snow. I rubbed my gloved—and still cold—hands together and wondered, why the heck do opossums live in Wisconsin?

When I got home, some Googling revealed an interesting fact: Wisconsin is at the limit of the opossum’s geographic range. In turn, this got me wondering—what governs the limits of a species’ range?

Ecologists typically classify range-limiting factors as either abiotic or biotic. Abiotic factors do not involve living organisms; climate is the quintessential example. Biotic factors are interactions with other organisms. A classic example is competition between organisms, which is a direct biotic interaction. However, biotic interactions can also be indirect, such as when one species improves or degrades habitat for another. Abiotic and biotic factors usually work in concert to limit an organism’s range.

The opossum I saw behind Olbrich Gardens bespeaks both. Opossums, with their naked tails and ears, have a difficult time surviving cold environments. And yet, opossums live in snowy Wisconsin! However, this is a relatively new phenomenon—opossums did not occur in Wisconsin until the 1850’s, when their range expanded northward. The opossum’s conquest of Wisconsin has been aided and abetted by another organism, namely Homo sapiens. Humans provide extra resources (like trash), which help opossums survive Wisconsin’s cold winters. A biotic interaction has helped opossums overcome an abiotic limitation.

Regardless of the exact cause, opossums reach the northern limit of their range in Wisconsin. Several other species reach range limits in the state, a fact that can come in handy while classifying Snapshot Wisconsin photos. Look a photo’s metadata—what county was it taken in? In some cases, this can narrow down identification possibilities. For example, any rabbit-looking creature in Waueksha County is likely an eastern cottontail, since snowshoe hares do not occur in southern Wisconsin. A good source for species range maps is NatureServe Explorer.

For more information about opossums, see this recent Snapshot Wisconsin blog post by Emily Buege.

For more information about the opossum’s range expansion northward, I recommend reading Walsh and Tucker (2017).

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Past Pieces: “You little weasel!”

Snapshot Wisconsin volunteers have identified a total of 493 triggers of weasels over the course of the project. Although we don’t distinguish between species in our classifications, Wisconsin is home to three distinct species: the long-tailed weasel, short-tailed weasel and least weasel. One of them is known as the smallest carnivore in North America, read through the post to find out which species it is!

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Long-tailed weasel from Jackson County

The article below, “You little weasel!” by Christian W. Cold, was originally published in Wisconsin Natural Resources magazine in February 1998. After reading, you may find that these mysterious critters will weasel their way into your liking.

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@ Gregory Scott

Survival is the objective of the day, every day, if you are little. And danger is always nearby when you are a weasel. Creatures larger than you are out there, listening, and watching with hungry eyes. You avoid their attention by remaining tentative; carefully choosing when to move. You travel about your 40-acre universe in a state of perpetual tension, keenly aware of every sound, every smell and every motion around you

Finding a meal is work. Avoiding becoming a meal is even tougher. As both hunter and hunted, you bear the risks by constantly moving. If you tarry, you die; if your prey hesitates, it dies. Your quarry takes many forms – most are smaller than you, but similar in appearance. Their scent lingers everywhere, but they vanish when you arrive. Your prey cringes in terror in your presence…with good reason

Weasels have an image problem. We are quick to condemn them as corrupt, greedy little villains who sneak around and kill with deadly efficiency for no reason whatsoever. We’ve historically viewed weasels as pests, varmints or scraps of fur only suitable for a decorative trim on collar or cuff. It’s a wonder that weasels have endured such a hostile world.In fact, weasels are marvelously successful. They persist by being alert, inquisitive, tenacious and most importantly, small.

weasels track mice through their winter runaways and lairs gregory scott

Weasels track mice through their winter runaways and lairs @Gregory Scott

Hunters of mice

Weasels are members of a large and diverse family of mammals known as mustelids, which include mink, martens, fishers and skunks. If you shake the family tree harder, badgers, wolverines and otters are also distant relatives. As a clan, the mustelids are typically slender, elongated animals with short legs, a small head and short fur. Their needle-like canine teeth are designed to pierce the throat and brain of small animals, particularly rodents.

The species name, Mustela, means “one who carries off mice,” and all weasels are accomplished mousers. However, to reap the benefits of their small, dynamic world, the weasels can’t afford to be picky eaters. The bill-of-fare includes chipmunks, ground squirrels, insects, small birds, frogs and snakes. Shrews form an important part of their winter diet. Though they occasionally eat fish, weasels are poor swimmers, paddling clumsily with their backs arched out of water.

Weasels have an earned reputation as fierce, efficient predators that will attack animals several times their own size. A four-ounce weasel can kill a four-pound rabbit. The weasel’s habit of killing larger prey and killing several animals at a time stems from its habit of storing or caching surplus food for later use.

Small caches scattered about can provide a series of small meals. Incidents of wholesale slaughter in poultry yards are likely triggered when a weasel goes into “hunt mode” in the unnatural setting of finding several confined prey with no avenue of escape

Weasel biology

Three species of true weasels live in North America. The largest, at 18 inches, is the long-tailed weasel. Long-tails occupy diverse habitats throughout the United States and into Central America. They appear to prefer patchy landscapes of mixed habitats intersected by streams and small rivers. The smaller short-tailed weasel or “ermine” is found in heavily-forested areas and brushy areas of Canada, the northeastern states, the Upper Midwest and Northwestern forests. Smallest of the tribe is the least weasel, a pugnacious dynamo who claims the title as North America’s smallest carnivore. The least weasel is infrequently observed in the marshes and damp meadow. I once inadvertently caught one in a repeating mouse trp at the Mead Wildlife Area in Marathon County in the Upper Great lakes region.

The pelts of all three weasels are brownish and are replaced by a winter white coat that begins to appear by the first of November. The long-tail and short-tail sport a black tip on their tails; an attribute which may confound the striking accuracy of avian predators. Hawks, owls, eagles, foxes, coyotes, bobcats, lynx and domestic cats will eat a weasel – provided they can catch one! The larger mustelids such as mink, marten and fisher pursue small weasels with frightful determination. Size and agility are not the weasel’s only defense. A pungent musk, secreted from an anal gland, may repel or nauseate all but the most persistent of predators.

Weasels sexually mature before their first birthday. A typical litter of six or seven young is born each year and is cared for by both parents. Weasels reaching five to six years are regarded as fully mature. Individuals as old as 10 are considered ancient

How to find a weasel

Weasels are easiest to see in winter when leaf cover is gone and a thin layer of tracking snow will show their whereabouts in the neighborhood. Weasels leave staggered pairs of little footprints placed in a bounding gait fashion. Look for sudden right-angle turns in the tracks that often disappear beneath the snow and reappear at a considerable distance.

Weasels hunt aggressively during the cold, hungry months of winter. Their intense curiosity and insatiable appetite leads them to range widely in a seemingly erratic fashion. They seldom travel far in any one direction. A weasel will stop to poke its little head into every hole, nook and cranny it can probe. Its slender body can squeeze into lairs and runways of mice. Within these subterranean passages the weasel’s sensitive nose and ears will quickly locate the tenants.

Weasels often den in an abandoned (or usurped!) home of a chipmunk or ground squirrel, in a hollow log or under a pile of rubble. The nest chamber is often lined with fine grasses, feathers or the fur of the “former” occupants. Weasels are remarkably clean animals that will not defecate in their quarters. They designate a separate latrine area.

These sleek, little hunters are both feared and respected. If you are a weasel among mice, is it merely “a terrible efficiency” to be the animal that eats its neighbors? I think not. The weasel was designed by the limits of its environment to eat the flesh of others. It must kill to see another day. As humans, we tend to leave that grisly chore to the butcher, but the weasel assumes considerable risk and expends considerable energy just surviving.

The next time you get to see these ambitious little bundles of energy, take a moment to view the world from their perspective. Be thankful that at your comparative size, you can afford to wish them “good hunting.”

To view the full posting of the article in the Wisconsin Natural Resources magazine, click this link.

Happy New Year!

Happy Near Year from the Snapshot Wisconsin team! 

2018 brought a year of tremendous growth for Snapshot Wisconsin, and we couldn’t have done it without you! Since the statewide launch in August this year, the project has reached every Wisconsin county with over 1500 volunteers. We cannot thank you enough for your help making Snapshot Wisconsin the success that it is today. Happy New Year, and we can’t wait to see what 2019 brings!

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Celebrate the Solstice with Snapshot Wisconsin

“Hoppy” Holidays from the Snapshot Wisconsin team!M2E59L175-175R399B388

 

Opossums – The Creatures You Didn’t Know Were Interesting.

One of the most incredible things about studying wildlife is that, no matter how much you think you know, something new and surprising will appear. Recently, I had the opportunity to review thousands of photos for an exciting project involving machine learning (which you can read more about in this blog post). A subset of the photos on my plate for review were of Virginia opossums (Didelphis virginiana).

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An opossum caught on a Snapshot Wisconsin camera that appears to be carrying young in its pouch.

Some might not draw a line between the words “exciting project” and “opossum,” but they truly are an interesting species.  For starters, they are North America’s only marsupial, meaning females carry their offspring in a pouch, especially when the young are newly born (see the photo above).  Additionally, those of us who live where ticks are a concern can thank opossums for consuming a fair number of these pests.

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Snapshot Wisconsin images of opossums with varying coat colors.

The first thing I learned about opossums from my time examining the photos is that they can vary widely in color.  Above is a small collage of opossums that range in color from almost entirely white (known as leucism) to predominantly dark grey, although the animal pictured in the middle is more representative of Wisconsin’s majority.

Morphology, or the set of physical characteristics an animal displays, is not easily disguised in trail camera photos when compared to something fleeting, like behavior.  Often, animals captured in the photos simply appear to be moving across the frame.  This expectation is what led me originally to overlook a fascinating opossum behavior.  As I flipped through the images, I noticed an infrared trigger in which the animal seemed to have debris stuck to its rear half.  I imagined that it had gotten stuck in mud, but when I saw the phenomenon a second time – this time in daylight – I realized that this was no accident.  In fact, these opossums were using their prehensile tails intentionally to carry bunches of leaves and twigs.

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A Snapshot Wisconsin opossum using its prehensile tail to transport a collection of leaves and twigs.

After doing some research on this behavior, I discovered that this has been documented before, albeit rarely.  The consensus on the reason for this behavior is that opossums take their hauls to a temporary den site to use as bedding material.  Of the over 3,000 opossum triggers that I was sorting, I only encountered nine in which this behavior was displayed.  If I were to randomly choose a photo from the set, I would be more than twice as likely to encounter a raccoon misclassified as an opossum than I would be to have selected a photo of an opossum carrying leaves with its tail.  Nine instances do not constitute a large enough sample size to do any major analyses.  However, according to this photo set, there does not seem to be any obvious seasonality, with photos spread somewhat evenly from January 2017 through June 2018.  Only one trigger was taken during the daytime – likely a product of opossums being primarily nocturnal.

If you stumble upon any interesting Snapshot photos – opossums or otherwise – please reach out to us.  You can share them by using the “Talk” function on Zooniverse or by emailing them to DNRSnapshotWisconsin@wisconsin.gov.

 

What to Wear If You’re a Snowshoe Hare?

Have you ever wondered what is responsible for the crimson shade of a fox’s coat, or the distinctive stripes that distinguish a raccoon tail? The answer, in short, is pigments! Pigments are chemical compounds that determine the color an object appears to the human eye based on how much light they absorb or reflect. Melanin is a major group of pigments naturally produced by most animals. Two types of melanin, eumelanin and pheomelanin, control the color that hair appears. This is true from the hair on your head to the coats of the critters you see in the wild!

While most species maintain the same coat coloration year-round, some swap out their coats seasonally for white, “ecologically fashionable” winter coats. This process is known as molting. You may recall some species around the world that do this, including Arctic fox (Vulpes lagopus), White-tailed ptarmigan (Lagopus leacura) and various weasel species. Changing coats is not only a terrific way to help avoid predation, but may also serve as an extra tool to keep warm during the frigid winter months. Because the white fur lacks pigment, it is believed that there is extra space in the hair shafts for air that can be warmed by the animal’s body heat (think of a bird ruffling its feathers during a cool morning to trap in warm air).

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An Arctic fox sports a summer and winter coat. Source: animalia.com

Although the exact mechanisms behind this wardrobe change are not fully understood, there is evidence that suggests that the length of daylight, also known as photoperiod, plays a key role in when animals switch their coat color. Receptors in the retina transfer messages to the brain that it’s time to get a new outfit for the next season. Once this process begins, the hair begins to change color starting with the extremities.

A local expert at swapping out coats is Wisconsin’s own Snowshoe hare (Lepus americanus). You can most commonly find these long jump champions in the northern forests of Wisconsin. Contrary to the common Cottontail rabbit, Snowshoe hare swap brown summer coats for bright white during the snowy winter months to camouflage with their surroundings.

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Snowshoe hares in their summer and winter coats. Source: massaudobon.com

Snapshot Wisconsin cameras capture images of Snowshoe hares year-round across the state. This provides a unique opportunity to not only pinpoint the time of year that snowshoes go through their wardrobe change, but also identify the surrounding area’s brown down or green up state. Because Snowshoe hares rely heavily on their coat color to stay camouflaged and avoid predation, any mismatch between coat and season can make a hare an easy target for lunch. Snapshot Wisconsin cameras can capture images of these mismatches to help understand interactions between Snowshoe hares and predation, as well as Snowshoe hare molting biology across time.

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A Snowshoe hare captured by a Snapshot Wisconsin trail camera.

Not Easy Seeing Green…

As many volunteers may be aware, Snapshot Wisconsin operates continuously and year-round. This is distinct from many other studies and monitoring efforts focused upon wildlife that typically evaluate population status during a particular time of year and look at changes between years. One belief that underpins Snapshot Wisconsin is that because the environment consistently changes across the year, how animals move, behave, live, die also changes. Consistent and continuous data collection provides the project much richer insights into animal habitat associations, and it also gives the project the ability to evaluate which time of year deserves most focus–when should we be monitoring things?

As discussed in previous posts, we heavily rely on spatial data produced by processing satellite imagery in order to quantify species’ habitats and estimate or predict where animals are. Recall that two important sensors or satellites are Landsat, which has fine spatial resolution but captures imagery of Wisconsin less frequently, and Modis (on the Aqua and Terra Satellites) that has coarser spatial resolution, but captures an image of Wisconsin daily. Data produced from Modis imagery is incredibly useful for capturing, say, the timing of larger scale phenomena like big snow events, or the onset of either long-term snow coverage in the winter or green up in the spring. The behaviors and activity of animal species are often connected to the timing of these environmental cues (or others, like temperature, or the length of daylight).

One animal species that is particularly sensitive to seasonality is the black bear: bears spend the winter in dens in a state of torpor. In brown bears, previous research has suggested that the timing of bear den entry is sensitive to environmental factors, while the timing of ending torpor is more related to individual physiology. One thing we are interested in is whether bear behavior (out and about, or in torpor) exhibits any correlation with the variation in the timing of plant green-up and senescence across space, and whether “mismatch” between when bears exit dens and when plants green up (plants like sedges are an important food source for bears early in the year) seems to have any population consequences.

 

Snapshot Wisconsin cameras capture bears growing and moving across seasons. 

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While Wisconsin vegetation greens up from late winter to mid-summer, bears also become increasingly present across the landscape.

BUT…..while satellite imagery provides an excellent overhead synopsis of plant activity, it is not always clear which plants are making the image green. Bears–and many other animal species–primarily eat green matter at ground level rather than leaves at the tops of trees. Snapshot Wisconsin’s cameras provide a ground view that we can relate to satellite images to get a sense of what airborne imagery is responding to.  In the long term, this will allow improved estimates of where animals are and at what time.

Wisconsin Wildlife: Generalists & Specialists

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Raccoons aren’t picky about what they eat! Source: stevehdc, Wikimedia Commons.

My brother Ian was a picky eater. Breakfast was always a bowl of Crispex. For lunch, he ate a PB&J and refused to eat the crusts. I was the opposite. Even as a young child, I loved proverbially “gross” foods like mushrooms and started drinking coffee when I was twelve.

Turns out that some animals are like Ian and some are like me. For example, monarch caterpillars only eat milkweed. We call animals like the monarch specialists. Conversely, some animals will eat, well, just about anything. Raccoons, for example, are equally happy eating crayfish from the creek or scraps from your garbage can. We call such species generalists.

Diet isn’t the only thing to be picky about! Some species exhibit preferences for precise habitat types. For example, the Kirtland’s Warbler breeds only in young jack pine barrens, primarily in Michigan, but also occasionally in Wisconsin. On the other hand, some species are ubiquitous. The coyote is an exemplar habitat generalist—you might spot one in the wilds of the Chequamegon-Nicolet National Forest or in a suburb of Milwaukee.

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A coyote in a forest captured by a Snapshot Wisconsin trail camera.

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A coyote in farmland captured by a Snapshot Wisconsin trail camera

Taken together, diet and habitat comprise what we call the ecological niche of a species. You can think of a niche as the “cubbyhole” that a species occupies within the broader tapestry of its environment. The breadth of a niche is a continuum from extreme specialists (like Kirtland’s Warblers) to extreme generalists (like raccoons). Some species fall between those extremes; deer are a great example. Deer are strict herbivores, but they can be found in many different habits, from forests to farmlands. So, not every species can be neatly classified as a generalist or a specialist.

Scientists are interested in generalists and specialists because they exhibit different responses to change. Like a trained craftsman whose job is replaced by a machine, the specialist has nowhere to go when the environment changes. Generalists, on the other hand, can capitalize on the vacant niche space and colonize altered landscapes. Given the widespread changes humans are exerting on the earth, we are seeing global proliferation of generalists while many specialists are disappearing, a process known as biotic homogenization.

This may seem dire, but the more we learn about generalists and specialists, the more we’ll be able to do to maintain biodiversity and lose fewer specialists. In the meantime, I encourage you to think about the animals you see on a regular basis. Is that squirrel outside your window an ecological jack-of-all-trades? Are there any habitat specialists that live on your property? And maybe even think about your own niche—are you a generalist, a specialist, or somewhere in between?

23 Million Photos: “How Do You Keep Up?”

Stop for a second and try to visualize 23 million of something. The number of species on the planet? That’s roughly 8.7 million. The number of residents in Wisconsin? Nah, that’s not even 8 million! How about photos collected by the Snapshot Wisconsin project since 2016? Ding ding ding! (well – 23,706,425 photos to be exact, not like we are counting or anything…)

Up until recently, Snapshot Wisconsin volunteers were uploading roughly one million photos total per month – but this number is bound to increase after nearly doubling our volunteer base! When we share this statistic during trainings and presentations, we always know to expect the question, “How do you keep up with all of those photos!?”
“Well, it’s a little complicated”, is how we generally start the answer. In this blog post we will dive into how the project has been maintaining this vast amount of data so far, and exciting prospects for the future of Snapshot Wisconsin.

Filtering Photos for Zooniverse:

Zooniverse is a crowdsourcing platform hosting sites for a large variety of projects, including Snapshot Wisconsin. Here anyone with internet access can go online and classify images collected by trail cameras in the project. Before photos are sent to Zooniverse, Snapshot Wisconsin trail camera hosts have the opportunity to view and classify their own photos.

While the volunteers are required to identify and remove human photos, classifying blanks and animals is extremely helpful! Why is this? We conducted an analysis to determine the accuracy of single classifications made by volunteers in their MySnapshot accounts, versus consensus classifications made by volunteers on Zooniverse. We were able to identify species that volunteers are really great at classifying (e.g. deer, squirrels, raccoons, turkeys, etc.)! When these photos are classified by volunteers in their MySnapshot accounts, we do not send them to Zooniverse. Instead we take the volunteer’s classification as the “final classification” for the image, which helps cut back on the number of photos for which we rely on Zooniverse classifications.

Crowd Sourced Classifications on Zooniverse:

The remainder of photos are uploaded to Zooniverse in “seasons”, with each season containing anywhere from 30,000 to 50,000 images. Once a season wraps up, staff members can be caught scrambling around the office getting a new season ready to go. To prepare for each season we need to upload photos and their information, as well as manually review photos to ensure no humans or excessive number of blanks get uploaded. On average, it has taken roughly 3 months for a season to be fully classified and a new season to be uploaded, which generally includes a break that lasts a few weeks.

On Zooniverse, multiple volunteers will view and classify each photo to produce a consensus classification. Photos are viewed by upwards of 11 volunteers if a consensus isn’t reached early on; if a consensus is never reached the photos will go on to expert review by staff members. Since launching in 2016, Zooniverse volunteers have helped the project move through 9 full seasons of photos. Season 10 just launched last week! Across these seasons, the project has had over 6,000 registered volunteers participate and classify 2,253,244 images, or an average of 2,596 photos per day.

Moving Forward – Machine Learning to Classify Animals:

Recently a team of researchers created a computer model using machine learning that classifies images captured by trail cameras. Machine learning is the science of getting computers to learn and act like humans do, and improve their learning over time in autonomous fashion buy feeding them data and information in the form of observations and real-world interactions¹. In this case, the computer model was provided over 3 million images of animals, each that had already been classified by a human, to aid the computer in “learning” to determine which species is which in trail camera images.

The trained model was able to classify approximately 2,000 images per minute at 98% accuracy on images of species collected in the United States. The Snapshot Wisconsin team recently prepared a large set of classified images to further improve the model, and to potentially be incorporated into the project to help keep up with the booming number of images collected. This isn’t to say that volunteer classifications (MySnapshot or Zooniverse) would be replaced, but using automated classifications could help ramp up the speed at which the project produces viable data.

To view the scientific paper, Machine learning to classify animal species in camera trap images: application in ecology, visit this link: https://www.biorxiv.org/content/biorxiv/early/2018/06/13/346809.full.pdf

Sources:

  1. Faggella, Daniel (2018, October 29). What is Machine Learning? Retried from https://www.techemergence.com

Science of Surveys

As graduate student on the Snapshot Wisconsin project, part of my role is to help the team better understand their volunteers and conduct research that will assist with program improvement.  One way I do this is by surveying trail camera hosts when they enter the program and after they have been participating in Snapshot Wisconsin for one year.

Developing a survey takes more work than you might expect!  Some things, like age or occupation, are relatively easy to measure.  However, abstract concepts like satisfaction or attitudes are much more difficult to capture in a survey.  These abstract concepts must be measured in more indirect ways, and typically social scientists develop a number of survey questions or items to measure a concept.

For example, let’s say I wanted to measure someone’s job satisfaction through a survey.  You could ask, “How happy are you overall with your job?” (Rate 1-5).

Surveys ready to be delivered to Snapshot trail camera hosts

In order to capture more aspects of job satisfaction, it would be better to ask: “How happy are you with each of the following parts of your job?  Autonomy, work load, salary, coworker relations, etc.” (Rate each 1-5).

Bear with me while I get theoretical for a moment…

Imagine you have a whole universe of survey items you could ask someone about job satisfaction.  If you choose just one question to ask them, that question is not likely to be a good representation of their job satisfaction as a whole.  However, if you ask them multiple questions, you get a much better representation of their job satisfaction.

Let me use an analogy.  If I want to know all the different species of mammals found in a particular county and I put out just one trail camera in that county, it isn’t likely to be sufficient.  I put out a whole bunch of cameras across the county, I’d get a much more accurate count.

Often, I get this question from people who take surveys: Why do some of these survey questions seem so similar to one another?  Can’t you ask this with just one question?

The answer is: if we are asking about an abstract concept in a survey, assessing it indirectly though multiple questions is the best way to go if we want valid scientific results.

Through email and the internet it is so easy to deliver surveys and if you are like me, you get a survey in your inbox from some business or organization just about every week.  Hopefully this sheds a little light on what goes on behind the scenes before you get that “new mail” notification.

For those of you who have completed a Snapshot Wisconsin survey, your responses are truly valued.  We are learning a lot; see here for some early results and keep your eyes on the blog for more. If you are interested in learning more about the science behind surveys, let me know in the comments!