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).
We are throwing it back to summer last year for this Snapshot Saturday featuring a bobcat and her three kittens caught on a Vilas County camera. Can you spot them all?
View and classify photos collected from Snapshot Wisconsin cameras across the state at https://www.SnapshotWisconsin.org.
February’s Volunteer of the Month is
Mike from Iowa County!
February’s Volunteer of the Month goes to Mike from Iowa County, one of the first two counties where Snapshot Wisconsin started recruiting volunteers. Mike was no stranger to trail cameras when he joined the project two years ago—he had spent his career as a biologist in the tropics where he used trail cameras as one technique to study and conserve wildlife.
“Camera trap techniques motivate me because the photos are a fantastic way to learn about wildlife. The pictures are a moment in time of critters’ daily movement that is captured forever,” Mike said.
Birds are among his favorite critters captured at his site, including sandhill cranes, pileated woodpeckers and a great horned owl (who Mike noted doesn’t appear to have caught the squirrel repeatedly triggering his camera). Check out this awesome photo below that Mike shared of a squabbling pileated woodpecker and crow. In addition to participating in Snapshot Wisconsin, he is also involved with wintertime roosting eagle counts with the Ferry Bluff Eagle Council.
Thank you, Mike! Thank you to all our trail camera hosts and Zooniverse volunteers for helping us discover our wildlife together.
Emerging from the polar vortex and a foot of snow, we couldn’t help but reminisce on summer days for January’s #SnapshotSaturday! This summertime buck from Iowa County was nominated by Zooniverse volunteer TJPer.
Continue classifying photos on Zooniverse and hashtagging your favorites for a chance to be featured in the next #SuperSnap blog post. Check out all of the nominations by searching “#SuperSnap” on the Snapshot Wisconsin Talk boards.
Whether you’re enjoying ice fishing or a fisher enjoying the ice, happy Snapshot Saturday!
Interested in hosting your own Snapshot Wisconsin camera? Visit our webpage to find out how to get involved: https://dnr.wi.gov/topic/research/projects/snapshot/.
For January’s Science Update, also featured in The Snapshot monthly e-newsletter, we explored the accumulation of Snapshot Wisconsin photos over time and how the number of photos taken fluctuates with the seasons. To date, our data set contains more than 24 million photos, and their content is a vital component of the Snapshot Wisconsin project.
The bar chart above indicates that over half of the photos are blank. This can be attributed to the fact that our cameras contain a motion trigger function, which is designed to capture wildlife as it moves through the frame. However, this mechanism only detects movement and cannot differentiate between animals and vegetation. This means that on windy days during the spring green up period, thousands of blank photos can be captured. Occasionally cameras will malfunction and continuously take blank photos without being triggered by motion. This issue was more prevalent with earlier versions of our cameras; the model we currently use does not take as many blank photos. Additionally, over time volunteers have learned that trimming vegetation in front of their camera helps prevent blank photos.
Every day at 10:40AM, the cameras are programmed to record a time lapse photo. This is not only to document the “spring green up” period and the “fall brown down” period, but also to sync ground-level measures of greenness with satellite data. These photos are primarily used by our partners at UW-Madison and compose 7% of our data set.
It is not uncommon for our trail camera hosts to trigger the camera themselves during check events, which is the cause of most of the 3% of photos that are tagged as human. Although these photos are removed from the data set prior to analysis, they can be helpful in instances where the camera has been recording photos with the wrong date and time. A photo of a hand in front of the camera combined with the date and time reported by the volunteer at each check event are enough for us to adjust the date and time for the whole set of photos.
Twenty percent of the Snapshot Wisconsin photos are untagged, meaning they have yet to be classified as blank, human or animal. Many of these photos will be sent to the crowd sourcing website, Zooniverse, for classification. We hope to implement a program to automatically classify photos to work through this backlog as well.
Finally, about 14% of Snapshot Wisconsin photos are of confirmed animals. In the graph above, we have broken down which species appear in these photos. Deer are by far the most common species, appearing in about two-thirds of photos, followed by squirrels, raccoons, turkey, cottontail rabbits, coyotes, and elk. The remaining 8 percent of animal tags are divided up across 34 categories including other bird, opossum, snowshoe hare, bear, crane, and fox. Elk may have a higher proportion of triggers than expected because Snapshot Wisconsin cameras are placed more densely in the elk reintroduction areas than in other areas of the state.
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!
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.
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.
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
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.