Counting critters with the Minnesota DNR

Who knew the red fox population in southern Minnesota has dropped in recent years?

And that bobcat numbers have risen in the northern forests, and coyotes have increased in the south?

Or that skunks in farm country have finally declined after several years of high populations?

John Erb knew.

Erb, a wildlife research scientist for the Department of Natural Resources, coordinates two annual surveys of carnivores used to monitor their populations. The surveys include admired species like wolves and bobcats, and other not-so-prized wildlife, including skunks, raccoons, feral cats and even roaming dogs.

Minnesota monitors the populations of 14 primary species literally by counting their tracks. They also include martens, fishers, weasels, opossums, gray fox and snowshoe hares.

"We may be the only state doing this on this scale,'' said Erb. "I think there's great value in understanding population trends over time. It allows us to assess how changes on the landscape are playing a role in numerous populations.''

Fall, winter surveys

The DNR uses two methods to monitor populations of critters that are highly secretive and relatively few in number. One, done each fall for nearly 40 years, is a called a "scent station'' survey, where discs about the size of a quarter, impregnated with scent, are placed in the center of a 3-foot circle of sifted soil. The tracks of animals attracted to the scent then are counted.

Some 2,500 scent stations on 273 routes from north to south were done last fall by about 100 DNR, county and tribal employees. The species are tracked in three geographic ranges: the farmland in the south, the transition zone where agriculture meets forest in the north-central region, and the forest in the north.

Identifying tracks isn't always easy. "Reading tracks on dirt is as much an art as a science,'' Erb said.

The other DNR survey, done for the past 20 years, takes place each winter only in northern Minnesota and doesn't involve scented discs. Instead wildlife officials monitor animal tracks on 60 10-mile survey routes after fresh snowfall. A few species — wolf, coyote, fox and bobcat — are counted in both surveys.

Neither survey attempts to estimate actual critter populations. Instead the surveys provide a population index that shows trends over time.

"The surveys are designed to detect changes in the population, whether increasing, decreasing or remaining stable,'' Erb said. Comparing one-year changes is less accurate than looking at longer-term trends.

The surveys allow the DNR to document the effects of harvest, habitat change and environmental variables. Some of the monitored species, including coyote, raccoon, opossum and fox, have no bag limits, so the population trends aren't used regularly to adjust harvest.

"Coyotes are unprotected, you can kill them year 'round,'' Erb said. "But wolves and bobcats are species we annually make decisions about what harvest should be,'' he said. The surveys give the DNR data to help decide safe harvest levels. Cats and coyotes

Among the trends reflected by the surveys:

• Coyote numbers in the farmland and transition areas remain at or near all-time highs.

"I think it shows the adaptability of coyotes,'' Erb said. "Wolves keep coyote populations suppressed. One theory is the absence of wolves in what is now farmland opened up a niche for coyotes.'' Meanwhile in the north, where the wolf population has flourished, coyote numbers have been below long-term averages since the late 1990s.

• Red fox numbers are down in the farmland and transition areas, a trend Erb said likely is related to the higher coyote populations. "There's a good case that the increase in coyotes contributed to the decline in fox,'' Erb said.

• The population of cats — likely including feral and semi-domesticated — has been at above-average levels for years in farm country, but fell last year. Cats commonly show up in the scent survey, more often than dogs. Comparing species population indexes to each other is difficult, Erb said, because cats, for example, may be less wary of approaching a scent disk than, say, a coyote. "That said, it appears there is no shortage of cats out there,'' he said.

• Raccoons and skunks have been at high levels in southern Minnesota, though the skunk index fell dramatically last year. Erb said it's possible increased trapping, spurred by higher pelt prices, might have boosted skunk mortality. It's also possible last year's long, cold winter could have hurt skunks.

• The bobcat population in northern Minnesota has increased significantly over the past decade, a trend reflected in both the surveys and in the harvest. Ten years ago, trappers were taking about 200 a year; two years ago, they trapped almost 1,900. Erb's theory on the reasons for the increase: mild winters (except the last two) combined with active timber harvesting, which produces a younger, more-dense forest, which bobcats like. That habitat also boosts deer and ruffed grouse numbers, main prey for bobcat.

• Fisher and marten, two northern furbearers, have declined in recent years, and in response the DNR has reduced the harvest.

• Both surveys showed wolf numbers generally increased over the years, then dropped last year to near the long-term average. It's too early to tell if that is a legitimate trend, and, if so, what the causes might be. The drop could be due to the wolf hunting-trapping season that began in 2012 or perhaps the decline in the deer herd.

Doug Smith • 612-673-7667