Predator Management Update - What We've Learned in 17 Years
By Dr. Frank Rohwer, Delta's Scientific Director and Dan Nelson, Editor of Delta Waterfowl Magazine
Delta President Rob Olson (R) speaks with trapper Brian Poncelet near Minnedosa, MB.
If we could use time-lapse photography to compress three decades of nesting seasons into a few minutes of video, the incredible volatility of the breeding grounds would be stunningly apparent.
In those few minutes, millions of acres would be converted from cropland to grass and back to grain; fox would appear in inestimable numbers only to vanish, and every few seconds parched wetlands would fill to overflowing, shrivel, then fill again.
If this ever-changing ecosystem presents a challenge for nesting ducks, it can be downright frustrating for waterfowl scientists working to cement their understanding of the breeding grounds while man and Mother Nature keep changing the rules.
While nesting conditions across the Prairie Pothole Region (PPR) are a moving target, the aim of waterfowl management hasn’t changed much in 50 years. Scientists have long recognized the critical importance of securing the small wetlands necessary to attract nesting ducks and finding ways to increase hatch rates. Nest success--how many eggs hatch--is the crucial limiting factor influencing duck production, and decades of research have shown nest predation is the reason most eggs never hatch.
The accepted method for reducing predation has been to secure large tracts of grass cover, thereby making upland nests as difficult for predators to find as the proverbial “needle in a haystack.” Unfortunately, the rising cost of land and a growing demand for farm commodities render this approach too costly to implement on a meaningful scale. The PPR encompasses some 275,000 square miles; even if it was for sale, which it isn’t, we couldn’t afford to buy even a small piece of it.
Managers needed other tools in their kit, and research conducted by Alan Sargeant of the Northern Prairie Wildlife Research Center in the 1980s clearly identified one of them. Sargeant’s research revealed that red fox were killing up to 900,000 ducks, most of them hens, on the breeding grounds every spring. Raccoon and skunk take a huge bite out of nest success too, but fox were particularly egregious because they also kill hens.
Sargeant’s work is one reason that in 1994, Delta took the politically incorrect fork in the road when we set out to reduce nest predation by trapping predators during the nesting season. Much of the management community was skeptical, but after 17 years of rigorous evaluation, independent university-led teams have demonstrated that predator reduction dramatically increases nest success.
Our early efforts at managing predators during the nesting season produced eye-popping results, lifting nest success to far above population-expanding levels.
But that’s not the end of our story, it’s only the first chapter. Even an approach as seemingly straight-forward as predator management is not immune to some “ifs” and “buts”, and along the way we learned that if maximizing duck production is the goal, we had to be adaptable.
The prairie breeding grounds underwent dramatic detours during the 1980s and ‘90s. In 1985 Congress got into the game when it authorized the Conservation Reserve Program (CRP), a U.S. Department of Agriculture (USDA) program that accomplished what state and federal agencies and conservation groups never could. At its peak, CRP provided some 5 million acres of undisturbed nesting cover embedded with millions of high-quality wetlands across the U.S. side of the PPR.
Not since duck stamp dollars were used to permanently protect more than 2 million acres of wetlands back in the 1960s has any program had such an impact on duck production. Research conducted by Ron Reynolds of the U.S. Fish and Wildlife Service’s Habitat and Population Evaluation Team (HAPET) shows that CRP has been responsible for 2.1 million incremental ducks every year since 1992.
The benefits of CRP weren’t readily apparent because the Dakotas and Montana were in the throes of a devastating drought, but the rains came after the 1993 breeding season, and when Delta launched its trapping program a year later, the U.S. side of the PPR was open for business.
That first year one of our trappers removed 120 red fox from a single 16-square-mile tract, and as a result nest success was an off-the-charts 42 percent compared to 23 percent on a non-trapped (control) block. From 1997 through 2008, trapping at 36-square-mile sites in North Dakota produced an average of 51.7 percent compared to 27.7 percent on non-trapped (control) sites.
A Look at the Future
Of Predator Management
Delta Waterfowl’s 17-year-old predator management program has produced some gratifying results, revealed several new challenges and raised some intriguing questions about the future.
On the prairie breeding grounds, where the only constant is change, waterfowl managers are keeping a close eye on several pivotal developments that will go a long way towards determining the direction of our future trapping efforts.
Easily the most critical is the fate of the Conservation Reserve Program (CRP), which at one point was responsible for 5 million acres of grass and high-quality wetlands across North Dakota, South Dakota and Montana, the key duck-producing states on the U.S. side of the Prairie Pothole Region.
According to research conducted by Ron Reynolds of the U.S. Fish and Wildlife Service, CRP has put more than 2 million extra ducks on the wing each year since 1992. But the program once called “Noah’s Arc for Wildlife” is losing steam. The 3.4 million acres enrolled in the program in North Dakota as recently as 2007 is now down to 2.7 million acres, with another 1.5 million acres set to expire by 2012.
In 2007 South Dakota lost nearly half of its 1.6 million CRP acres. South Dakota also has lost hundreds of thousands of acres of native prairie in recent years.
Research by Reynolds confirmed that ducks can maintain population-expanding nest success where township-sized areas have at least 40 percent grass cover. Large blocks of undisturbed grass like those provided by CRP buffer duck nests from marauding predators, making them a “needle in a haystack”.
In low-grass areas, the only available nesting cover is often found in road ditches, fence lines and wetland margins too muddy to plant. Some scientists call these “predator corridors”, while others prefer the more descriptive “death traps” because they are so easy for predators to hunt.
In the future, we’ll focus our trapping efforts where they’ll do the most good: areas where the percentage of grass cover is low. In the absence of CRP cover, managing predators is one of the most effective tools for increasing nest success.
The future of fox across the U.S. side of the region is another question mark. During the late 1990s and early 2000s, an outbreak of mange nearly wiped out fox numbers across the Dakotas. Coyotes greatly expanded their range the last decade, presumably in response to CRP, which provides ample food sources for coyotes.
Mange usually runs its course in 10 years or so, which means fox numbers are slowly coming back. Some scientists believe coyotes will keep fox numbers in check; others aren’t so sure. But as more CRP cover is lost, coyotes likely will decline, and if that happens fox will fill the void--a double whammy for nesting ducks.
Managing predators like raccoons and skunks has given nest success a boost, but increasing brood survival will mean controlling mink populations as well. Delta is already perfecting specific techniques for reducing mink populations during prolonged wet periods when, thanks to increased wetlands, they greatly expand their range.
Dealing with changing habitat conditions isn’t our only challenge; we’ve learned that predator populations can vary from area to area. For instance, opossum and badger are far more common in South Dakota than anyplace else in the PPR, and fox, ravens and magpies are a bigger problem in the Canadian parklands than they are on the prairies.
Understanding the impact of each will require the use of new tools like the use of trail cameras to document which of these predators is doing the most damage. We started using cameras this year.
We also tested duckling survival with encouraging results. In North Dakota predator management elevated duckling survival from 50 to 71 percent; in Saskatchewan a more exhaustive study using radio-marked mallards showed duckling survival went from background rates of 36 percent to 57 percent.
These results begged some intriguing questions. Back in the 1940s and ‘50s, Delta researchers Al Hochbaum and Lyle Sowls had documented a behavior called homing. As the name implies, female ducks tend to return to nest where they were raised.
We suspected homing would result in an increasing number of hens settling on trapped blocks, which would magnify the benefits of trapping by attracting hens to a secure environment. We didn’t closely monitor nest density in those days because our focus was to measure nest success, but our year-over-year observations suggested nest densities were increasing.
Remember what we said about the instability of the breeding grounds? Mother Nature has a fickle streak, and shortly after we launched our trapping program she threw us a wicked changeup--an outbreak of Sarcoptic mange that swept across much of the U.S. breeding grounds causing fox numbers to crash.
The manage changed everything. With fox numbers down, wetland counts at all-time highs and CRP cover peaking, nest success soared on our trap blocks. At the same time, nest success on the non-trapped control sites also rose well above population-expanding levels.
At that point, we probably should have shifted our efforts to areas with less grass, where the predator management would have made a more significant difference in nest success. We didn’t, and one reason was that nest densities were so high they provided us with a rare opportunity to conduct research not feasible anywhere else. Two of those studies, conducted in the mid-2000s by Delta students Matt Pieron and Courtney Amundson, signaled it was time to chart a new course.
When we started trapping, few researchers would have believed 40 percent nest success was an attainable goal, but Matt’s research showed 42 percent nest success even on control sites. No real surprise there: fox numbers remained low, there was plenty of CRP, we were still removing predators and even after the record wet cycle of the ‘90s ended, wetland counts across the U.S. side of the pothole region were as high as they were in the 1970s.
We suspected that given these conditions, ducks nesting in high-grass landscapes no longer needed our help, and Matt and Courtney’s research confirmed it. Matt’s work showed that while nest success was 20 points higher on trap blocks, the net increase in mallard numbers--what we call incremental ducks--was lower than we’d expected.
How can that be? When the prairies are wet, mallards are prolific re-nesters. Nest success measures the success of a single nest but doesn’t take into account the probability that when her nest is destroyed, the hen will be successful in subsequent attempts--we call it hen success.
Hen success accounts for the fact that we were producing almost as many incremental mallards as the trap blocks despite lower nest success. It’s also possible that after years of excellent production, mallard numbers approached the saturation point for the existing wetland base, a phenomena scientists refer to as density dependence. Translation: Squeezing any more ducks into those sites was like trying to stuff 15 pounds of potatoes in a 10-pound bag.
When Courtney summarized her mallard brood survival data in 2008, we were surprised to find that trapping has no apparent impact on brood survival. One possible explanation is that the unprecedented string of wet years allowed the population of mink, the primary predator of ducklings, to rise. Because our program didn’t specifically target mink--we were more focused on nest success than brood survival--trapping had less impact on mink numbers.
During one year of Courtney’s research the region got dry early in the summer, concentrating mink and vulnerable ducklings on the remaining wetland basins with a predictable outcome.
Had her research been conducted in the early ‘90s when mink populations were low, Courtney’s results might well have been more consistent with earlier brood survival studies. Still, those results suggested trapping had outlived its usefulness on high-grass areas, and Team Delta did what a small, honest and nimble organization does best: We moved the trapping program to areas with an abundance of duck-attracting wetlands but precious little grass to buffer nests from predators. Unlike the CRP-rich landscapes we left behind, these “moonscapes” needed a helping hand.
Reynolds’ HAPET team identified two likely sites, and in 2009 our trapped blocks enjoyed 21.2 and 49.9 percent nest success compared to 3.2 and 6.6 percent on the control sites. This year, for the sake of scientific rigor, we’re trapping two different sites.
Dealing with the seasonal changes that occur on the breeding grounds is a huge challenge, but it’s not the only one. As we expanded our predator management program into South Dakota, Saskatchewan and Manitoba, we realized that each new area presents its own unique challenges.
The Canadian parklands are a wide belt of habitats that comprise the transition from prairie grasslands to the south and the closed spruce forests of northern Canada. The parklands have an abundance of mostly stable wetlands and trees interspersed with grasslands. The grassland areas have been intensively farmed, and in some cases trees have been removed and wetlands drained to gain tillable acres.
Because parkland wetlands are more stable than prairie wetlands, the parklands have great ability to consistently attract breeding ducks, yet intensive research conducted by Ducks Unlimited shows nest success is consistently lower than prairie habitats with equal cover--in some instances, absurdly low.
Research conducted by Ph.D. student Dan Coulton in the Minnedosa area of Manitoba showed mallard nest success was less than one percent for two years and averaged two percent over four field seasons. Given the millions of dollars spent on habitat conservation in this eco-region over many decades, it’s apparent other techniques for improving nest success are necessary.
In 2007 Delta student John Dassow evaluated trapping in the parklands of Saskatchewan. His initial results were encouraging: 38 and 29 percent nest success on the trapped blocks compared to 5 and 6 percent on the non-trapped blocks. In 2008 John switched sites with the trapped blocks becoming the non-trapped blocks, and two more years of collecting data showed the trapped blocks had 13.3 percent overall nest success compared to 4.3 percent on the non-trapped blocks--not a statistically significant outcome.
There are numerous differences between the prairies of North Dakota and the parklands of Saskatchewan that might explain the results of John‘s evaluation. One is the aspen bluffs that provide refuge to predators while at the same time decreasing our effectiveness at reducing predator numbers. Fox are far more abundant in Saskatchewan, and avian predators like black-billed magpies, crows and ravens--all of which have been observed taking eggs--are far more common in Canada.
Last year we started trapping at Minnedosa with results similar to what Dassow found in Saskatchewan--13 percent in trapped areas and 7 percent in non-trapped.
While we don’t yet understand why trapping didn’t increase nest success significantly in the parklands, we are certain predation is the overwhelming cause of nest failure, and have made a long-term commitment to find effective ways to manage predators there.
About the same time we moved into the parklands, we accepted an invitation from the South Dakota Department of Game, Fish and Parks to trap the southern end of the PPR. Delta student Nick Docken was tasked with monitoring nest success for upland ducks and ring-necked pheasants, the latter proving more difficult because pheasant hens run from nests rather than flushing.
Three years of work showed duck nest success on seven trapped blocks was 38 percent for ducks and 21 percent for pheasants, while on seven non-trapped blocks nest success was 29 percent for ducks and 14 percent for pheasants.
The mix and density of nest predators was far different from what we encountered in North Dakota. One of our South Dakota trappers removed more than 700 predators, including badgers in numbers we never experienced in North Dakota and plenty of opossum, a first in our experience. Franklin’s ground squirrels are abundant, along with more fox, raccoon and skunk than we had previously experienced.
Due to the volume of research required to handle North Dakota and Canada, we are not continuing trapping in South Dakota, but we do anticipate returning in the immediate future.
