Remember the Mange: Did Drop in Fox Numbers Give Ducks a Shot in the Arm?
Posted on 07/28/2003
Editor’s Note: The following article is not intended to be a scientific analysis of the impact of mange on the continental duck population, but rather a thought-provoking essay that examines the possible connection between mange and burgeoning duck numbers during the second half of the 1990s. Readers can draw their own conclusions based on the scientific and anecdotal evidence contained in this piece.
When the long-dry prairie breeding grounds got a soaking in the middle of the 1990s, the continental duck population predictably soared. Duck numbers always rise during wet cycles, always fall during periods of drought.
By 1999 the spring breeding population of mallards–our most accurate assessment of duck numbers–had climbed to its highest level since the 1950s, and the fall harvest appeared to confirm what the spring survey suggested.
The fall flight of ducks that year was estimated at 105 million birds, a figure some gleefully hailed as an “all-time record” and others dismissed as shameless hyperbole based more on assumptions than science.
Whatever the actual figure, the undeniable jump in duck numbers led to the inevitable claims that the 70-year-old template for waterfowl management–creating, restoring and preserving wetlands on a continent-wide scale–was the primary reason for the late-‘90s spike in duck numbers.
Such an assumption requires a Michael Jordan-like leap of faith. While there’s no question that wetlands are critical for duck production, science tells us the habitat needs of nesting ducks are so specific they often aren’t met by postage stamp-sized projects pasted on an otherwise fragmented landscape.
Nesting puddle ducks don’t need just any wetlands, they need seasonal and temporary wetlands, the little one-, two- and three-acre wet spots that often dry up by the end of the breeding season. Our best science also tells us that puddle ducks need very large blocks of grass nesting cover if they’re to produce ducklings at population-expanding levels.
In fact, the needs of nesting ducks are so specific that on many of the habitat projects across the breeding grounds, nest success actually falls below population-expanding levels.
Further, management dollars spent outside the pothole region–while they provide for the birds’ needs at other times of year–directly contribute little to production and therefore deserve minimal credit when duck populations expand.
A study by Steven Hoekman, funded by Ducks Unlimited-Canada’s Institute for Waterfowl and Wetland Research, confirmed that most of the variation in the mallard population is explained by nest success (43 percent), survival of adult females during the breeding season (19 percent) and survival of ducklings (14 percent). Hoekman further noted that more than 65 percent of hen mortality occurs on the breeding grounds.
Besides, if habitat projects alone were responsible for the increase in ducks, why did duck numbers plummet to all-time lows in the 1980s? If the habitat projects couldn’t slow the decline during the last drought, does it seem logical they deserved credit when the population skyrocketed during the wet cycle?
But if habitat projects weren’t the primary reason the continental duck population spiked in the second half of the ‘90s, what was?
It Started with CRP
To answer that question we must go back to 1985, the year Congress authorized the Conservation Reserve Program (CRP) converting millions of acres of cropland to grass cover across the US portion of the prairie pothole region.
The benefits of CRP nesting cover weren’t immediately apparent because the pothole region was in the throes of a nasty drought. Puddle ducks like mallards need large blocks of dense grass cover to nest successfully, but science also has shown they also need the seasonal and temporary wetlands that attract nesting hens. One without the other is of little benefit to nesting ducks.
Late in 1993 the prairie breeding grounds went into one of the most prolonged wet cycles of the 20th century, and the combination of large blocks of CRP grass cover and excellent water conditions was good news for ducks and duck hunters.
Drought is an ugly but necessary reality on the prairies. Drought allows seasonal and temporary wetlands to regenerate, to become productive again, and following the Dust Bowl-like drought of the late 1980s and early ‘90s those little wet spots were hitting on all cylinders.
A study by Ron Reynolds of the US Fish and Wildlife Service published in the Journal of Wildlife Management in 2001 showed that CRP acres in North and South Dakota contributed an additional two million ducks to the annual fall flight. That’s a net gain of two million ducks each year, birds that wouldn’t have been there without CRP.
It would seem that Mother Nature and Congress deserve most of the credit for the burgeoning fall flights of the late-‘90s. But Mother Nature may have contributed more than we realized.
Fox Numbers Plummeted
There was another, often overlooked, factor that came into play in the second half of the ‘90s. About the same time the prairies were getting wet, Mother Nature threw in a bonus by leveling the red fox population across much of the Dakotas with an outbreak of sarcoptic mange.
Delta Waterfowl’s trappers keep detailed accounts of animals harvested during predator management activities, and during the ‘90s the catch of red fox decreased dramatically.
That’s what scientists would call “anecdotal evidence”, but those numbers would seem to suggest that something was going on with fox populations. The numerous biologists we contacted across the region said that while they could provide no scientific evidence to confirm a drop in fox numbers–no one officially monitors fox populations–all agreed mange significantly reduced the population.
The timing couldn’t have been more fortuitous. According to a study conducted by Al Sargeant of the Northern Prairie Wildlife Research Center between 1968 and 1973, red fox were killing an estimated 900,000 ducks, mostly nesting hens, in the prairie pothole region every spring. Fox also cache (bury) eggs and eat them later.
Sargeant wrote, “…if fox density in Canada had been equal to that in east river North Dakota, our estimate of average number of adult ducks taken annually by foxes…would have more than doubled. Conservesly, during drought years when nesting is curtailed or during periods of low fox density, mortality from foxes would be much less than our estimated average.”
At the time Sargeant conducted his study, the majority of the ducks nesting in the pothole region settled in Canada. By the ‘90s the distribution of nesting ducks had shifted dramatically. More than half the ducks nesting in the pothole region were settling on the US side of the border, where fox densities at the time of Sargeant’s study had been high.
No research has been conducted to determine how many hens survived the wet cycle thanks to the outbreak of mange or how many ducklings those hens contributed to the fall flight, but if one believes Sargeant’s research, the impact could have been significant
Nature abhors a vacuum, and fox were likely replaced by increases in other species of predators, primarily skunks and raccoons. While skunks and raccoons eat eggs, they do not pose a serious threat to hens.
Some scientists say that during the abundant-water period of 1994 to 1999, mallards likely overcame low nest success through persistent re-nesting attempts, but persistent re-nesting increases a hen’s exposure to fox predation.
The above-mentioned studies beg several intriguing questions. First, what would the duck population have been in 1999 without six million acres of CRP across the US side of the pothole region and the small-wetland protections afforded by the Clean Water Act and the Swampbuster provisions of the farm bill? Would the habitat projects scattered across the continent have lifted duck numbers to their 1999 high without CRP?
Second, how much did the outbreak of mange across much of the breeding grounds contribute to the rise in duck numbers?
Even more compelling: What might happen during the next wet cycle if fox numbers rebound?
The Habitat ‘Threshold’
Reynolds’ study showed that unless 40 percent of the landscape contains grass nesting cover, ducks typically do not achieve population-expanding nest-success levels. One possible reason: “…idle grass cover provided nesting ducks and predators with increased nesting and foraging options, respectively, that reduced predator contact with nests.”
Reynolds’ findings explain why many habitat projects–those in areas that don’t achieve the habitat threshold–don’tenjoy population-expanding nest success.
In an interview, Reynolds told us, “Where 40 percent of the landscape has grass nesting cover, ducks are able to achieve population-expanding nest success levels. Where less than 40 percent of the landscape has grass, other management tools are necessary.”
Obviously, waterfowl management doesn’t have the resources to elevate significant portions of the 224 million-acre prairie pothole region to the habitat threshold.
Congress does, and the six million acres of CRP in the Dakotas–coupled with favorable water conditions–deserve most of the credit for jump-starting duck production in the 1990s.
The prairies will get wet again, but if duck populations are to respond as they did in the ‘90s, we must rely on other management tools–predator management and Hen Houses–that supplement habitat in areas where management doesn’t have the dollars to achieve the 40 percent threshold on a landscape level.
The Drift Prairie of eastern North Dakota is a prime example. Despite the fact that the Drift Prairie is intensively farmed, half the ducks that nest in North Dakota settle there. Those ducks have little chance of achieving population-expanding nest success because most of the region falls short of the habitat threshold.
What role did mange play in the population boom of the ‘90s? We may never know the answer to that question. But based on the research by Sargeant, Reynolds and Hoekman, it’s something waterfowl managers might want to consider before the current outbreak of mange runs its course.