Predator Management Ecology and Management of Predators

Serge Lariviere, Scientific Director, Delta Waterfowl
Rob Olson, Assistant Scientific Director, Delta Waterfowl

Changes in the prairies: new challenges for breeding ducks

Changes to the prairies because of agriculture have had two major changes on the relationship between nesting ducks and their predators. First, many species of specialist predators (prairie wolves, swift fox, black-footed ferrets, etc.) have been replaced by more adaptable generalist species such as red fox, striped skunk and raccoon. These species can achieve higher densities, and thus may depredate more nests simply because they are more abundant. Secondly, the landscape architecture is such that it may now be easier for foraging predators to find duck nests, especially because of the overall low amount of nesting cover remaining and the shape and size of remaining patches (e.g., fencelines are easy to search by foraging predators). Third, the creation of den sites, such as abandoned farm buildings, and increased food resources from waste grain have undoubtedly benefited the "new" prairie predator community.Predator Ecology: what do we know?

In the last few years, several projects have addressed components of predator ecology in the Prairie Pothole Region – the continent’s duck factory. A quick review of these projects highlights some interesting findings.

Skunk work near Redberry Lake, Saskatchewan (1993-1995)
Striped skunks were radio-tracked to determine their use of farm buildings as denning sites and also their use of different nesting cover types when foraging. Skunks used mostly abandoned farm buildings, preferred wetlands and avoided upland nesting cover when foraging, and encountered duck nests opportunistically. The presence of buildings and the distribution of wetlands inside fields of upland nesting cover affected their use by skunks.

Skunk work near Yorkton, Saskatchewan (1999-current)
Populations of skunks were systematically captured and recaptured to determine density and animals were radio-collared to determine limiting factors. Although only preliminary results are available, density of striped skunks decreased 3-fold from 1999 (3/km2) to 2000 (1/km2) because of high rabies mortality. Rabies is the primary limiting factor for this striped skunk population.

Small mammal abundance and nest success: any relationships?
The impact of predators on duck nests may vary according to availability of main foods such as small mammals (voles, mice, etc.). The prevailing hypothesis is that in years of high small mammal abundance, predators reach satiation faster, so they travel less and encounter fewer duck nests. So far, only 1 study in prairie Canada has examined this question (Arts, Clark and Messier). Results suggest that nest success increases as small mammals increase. Results from a study done in grasslands of California (Ackerman and Eadie) also indicate that nest success increase with increasing abundance of small mammals.

Predator Management: a summary of Delta Waterfowl’s research
In 1994, Delta initiated research on the effects of predator removal on duck production. When this work was proposed, many in the waterfowl community said it wouldn’t work, they said that new predators would move in faster than they could be caught. Well, turns out that predator management is quite effective in raising ducks. Two studies have been completed so far. Pam Garrettson studied predator control on 16 mi2 blocks while Mike Hoff looked at 36 mi2 blocks in North Dakota where CRP (Conservation Reserve Program) cover was abundant. These students monitored nest success. Results are summarized below.

Student	Location	Block size	Predator removal	Nest Success Control blocks
Pam Garrettson	ND	16 mi2	46%	16%
Mike Hoff	ND	36 mi2	35%	15%

Once it became apparent that predator removal was effective in areas of North Dakota’s CRP country, Delta set out to determine if the technique would work in the highly cultivated areas (i.e., moonscape) typical of prairie Canada. In the spring of 2000, Vance Lester (University of Saskatchewan) and Aaron Pearse (Idaho State) kicked off Delta’s predator removal research in the "moonscape" of southern Saskatchewan – one of the continent’s most important pintail nesting areas. As most waterfowlers know, pintails are a species on the decline so Delta was interested in conducting the moonscape predator removal work in a key pintail nesting area.

Vance and Aaron’s study sites are 16 mi2 blocks located near the town of Ceylon, Saskatchewan (about one hour south of Regina, Sk.). Vance Lester is measuring nest success while Aaron Pearse is measuring duckling survival. Results from the first year are summarized below.

Student	Location	Block Size	Nest Success
Vance Lester	SK	16 mi2	28%	11%
			Duckling Survival
Aaron Pearse	SK	16 mi2	50%	28%

Keep in mind that this is only one year of data and that there is still one more field season to go. Nevertheless, predator removal appears to be effective in highly disturbed areas such as southern Saskatchewan, although perhaps not as effective as in areas with more nesting cover.

Why do predator management?
Recall that the predator community of the prairie pothole region has drastically changed as habitat was modified by agriculture. The resulting highly cultivated and heavily fragmented landscape is more conducive to smaller predators such as fox, raccoon and skunk than it is for wolves and grizzlies. Smaller predators now occur at very high densities across the prairie breeding grounds. In fact, raccoons are essentially non-native to the prairies! These smaller predators prey heavily on all ground nesting birds - including ducks. Keep in mind, this is a HUMAN-CAUSED problem. The result is that we now rarely observe nesting success in ducks over 15%, which is likely the break even point for most populations of waterfowl. The waterfowl conservation community (i.e., DU, Delta, USFWS) have tried everything to increase duck nest success in the face of this swarm of egg eating animals. The following three approaches are about all we have found to work.

Large Expanses of Grass Cover
Habitat programs only raise significant numbers of extra ducks if they come in large, contiguous blocks - GO BIG OR STAY HOME. Commonly, duck organizations have established relatively small blocks of nesting cover (often less than 640 acres in size) across the prairies in an attempt to provide safe places for nesting ducks to hatch a brood. Research by groups such as DU and Delta Waterfowl has shown that this "postage stamp" approach to habitat establishment is not effective in raising significant numbers of extra ducks.

The Conservation Reserve Program (CRP) in the Dakotas has shown us that you really need a large area covered up in grass before the ducks actually show increased nest success as compared to an unmanaged landscape (i.e., no extra grass planted). Our best guess right now is that you’d need to establish approximately at least 30% grass cover over an area the size of a county to significantly increase nest success. Research is currently being conducted to better determine what the "threshold" for habitat management is. The bottom line is that it really takes a landscape level government program like CRP to pull off the number of acres required to establish enough grass cover on the prairie landscape to raise extra ducks.

Hen Houses
Delta Waterfowl developed these horizontal nesting structures which raise mallards like nothing else on this continent. We've been experimenting with various designs for these nesting structures for years. We often see greater than 75% occupancy of our structures in many areas of the Prairie Pothole Region with approx. an 80% hatch rate. Unfortunately, it is mainly only Mallards that use Hen Houses, but hey, hunters annually harvest a bunch of Mallards so it is a good thing to raise some. Waterfowlers understand that if we are going to harvest ducks such as mallards, we need to put back too, and Hen Houses are one of the best ways to do that. Basically, Hen Houses are cheap to build and maintain and they keep the mallards off of the ground where the predators are - they work!

Predator Removal
The options at this point are relatively limited. Other than huge blocks of habitat and Hen Houses, the only other thing that we can do as waterfowl managers to increase duck production is to control predators.

Some Common Myths Regarding Predator Removal

1.Its not cost effective, you can't possibly trap the whole Prairie Pothole Region.
Noboby wants to. The whole idea is that you would only trap the best of the best areas where there are enormous numbers of nesting waterfowl currently experiencing low nest success without predator removal. The beauty of predator control is that you can move it around to where the highest densities of nesting ducks are located. Compared to other forms of waterfowl management such as habitat creation, predator removal is very, very cheap. When you factor in the cost to plant and manage the nesting cover, and low return of ducks produced, predator removal becomes very attractive.

2.Predator Removal is a waste of money because you have to do it every year.
Predator removal must be done every year, no question. However, every duck management effort requires annual expenditures. Nesting cover can take years of effort to establish. It has to be sprayed, fire guards must often be plowed every year around the edge, site evaluations need to be done to determine how the cover is doing, etc. We have to realize that raising ducks requires annual effort, no matter what we do.

3. Habitat conservation is critical and predator removal only distracts us from those efforts.
Nobody argues that habitat conservation is not critical. Delta Waterfowl believes that the majority of the waterfowl world's money and resources should go to protecting waterfowl habitat. But wait a minute. Habitat conservation is a long-term investment in conserving waterfowl. Predator removal is a short-term investment that produces lots of duck today. When you invest your own money, don't you diversify your resources? Why don't we do the same in waterfowl management? Continentally, let's put most of our efforts into habitat, but let's put SOME of it into predator removal too so that we help duck populations now.

4. The public won't accept predator removal programs.
Certainly a big unknown. Its possible that anti-trapping and anti-hunting groups will fight against predator removal and get legislation passed that will stop it. However, keep in mind that in key duck production jurisdictions like Saskatchewan, North Dakota and South Dakota, there are no really large urban centers. We all know that the well organized, effective animal rights groups typically launch out of large, urban areas. In fact, in the Dakotas and Saskatchewan, we've found that rural residents are ecstatic to have us trap skunks and coons off of their land because of the damage they do. Skunks are major transmitters of rabies, a disease which appears to be on the rise in North America. Coons are very destructive on buildings, frequently ripping shingles off of roofs. Be aware that Audubon Society supports trapping coons off of colonial bird nesting islands and seems to be supported for doing it. Coons are clearly not supposed to be on those islands - they aren't supposed to be on the prairies either! The current predator community of the prairies is completely unnatural, its HUMAN CAUSED AND REQUIRES A HUMAN SOLUTION. Delta recognizes that there is a real need to conduct social research on societal attitudes towards predator removal.

5. It is illegal to kill raptors, which will limit the effectiveness of predator removal.
Reality is that so far, predator removal has been highly effective and raptors have obviously NOT been killed on any of our study sites since 1994 in North Dakota or Saskatchewan. However, it may be that in some areas like the Aspen Parkland of the northern, Canadian portion of the Prairie Pothole Region, there will be higher populations of raptors which will limit the effectiveness of the technique. We’ll find that out eventually and if that is indeed the case, we may suggest that predator removal not be conducted in areas of high raptor populations.

6. Why do it when duck populations are near record levels?
Near record levels? Tell that to the pintail and scaup lovers out there. Predator removal certainly holds promise for waterfowl populations at risk such as prairie nesting pintails. We recognize we need to do more work to determine the benefits of predator removal for pintails. Let’s not forget that other prairie bird populations are also at risk, such as shorebirds, and predator removal in waterfowl nesting areas may have important benefits for non-game species as well.

Summary

Predator removal is best described as a supplement, NOT a substitute for habitat management and conservation. It may be our only tool to help all species of prairie nesting waterfowl in many areas because of our inability to acquire enough habitat to benefit waterfowl.

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