Predators Of Weasels

Tame weasels of any species, well settled in captivity, can live up to 10 years. Why do most wild weasels achieve only a tenth or less of their potential life span? The most obvious explanation, considering that weasels are obligate predators dependent on wildly variable food supplies, is shortage of food, but there are others.

Stoats and longtails are often killed on the road, especially males in spring (Buchanan 1987; Sleeman 1988a). The active lifestyle of weasels and their insatiable curiosity about dark and inviting-looking holes make them vulnerable to sudden death in kill traps (Chapter 12) and other dangerous places. One unlucky, rain-soaked common weasel crawled into a railway signal box to get out of a storm, short-circuited all the signals, and stopped the trains (Anon. 1998).

In addition, weasels suffer from more natural hazards, including intense persecution from larger predators. Weasels of all species are small enough to be regarded as, or confused with, the normal prey of foxes, coyotes, feral cats, minks and ferrets, plus owls and hawks (Hellstedt & Kallio 2005). Weasels are believed to have somewhat distasteful flesh, so these predators do not necessarily eat a weasel once they have killed it, but that is hardly a comfort. The question is, do these encounters happen often enough to affect the weasel populations?

One of the earliest researchers who believed they do was Latham (1952). He examined the bounty records of the Pennsylvania Game Commission during the 1930s and 1940s, when hunters and trappers were paid to turn in dead foxes, both red and gray, and weasels of all three species. In the early 1930s the total number of weasels killed per year was over 100,000, and of foxes, under 40,000. Then in the late 1930s and early 1940s, the number of weasels decreased, dropping below 20,000 by 1946, while the number of foxes over the same period increased to over 80,000; their relative proportions in the records became nearly reversed. Between 1946 and 1949, the numbers of foxes went down again, which Latham attributed to the heavy toll from the bounty hunters; at the same time, the weasels were apparently recovering.

This inverse relationship was repeated in the relative numbers of foxes and of weasels killed in each county, as shown in Table 11.5. It seems logical to assume, concluded Latham, that foxes reduce and control the numbers of weasels in Pennsylvania.

A single historical event observed and described in unusual detail by Mulder (1990) supports Latham's view. The North Sea coast of the Netherlands from Den Haag to Den Helder is lined with a strip of sand dunes up to 5 km wide, long famous for its rabbits and for the stoats, weasels, and polecats that hunted them. The local game wardens knew their areas and the animals well, and they compiled field notes and annual reports on their activities. The wardens also assisted with several research projects on the stoat populations during the 1960s (Heitkamp & van der Schoot 1966; van Soest & van Bree 1970).

The detailed local knowledge of the wardens documented incidentally the unexpected disappearance of stoats from the dunelands from the mid- to late 1980s onward, following the arrival of foxes between 1968 and 1977. Mulder searched the records, conducted interviews with the wardens, and plotted the acceleration in the recovery of the rabbits from myxomatosis throughout the 1970s and early 1980s. He concluded, on circumstantial but consistent evidence, that the formerly abundant stoats were driven to extinction by foxes throughout the dunelands. The only area not to report this pattern of events, Zwanenwater, apparently proved the rule. There, foxes were shot on sight to protect a nesting colony of spoonbills, and the stoats remained.

Table 11.5 Apparent Reciprocal Distribution of Foxes and Weasels (All Three Species, but Mostly Stoats and Longtails) in Bounty Records From Pennslyvania, 1930-1951

Annual weasel kill per county

Weasel-to-fox ratio

1-99

1:9.33

100-199

1:4.06

200-299

1:2.75

300-399

1:1.86

400-499

1:1.24

500+

1:0.99

(From Latham 1952.)

(From Latham 1952.)

Another data set, collected before Latham's idea was published and without any special attention to weasel populations, further supports the hypothesis that predation can be an important force, at least sometimes. It stimulated Powell (1973) to explore the situation further. Craighead and Craighead (1956) had studied the ecology and predator-prey relationships of raptors in Michigan in the 1940s. They estimated the densities and food habits of all the local predators, and censused the numbers of their prey available. They reckoned the density of weasels in 1942 was 27 to 36 per township (36 square miles, 93 km2).

Powell took the maximum value of one per square mile, that is, one per 259 ha, or 0.38/km2. The Craigheads published extensive tables of data on the contents of nearly 5,000 raptor pellets analyzed, in which weasels appeared consistently as 1% or less of the items listed. Powell calculated that the raptors had killed roughly 70% of the summer (postbreeding) weasel population available in 1942. Powell then constructed a computer simulation model of the weasel-raptor-vole ecosystem, assuming that (1) the area was large enough to operate effectively as a closed system; (2) weasel populations were not limited by food; (3) the reproduction of weasels and of voles was density independent; (4) populations of raptors and of voles were linearly related (this idea was supported by the Craigheads' data); and (5) all mortality of voles and of weasels was due to predation.

If a simulated weasel population could be controlled by predation under these extreme conditions, Powell argued, real predation could probably control real weasel populations. He ran simulations using a variety of starting conditions and ranges of variation in both the weasel and the vole populations. The results always showed that predation kept the weasels below the number that would have been limited by the available voles. Powell concluded that "under certain conditions, a limiting factor for weasel populations is predation by other predators. This point has been given only limited consideration."

Some of the rather obvious problems with this model are explicable. For example, the Craigheads' field data were designed to count raptors, and detected weasels only incidentally, which may be why their estimates of weasel density appears low, compared with those quoted in Table 10.2. But Powell also ran the model starting with weasel densities that were certainly too high, and the results came out the same. He also deliberately included assumptions he well knew were not supported by field evidence, especially that weasel populations are not limited by food and that weasel mortality is due mainly to predation, but only so as to test the question he was asking. The model was designed so that limitation by food and by predation would lead to different outcomes: One set of results would rule out the other, and vice versa. His conclusion was that predation could limit weasel populations, sometimes.

The questions of how often and to what extent raptors might limit weasel populations in real life, as opposed to in a computer, can be addressed only from real data. Fortunately, Erkki Korpimaki and his team in Finland have provided some. They collected detailed information on the diets of 10 species of raptors and 10 of owls, and found that 16 of these species ate, during the breeding season, a range of items including at least 10% small mammals. Their extensive field data also showed that populations of least weasels varied with those of the rodents, and that predatory birds ate weasels along with the rodents, but not to the same extent every year.

Korpimaki and his coworkers became curious when one of us (King 1983c) published a paper suggesting that mustelid populations are controlled mainly by shortage of food, contradicting the raptor predation model proposed by the other (Powell 1973). They used their extensive data (Korpimaki & Norrdahl 1989a, 1989b) to ask which of us was right.

Over the years 1984-1987, the team estimated (1) in late February and March, at the beginning of each breeding season, the densities of least weasels using data collected by snow tracking over 50 km2; (2) from mid-April to the end of July, the number of least weasels that could have been eaten by the birds of prey; and (3) in May and September, the abundance of small mammals by index snap trapping. Buzzards, kestrels, and three species of owls (eagle, Ural, and short-eared owls) normally ate few small mustelids, on average less than 1% of all the food items identified for these birds. Mustelid skulls did, however, appear in the pellets more often (up to 2%) during the period when voles were decreasing in numbers.

This pattern is quite reasonable, because more weasels are always present after a period of high rodent densities, and predators often have to be content with alternative prey when their favorites are off the menu. Furthermore, both least weasels and stoats might be more exposed to predation in years when rodent populations are decreasing or low, because hunting is harder then and the weasels have to travel further to find the few rodents left (Klemola et al. 1999).

Korpimaki and his coauthors estimated that kestrels took only 20% of 190 available least weasels in the vole peak year of 1985, but kestrels plus short-eared owls took 80% of 650 least weasels available in the vole decline year of 1986. The authors were, of course, quite well aware of the multiple possible sources of error that dog any such exercise. Nonetheless, they listed several good reasons for supposing that their estimates of the predatory impact on weasels were not exaggerated. For example, they sampled the diets only of resident birds of prey, ignoring transient birds of prey and mammalian carnivores.

Other observations in the northern hemisphere confirm that predation is a serious hazard for weasels. Dead weasels or their remains are sometimes found at the dens of foxes and in the nests of raptors; two of nine male longtails radio collared by Gehring and Swihart (2000) were killed by other predators, and Ratz (2000) found a remarkably constant reciprocal distribution of tracking tunnels marked with footprints of stoats versus ferrets. Of eight collared common weasels radio tracked in Bialowieza in 1990, one was killed by a fox and a second by another weasel (Jgdrzejwski et al. 1995), and the remains of other unmarked weasels were found in the pellets and scats of larger preda tors. Jgdrzejwska and Jgdrzejwski (1998) concluded that predation accounted for some 65% of the mortality of common weasels.

Calculations like these suggest that weasel populations can be controlled by larger predators under certain circumstances. Nevertheless, distinguishing between the various possible factors that limit weasel populations, including habitat requirements, predation, and shortage of food, is hard to do, especially when they undoubtedly work together. For example, in Belarus, the invasion of American minks starting in 1988 is staging an as yet unfinished experiment with different results for stoats (a 16-year steady decline in winter snow tracks) compared with common weasels (a corresponding but even steeper increase) (Sidorovich et al. unpublished). Until the arrival of the minks, the Belarus stoats had preferred to hunt water voles in marshlands (as they do in Sweden: p. 251), while weasels preferred forest habitats; since then, stoats have suffered from increased competition with the minks, which weasels can avoid.

King and Moors (1979a) had predicted that body size for weasels is a tradeoff between greater foraging efficiency and aggressive power. Indeed, large weasels can dominate small weasels, but they come off badly against larger carnivores and have fewer escape options than small weasels do. Caught between the two, any increased exposure to predation is a greater risk for the larger of the two weasels of a sympatric pair. The same idea could explain why longtails in northern Quebec were more at risk of predation, compared with stoats, in areas where larger furbearers were protected (St.-Pierre et al. in press-a).

To understand what limits a particular population at a particular time, we need a statistically rigorous model that can eliminate alternative hypotheses under different conditions. Such a model may be ideal in theory, but in practice, extremely difficult to construct.

Raptors certainly kill weasels, perhaps many of them, especially after a vole peak when weasels and raptors are more numerous than usual and voles are becoming scarce. At such times, a weasel population will be decreasing rapidly, whether any are killed by raptors or not. Latham's correlations could well be evidence of active predation on weasels by foxes, but they could equally well be evidence that the habitats in counties supporting large populations of weasels are less favorable for foxes, and vice versa. Stoat populations are prone to local extinction with or without the help of foxes, so we cannot exclude the alternative explanation that Mulder's observations could have been mere coincidence. In part, the question of whether carnivores limit their prey or are limited by them is a matter of scale, and has no simple, general answer (Powell 2001).

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