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Distribution and Abundance of Predators that Affect
Duck Production: Prairie Pothole Region

Annual Variation in Size of Predator Populations


Populations of predator species are subject to annual changes in abundance. The magnitude of such changes was important to interpretation of our findings because we studied populations in each area 1-3 years.

Annual variation in size of predator populations is caused by mortality, nasality, emigration, and immigration. For the species we studied (except northern harrier), mortality rate was the factor most likely to cause major changes in annual abundance. For example, an epizootic or severe human-inflicted mortality could decimate a population in 1 year. In contrast, more gradual changes are expected from (1) the relatively low recruitment rates of most predator species; (2) the tendency of adult predators, especially females, to remain in or return to the same area during successive years; and (3) the tendency of juveniles to disperse. Local populations of northern harriers may fluctuate greatly from year to year from emigration or immigration in response to availability of prey, especially meadow voles (Clark 1972; Hamerstrom 1979).

During our study, we were unaware of major programs to reduce abundance of predators in our study areas other than the removal we conducted in three of the small unit management study areas. However, coyotes may have been removed from or near some study areas, especially in Montana and western North Dakota, to protect livestock. There was evidence of limited killing of certain predators by local residents in a few study areas. For example, several recently killed adult striped skunks were found in one study area (Shamrock) in 1985, one dead adult and two kit raccoons were found (seemingly killed by humans) in another study area (Ceylon) in 1983, and a freshly shot American crow was found in a third study area (Hanley) in 1985. Although most of the studied avian species were protected by law, remains of two red-tailed hawks (in Penhold) in 1985 and a dead northern harrier (in Litchville) in 1988 were found; all were probably killed by humans. Fur prices were high throughout the study (most studied carnivores were economically important forbearers), but harvests in all areas were regulated by provincial, state, and federal agencies.

We were unaware of epizootics among predators or major environmental calamities that could have had a major effect on the abundance of predators in our study areas. However, most data were collected during a drought, which probably contributed greatly to the scarcity of minks and may have affected the abundance of some other species (e.g.,northern harrier). The abundance of snowshoe hares (Lepus americanus) was declining during our study and probably reduced nesting by great horned owls in the aspen parkland (Rusch et al. 1972; Houston 1987).

A maximum of 18 (number varied by survey type) between-year comparisons of indices was available for predator populations in 16 study areas (10 in Canada and 6 in the United States; Appendix Table 1). In general, the index values for individual species in individual areas changed little between years. For example, the average between-year change (increase or decrease) in the percentage of searched quarter sections with tracks of a particular carnivore species in a study area was 9.2% (SD = 9.74; calculated from data in Appendix Table 5). (The total between-year comparisons for all carnivore species combined was 108, excluding the gray wolf because of its rareness and weasels for which no track searches were conducted.) The average between-year change in the percentage of quarter sections in which black-billed magpies were detected in a study area (12 comparisons) was 1.2% (SD = 2.51) and in which American crows (12 comparisons) were detected, 4.6% (SD = 2.98; calculated from data in Appendix Table 4). The average between-year change in density of American crow nests in a study area (13 comparisons) was 0.14 (SD = 0.15) nests/km2 (calculated from data in Appendix Table 12). The average between-year change in number of occupied nests by a single species of large hawks (except the northern harrier, for which all nest searches were incomplete, and the ferruginous hawk, which was excluded because of its general scarcity) and the great horned owl combined in a study area was 0.03 (SD = 0.05) nest/km2 (calculated from data in Appendix Table 15). Although we were unable to establish meaningful criteria for a major change in each species' abundance or to determine the extent of measurement error, we interpret these and similar findings from other surveys as indicative of little change in species abundance and little measurement error. We believe that for most species our findings reflected abundance during several years.

Some major between-year changes in indices of species abundance occurred in a few study areas. In one (Moore Park), the coyote track index increased from 22 to 72% of searched quarter sections from 1983 to 1984 (Appendix Table 5). An occupied coyote den was in a central portion of that area in 1984 where no evidence of a coyote den and few quarter sections with coyote tracks were found in 1983. Red fox tracks were in several quarter sections in the vicinity of that den in 1983, but none were found in those quarter sections in 1984. The percentage of searched quarter sections with red fox tracks decreased from 60% in 1983 to 44% in 1984. In one study area (Craik), no evidence of red foxes was found in the 8 western-most quarter sections in 1984, but red fox tracks were found in all but one of those quarter sections in 1985. This change accounted for most of the increase from 64% of searched quarter sections with tracks in 1984 to 92% with tracks in 1985 (Appendix Table 5) and indicated that a new red fox family had become established in the western portion of the study area. Coyote tracks were found in only two of the 8 western-most quarter sections each year.

The decline in one study area (Moore Park) from 70% of quarter sections with raccoon tracks in 1983 to 18% with tracks in 1984 (Appendix Table 5) seemed to reflect a major population decline. In 1984, raccoon tracks were found at only a few locations throughout that study area despite improved conditions for finding raccoon tracks. The cause of the decline was undetermined.

Measurement error from variable tracking conditions probably had the most influence on track indices for striped skunks, badgers, and minks. The greatest annual change in the track index for the striped skunk (Shamrock from 1984 to 1985; Appendix Table 5) was not supported by a comparable change in capture rates (Appendix Table 9). Although several dead striped skunks were found discarded in a pile there in 1985, more important to interpretation of the track data was the very poor tracking conditions from drought and untimely strong winds.

The indices of the annual observation (Appendix Table 10) and capture rates (Appendix Table 11) of Franklin's ground squirrels provided similar indicators of relative abundance except where (Moore Park) the large decline in the observation rate from 1983 to 1984 was inconsistent with the substantial increase in the capture rate. Measurement error was the suspected cause of the discrepancy. Nevertheless, both indices in both years indicated that Franklin's ground squirrels were much more abundant there than in most other study areas.

Indices of avian species except the black-billed magpie, northern harrier, and great horned owl were the number of occupied nests that were subject to little measurement error. The largest annual variations of avian species occurred where (Penhold) most bird indices declined from 1984 to 1985 (Appendix Tables 4, 12, 13, and 15). No reasons were found for the apparent declines, although human-inflicted mortality of red-tailed hawks was suspected. Northern harriers are noted for large variation in annual abundance (Clark 1972; Hamerstrom 1979, 1986). Where substantial changes in the abundance of northern harriers occurred (Hay Lakes, Fredonia; Appendix Table 15), they were probably in response to changes in availability of meadow voles.


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