OIKOS 93: 365 – 375. Copenhagen 2001 Spatial variation in mink and muskrat interactions in Canada John Erb, Mark S. Boyce and Nils Chr. Stenseth Erb, J., Boyce, M. S. and Stenseth, N. C. 2001. Spatial variation in mink and muskrat interactions in Canada. – Oikos 93: 365 – 375. We investigated the spatial attributes of mink (Mustela ison ) and muskrat (Ondatra zibethicus ) interactions in Canada using 160 geographically paired historic time series of mink (n =80) and muskrat (n =80) harvest data obtained from Hudson’s Bay Co. Archives. All series were 25 years in length (1925 – 1949) and were distributed primarily throughout five ecozones. We used autoregressive models and cross-correla- tion analysis to characterize the interactions between mink and muskrat. Model selection results did not differ among ecozones, and indicated that a predator-prey autoregressive model incorporating a delayed density-dependent term best described both the mink and muskrat harvest time series. Subsequent analysis of autoregressive coefficients and estimated lags indicated that mink and muskrat interactions vary throughout Canada. In western Canada, the trophic interactions appear to be strong, and mink population cycles lag behind muskrats 2 – 3 years. In central Canada, mink harvests lagged behind muskrats 1 year, and mink and muskrat interactions in central Canada, with the exception of the Hudson Plains ecozone, were intermediate. In eastern Canada, the trophic interactions appeared weakest, and there were no distinct time lags between mink and muskrat. Stronger interactions in western Canada may be a result of decreased prey diversity, forcing mink to specialize more on muskrats, whereas comparatively stronger perturbations stemming from other trophic interac- tions may alter the estimated interaction between mink and muskrat in eastern Canada. J. Erb, Dept of Zoology and Physiology, Uni. of Wyoming, Laramie, WY 82071, USA (present address: Minnesota Dept of Natural Resources, RR1 Box 181, Madelia, MN 56062, USA [ john.erb@dnr.state.mn.us]).– M. S. Boyce, Dept of Biological Sciences, Uni. of Alberta, Edmonton, AB, Canada T6G 2E9.– N. Chr. Stenseth, Di. of Zoology, Dept of Biology, Uni. of Oslo, P.O. Box 1050 Blindern, N-0316 Oslo, Norway. Documenting and understanding periodic multiannual fluctuations in abundance of microtine (or arvicoline) rodents and their predators has been a long-standing pursuit among ecologists (see, e.g., Elton 1924, Elton and Nicholson 1942a, Krebs and Myers 1974, Finerty 1980, Norrdahl 1995, Stenseth et al. 1996a, 1998a, Stenseth 1999). Additionally, researchers have studied snowshoe hare – lynx cycles (e.g., Elton and Nicholson 1942b, Keith 1963, Krebs et al. 1995, Stenseth et al. 1997) and a host of other species whose populations have been shown to fluctuate periodically (Keith 1963, Finerty 1980, Angelstam et al. 1984, Linden 1988). Although research continues regarding the underlying mechanisms producing observed fluctuations, predator- prey interactions clearly seem to contribute to observed patterns of fluctuation in cyclic mammals (Henttonen et al. 1987, Hanski et al. 1991, 1993, Korpima ¨ki et al. 1991, Krebs et al. 1995, Korpima ¨ki and Krebs 1996, Stenseth et al. 1997). All predators can influence the abundance of their prey; however, only predation by specialist predators is expected to produce cyclical fluctuations in their prey (Andersson and Erlinge 1977, Hanski et al. 1991). Essentially, specialist predators respond numerically (rather than strictly behaviorally) to changes in prey abundance, thereby producing a lag in predator popu- lation responses (e.g., predator reproduction cannot respond instantaneously to prey increases). Such lags Accepted 27 December 2000 Copyright © OIKOS 2001 ISSN 0030-1299 Printed in Ireland – all rights reserved OIKOS 93:3 (2001) 365