Population regulation in snowshoe hare and Canadian lynx: asymmetric food web configurations between hare and lynx

Abstract

The snowshoe hare and the Canadian lynx in the boreal forests of North America show 9- to 11-year density cycles. These are generally assumed to be linked to each other because lynx are specialist predators on hares. Based on time series data for hare and lynx, we show that the dominant dimensional structure of the hare series appears to be three whereas that of the lynx is two. The three-dimensional structure of the hare time series is hypothesized to be due to a three-trophic level model in which the hare may be seen as simultaneously regulated from below and above. The plant species in the hare diet appear compensatory to one another, and the predator species may, likewise, be seen as an internally compensatory guild. The lynx time series are, in contrast, consistent with a model of donor control in which their populations are regulated from below by prey availability. Thus our analysis suggests that the classic view of a symmetric hare-lynx interaction is too simplistic. Specifically, we argue that the classic food chain structure is inappropriate: the hare is influenced by many predators other than the lynx, and the lynx is primarily influenced by the snowshoe hare.

Figure 1

(A) Food web of the boreal forest ecosystem: the food web links directly influencing the hare population are highlighted [based upon Boutin et al. (16), Krebs et al. (4), and C. J. Krebs/Kluane team, unpublished results]. Only the major dietary items are shown. (B) Yearly hare abundances (log-transformed and standardized; see text) for the main drainage of the Hudson Bay (17): 1844–1904 represents data on fur returns and 1905–1935 represents data from trapper questionnaires.

Figure 2

(A) Food web of the boreal forest ecosystem: the food web links directly influencing the lynx are highlighted (see Fig. 1). (B) Yearly lynx abundances (log-transformed and standardized; see text) (2). Data for 1821–1891 represent fur returns from the North Central district (L10) and for 1897–1938 represent fur returns from the combined James Bay and Lake Districts (L13) (which corresponds to the geographic location of the hare series shown in Fig. 1B).

Figure 3

The f_i functions (as given by Eq. 1; i = 1, . . . , d) for the hare series (H2) with d = 3 (A) and the lynx series (L10) with d = 2 (B). See the main text for interpretation. All f_i functions have been smoothed by natural cubic splines with 2 degrees of freedom using the backfitting algorithm implemented in the function generalized additive models in s-plus (50). Estimated confidence intervals and residuals are also shown.