3.5
Discussion
The analysis in this chapter reconstructs population trends by assuming four different models. I project trends into the future and perform a qualitative analysis of the interactions between predation rates and demographic rates. The models describe four hypotheses on how caribou can co-exist (or not) in a system where wolves prey predominantly on moose. I find that all four models fit the data equally well, but that two models predict extirpation of caribou within approximately 100 years. I do not compare the model fits statistically because small variation in parameter values can cause large enough changes to the likelihood objective function that an examination of the qualitative behaviour of the models is more meaningful. In addition, the uncertainty shown in Figures 3.9-3.12 demonstrates that a range of variation should be considered. Model 1 only allows caribou to persist if the effective rate of search is low enough, which implies some low encounter rate (両) value. It requires high spatial and/or temporal separation of caribou from wolves. Once caribou are extirpated, wolves and moose either stabilize or enter a periodic cycle. The empirical model fit suggests that separation is not currently occurring adequately enough to sustain caribou densities. Model 2 does not allow a species to go extinct unless the natural mortality rate of adults is higher than recruitment. This occurs because predation rates under Type 3 functional responses are very low at low densities. It implies switching away from caribou when they are at low densities, which precludes allowing incidental encounters to result in a kill. Given the precedence of high predation rates of wolves on caribou when caribou are at low densities (Wittmer 2004), this does not seem reasonable. It is possible, however, that a strict Type 3 form may relax predation too much at low densities. It may be
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