Abstract
Woodland caribou populations are considered threatened in Alberta and have declined in the Canadian Rocky Mountain National Parks of Banff and Jasper despite protection from factors causing caribou populations to decline outside of parks. Recent research emphasizes the importance of the numeric response of wolves to moose in moose-caribou-wolf systems to caribou persistence. Moose are rare in the Canadian Rockies, where the dominant ungulate prey for wolves is elk. Few studies have explored wolf-elk dynamics and none have examined implications for caribou. We used data collected in Banff to estimate the numeric response of wolves to elk from 1985 to 2005. Because no caribou kill-rate data exist for the Rockies, we explore the consequences of a range of hypothetical kill-rates based on kill-rates of alternate prey collected from 1985 to 2000 in Banff. We then multiplied the numeric response of wolves by the estimated caribou kill-rates to estimate the wolf predation response on caribou as a function of elk density. Caribou predation rates were inversely density dependent because wolf numbers depend on prey species besides caribou in multiple prey species systems. We then combined this simple wolf-elk-caribou model with observed demographic and population estimates for Banff and Jasper caribou from 2003-2004 and solved for the critical kill-rate thresholds above which caribou populations would decline. Using these critical kill-rate thresholds, Jasper caribou are likely to persist when wolf densities are below 2.1 - 4.3 wolves/1000km2 and/or when elk densities are below 0.015- 0.033 elk/km2. Thresholds for Banff caribou persistence are much lower because of inverse density dependence. Future research is needed on some of the necessary assumptions underlying our modeling including multi-prey wolf numeric responses, wolf kill-rates of caribou, caribou mortality by other predators, and spatial aspects of wolf-elk-caribou dynamics.
Highlights
Common causes of species endangerment include habitat loss, fragmentation or alteration, over-hunting, and competition by invasive species, all of which can alter trophic relationships (Sinclair & Byrom, 2006)
Wolf numbers were assumed to respond only to the density of their primary prey, elk, not alternate prey. This approach was used instead of using a wolf- total ungulate biomass equation (Fuller, 1989) for the following reasons; 1) wolf abundance in Canadian Rockies is largely driven by elk density (Hebblewhite, 2000), 2) wolves in the Rockies are highly selective for elk (Huggard, 1993), and other studies confirmed the density of preferred prey strongly influences the multi-species wolf numeric response (Dale et al, 1995, Mech et al, 1998); and 3) the rugged terrain of the Rockies allows strong spatial separation of some relatively abundant secondary prey species from elk
A linear numeric response was the best fitting of the three models fit to the wolf and elk density data; linear ΔAICc =0, decelerating type II ΔAICc =
Summary
Common causes of species endangerment include habitat loss, fragmentation or alteration, over-hunting, and competition by invasive species, all of which can alter trophic relationships (Sinclair & Byrom, 2006). A brief review of predator-prey theory for alternate prey reveals why Prey density influences both kill-rates (the functional response) and densities (the numeric response) of predators (Holling, 1959, Fig. 1). This implies a lowdensity state for alternate prey at P* is possible. Both illustrate that given a Y-intercept, once alternate prey species decline past some threshold, regardless of the functional response type, further population declines are likely
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