Wolf-Moose Populations on Isle Royale Modeling potential impacts of climate change

The Island Isle Royale is an island in Lake Superior, 14 mi from Canada. Its 544 km² of land is home to 165 miles of trail and a wide array of wild- life. It was among the first national parks in the United States and is now the home of the world’s longest predator- prey population study.

The Study  In the late 1940s, an ice bridge over Lake Superior facilitated the colonization of Isle Royale by a pack of gray wolves (Canis lupus).  Over 90% of the wolves’ diet now consists of the moose (Alces alces) already inhabiting the island (Peterson and Page, 1988), and they are the only predator capable of hunting the moose.  Since 1958, the populations have been monitored and studied, and are a leading example of predator-prey relations in a three trophic level system.

The role of Winter Ticks  Winter ticks (Dermacentor albipictus) cause excessive moose grooming, leading to alopecia and reduced heat-trapping ability, making moose vulnerable in the winter months. Preoccupation with grooming, if infestation is severe enough, can also lead to reduced grazing and starvation (McLaughlin and Addison, 1986 and Mooring and Samuel, 1999).

Our Question  In the past, outbreaks of winter ticks (Dermacentor albipictus) have contributed to declines in moose populations.  As a result of climate change, increasing frequency of warm springs is expected, leading to a rise in tick populations.  How might climate change, with respect to impacts of ticks on moose populations, affect the wolf-moose population dynamic on Isle Royale?

Methods  Using STELLA, we modeled the interactions between moose and wolf with and without the influence of ticks.  We assume the absence of other factors such as disease, stochastic events, and other results of climate increase. Wolf predation and tick population are taken to be the only causes of moose mortality.  Tick population density is assumed to be directly correlated with temperature, with little activity at temperatures less than 4°C (DelGiudice, et al., 1997) and increasing impact as temperature rises thereafter.  Tick impact on moose mortality is also taken to be in direct correlation with population of ticks.

Methods, continued

Results  The first graph depicts a generalization of the moose-wolf population in the absence of ticks. Rate of predation is based on a ratio dependent equation proposed by Vucetich, et al. (2002).  The second graph depicts the impact of increased tick population as a result of average April temperature according to projected changes in temperature (Boer, et al. 2000)

Summary  Because of its distance from the mainland and the decreasing odds of ice bridge formation over Lake Superior, additional colonization of the island is unlikely, as is the potential for northward migration of moose in response to pressure from higher tick populations.  Thus, climate change has the potential to significantly impact the population dynamic between wolves and moose on Isle Royale, MI, the subject of a population study nearly half a century long.

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