Parasite‐Mediated Competition in Deer and Moose: How Strong is the Effect of Meningeal Worm on Moose?
Authors: Schmitz Oswald J. , Nudds Thomas D. 
Published: April 2006

It is widely acknowledged (1) that meningeal helminths (Parelaphostrongylus tenuis) carried by white—tailed deer (Odocoileus virginianus) but fatal to moose (Alces alces) and other cervids, have caused widespread population declines where deer have invaded the ranges of other cervids, (2) that meningeal worms can alter the outcome of interspecific competition between cervid hosts, and (3) that moose populations persist on enzootic range in habitat refuges not frequented by deer. Further, some attempts to reintroduce other cervids to range now occupied by deer have failed. However, moose have been observed to persist on range sympatric with deer, there is evidence that at least one moose population introduced to range with deer is growing, and other events (such as habitat change) are confounded with the presumed effects of meningeal worms. Thus, it is still not clear whether the mortality of individual cervids attributed to meningeal worms necessarily has the effects on population dynamics ascribed to it. Using conventional host—macroparasite models, we describe the population dynamics of deer, moose, intermediate gastropod hosts, and the parasite to determine conditions under which the meningeal worm might be implicated in widespread declines of moose populations. The analysis resulted in three general scenarios depending on (1) the magnitude of parameters that affect transmission rates between different phases of the parasite life cycle, (2) the parasite—induced mortality rate of moose, and (3) the difference in comptitive ability between deer and moose. First, when larval transmission to intermediate hosts, consumption of intermediate hosts by definitive hosts, and parasite—induced mortality of moose were all small, and the competitive difference between definitive hosts was large, moose excluded deer. Second, moose and deer could coexist at either a stable or unstable equilibrium. Stable coexistence occurred when transmission rates, mortality rates, and competitive differences were all large; then deer outnumbered moose. Unstable coexistence at equilibrium occurred under the opposite conditions; moose then were more abundant than deer. Finally, deer could competitively exclude moose. To determine which of these cases was most plausible, we substituted available estimates for some parameters from published literature, solved the equations numerically, and conducted a sensitivity analysis. Overall, model solutions were most sensitive to the magnitude of the competitive effect of moose on deer, the parasite—induced mortality rate of moose, and the intrinsic rate of increase of the intermediate host–all parameters for which there is little empirical information. We conclude that, with the available empirical information, meningeal worm cannot be ruled out as a factor contributing to declines of moose populations. However, our analyses suggest that moose populations sympatric with deer, in many cases, should not become extinct. This implies that reintroductions of moose (and perhaps other cervids) to range now occupied by deer need not be presumed to fail necessarily. Moose may be able to coexist with deer, albeit at lower densities, even in the absence of habitat refuges from the disease.