Population Dynamics

In the 1970s R. Campbell developed the first comprehensive theory of gypsy moth dynamics, wherein populations alternate between low-density and high-density phases, each maintained by different factors and sources of mortality. At low density, predators, particularly mice, maintain gypsy moth populations indefinitely at a low-density equilibrium. The concept of an equilibrium implies that predation is density dependent, which means that it increases as gypsy moth density increases until total mortality balances fecundity of gypsy moth and the population density stops growing. The equilibrium is an unstable one, however, because the density-dependent response of most natural enemies is constrained by a variety of factors. For example, most predators or parasitoids have their own natural enemies. These constraints produce a threshold density above which gypsy moth population growth outpaces the mortality caused by natural enemies, and as a result densities of gypsy moth increase rapidly into an outbreak phase. At the much higher outbreak densities, a different set of natural enemies becomes predominant. These natural enemies, coupled with competition among gypsy moths for available foliage, limit further increases in gypsy moth density. The outbreak population either persists for several generations at high density or collapses back to the low-density phase. The model is particularly appropriate when the principal mortality factors maintaining the low-density equilibrium are generalist predators, such as mice. Unlike specialist natural enemies, whose densities often track those of their hosts, the densities of mice are not determined by gypsy moths. Instead their densities are determined by their overwintering food supply, mainly acorns. Whether this conceptual model is an accurate description of the gypsy moth system remains to be demonstrated.

Gypsy moth outbreaks are spatially synchronized (Fig. 2). Outbreaks tend to occur simultaneously over a large region. A cause of this pattern may be acorn crops, which are heavy or meager over a region in response to regional weather patterns. Indeed, as indicated by P. Moran, populations of many species are synchronized by a variety of weather-related influences. Weather may synchronize gypsy moth populations by exerting a common influence across populations on many of the factors that affect gypsy moth growth and survival. Many gypsy moth researchers believe that there is a 10-year cycle of abundance of gypsy moth embedded in the erratic temporal pattern evident in Fig. 2. Analyses of these data by D. Williams and A. Liebhold provide some support for this view. If these cycles exist, the factors that cause them remain unknown. Analyses by P. Turchin of decade-long records of

FIGURE 2 Historical record of area defoliated by gypsy moth in each of five states in northeastern United States from 1924 to 1996. (Reproduced from Liebhold et al., 2000, Pop. Ecol. 42, 257-266, © Springer-Verlag, with permission.)

Year

FIGURE 2 Historical record of area defoliated by gypsy moth in each of five states in northeastern United States from 1924 to 1996. (Reproduced from Liebhold et al., 2000, Pop. Ecol. 42, 257-266, © Springer-Verlag, with permission.)

gypsy moth egg-mass density in Yugoslavia provide more convincing support for regular cycles in that region, evidently caused by fluctuations in parasitism.

Adding to the complexity of gypsy moth dynamics in North America is the recent appearance of the fungal pathogen E. maimaiga. In the northeastern United States, epizootics of this fungus have occurred nearly every year since 1989, except in years that were extremely dry. We still have much to learn about this agent, but it causes substantial mortality in both low- and high-density populations and appears to have prevented several incipient outbreaks. It appears that incidence of E. maimaiga, which is largely determined by rainfall in May and June, is now a prime determinant of whether outbreaks occur.

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