Spatial dynamics of gypsy moth outbreaks
By Andrew Liebhold.
The gypsy moth is a non-indigenous foliage-feeding insect that periodically reaches outbreak levels, causing widespread defoliation of broadleaf forests across much of eastern North America. This insect serves as a remarkable model system for understanding the spatial dynamics of outbreaking insect populations; there exist extensive historical map data delineating outbreaks and these can be used to characterize both temporal and spatial patterns. As is the case for many other animal populations, gypsy moth outbreaks exhibit considerable spatial synchrony: geographically disjunct populations oscillate in synch over hundreds of kilometers. Gypsy moth populations also exhibit periodicity though the character of this periodicity varies geographically. In oak-dominated forests growing in dry locations, populations exhibit a dominant period of ca. 4-5 years but in more mesic sites, populations are dominated by a periodicity of 8-10 years. In addition the strength of this periodicity is related to both elevation and latitude.
We use a population model that incorporates disease dynamics and predator effects to illustrate that this geographical variation in periodicity may be caused by variability in carrying capacities of small mammal generalist predators which are key mortality agents at low densities. Furthermore, mast seeding in oaks affects the dynamics of predator populations and thereby also influences gypsy moth populations. We hypothesize that synchrony in mast seeding leads to synchronous dynamics in predator populations and in turn contributes to synchrony in gypsy moth populations.
Andrew Liebhold, US Forest Service Northern Research Station, Morgantown, WV, USA.
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