Spatial synchrony of gypsy moth outbreaks affected by structure of forest food webs

Background/Question/Methods

Spatial synchrony of population fluctuations is a pervasive phenomenon, yet the causal mechanisms are rarely known for a given system.  Previous work suggests that spatial synchrony in outbreaks of the gypsy moth in North America is a result of indirect synchronization due to resource pulses caused by acorn masting.  A mechanistic model showed that synchrony in populations of the white-footed mouse (the gypsy moth's chief predator) driven by masting of acorns of the red-oak subgenus may be the main factor leading to synchrony in gypsy moth populations.  However, the white-oak subgenus is dominant to the red-oak subgenus in one of the forest types (oak-pine) invaded by gypsy moths.  Mast fruiting of red-oak acorns is highly periodic, with peak fruiting occurring every 4-5 years.  In contrast, the production of white-oak acorns is aperiodic.  In this study, we investigated how heterogeneity in acorn crops among forest types affects levels of spatial synchrony in gypsy moth populations.  The effects of the periodicity of acorn production on levels of synchrony in gypsy moth populations were examined using a mechanistic model of multi-trophic interactions.  We then compared levels of spatial synchrony in gypsy moth populations occupying forest types dominated by white (oak-pine) and red oaks (oak-hickory, maple-beech-birch) using spatially referenced time series of forest defoliation.

Results/Conclusions

Spatial synchrony among simulated gypsy moth populations was enhanced when mast fruiting of acorns was highly periodic and the period length of the mast cycle was relatively long (e.g., 5 yrs), as is seen in trees of the red-oak subgenus.  Consistent with this theoretical prediction, defoliation caused by gypsy moths was much more synchronous in forest types dominated by red oaks (oak-hickory, maple-beech-birch) than in a forest type dominated by white oaks (oak-pine).  These findings provide further support for the hypothesis that synchrony in mast seeding can propagate across trophic levels, thus explaining observed levels of synchrony in white-footed mice and gypsy moth populations.  Additionally, our study indicates that mechanistic modeling, coupled with data on population spatiotemporal patterns, offers a powerful tool for determining the mechanisms responsible for spatial synchrony of population fluctuations.