How topography induces reproductive asynchrony and alters gypsy moth invasion dynamics

Background/Question/Methods

Reproductive asynchrony, a temporal mismatch in reproductive maturation between an individual and potential mates, may lead to mate-finding failure and contribute to Allee effects that influence the establishment and spread of invasive species.  Geographic variation in elevation is likely to promote variability in maturation times for species with temperature-dependent development, but it is not known how strongly this influences reproductive asynchrony or the population growth of invasive species.  We examined whether spatial variation in reproductive asynchrony, due to differences in elevation and local heterogeneity in elevation (hilliness), can explain spatial heterogeneity in the population growth rate of the gypsy moth, Lymantria dispar (L.), along its invasion front in Virginia and West Virginia, USA.  We used a spatially explicit model of the effects of reproductive asynchrony on mating success to develop predictions of the influences of elevation and elevational heterogeneity on local population growth rates.  Then, we connected our model results to real-world populations by examining population growth rates in 22 years of field data. 

Results/Conclusions

In our model, population growth rates declined with increases in both elevation and heterogeneity in elevation.  We also found a positive relationship between the population growth rate and the number of introduced egg masses, indicative of a demographic Allee effect.  At high elevations and high heterogeneity in elevation, the population growth rate was lowest and the density at which the population tended to replace itself (i.e., the Allee threshold) was highest.  An analysis of 22 years of field data also showed decreases in population growth rates with elevation and heterogeneity in elevation that were largely consistent with the model predictions.  These results collectively highlight how topographic characteristics can affect reproductive asynchrony and influence mate-finding Allee effects in an invading non-native insect population.  Given the dependence of developmental rates on temperature in poikilotherms, topographic effects on reproductive success could potentially be important to the population dynamics of a number of organisms.