How topography induces reproductive asynchrony and alters gypsy moth invasion dynamics

Abstract

Summary 1.Reproductive asynchrony, a temporal mismatch in reproductive maturation between an individual and potential mates, may contribute to mate-finding failure and Allee effects that influence the establishment and spread of invasive species. 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. 2. 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. 3. 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. Population growth rates declined with increased elevation and more modestly with increased elevational heterogeneity. As in earlier work, we found a positive relationship between the population growth rate and the number of introduced egg masses, indicating 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.4. 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. 5. These results 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 many organisms.

Spatial variation in Allee effects influences patterns of range expansion

“…Consistent with many studies (Keitt et al , Wang and Kot , Wang et al ), increasing the Allee parameter slowed the mean rate of spread. These and other studies, however, represented Allee effects using a constant value and our findings, motivated by growing recognition of spatiotemporal variability in Allee effects (Tobin et al , Kramer and Drake , Kramer et al , Walter et al ), add nuance to predictions of range expansion in heterogeneous landscapes. We found that spatial variations in Allee effects, and the structure of the variation, can drive deviations from predictions generated assuming a spatially constant Allee effect (Table , Fig.…”

“…climate), and also vary in response to habitat patchiness. Gradients in Allee effects could result if, for example, there are elevational or latitudinal gradients in mating success (Rhainds and Fagan , Lynch et al , Walter et al ), predation pressure (McKinnon et al ), or other biotic interactions (Schemske et al ). Other types of landscape heterogeneity, such as variation in habitat type, often display patchiness, and the spatial scale of habitat patchiness – relative to the scale of dispersal – may impact habitat occupancy (Hanski et al ) and rates of movement (With and Crist ) of a focal species as well as its natural enemies (Yahner and Smith , Cronin and Reeve ).…”

Invasion in patchy landscapes is affected by dispersal mortality and mate‐finding failure

“…, Walter et al. ). In its original form, the model was used to assess how mate finding and Allee effects were affected by reproductive asynchrony, i.e., mismatches in the timing of reproductive activity among potential mates.…”

“…, Walter et al. ). In addition, long‐term spatially explicit data on gypsy moth abundance over large areas permit uniquely detailed analysis of spread in real landscapes.…”

Novel insights on population and range edge dynamics using an unparalleled spatiotemporal record of species invasion

“…Spread rates in this region were found to be negatively correlated to hours of spring/summer temperature above optimal for gypsy moth growth (Tobin et al., ). Elevational gradients may also play a role by creating phenological differences between adjacent populations, which can affect spread rates (Walter, Meixler, et al., ). Research that partitioned potential anthropogenic factors found that personal firewood use had the strongest relationship to gypsy moth spread rates (Bigsby, Tobin, & Sills, ).…”