Geographic variation in forest composition and precipitation predict the synchrony of forest insect outbreaks

Abstract

Evaluating the causes of spatial synchrony in population dynamics in nature is notoriously difficult due to a lack of data and appropriate statistical methods. Here, we use a recently developed method, a multivariate extension of the local indicators of spatial autocorrelation statistic, to map geographic variation in the synchrony of gypsy moth outbreaks. Regression analyses indicated that local synchrony of gypsy moth defoliation increased with the local synchrony of precipitation and the proportion of host tree density composed of oaks, especially those in the Lobatae (red oak) section. This may be the first study that demonstrates a relationship between defoliator population synchrony and host tree composition. More broadly, this study contributes to a small body of recent work that illustrates how mapping hotspots of unusually high or low synchrony facilitates an improved understanding of factors influencing spatially synchronous population dynamics generally, and triggers of pest insect outbreaks, more specifically.

Population spatial synchrony enhanced by periodicity and low detuning with environmental forcing

“…In comparison, the detuning between the dominant timescales of gypsy moth outbreaks (approx. 7-11 years) and Partially synchronized fluctuations in precipitation at the 7-11 year timescales appears to be the main abiotic driver of population synchrony in our study, but we also found that the 2-4 year synchrony of gypsy moth outbreaks was No previous studies have reported effects of temperature on the synchrony of gypsy moth outbreaks [20,21,42], we found no evidence that temperature affects the phase synchrony of defoliation (table 2), and we are unaware of any processes whereby synchrony in the fluctuations of an abiotic factor at a given timescale would reduce the synchrony of population fluctuations at the same timescale. For these reasons, we suspect the negative relationship between the synchronies of temperature and defoliation that we detected was spurious.…”

“…(See a flowchart describing the workflow we followed in our analyses in electronic supplementary material, figure S2.) Given the low dispersal ability of gypsy moths-adult females are flightless and ballooning 1st instar larvae typically disperse only tens of metres [31]-we examined this aerial survey data at a finer spatial scale (8 Â 8 km grid cells) than previous studies of synchrony in gypsy moth dynamics [20,21,25,26]. Based on previous research showing a positive correlation between the area of forest defoliated and gypsy moth egg masses [32,33], we used the proportion of defoliated area (km 2 ) in a given year as a proxy of gypsy moth abundance within each cell.…”

“…Gypsy moth populations can be characterized as cyclic, nonlinear oscillators in that they exhibit periodic abundance fluctuations which are thought to arise primarily from host -pathogen dynamics [18,19]; there is no evidence that periodicity in meteorological factors are significant drivers of periodicity of gypsy moth populations. However, previous work showed that synchrony in precipitation is a likely driver of synchrony in gypsy moth outbreaks [20,21]. Therefore, in this study we examined the combined effects of synchrony in weather and population cycling on the synchrony of gypsy moth outbreaks.…”

Spatial synchrony of wader populations in inland lakes of the Iberian Peninsula

“…On the other hand, 88 some distant sites may show high spatial synchrony of 89 populations as a response to correlated environmental 90 conditions, a process known as Moran effect (Koenig 91 2002). Haynes et al (2013), for instance, noted that 92 precipitation acted as synchronizer factor of populations 93 of moths due to the similar effect on the survival and 94 reproduction of individuals. In this case, as in many 95 others, the Moran effect is mainly associated with 96 environmental conditions extrinsic (or exogenous), 97 which are common over large spatial scales (e.g.…”

Spatial synchrony is related to environmental change in Finnish moth communities

“…One way to assess the importance of individual populations to spatial synchrony across the entire spatial network is by combining the geography of synchrony approach [7] with measures from graph theory. Similar approaches have also been developed to examine variation in site-level contributions to spatial synchrony [28,29]. The geography of synchrony approach-specifically with respect to the incorporation of graph theory-builds a spatial network of local populations, which are connected to other populations based on their degree of synchrony (anti-synchrony), which can be measured given time series or across a rolling window.…”