Research Professor University of Virginia, United States
Abstract: There is widespread evidence that population cycling (i.e., where abundances of a population oscillate at regular intervals) can be altered by shifts in climate. Similarly, climatic fluctuations could also produce cyclic patterns in the movement of a species’ invasion front; however, empirical evidence of periodicity in invasive spread (i.e., fluctuations at regular intervals) is scarce. One exception is periodicity in invasive spread rates of the spongy moth in North America, with peaks in invasion occurring every 4-6 years. It is unknown whether multi-annual climatic fluctuations contribute to this observed periodicity. In this study we investigated whether multi-annual cycles in mean temperature and precipitation during the springtime larval period, and minimum temperature and snow depth during the winter, are causal factors for observed cycles of spongy moth spread in four ecoregions within the U.S. We used robust spatiotemporal data on spongy moth abundances and the abiotic variables for the period of 1989-2020. From the spongy moth abundance data, we estimated annual spread rates within each ecoregion by determining the difference in distance between 10-moth population abundance isoclines for pairs of successive years. The effects of periodicity in the abiotic variables on periodicity in rates of invasive spread were assessed using wavelet multiple regression.
We found that climatic extremes associated with multi-annual oscillations of climate were related to cyclical patterns of spread in the spongy moth. However, effects of periodicity in climate on periodicity in spread rates, were only present in the more northerly ecoregions (located just west of the Great Lakes). Additionally, an effect of multi-annual fluctuations in climate on periodicity in spread was always explained by only one of the four abiotic variables, but the abiotic variable was not the same across ecoregions. For example, at long timescales (4-8 years) in the most midwestern ecoregion, the Central USA Plains, precipitation cycles during the larval period explained nearly 90% of periodicity in rates of spread. At the same long timescale, but in the northernmost ecoregion, periodicity in minimum temperature during the wintertime explained over 90% of the synchrony in spongy moth spread. The findings of this study demonstrate that multi-annual climatic cycles are key drivers of temporal variation in rates of spongy moth invasive spread. Expanding this work to investigate how sources of these cycles (e.g., teleconnection patterns) affect other invasive species, represents an important new avenue of research.
