Abstract
Cyclic or fluctuating populations experience regular periods of low population density. Genetic bottlenecks during these periods could give rise to temporal or spatial genetic differentiation of populations. High levels of movement among increasing populations, however, could ameliorate any differences and could also synchronize the dynamics of geographically separated populations. We use microsatellite markers to investigate the genetic differentiation of four island and one mainland population of western tent caterpillars, Malacosoma californicum pluviale, in two periods of peak or pre-peak density separated by 8 years. Populations showed high levels of genetic variation and little genetic differentiation either temporally between peaks or spatially among sites. Mitochondrial haplotypes were also shared between one island population and one mainland population in the two years studied. An isolation-by-distance analysis showed the FST values of the two geographically closest populations to have the highest level of differentiation in both years. We conclude that high levels of dispersal among populations maintain both synchrony of population dynamics and override potential genetic differentiation that might occur during population troughs. As far we are aware, this is the first time that genetic similarity between temporally separated population outbreaks in insects has been investigated. A review of genetic data for both vertebrate and invertebrate species of cyclic animals shows that a lack of spatial genetic differentiation is typical, and may result from high levels of dispersal associated with fluctuating dynamics.
Highlights
The geographic structure and genetic differentiation of populations are determined by demographic processes and dispersal [1]
Three mechanisms have been proposed as potential causes of the synchronous dynamics among geographically separated populations of cyclic species: (1) sufficient movement of individuals synchronizes populations, (2) widespread weather patterns influence the populations, and/or (3) widespread movement of natural enemies synchronize population declines [4]
Field Collections Late instar tent caterpillar larvae and pupae were collected during the spring and summer of 2003 and 2011 when population sizes were increasing or had reached peak density (Saturna Island), on four islands in southern British Columbia (BC) Canada including: Galiano Island (48u 559 190, 2123u 259 130), Mandarte Island (48u 389 010, 2123u 179 140), Pender Island (48u 459 070, 2 123u 139 110) and Saturna Island (48u 469 320, 2123u 079 550)
Summary
The geographic structure and genetic differentiation of populations are determined by demographic processes and dispersal [1]. Of particular interest in this regard are populations that regularly fluctuate in density: cyclic species. These include a number of species of voles and lemmings, snowshoe hares, grouse and some forest Lepidoptera. For these species, population densities can fluctuate over orders of magnitude with peak densities occurring every 3 (small mammals) to 11 (snowshoe hares and forest Lepidoptera) years. Mechanisms 2 and 3 would not be as likely to influence genetic structure of populations
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