The unpredictably eruptive dynamics of spruce budworm populations in eastern Canada

Abstract We examine four historical population data sets, each spanning 28–46 years in length, from three locations in eastern Canada, showing that spruce budworm through the period 1930–1997 has exhibited eruptive (i.e., nonlinear) growth dynamics. According to our analysis of density‐dependent recruitment curves, any of several factors, both deterministic and stochastic, could be sufficient to trigger outbreaks. For this reason, eruption timing is inherently unpredictable. There is little evidence that spruce budworm population growth rates are near‐linear second‐order density‐dependent, as required by Moran's Theorem for cycle synchronization. Rather, growth rates are nonlinear first‐order density‐dependent, which makes population eruptions difficult to synchronize by any mechanism, including spatially autocorrelated weather effects and dispersal.