Increased outbreak frequency associated with changes in the dynamic behaviour of populations of two aphid species

Sudden changes in the variability of natural populations can result in increased likelihood of extinction or in greater
frequency and intensity of pest outbreaks. These changes could be associated with changes in some relevant population
parameters such as the equilibrium density or the maximum population growth rate. However, changes in these parameters
have very different consequences. An increase in equilibrium density results in a higher variance in population fluctuations according to the relationship between mean and variance described by Taylor’s power law, but does not modify the
stability properties of the system. On the other hand, changes in the maximum growth rate induce changes in the dynamic
regimes and stability properties of the population. In this study, using statistical and mathematical methods borrowed from
econometrics and engineering, we identify structural changes to the variance in the population dynamics of the sycamore
aphid Drepanosiphum platanoidis and the green spruce aphid Elatobium abietinum in the UK. Some localities showed
strong changes in their population parameters, such that their dynamic regime changed completely. These changes in the
population dynamic regimes increase the expected frequency of outbreaks, which has enormous economic and ecological
consequences. Through this study we show the application of methods that could be helpful to pest and wildlife managers in the task of evaluating changes in the risk of outbreaks or extinction of animal populations under changing global
environmental scenarios.