How to invade a Mediterranean forest ecosystem ? A lesson from seed insects in French Cedrus atlantica (Pinaceae) forests

Abstract

Worldwide exchange of tree materials is a major factor favouring insects’ invasions in forest ecosystems through considerable extensions of their distribution ranges. Here we describe the biological and ecological mechanisms involved in the recent invasion of southeastern French cedar (Cedrus atlantica) stands by the invasive seed predator Megastigmus schimitscheki (Hymenoptera: Torymidae). Despite the presence of resident direct competitor (M. pinsapinis), French M. schimitscheki populations display a continuous increase in abundance due to multiple adequations of its life cycle with its new environment. Specifically, adults emergence is well synchronized with the timing of cedar’s fructification and interannual variations in resource abundance may be counterbalanced by prolonged diapause, temporally dispersing individuals of a cohort. Adult emergence and realized fecundity in M. schimitscheki were found significantly earlier and higher (respectively) than in M. pinsapinis, suggesting enhanced abilities of M. schimitscheki to exploit local resources compared to M. pinsapinis. Consequently, we observe recurrent local exclusion by the invasive species. Population genetics of M. schimitscheki also suggest high long distance dispersal abilities. Our analysis of key components of both local and regional dynamics of this species shed a critical light on the processes involved in a successful insect invasion in a French Mediterranean ecosyste