Assessing the consequences of biological invasions on species with complex life cycles: Impact of the alien crayfish Procambarus clarkii on Odonata

The temporal dimension is a key parameter when analysing the impact of invasive alien species. Studies on early invasion stages allow a better understanding of how ongoing processes modify native communities, helping to plan effective management actions. Procambarus clarkii is an invasive crayfish influencing multiple features of invaded wetlands, but unravelling its impact on organisms with complex life cycles is difficult. We monitored 107 wetlands in Northern Italy, and evaluated the relationships between P. clarkii and the richness of three life history stages of odonates: adults, larvae and exuviae. We measured environmental features of each wetland and the natural vegetation in the surrounding landscape. We used an information-theoretic approach to relate species richness of the three life history stages of odonates to: wetland features, features of the surrounding landscape; crayfish presence. We used a spatially explicit technique (Moran Eigenvector Mapping) allowing the integration of spatial autocorrelation into analyses. Wetland and landscape features explained a significant amount of community richness. Wetland hydroperiod, canopy cover and stream velocity were the variables most strongly related to odonate richness. Furthermore, we observed significant relationships between P. clarkii and the richness of odonate communities, but the effect of the crayfish on the three odonate stages was different. Species richness measured using both larvae and exuviae was negatively related to the crayfish presence, while negative effects on adults were not evident. Furthermore, negative relationships were observed for Anisoptera (dragonflies) but not for Zygoptera (damselflies). A significant effect of eigenvectors representing spatial configuration suggests an important role of dispersal-related mechanisms in maintaining species richness in invaded wetlands, where fitness is likely lower. Larvae and exuviae may be more helpful for the assessment of the impact of invasive species at early stages of the invasions, while adults may better describe the long term consequences of the invasion at the landscape scale. Considering multiple life-history stages improves our understanding of the impact of biological invasions in freshwaters.