Dispersal and demography contributions to population spread of Carduus nutans in its native and invaded ranges

Invasive species usually exhibit different spatial population dynamics in their native and invaded range. This is often attributed to demographic differences, but may be due to differences in dispersal as well. Here we investigate what demographic and dispersal variables of the invasive thistle Carduus nutans drive differences in spread rates in different invaded ranges compared to spatial population dynamics in the native range (Eurasia). We construct and analyze spatial population models that combine structured, local population models with mechanistic models of seed dispersal by wind. Published and new demographic and dispersal rates from invaded and the native ranges are used to parameterize these spatial models. We analyze the contributions of the shifts in the vital rates to variation in the invasion wave speed estimates for the different invaded ranges compared to that for the native range. This analysis assesses the relative contributions of changes in demographic vital rates such as survival, growth and seed production, and changes in particular plant and seed variables that influence seed dispersal distances, to spread potential of C. nutans. For instance, the wind-dispersal capacity of seeds appears to be greater in the invaded ranges. Thus, by incorporating a mechanistic dispersal model with a structured population model, and by linking this joint model to field data from several continents, we improve our understanding of how dispersal contributes to invasion success.