Is the Giant Hogweed still a threat? An individual-based modelling approach for local invasion dynamics of Heracleum mantegazzianum

The spread of invasive plant species is an increasing concern in many parts of the world. Negative effects on biodiversity, public health concerns, or economic damage are reported from the most vigorous of them. The monocarpic Heracleum mantegazzianum (Giant Hogweed, Apiaceae) is one of these species in Central Europe. The aim of the individual-based model (IBM) presented here was to assess the invasion status of H. mantegazzianum. This research was motivated by a recent study conducted by [Hüls, J., 2005. Populationsbiologische Untersuchung von Heracleum mantegazzianum Somm. & Lev. in Subpopulationen unterschiedlicher Individuendichte. Dissertation. Justus-Liebig-Universität Giessen, Germany (in German)], which predicted declines in a number of German Hogweed populations. This result contradicted many current, as well as past observations. In the presented study, we intend to resolve this controversy.First, we show that the developed IBM is based on the same data set as the matrix model developed by [Hüls, J., 2005. Populationsbiologische Untersuchung von Heracleum mantegazzianum Somm. & Lev. in Subpopulationen unterschiedlicher Individuendichte. Dissertation. Justus-Liebig-Universität Giessen, Germany (in German)]. Yet, our results illustrate that the invasion status of H. mantegazzianum has not changed and that populations are still expanding in space.Second, the reason for this opposite result is analyzed. Results from the IBM were compared with those of the transition matrix models and with a reduced version of the IBM. We identify individual variability as the main cause, which is accounted for in the original IBM but missing in the original matrix model and the reduced IBM.Our studies also show that, although the long-term average of the population growth rate is larger than one and populations generally expand, there are years in which populations decline (actual growth rates R < 1).This highlights a need for longer-term monitoring of Giant Hogweed populations if matrix models are to be used to assess this species’ invasion status. Results of IBMs, to the contrary, are insensitive to parameters estimated from “expansive” or “declining” years.