Simulating the spread and management of alien riparian weeds: are they out of control?

1. This paper examines the circumstances under which control programmes may reduce the range of two widespread invasive weeds of riparian habitats: Impatiens glandulifera (Himalayan balsam) and Heracleum mantegazzianum (giant hogweed). 2. The spread of both species was modelled using MIGRATE, a spatially explicit model that incorporates realistic demographic parameters and multiple dispersal mechanisms. Simulations of a range of control scenarios were run within a geographical information system (GIS) using authentic landscapes based on topographic, hydrological and land cover maps of County Durham, UK. Results were interpreted at both a catchment and a regional scale. 3. Six representative strategies were explored that prioritized control as follows: at random, in relation to human population density, or by the size, age (new and old) or spatial distribution of weed populations. These strategies were assessed at different intensities of management (area treated per year) and for varying efficiencies (proportion of plants destroyed) as well as the timeliness (how long since the species became established) of implementations. 4. Strategies that prioritized control based on weed population and spatial characteristics were most effective, with plant population size and spatial distribution being the key parameters. The reduction in geographical range within a catchment or region following control was always greater for H. mantegazzianum than I. glandulifera due to its slower rate of spread. 5. Successful control of both species at a regional scale is only possible for strategies based on species distribution data, undertaken at relatively high intensities and efficiencies. The importance of understanding the spatial structure of the population and potential habitat available, as well as being able to monitor the progress of the eradication programme, is highlighted. Tentative conclusions an offered as to the feasibility of eradicating these species at a regional scale.