Factors affecting the distribution of three non-indigenous riparian weeds in north-east England

Abstract

The work presented uses a multi-disciphnary approach to examine the factors important in determining the distribution of the non-indigenous species, Impatiens glandulifera, Heracleum mantegazzianum and Fallopia japonica at a river catchment level. Distribution data for all three species along the Wear catchment, Co. Durham, were initially collected and the distribution of the species, in terms of density and abundance in different zones and habitats of the riparian system, were investigated. This work concluded that zones of the riverbank were used to differing extents by the three species. For all three species the lower riparian zone was the most important for the occurrence of populations. Data extracted from the Environment Agency's River Corridor Survey were used to provide information on characteristics of two river catchments. Examination of these data in association with the alien species distribution data highlighted differences in distribution patterns related to factors such as woodland, ruderal vegetation and bank management. Modelling species occurrences using the RCS data produced good predictive models for the two seed producing species {Impatiens and Heracleum) within a catchment but only poor models for Fallopia, with its solely vegetative method of spread. However testing such models on alternative catchments resulted in a reduction in predictive ability; the best overall models being derived from data amalgamated from both catchments. Variables selected in the models were found to concord with habitat preferences given elsewhere and also highlighted the importance of climate. Increasing the resolution of the collected data from 500m to 50m sections was found not to greatly improve the ability to predict species presence, though these data did allow predictions of Impatiens abundance to be made. Demographic analyses in different habitat types emphasised the importance of herb/ruderal vegetation, though all three species were found to persist in woodland areas despite reduced productivity. Other experiments examining the effects of climate, as represented by altitude, on the performance of the study species indicated that factors such as seed production and plant biomass varied with altitude, whereas germination did not. The thesis highlights potential shortfalls in producing predictive models for non-indigenous species based on non-equilibrium distributions and demonstrates interesting scale- dependent phenomena. It is suggested that whilst Impatiens may be largely climatically limited, Heracleum and Fallopia are more likely to be dispersal limited. [brace not closed]