Predicting the spatial distribution of non-indigenous riparian weeds: issues of spatial scale and extent

1. The existence of a hierarchical scheme of environmental controls on the spatial distribution of plant species was explored fbr three non-indigenous weeds, Fallopia japonica, Heracleum mantegazzianum and Impatiens glandulif era, in the British Isles. 2. Logistic regression analyses of the presence/absence of the three weed s pecies examined the relative importance of 60 environmental variables, enco mpassing land cover, geology and climate. Analyses were undertaken using va riables assessed at a hectad (10 x 10 km) or tetrad (2 x 2 km) resolution a t national (England and Wales) and regional (County Durham, UK) spatial ext ents. 3. The ranges of all three species in the British Isles are currently incre asing, and the non-equilibrium nature of their distribution limited the goo dness-of-fit of logistic models. Interpretation of whether a species has ex panded to occupy entirely its potential spatial range was scale-dependent, and species' distributions, when viewed at coarser spatial scales, may be m ore likely to be interpreted as having reached stasis. 4. Spatial autocorrelation was more evident at the finer tetrad spatial res olution for both F. japonica and I. glandulifera, but not evident at all fo r H. mantegazzianum. Only the distribution of I. glandulifera revealed sign ificant spatial autocorrelation among hectads at the national scale. These patterns appear related to the different dispersal mechanisms of the three species. 5. The majority of the environmental variables identified as important at t he tetrad resolution for County Durham were also important at the hectad re solution for England and Wales for both F. japonica and I. glandulifera, bu t not for H. mantegazzianum. However, for all three species the environment al variables identified as significant were consistent with qualitative des criptions of the species' habitat characteristics. There was no evidence of a hierarchy of environmental controls. 6. At the regional extent, scaling-up species' distributions from tetrads t o hectads was relatively successful, but scaling-down was not. The coarser resolution models were too unrefined to model fine-scale distributions succ essfully. Similarly, at a coarse hectad resolution, regional models were po or predictors of national species' distributions. It therefore appears that scaling-up from fine to coarse resolution is appropriate when spatial exte nt is held constant, and focusing-down from large to small spatial extents is appropriate when data resolution is held constant.